Lightning Protection

5. External lightning protection 5.1 Air-termination systems The function of the air-termination sys-tems of a lightning protection system isto prevent direct lightning strokes fromdamaging the volume to be protected.They must be designed to prevent uncon-trolled lightning strokes to the structureto be protected.By correct dimensioning of the air-termi-nation systems, the effects of a lightningstroke to a structure can be reduced in acontrolled way.

Air-termination systems can consist ofthe following components and be com-bined with each other as required:

⇒ Rods

⇒ Spanned wires and cables

⇒ Intermeshed conductors

When determining the siting of the air-termination systems of the lightning pro-tection system, special attention must bepaid to the protection of corners andedges of the structure to be protected.This applies particularly to air-termina-tion systems on the surfaces of roofs andthe upper parts of façades. Most impor-tantly, air-termination systems must bemounted at corners and edges.

Three methods can be used to determinethe arrangement and the siting of theair-termination systems:

⇒ Rolling sphere method

⇒ Mesh method

⇒ Protective angle method

The rolling sphere method is the univer-sal method of design particularly recom-

mended for geometrically complicatedapplications.

The three different methods aredescribed below.

5.1.1 Installation methods and typesof air-termination systems

The rolling sphere method – “geometric-electrical model“

For lightning flashes to earth, a down-ward leader grows step-by-step in aseries of jerks from the cloud towards theearth. When the leader has got close tothe earth within a few tens, to a fewhundreds of metres, the electrical insu-lating strength of the air near theground is exceeded. A further “leader“discharge similar to the downwardleader begins to grow towards the headof the downward leader: the upwardleader. This defines the point of strike ofthe lightning stroke (Fig. 5.1.1.1).The starting point of the upward leaderand hence the subsequent point of strikeis determined mainly by the head of thedownward leader. The head of thedownward leader can only approach theearth to within a certain distance. Thisdistance is defined by the continuouslyincreasing electrical field strength of theground as the head of the downwardleader approaches. The smallest distancebetween the head of the downwardleader and the starting point of theupward leader is called the final strikingdistance hB (corresponds to the radius ofthe rolling sphere).Immediately after the electrical insulat-ing strength is exceeded at one point,the upward leader which leads to thefinal strike and manages to cross the finalstriking distance, is formed. Observationsof the protective effect of overhead

earth wires and high voltage towerswere used as the basis for the so-called“geometric-electrical model“.This is based on the hypothesis that thehead of the downward leader approach-es the objects on the ground in an arbi-trary way, unaffected by anything, untilit reaches the final striking distance.The point of strike is then determined bythe object closest to the head of thedownward leader. The upward leaderstarting from this point “forces its waythrough“ (Fig. 5.1.1.2).

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If there are external areas of the structure situatedin heights which are higher than the radius of thecorresponding rolling sphere (Tab. 5.1.1.3), an air-termination system has to be installed applying e.g.the mesh method.

air-termination system

r

heig

ht a

cc. t

oty

pe o

f LPS

α

protective angleacc. to type of LPS

mesh size androlling sphereradius corresp. tothe type of LPS

Fig. 5.1.1 Air-termination system for high buildings (h ≥ 60 m) – Mesh method

final striking

distance hB

point afar fromthe head of thedownward leader

startingupward leader

downward leader

head of the downward leader

startingupward leader

closest point tothe head of thedownward leader

rolling sphere

Fig. 5.1.1.1 Starting upward leader defining the point of strike

Fig. 5.1.1.2 As this model examination shows, a rol-ling sphere can touch not only the stee-ple, but also the nave of the church atseveral points. All points touched arepotential points of strikeRef.: Prof. Dr. A. Kern, Aachen

Classification of the type of lightningprotection system and radius of therolling sphereAs a first approximation, a proportionali-ty exists between the peak value of thelightning current and the electricalcharge stored in the downward leader.Furthermore, the electrical field strengthof the ground as the downward leaderapproaches is also linearly dependent onthe charge stored in the downwardleader, to a first approximation.There is therefore a proportionalitybetween the peak value I of the light-ning current and the final striking dis-tance R/radius of the rolling sphere:

R in m

I in kA

The protection of structures againstlightning is described in DIN V VDE V0185-1. Among other things, this stan-dard defines the classification into indi-vidual types of lightning protection sys-tem and stipulates the resulting light-ning protection measures.It differentiates between four types oflightning protection system. A Type Ilightning protection system provides themost protection and a Type IV, by com-parison, the least. The interception effec-tiveness Ei of the air-termination systemsis concomitant with the type of lightningprotection system, i. e. which percentageof the prospective lightning strokes issafely controlled by the air-terminationsystems. From this results the final strik-ing distance and hence the radius of the“rolling sphere“. The correlationsbetween type of lightning protection sys-tem, interception effectiveness Ei of theair-termination systems, final striking dis-tance/radius of the “rolling sphere“ andcurrent peak value are shown inTable 5.1.1.1.

Taking as a basis the hypothesis of the“geometric-electrical model“ that thehead of the downward leader approach-

es the objects on the earth in an arbitraryway, unaffected by anything, until itreaches the final striking distance, a gen-eral method can be derived which allowsthe volume to be protected of anyarrangement to be inspected. Carryingout the rolling sphere method requires ascale model (e. g. on a scale of 1:100) ofthe building/structure to be protected,which includes the external contoursand, where applicable, the air-termina-tion systems. Depending on the locationof the object under investigation, it isalso necessary to include the surroundingstructures and objects, since these couldact as a “natural protective measure “ forthe object under examination.Furthermore, a true-to-scale sphere isrequired according to the type of light-ning protection system with a radiuscorresponding to the final striking dis-tance (depending on the type of light-ning protection system, the radius R ofthe “rolling sphere“ must correspondtrue-to-scale to the radii 20, 30, 45 or60 m). The centre of the “rolling sphere“used corresponds to the head of thedownward leader formed by the respec-tive upward leader.

The “rolling sphere“ is now rolledaround the object under examinationand the contact points representingpotential points of strike are marked ineach case. The “rolling sphere“ is thenrolled over the object in all directions. Allcontact points are marked again. Allpotential points of strike are thus shownon the model; it is also possible to deter-mine the areas which can be hit by later-al strokes. The naturally protected zonesresulting from the geometry of theobject to be protected and its surround-ings can also be clearly seen. Air-termina-tion conductors are not required at thesepoints (Fig. 5.1.1.3).

It must be borne in mind, however, thatlightning footprints have also beenfound on steeples in places not directlytouched as the “rolling sphere“ rolledover. This is traced to the fact that,

among other things, in the event of mul-tiple lightning flashes, the base of thelightning flash moves because of thewind conditions. Consequently, an areaof approx. one metre can come uparound the points of strike determinedwhere lightning strokes can also occur.

Example 1: New administration buildingin MunichDuring the design phase of the newadministration building, the complexgeometry led to the decision to use therolling sphere method to identify theareas threatened by lightning strokes.This was possible because an architectur-al model of the new building was avail-able on a scale of 1:100.It was determined that a Iightning pro-tection system Type I was required, i. e.the radius of the rolling sphere in themodel was 20 cm (Fig. 5.1.1.4).

The points where the “rolling sphere“touches parts of the building, can be hitby a direct lightning stroke with a corres-ponding minimum current peak value of2.9 kA (Fig. 5.1.1.5). Consequently, thesepoints required adequate air-termin-ation systems. If, in addition, electricalinstallations were localised at thesepoints or in their immediate vicinity (e. g.on the roof of the building), these loca-

R I= 10 0 65 i ,

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Lightning Interception Radius of the rolling Min. peakprotection level criterion sphere (final striking value of current

distance hB)Ei R in m I in kA

IV 0.84 60 15.7

III 0.91 45 10.1

II 0.97 30 5.4

I 0.99 20 2.9

Table 5.1.1.1 Relations between ligtning protection level, interception criterion Ei, final striking distance Rand min. peak value of current IRef.: Table 5 and 6 of DIN V VDE V 0185-1

R

R

R

R

RR

building

rolling sphere

Fig. 5.1.1.3 Schematic application of the “rollingsphere” method at a building with con-siderably structured surface

Fig. 5.1.1.4 Construction of a new administrationbuilding: Model with “rolling sphere”acc. to lightning protection system Type IRef.: WBG Wiesinger

tions were equipped with additional airtermination measures.The application of the rolling spheremethod meant that air-termination sys-tems were not installed where protectionwas not required. On the other hand,locations in need of more protectioncould be equipped accordingly, wherenecessary. (Fig. 5.1.1.5).

Example 2: Aachen CathedralThe cathedral stands in the midst of theold town of Aachen surrounded by sev-eral high buildings.Adjacent to the cathedral there is a scalemodel (1:100) whose purpose is to makeit easier for visitors to understand thegeometry of the building.The buildings surrounding the AachenCathedral provide a partial natural pro-tection against lightning strokes.Therefore, and to demonstrate the effec-tiveness of lightning protection meas-ures, models of the most important ele-ments of the surrounding buildings weremade according to the same scale (1:100)(Fig. 5.1.1.6).Fig. 5.1.1.6 also shows “rolling spheres“for lightning protection systems Types IIand III (i. e. with radii of 30 cm and 45 cm)on the model.The aim here was to demonstrate theincreasing requirements on the air-termi-nation systems as the radius of the rolling

sphere decreases, i. e. which areas ofAachen Cathedral had additionally to beconsidered at risk of being hit by light-ning strokes, if a lightning protection sys-tem Type II with a higher degree of pro-tection was used. The “rolling sphere“ with the smallerradius (according to a type of lightningprotection system with a higher light-ning protection level) naturally touchesalso the model at all points alreadytouched by the “rolling sphere“ with thelarger radius. Thus, it is only necessary todetermine the additional contact points.As demonstrated, when dimensioningthe air-termination system for a struc-ture, or a structure mounted on the roof,the sag of the rolling sphere is decisive.

The following formula can be used to cal-culate the penetration depth p of therolling sphere when the rolling sphererolls “on rails“, for example . This can beachieved by using two spanned wires, forexample .

R Radius of the rolling sphere

d Distance between two air-termina-tion rods or two parallel air-termina-tion conductors

Fig. 5.1.1.7 illustrates this consideration.Air-termination rods are frequently usedto protect the surface of a roof, or instal-lations mounted on the roof, against adirect lightning stroke. The squarearrangement of the air-termination rods,over which no cable is normally spanned,means that the sphere does not “roll onrails“ but “sits deeper“ instead, thus

increasing the penetration depth of thesphere (Fig. 5.1.1.8).The height of the air-termination rods ∆hshould always be greater than the valueof the penetration depth p determined,and hence greater than the sag of therolling sphere. This additional height ofthe air-termination rod ensures that therolling sphere does not touch the struc-ture to be protected.

Another way of determining the heightof the air-termination rods is using Table5.1.1.2. The penetration depth of therolling sphere is governed by the largestdistance of the air-termination rods fromeach other. Using the greatest distance,the penetration depth p (sag) can be taken from the table. The air-termina-tion rods must be dimensioned accordingto the height of the structures mountedon the roof (in relation to the location of

p R Rd

= − − ⎛⎝⎜

⎞⎠⎟

22

2


5555Fig. 5.1.1.5 Construction of a DAS administration

building: Top view (excerpt) on thezones threatened by lightning strokesfor lightning protection system type IRef.: WBG Wiesinger

Fig. 5.1.1.6 Aachen Cathedral: Model with environ-ment and “rolling spheres” for lightningprotection systems type II and IIIRef.: Prof. Dr. A. Kern, Aachen

∆h

d

R

air-terminationconductor

pene

trat

ion

dept

h p

Fig. 5.1.1.7 Penetration depth p of the rolling sphere

d

∆h

R

p

cuboidal protective area bet-ween four air-termination rods

Type of LPSI II III IV

R 20 30 45 60

Fig. 5.1.1.8 Air-termination system for installationsmounted on the roof with their protec-tive area

the air-termination rod) and also thepenetration depth (Fig. 5.1.1.9).If, for example, a total height of an air-termination rod of 1.15 m is either calcu-lated or obtained from the table, an air-termination rod with a standard lengthof 1.5 m is normally used

Mesh method

A “meshed“ air-termination system canbe used universally regardless of theheight of the structure and shape of theroof. A reticulated air-termination net-work with a mesh size according to the

type of lightning protection system isarranged on the roofing (Table 5.1.1.3).To simplify matters, the sag of the rollingsphere is assumed to be zero for ameshed air-termination system.

By using the ridge and the outer edges ofthe structure, as well as the metal naturalparts of the structure serving as an air-termination system, the individual cellscan be sited as desired.The air-termination conductors on theouter edges of the structure must be laidas close to the edges as possible.

A metal attic can serve as an air-termina-tion conductor and/or a down-conductorsystem if the required minimum dimen-sions for natural components of the air-termination system are complied with(Fig. 5.1.1.10).

Protective angle methodThe protective angle method is derivedfrom the electric-geometrical lightningmodel. The protective angle is deter-mined by the radius of the rolling sphere.The comparable protective angle withthe radius of the rolling sphere is givenwhen a slope intersects the rolling spherein such a way that the resulting areashave the same size (Fig. 5.1.1.11).This method must be used for structureswith symmetrical dimensions (e. g. steeproof) or roof-mounted structures (e. g.antennas, ventilation pipes).The protective angle depends on thetype of lightning protection system andthe height of the air-termination system


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Fig. 5.1.1.9 Calculation ∆h for several air-termination rods according to rolling sphere method

d diagonal

∆ h

domelight installedon the roof

d Sag of the rolling sphere [m] (rounded up)

Distance Type of LPS with rolling sphere radius in metresbetween air-

termination rods I (20 m) II (30 m) III (45 m) IV (60 m)

2 0.03 0.02 0.01 0.01

4 0.10 0.07 0.04 0.03

6 0.23 0.15 0.10 0.08

8 0.40 0.27 0.18 0.13

10 0.64 0.42 0.28 0.21

12 0.92 0.61 0.40 0.30

14 1.27 0.83 0.55 0.41

16 1.67 1.09 0.72 0.54

18 2.14 1.38 0.91 0.68

20 2.68 1.72 1.13 0.84

23 3.64 2.29 1.49 1.11

26 4.80 2.96 1.92 1.43

29 6.23 3.74 2.40 1.78

32 8.00 4.62 2.94 2.17

35 10.32 5.63 3.54 2.61

Table 5.1.1.2 Sag of the rolling sphere over two air-termination rods or two parallel air-termination conductors

e.g. gutter

Fig. 5.1.1.10 Meshed air-termination system

Type of LPS Mesh size

I 5 x 5 m

II 10 x 10 m

III 15 x 15 m

IV 20 x 20 m

Table 5.1.1.3 Mesh size

above the reference plane (Fig. 5.1.1.12).

Air-termination conductors, air-termina-tion rods, masts and wires should bearranged to ensure that all parts of thebuilding to be protected are situatedwithin the volume of protection of theair-termination system.The protection zone can be “cone-shaped“ or “tent-shaped“, if a cable, forexample, is spanned over it (Figs. 5.1.1.13to 5.1.1.15).If air-termination rods are installed onthe surface of the roof to protect struc-tures mounted thereon, the protective

angle α can be different. In Fig. 5.1.1.16,the roof surface is the reference planefor protective angle α1. The ground is thereference plane for the protective angleα2. Therefore the angle α2 according toFig. 5.1.1.12 and Table 5.1.1.4 is less thanα1.

Table 5.1.1.4 provides the correspondingprotective angle for each type of light-

ning protection system and the corres-ponding distance (zone of protection).

Protective angle method for isolatedair-termination systems on roof-mounted structures

Special problems may occur when roof-mounted structures, which are ofteninstalled at a later date, protrude fromzones of protection, e. g. the mesh. If, inaddition, these roof-mounted structurescontain electrical or electronic equip-ment, such as roof-mounted fans, anten-nas, measuring systems or TV cameras,additional protective measures arerequired.

If such equipment is connected directlyto the external lightning protection sys-tem, then, in the event of a lightningstroke, partial currents are conductedinto the structure. This could result in thedestruction of surge sensitive equipment.Direct lightning strokes to such structuresprotruding above the roof can be pre-vented by having isolated air-termina-tion systems.Air-termination rods as shown in Fig.5.1.1.17 are suitable for protecting smaller roof-mounted structures (withelectrical equipment). They form a “cone-shaped“ zone of pro-tection and thus prevent a direct light-ning stroke to the structure mounted onthe roof.

The separation distance s must be takeninto account when dimensioning theheight of the air-termination rod (seeChapter 5.6).


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rolling sphere

equal surface areas

air-termi-nation rod

baseprotective angle

R

Fig. 5.1.1.11 Protective angle and comparable radi-us of the rolling sphere

h (m)

αϒ

I II III

Protective angle method

80

70

60

50

40

30

20

10

00 2 10 20 30 40 50 60

IV

Fig. 5.1.1.12 Protective angle α as a function of height h depending on the type of lightning protection system

Fig. 5.1.1.13 Cone-shaped protection zone

angle α angle α

angle α angle α

Fig. 5.1.1.14 Expamle of air-termination systemswith protective angle α

α°

h t


Angle α depends on the type of lightning protectionsystem and the height of the air-termnation con-ductor above ground

Fig. 5.1.1.15 Area protected by an air-terminationconductor

h 1

α1

α2

h 2

hh 1

Note:Protective angle α1 refers to theheight of the air-termination systemh1 above the roof surface to be pro-tected (reference plane);Protective α2 refers to the heighth2 = h1 + h,while the earth surfaceis the reference plane.

h1: Physical height of the air-termination rod

Fig. 5.1.1.16 External lightning protection system,volume protected by a vertical air-ter-mination rod


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Height of the air- Type of LPS I Type of LPS II Type of LPS III Type of LPS IVtermination rod Angle Distance Angle Distance Angle Distance Angle Distance

h in m α a in m α a in m α a in m α a in m

1 67 2.36 71 2.90 74 3.49 78 4.702 67 4.71 71 5.81 74 6.97 78 9.413 67 7.07 71 8.71 74 10.46 78 14.114 65 6.43 69 10.42 72 12.31 76 16.045 59 6.66 65 10.72 70 13.74 73 16.356 57 7.70 62 11.28 68 14.85 71 17.437 54 8.26 60 12.12 66 15.72 69 18.248 52 8.96 58 12.80 64 16.40 68 19.809 49 9.20 56 13.34 62 16.93 66 20.21

10 47 9.65 54 13.76 61 18.04 65 21.4511 45 10.00 52 14.08 59 18.31 64 22.5512 42 9.90 50 14.30 58 19.20 62 22.5713 40 10.07 49 14.95 57 20.02 61 23.4514 37 9.80 47 15.01 55 19.99 60 24.2515 35 9.80 45 15.00 54 20.65 59 24.9616 33 9.74 44 15.45 53 21.23 58 25.6117 30 9.24 42 15.31 52 21.76 57 26.1818 28 9.04 40 15.10 50 21.45 56 26.6919 25 8.39 39 15.39 49 21.86 55 27.1320 23 8.07 37 15.07 48 22.21 54 27.5321 36 15.26 47 22.52 53 27.8722 35 15.40 46 22.78 52 28.1623 33 14.94 45 23.00 51 28.4024 32 15.00 44 23.18 50 28.6025 30 14.43 43 23.31 49 28.7626 29 14.41 42 23.41 49 29.9127 27 13.76 40 22.66 48 29.9928 26 13.66 39 22.67 47 30.0329 25 13.52 38 22.66 46 30.0330 23 12.73 37 22.61 45 30.0031 36 22.52 44 29.9432 35 22.41 44 30.9033 35 23.11 43 30.7734 34 22.93 42 30.6135 33 22.73 41 30.4336 32 22.50 40 30.2137 31 22.23 40 31.0538 30 21.94 39 30.7739 29 21.62 38 30.4740 28 21.27 37 30.1441 27 20.89 37 30.9042 26 20.48 36 30.5143 25 20.05 35 30.1144 24 19.59 35 30.8145 angle α 23 19.10 34 30.3546 33 29.8747 32 29.3748 32 29.9949 31 29.4450 30 28.8751 30 29.4452 29 28.8253 height h 28 28.1854 of the 27 27.5155 air-termination rod 27 28.0256 26 27.3157 25 26.5858 25 27.0559 distance a 24 26.2760 23 25.47

Table 5.1.1.4 Protective angle α depending on the types of lightning protection system

α

Isolated and non-isolated air-termina-tion systems

When designing the external lightningprotection system of a structure, we dis-tinguish between two types of air-termi-nation system:

⇒ isolated

⇒ non-isolated

The two types can be combined.

Air-termination systems of a non-isol-ated external lightning protection sys-tem for protection of a structure can beinstalled in the following ways:

If the roof is made of non-flammablematerial, the conductors of the air-termi-nation system can be installed on the sur-face of the structure (e. g. gable or flatroof). Normally non-flammable buildingmaterials are used. The components ofthe external lightning protection systemcan therefore be mounted directly on thestructure (Figs. 5.1.1.18 and 5.1.1.19).

If the roof is made of easily inflammablematerial (building material class B 3, seeAnnex E of DIN V VDE V 0185-3) e. g.thatched roofs, then the distance

between the flammable parts of the roofand the air-termination rods, air-termi-nation conductors or air-terminationmeshes of the air-termination systemmust not be less than 0.4 m.

Easily inflammable parts of the structureto be protected must not be in directcontact with parts of the external light-ning protection system. Neither may theybe located under the roofing, which canbe punctured in the event of a lightningstroke (see also Chapter 5.1.5 Thatchedroofs).

With isolated air-termination systems,the complete structure is protectedagainst a direct lightning stroke via air-termination rods, air-termination mastsor masts with cables spanned over them.When installing the air-termination sys-tems, the separation distance s to thestructure must be kept.

Figs. 5.1.1.20 and 5.1.1.21 illustrate onetype of air-termination system which isisolated from the structure

The separation distance s between theair-termination system and the structuremust be kept.

Air-termination systems isolated fromthe structure are frequently used, whenthe roof is covered with inflammablematerial, e. g. thatch or also for ex-instal-lations, e. g. tank installations.

See also Chapter 5.1.5 “Air-terminationsystem for structures with thatchedroofs“.


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Fig. 5.1.1.17 Protection of small-sized installations on roofs against direct lightning strokes by means of air-termination rods

Fig. 5.1.1.18 Gable roof with conductor holder

Fig. 5.1.1.19 Flat roof with conductor holder

s s

α α

2

1 1

3

4

Air-termination mast

Protected structure

Reference plane

Separation distance s corresponding to 5.3,main section 1 of DIN V VDE V 0185-3

α Protective angle acc. to Table 5.1.1.3

12

3

4

Fig. 5.1.1.20 Isolated external lightning protectionsystem with two separate air-termina-tion masts according to the protectiveangle method; Projection on a verticalarea

Fig. 5.1.1.21 Isolated external lightning protectionsystem, consisting of two separate air-termination masts, connected througha horizontal air-termination conductor:Projection on a vertical surface via thetwo masts (vertical section)

s2 s2

s 1

1

2

31

Air-termination mast

Horizontal air-termination conductor

Protected structure

s1, s2 separation distance

acc. to DIN V VDE V 0185-3

12

3

A further method of designing isolatedair-termination systems consists in secur-ing the air-termination systems (air-ter-mination rods, conductors or cables) withelectrically insulating materials such asGRP (glass fibre-reinforced plastic).This form of isolation can be limited tolocal use or applied to whole parts of theinstallation. It is often used for roof-mounted structures such as fan systemsor heat exchangers with an electricallyconductive connection into the structure(see also chapter 5.1.8).

Natural components of air-terminationsystems

Metal structural parts such as attics, gut-tering, railings or cladding can be used asnatural components of an air-termina-tion system.

If a structure has a steel skeleton con-struction with a metal roof and façademade of conductive material, these canbe used for the external lightning protec-tion system, under certain circumstances.

Sheet metal cladding on the walls or roofof the structure to be protected can beused if the electrical connection betweenthe different parts is permanent.These permanent electrical connectionscan be made by e.g. brazing, welding,pressing, screwing or riveting, for exam-ple.If there is no electrical connection, a sup-plementary connection must be madefor these elements e. g. with bridgingbraids or bridging cables.

If the thickness of the sheet metal is notless than the value t' in Table 5.1.1.5, andif there is no requirement to takeaccount of a through-melting of thesheets at the point of strike or the igni-tion of flammable material under thecladding, then such sheets can be used asan air-termination system.

The material thicknesses are not distin-guished according to the type of light-ning protection system.It is, however, necessary to take precau-tionary measures against through-melt-ing or intolerable heating-up at thepoint of strike, if the thickness of the

sheet metal shall not be less than value tin Table 5.1.1.6.

The required thickness t of the materialscan generally not be complied with, forexample, for metal roofs, For pipes or containers, however, it ispossible to meet the requirements forthese minimum thicknesses (wall thick-ness). If, though, the temperature rise(heating-up) on the inside of the pipe ortank represents a hazard for the mediumcontained therein (risk of fire or explo-sion), then these must not be used as air-termination systems (see also chapter5.1.4).

If the requirements on the appropriateminimum thickness are not met, the com-ponents, e. g. conduits or containers,must be situated in an area protectedfrom direct lightning strokes. These natu-ral components can nevertheless still bein a position to conduct lightning cur-rents and can therefore be used as aninterconnecting conductor or down-con-ductor system.

A thin coat of paint, 1 mm bitumen or0.5 mm PVC cannot be regarded as insu-lation in the event of a direct lightningstroke. Such coatings break down whensubjected to the high energies depositedduring a direct lightning stroke.There must be no coatings on the jointsof the natural components of the down-conductor systems.

If conductive parts are located on the sur-face of the roof, they can be used as anatural air-termination system if there isno conductive connection into the struc-ture. By connecting, e.g., pipes or electricalconductors into the structure, partiallightning currents can enter the structureand affect or even destroy sensitive elec-trical/electronic equipment.In order to prevent these partial light-ning currents from penetrating, isolatedair-termination systems shall be installedfor the aforementioned roof-mountedstructures.The isolated air-termination system canbe designed using the rolling sphere or

protective angle method. An air-termina-tion system with a mesh size according tothe type of lightning protection systemused can be installed if the wholearrangement is isolated (elevated) fromthe structure to be protected by at leastthe required separation distance s.

A universal system of components for theinstallation of isolated air-terminationsystems is described in chapter 5.1.8.

5.1.2 Air-termination systems forstructures with gable roofs

Air-termination systems on roofs are themetal components in their entirety, e. g.air-termination conductors, air-termina-tion rods, air-termination tips.The parts of the structure usually hit bylightning strokes, such as the top of thegable, chimneys, ridges and arrises, theedges of gables and eaves, parapets andantennas and other protruding struc-tures mounted on the roof, must beequipped with air-termination systems.Normally, a reticulated air-terminationnetwork is installed on the surface ofgabled roofs, said network corres-ponding to the mesh size of the appro-priate type of lightning protection sys-tem (e. g. 15 m x 15 m for a lightning pro-tection system Type III) (Fig. 5.1.2.1).

By using the ridge and the outer edges ofthe structure, as well as the metal partsof the structure serving as an air-termina-tion system, the individual meshes can besited as prefered. The air-terminationconductors on the outer edges of thestructure must be installed as close to theedges as possible.Generally, the metal gutter is used forclosing the “mesh“ of the air-termin-ation system on the roof surface. If thegutter itself is connected so as to be elec-trically conductive, a gutter clamp is


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Material Thickness t‘

Fe 0.5 mm

Cu 0.5 mm

Al / StSt 0.7 mm

Table 5.1.1.5 Min. thickness of metal plates (ifmelt-through is allowed)

Material Thickness t

Fe 4 mm

Cu 5 mm

Al / StSt 7 mm

Table 5.1.1.6 Min. thickness of metal plates (ifmelt-through is not allowed)

Fig. 5.1.2.1 Air-termination system on a gable roof

mounted at the crossover of the air-ter-mination system and the gutter.

Roof-mounted structures made of elec-trically non-conductive material (e. g.PVC vent pipes) are considered to be suf-ficiently protected if they do not pro-trude more than h = 0.3 m from the planeof the mesh (Fig. 5.1.2.2).

If the protrusion is h > 0.3 m, the struc-ture must be equipped with an air-termi-nation system (e. g. interception tip) andconnected to the nearest air-terminationconductor. One way of doing this wouldbe to use a wire with a diameter of 8 mmup to a maximum free length of 0.5 m, asshown in Fig. 5.1.2.3.

Metal structures mounted on the roofwithout conductive connection into thestructure do not need to be connected tothe air-termination system if all the fol-lowing conditions are met:

⇒ Structures mounted on the roof mayprotrude a maximum distance of0.3 m from the plane of the mesh.

⇒ Structures mounted on the roof mayhave a maximum enclosed area of1 m2, (e. g. dormer windows)

⇒ Structures mounted on the roof mayhave a maximum length of 2 m (e. g.sheet metal roofing parts)

Only if all three conditions are met, noterminal is required.Furthermore, with the conditions statedabove, the separation distance to the air-termination conductors and down-

conductor systems must be maintained(Fig. 5.1.2.4).

Air-termination rods for chimneys mustbe erected to ensure that the wholechimney is in the zone of protection. Theprotective angle method is applied whendimensioning the air-termination rods.If the stack is brick-built or constructedwith preformed sections, the air-termina-tion rod can be mounted directly on thestack. If there is a conductive pipe in the interiorof the stack, e. g. as found when redevel-oping old buildings, the separation dis-tance to this conductive component mustbe kept. This is an example where isol-ated air-termination systems are usedand the air-termination rods are erectedwith distance holders.

The assembly to protect parabolic anten-nas in particular is similar to that to pro-tect stacks with an internal stainless steelpipe.In the event of a direct lightning stroketo antennas, partial lightning currentscan enter the structure to be protectedvia the shields of the coaxial cables andcause the effects and destruction previ-ously described. To prevent this, anten-nas are equipped with isolated air-termi-nation systems (e. g. air-termination rods)(Fig. 5.1.2.5).

Air-termination systems on the ridgehave a tent-shaped zone of protection(according to the protective anglemethod). The angle depends on theheight above the reference plane (e. g.surface of the earth) and the type oflightning protection system chosen.

5.1.3 Air-termination systems forflat-roofed structures

An air-termination system for structureswith flat roofs (Figs. 5.1.3.1 and 5.1.3.2) isdesigned using the mesh method. Amesh-type air-termination system with amesh size corresponding to the type oflightning protection system is installedon the roof (Table 5.1.1.3).

Fig. 5.1.3.3 illustrates the practical appli-cation of the meshed air-termination sys-tem in combination with air-terminationrods to protect the structures mountedon the roof, e. g. domelights, photovolta-ic cells or fans. Chapter 5.1.8 shows howto deal with these roof-mounted struc-tures.

Roof conductor holders on flat roofs arelaid at intervals of approx. 1 m. The air-termination conductors are connectedwith the attic, this being a natural com-ponent of the air-termination system. Asthe temperature changes, so does thelength of the materials used for the attic,and hence the individual segments mustbe equipped with “slide plates“. If theattic is used as an air-termination system,these individual segments must be per-manently interconnected so as to be elec-trically conductive without restrictingtheir ability to expand. This can beachieved by means of bridging braids,straps or cables (Fig. 5.1.3.4).

The changes in length caused by changesin temperature must also be taken intoaccount with air-termination conductorsand down-conductor systems (see Chapter 5.4).

A lightning stroke to the attic can causethe materials used to melt through. Ifthis is unacceptable, a supplementary air-termination system, e. g. with air-termi-nation tips, must be installed, its locationbeing determined by using the rollingsphere method.


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Fig. 5.1.2.2 Height of a roof superstructure made ofelectrically non-conductive material (e.g. PVC), h ≤ 0.3 m

Fig. 5.1.2.3 Additional air-termination system forventilation pipes

Fig. 5.1.2.4 Building with photovoltaic systemRef.: Wettingfeld Lightning Protection,Krefeld, Germany

Fig. 5.1.2.5 Antenna with air-termination rod Ref.: Upper Austrian Lightning Protec-tion, Linz, Austria

Conductor holders for flat roofs, homogeneously welded

In the wind, roof sheetings can moveacross the roof surface horizontally, ifthey are only fixed mechanically/laid onthe surface. A special position fixing isrequired for the air-termination conduct-or for preventing the conductor holdersfor air-termination systems from beingdisplaced on the smooth surface. Con-ventional roof conductor holders cannotbe permanently bonded to roof sheet-ings since the latter do not usually permitthe application of adhesives.A simple and safe way of fixing the posi-tion is to use roof conductor holders TypeKF in combination with straps (cut thestrips to fit) made of the roof sheetingmaterial. The strap is clamped into theplastic holder and both sides are weldedonto the seal. Holder and strap should bepositioned immediately next to a roofsheeting joint at a distance of approx.1 m. The strip of foil is welded to the roofsheeting according to the manufacturerof the roof sheeting. This prevents air-termination conductors on flat roofsfrom being displaced.

If the slope of the roof is greater then 5°,each roof conductor holder must beequipped with a position fixing element.If the synthetic roof sheetings aresecured by mechanical means, the roofconductor holders must be arranged inthe immediate vicinity of the mechanicalfixing elements.

When carrying out this work, it must beconsidered that welding and bondingwork on the seal affect the guaranteeprovided by the roofer.The work to be carried out must there-fore only be done with the agreement ofthe roofer responsible for the particularroof, or be carried out by him himself(Fig. 5.1.3.5).


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Fig. 5.1.3.2 Air-termination system on a flat roof

Fig. 5.1.3.3 Use of air-termination rods Fig. 5.1.3.4 Bridged attic

expansion piece

distance between theroof conductor holdersapprox. 1 m

flexible connection

Fig. 5.1.3.1 Air-termination system

Bridging braidPart No. 377 015

Roof conductor holderType FB2Part No. 253 050

Roof conductor holderType FBPart No. 253 015

5.1.4 Air-termination systems onmetal roofs

Modern industrial and commercial pur-pose-built structures often have metalroofs and façades. The metal sheets andplates on the roofs are usually 0.7 –1.2 mm thick.

Fig. 5.1.4.1 shows an example of the con-struction of a metal roof.

When the roof is hit by a direct lightningstroke, melting through or vaporisationcan cause a hole formed at the point ofstrike. The size of the hole depends onthe energy of the lightning stroke andthe characteristics of the material, (e. g.thickness). The biggest problem here isthe subsequent damage, e. g. waterentering at this point. Days or weeks canpass before this damage is noticed. The

roof insulation becomes damp and/or theceiling becomes wet.Protection against the rain is no longerguaranteed to be provided.

One example of damage, assessed usingthe Lightning-Information Service fromSiemens (BLIDS) illustrates this problem(Fig. 5.1.4.2). A current of approx.20,000 A struck the sheet metal roof andmade a hole (Fig. 5.1.4.2: Detail A). Sincethe sheet metal roof was not earthedwith a down-conductor system,flashovers to natural metal componentsin the wall occurred in the area aroundthe fascia (Fig. 5.1.4.2: Detail B), whichalso caused a hole.

To prevent such kind of damage, a suit-able external lightning protection systemwith wires and clamps capable of carry-ing lightning currents must be installedeven on a “thin“ metal roof. The DIN V

VDE V 0185-3 lightning protection stan-dard clearly illustrates the risk of damageto metal roofs. Where an external light-ning protection system is required, themetal sheets must have the minimum val-ues stated in Table 5.1.4.1.

The thicknesses t are not relevant forroofing materials. Metal sheets with athickness t’ may only be used as a naturalair-termination system if puncturing,overheating and melting is tolerated.The owner of the structure must agree totolerate this type of roof damage, sincethere is no longer any guarantee that theroof will offer protection from the rain.Also the Rules of the German RoofingTrade concerning lightning protection onand attached to roofs require the agree-ment of the owner.

If the owner is not prepared to toleratedamage to the roof in the event of alightning stroke, then a separate air-ter-mination system must be installed on ametal roof. The air-termination systemmust be installed to ensure that therolling sphere (radius R which corre-sponds to the type of lightning protec-tion system chosen) does not touch themetal roof (Fig. 5.1.4.3).

When mounting the air-termination sys-tem it is recommended to install a so-called “hedgehog roof“ with longitudi-nal cables and air-termination tips.

In practice, the heights of air-terminationtips according to Table 5.1.4.2 are triedand tested, regardless of the type oflightning protection system involved.Holes must not be drilled into the metalroof when fixing the conductors and air-termination tips. Various conductor hold-ers are available for the different types ofmetal roofs (round standing seam, stand-ing seam, trapezoidal). Fig. 5.1.4.4ashows one possible design for a metalroof with round standing seam.

~300

~ 300

~90

~70

distance between theroof conductor holdersapprox. 1 m

flexible connection


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Fig. 5.1.3.5 Highly polymeric roof sheetings for flat roofs - Roof conductor holder Type KF / KF2

Roof conductor holderType KFPart No. 253 030

Fig. 5.1.4.1 Types of metal roofs, e.g. roofs withround standing seam

Fig. 5.1.4.2 Example of damage: Metal plate cover

Evaluation: BLIDS – SIEMENSI = 20400 A

Residential building

Detail B

Detail A

When installing the cables, care must betaken that the conductor holder locatedat the highest point of the roof must bedesigned with a fixed conductor leading,whereas all other conductor holdersmust be designed with a loose conductorleading because of the linear compensa-tion caused by changes in temperature(Fig. 5.1.4.4b).

The conductor holder with fixed con-ductor leading is illustrated in Fig. 5.1.4.5using the example of a trapezoidal sheetroof.Fig. 5.1.4.5 also shows an air-terminationtip next to the conductor holder. Theconductor holder must be hooked intothe fixing screw above the covering platefor the drill hole to prevent any enteringof water.

Fig. 5.1.4.6 uses the example of a roundstanding seam roof to illustrate the looseconductor leading.Fig. 5.1.4.6 also shows the connection tothe roof with round standing seam at the


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Table 5.1.4.1 Natural components of an air-termination system

Metal sheetings can be used as “natural” components of the air-terminationsystem, if the thickness t / t’ of the metal plate is not less than:

Materials If the melting through or the ignition of adjacentmaterial below the sheeting is

impermissible: permissible:Thickness t Thickness t’

Galvanised steel 4 mm 0.5 mm

Copper 5 mm 0.5 mm

Aluminium / StSt 7 mm 0.7 mm

Fig. 5.1.4.3 Air-termination system on a metal roof - Protection against holing

rolling sphere with aradius acc. to type of LPS

air-termination tip

Table 5.1.4.2 Lightning protection for metal roofs -Height of the air-termination tips

Suitable for all types of lightning protection systems

Distance of the Height of the air-horizontal termination tip*)

conductors

3 m 0,15 m

4 m 0.25 m

5 m 0.35 m

6 m 0.45 m

*) recommended values

Fig. 5.1.4.4a Conductor holders for metal roofs - Round standing seam

Parallel connector

St/tZn Part No. 307 000

Roof conductor holderfor metal roofs, loose conductorleading, DEHNgrip conductor holder

StSt Part No. 223 011Al Part No. 223 041

Roof conductor holder for metalroofs fixed conductor leading withclamping frame

StSt Part No. 223 010Al Part No. 223 040

2

1

1

3

2

3

Fig. 5.1.4.4b Conductor holder for metal roofs withround standing seam

roof connection

bridging braid

conductor holder withloose conductor leading

bridging cable

KS connector

air-termination tip

roof edge, which is capable of carryingcurrents.Unprotected installations projectingabove the roof, e. g. domelights andchimney covers, are exposed points ofstrike for a lightning discharge. In orderto prevent these installations from beingstruck by a direct lightning stroke, air-ter-mination rods must be installed adjacentto the installations projecting above theroof. The height of the air-terminationrod results from the protective angle α.

5.1.5 Principle of an air-terminationsystem for structures withthatched roof

The design of lightning protection sys-tems Type III generally meets the require-ments of such a structure. In particularindividual cases, a risk analysis based onDIN V VDE V 0185-2 can be carried out.

Subclause 3.1.2 of DIN V VDE V 0185-3stipulates a special installation procedurefor an air-termination system for a struc-ture with thatched roof.The air-termination conductors on suchroofs (made of thatch, straw or rushes)must be fastened across isolating sup-ports to be free to move. Certain dis-tances must also be maintained aroundthe eaves.

When a lightning protection system isinstalled on a roof at a later date, the dis-tances must be increased. This allowsmaintain the necessary minimum dis-tances when re-roofing is carried out.

For a lightning protection system Type III,the typical distance of the down-con-ductor system is 15 m.The exact distance of the down-conduct-or systems from each other resuslts fromcalculating the separation distance s inaccordance with DIN V VDE V 0185-3Clause 1, Subclause 5.3.

Chapter 5.6 explains how to calculate theseparation distance.

Ideally, ridge conductors should havespans up to around 15 m, and down-con-ductor systems up to around 10 m with-out additional supports.

Fastening posts must be tightly connect-ed to the roof structure (rafters and rails)by means of bolts and washers.

Metal components situated above theroof surface (such as weather vanes, irri-gation systems, conductors) must besecured, e. g. on non-conductive supportsso that a large enough separation dis-tance s is maintained, in accordance with5.3 Clause 1. Irrigation system feeds inthe vicinity of the duct through the skinof the roof, which are at least 0.6 mabove and below it, may only be made ofplastic (Figs. 5.1.5.1 to 5.1.5.3).

The previously described system for pro-tection against lightning is not effectivefor thatched roofs covered with a metalwire mesh. The metal wire meshworkmust be removed or substituted with aUV-resistant plastic mesh. Similarly, effec-tive protection against lightning is notpossible if metal covers, irrigation sys-tems, vent pipes, chimney skirtings,dormer windows, skylights and the likeare present. In such cases, effective pro-tection against lightning can only beachieved with an isolated external light-ning protection system with air-termina-tion rods near the structure, or with air-termination nets between masts adja-cent to the structure.


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Fig. 5.1.4.5 Model construction of a trapezoidalsheet roof, conductor holder with clam-ping frame

Fig. 5.1.4.6 Model construction of a roof with stan-ding seam

Fig. 5.1.4.7 Air-termination rod for a domelight on aroof with round standing seam

Signs and symbolsAir-termination conductorConnecting pointIsolating point /Measuring pointEarth conductorDown conductor

Important distances (min. values)a 0.6 m Air-term. conductor / Gableb 0.4 m Air-term. conductor / Roofingc 0.15 m Eaves / Eaves supportd 2.0 m Air-termination conductor /

Branches of trees

Fig. 5.1.5.1 Air-termination system for buildings with thatched roofs

If a thatched roof borders onto metalroofing material, and if the structure hasto be equipped with an external light-ning protection system, then an electri-cally non-conductive roofing material atleast 1 m wide, e. g. in plastic, must beinserted between the thatched roof andthe other roof.Tree branches must be kept at least 2 maway from a thatched roof. If trees arevery close to, and higher than, a struc-ture, then an air-termination conductormust be mounted on the edge of theroof facing the trees (edge of the eaves,gable) and connected to the lightningprotection system. The necessary dis-tances must be maintained.A further way of protecting structureswith thatched roofs against a stroke oflightning is to erect air-termination mastsso that the whole structure is in the pro-tected volume.This method can be found in Chapter5.1.8 Isolated air-termination system(steel telescopic lightning protectionmasts).

5.1.6 Walkable and trafficableroofs

It is not possible to mount air-termina-tion conductors (e. g. with concreteblocks) on drive-over roofs. One possiblesolution is to install the air-terminationconductors in either concrete or thejoints between the sections of the road-way. If the air-termination conductor isinstalled in these joints, mushroom-typecollectors are installed at the intersec-tions of the mesh as defined points ofstrike.

The mesh size must not exceed the valueaccording to the type of lightning pro-

tection system (see Chapter 5.1.1, Table5.1.1.3).

If it can be guaranteed that no personswill be on this area during a thunder-storm, then it is sufficient to install themeasures described above.Persons who can go onto this storey ofthe car park must be informed by meansof a sign that they must immediatelyclear this storey when a thunderstormoccurs, and not return for the duration ofthe storm (Fig. 5.1.6.1).

If it is also possible that persons are onthe roof during a thunderstorm, then theair-termination system must be designedto protect these persons, assuming theyhave a height of 2.5 m (with outstretchedarm) from direct lightning strokes.

The air-termination system can bedimensioned using the rolling sphere orthe protective angle method accordingto the type of lightning protection sys-tem (Fig. 5.1.6.2).

These air-termination systems can also beconstructed from spanned cables or air-termination rods. These air-terminationrods are secured to structural elementssuch as parapets or the like, for example . Furthermore, lighting masts, for exam-ple, can also act as air-termination rodsto prevent life hazards. With this version,however, attention must be paid to thepartial lightning currents which can beconducted into the structure via thepower lines. It is imperative to have light-ning equipotential bonding measures forthese lines.


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1

2 6

3

4

5

Pos Description DIN Part No.1 Clamping cap with 48811 A 145 309

air-termination rod2 Wood pile 48812 145 2413 Support for roof conductors − 240 0004 Eaves support 48827 239 0005 Tensioning block 48827 B 241 0026 Air-term. conductor, e.g. Al cable − 840 050

Fig. 5.1.5.2 Components for thatched roofs

4

5

3

1 2

6

Fig. 5.1.5.3 Thatched roof

down conducting viasteel reinforcement

conductors installed withinconcrete or the joints of theroadway (plates)

Warning!Keep off the car parkduring thunderstorms

Fig. 5.1.6.1 Lightning protection for car park roofs - Building protection

Mushroom-typecollector afterasphalting

Mushroom-type collectorPart No. 108 001

5.1.7 Air-termination system forplanted and flat roofs

A planted roof can make economic andecological sense. This is because it pro-vides noise insulation, protects the roofskin, suppresses dust from the ambientair, provides additional heat insulation,filters and retains rainwater and is a nat-ural way of improving the living andworking conditions. Moreover, in manyregions it is possible to obtain grantsfrom public funds for cultivating plantson the roof. A distinction is madebetween so-called extensive and inten-sive cultivation. An extensive plantedarea requires little care, in contrast to anintensive planted area which requiresfertiliser, irrigation and cutting. For bothtypes of planted area, either earth sub-strate or granulate must be laid on theroof.

It is even more expensive if the granulateor substrate has to be removed becauseof a direct lightning stroke.

If there is no external lightning protec-tion system, the roof seal can be dam-aged at the point of strike.

Experience has shown that, regardless ofthe type of care required, the air-termi-nation system of an external lightningprotection system can, and should, alsobe installed on the surface of a plantedroof.

For a meshed air-termination system, theDIN V VDE V 0185-3 lightning protectionstandard prescribes a mesh size whichdepends on the type of lightning protec-

tion system chosen (see Chapter 5.1.1,Table 5.1.1.3). An air-termination con-ductor installed inside the covering layeris difficult to inspect after a number ofyears because the air-termination tips ormushroom-type collectors are over-grown and no longer recognisable, andfrequently damaged by maintenancework. Moreover, air-termination conduc-tors installed inside the covering layer aremore susceptible to corrosion. Conduc-tors of air-termination meshes installeduniformly on top of the covering layerare easier to inspect even if they becomeovergrown, and the height of the inter-ception system can be lifted up by meansof air-termination tips and rods and“grown“ with the plants on the roof. Air-termination systems can be designed indifferent ways. The usual way is to installa meshed air-termination net with amesh size of 5 x 5 m (lightning protectionsystem Type I) up to a max. mesh size of15 x 15 m (lightning protection systemType III) on the roof surface, regardless ofthe height of the structure. It is prefer-able to determine the installation site ofthe mesh considering the external edgesof the roof and any metal structures act-ing as an air-termination system.Stainless steel (Material No. 1.4571) hasproven to be a good material for the con-ductors of air-termination systems onplanted roofs.Aluminium wire must not be used forinstalling conductors in the covering layer (in the earth substrate or granu-late), (Figs. 5.1.7.1 to 5.1.7.3).


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height of the air-termination roddimensioned according to therequired protective angle

additional air-termination cable

h

h = 2.5 m + s

Fig. 5.1.6.2 Lightning protection for car park roofs - Building and life protection

Fig. 5.1.7.1 Planted roof

Fig. 5.1.7.2 Air-termination system on a planted roof

Fig. 5.1.7.3 Conductor leading on the covering layer

5.1.8 Isolated air-termination sys-tems

Roof-mounted structures such as air con-ditioning and cooling systems, e. g. formainframes, are nowadays used on theroofs of larger office blocks and indus-trial structures. Antennas, electricallycontrolled domelights, advertising signswith integrated lighting and all otherprotruding roof-mounted structures hav-ing a conductive connection, e. g. viaelectrical cables or ducts, into the struc-ture, must be treated in a similar way.According to the State of the Art forlightning protection, such roof-mountedstructures are protected against directlightning strokes by means of separatelymounted air-termination systems. Thisprevents partial lightning currents fromentering the structure, where they wouldaffect or even destroy the sensitive elec-trical/electronic installations.In the past, these roof-mounted struc-tures were connected directly. This direct connection meant that partsof the lightning current were conductedinto the structure. Later, “indirect con-nection“ via a spark gaps was intro-duced. This meant that direct lightningstrokes to the roof-mounted structurecould also flow away via the “internalconductors“ to some extent, and in theevent of a more distant lightning stroketo the structure, the spark gap shouldnot operate. The operating voltage ofapprox. 4 kV was almost always attainedand hence a partial lightning current wasalso carried into the structure via theelectrical cable, for example . This can

affect or even destroy electrical or elec-tronic installations inside the structure.The only way of preventing these coupled currents is to use isolated air-ter-mination systems which maintain theseparation distance, previously alsoknown as safety distance.Fig. 5.1.8.1 shows a partial lightning current penetrating the inside of thestructure.These widely different roof-mountedstructures can be protected by variousdesigns of isolated air-termination sys-tems.

Air-termination rodsFor smaller roof-mounted structures (e. gsmall fans) the protection can beachieved by using individual, or a combi-nation of several, air-termination rods.Air-termination rods up to a height of2.0 m can be fixed with one or two con-crete bases piled on top of each other(e. g. Part No. 102 010) to be isolated(Fig. 5.1.8.2).

If an air-termination rod is higher than2.5 m to 3.0 m, the air-termination rodsmust be secured to the property to beprotected with distance holders made ofelectrically insulating material (e. g.DEHNiso distance holder) (Fig. 5.1.8.3).

Angled supports are a practical solutionwhen air-termination rods also have tobe secured against the effects of sidewinds (Figs. 5.1.8.4 and 5.1.8.5).

If higher air-termination rods arerequired, e. g. for larger roof-mountedstructures, which nothing can be securedto, the air-termination rods can beinstalled by using special supports.Self-supporting air-termination rods upto a height of 8.5 m can be installed byusing a tripod. These supports aresecured to the floor with standard con-crete bases (one on top of another).


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Roof

1st Floor

Ground floor

Basement

connection viaisolating spark gapdirect connection

EB

data lines

Fig. 5.1.8.1 Connection of roof mounted structures

Fig. 5.1.8.2 Isolated air-termination system, protec-tion provided by an air-termination rod

Fig. 5.1.8.3 Air-termination rod with distance holder

Fig. 5.1.8.5 Supporting element for the air-termination rod

Fig. 5.1.8.4 Angled support for air-termination rods

Additional guy lines are required above afree height of 6 m in order withstand thestresses caused by the wind.

These self-supporting air-terminationrods can be used for a wide variety ofapplications (e. g. antennas, PV installa-tions). The special feature of this type ofair-termination system is its short installa-tion time as no holes need to be drilledand only few elements need to bescrewed together (Figs. 5.1.8.6 to 5.1.8.7).

For protecting complete structures orinstallations (e. g. PV installations,ammunition depots) with air-termina-tion rods, lightning protection masts areused. These masts are installed in naturalsoil or in a concrete foundation. Freeheights of 19 m above ground level canbe achieved, even higher, if custom-made ones are used. It is also possible tospan a cable between these masts if theyare especially designed for this purpose.The standard lengths of the steel tele-scopic lightning protection masts aresupplied in sections of 2 m, offering enor-mous advantages for transportation.Further information (e. g. installation,assembly) about these steel telescopiclightning protection masts can be foundin installation instructions No. 1489 (Figs.5.1.8.8 and 5.1.8.9).

Spanned over by cables or conductorsAccording to DIN V VDE V 0185-3, air-ter-mination conductors can be installedabove the structure to be protected.The air-termination conductors generatea tent-shaped zone of protection at thesides, and a cone-shaped one at the ends.The protective angle α depends on thetype of lightning protection system andthe height of the air-termination systemabove the reference plane.The rolling sphere method with itscorresponding radius (according to thetype of lightning protection system) canalso be used to dimension the conductorsor cables.

The “mesh“ type of air-termination sys-tem can also be used if an appropriateseparation distance s between the com-ponents of the installation and the air-termination system must be maintained.In such cases, isolating distance holders inconcrete bases are installed vertically, forexample, for guiding the “mesh“ on anelevated level (Fig. 5.1.8.10).

DEHNiso-CombiA user-friendly way of installing conduc-tors or cables in accordance with thethree different design methods for air-termination systems (rolling sphere, pro-

tective angle, mesh) is provided by theDEHNiso-Combi programme of products.

The aluminium insulating pipes with “iso-lating distance“ (GRP – glass-fibre-rein-forced plastic), which are fixed to theobject to be protected, provide an indirectway of guiding the cables. The cables aresubsequently guided separately to thedown-conductor systems or supplemen-tary air-termination systems (e. g. mesh)by means of GRP distance holders.

Further information about the applica-tion is contained in the publicationsDS 123E “DEHNiso-Combi System for isol-ated Air-termination Systems“, DS 111“DEHNiso Distance Holder: The ModularLightning Protection System“ and in theset of installation instructions No. 1475.The types of design described can becombined with each other as desired toadapt the isolated air-termination sys-tems to the local conditions (Fig. 5.1.8.11to 5.1.8.14).


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Fig. 5.1.8.6 Isolated air-termination system for pho-tovoltaic system

Fig. 5.1.8.7 Isolated air-termination system for terre-strial antenna

Fig. 5.1.8.8 Additional corrosion protection in thetransition area by anticorrosive band forunderground application

Fig. 5.1.8.9 Installation of a steel telescopic light-ning protection mast

Fig. 5.1.8.10 Installed air-termination system,Ref.: Wettingfeld Lightning Protection,Krefeld, Germany

Fig. 5.1.8.11 Tripod support for self-supporting insula-ting pipes

5.1.9 Air-termination system forsteeples and churches

External lightning protection systemAccording to DIN V VDE V 0185-3, Sub-clause 7.1 a, lightning protection systemsType III meet the normal requirementsfor churches and steeples. In particularindividual cases, for example in the caseof culturally significant structures, a spe-cial risk analysis in accordance with DIN VVDE V 0185-2 must be carried out.

NaveAccording to DIN V VDE V 0185-3, Sub-clause 7.5, the nave must have its ownlightning protection system and, if asteeple is attached, this system must beconnected by the shortest route with adown-conductor system of the steeple. Inthe transept, the air-termination con-ductor along the transverse ridge mustbe equipped with a down-conductor sys-tem at each end.

SteepleSteeples up to a height of 20 m must beequipped with a down-conductor sys-tem. If steeple and nave are joined, thenthis down-conductor system must be con-nected to the external lightning protec-tion system of the nave by the shortestroute (Fig. 5.1.9.1). If the down-conduct-or system of the steeple coincides with adown-conductor system of the nave,then a common down-conductor systemcan be used at this location. According toDIN V VDE V 0185-3, Subclause 7.3,steeples above 20 m in height must beprovided with at least two down conductors. At least one of these downconductors must be connected with theexternal lightning protection system ofthe nave via the shortest route.


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Fig. 5.1.8.12 Isolated air-termination systems with DEHNiso-Combi

Fig. 5.1.9.1 Installing the down-conductor system ata steeple

Fig. 5.1.8.13 Detail picture of DEHNiso-Combi

Fig. 5.1.8.14 Isolated air-termination system withDEHNiso-Combi

Down-conductor systems on steeplesmust always be guided to the ground onthe outside of the steeple. They must notbe installed inside the steeple (DIN V VDEV 0185-3, Subclause 7.2). Further, the sep-aration distance s to metal componentsand electrical installations in the steeple(e. g. clock mechanisms, belfry) andunder the roof (e. g. air conditioning,ventilation and heating systems) must bemaintained by suitable arrangement ofthe external lightning protection system.The required separation distance canbecome a problem especially at the clock.In this case, the conductive connectioninto the structure can be replaced withan isolating connector (e. g. a GRP pipe)to prevent hazardous sparking in parts ofthe external lightning protection system.

In more modern churches built with rein-forced concrete, the reinforcement steelscan be used as down-conductor systemsif it can be ensured that they provide acontinuous conductive connection. Ifpre-cast reinforced concrete parts areused, the reinforcement may be used as adown-conductor system if terminals toconnect the reinforcement continuouslyare provided on the pre-cast concreteparts.

5.1.10 Air-termination systems forwind turbines (WT)

Requirement for protection againstlightningE DIN VDE 0127-24 describes measures toprotect wind turbines against lightning.Lightning protection measures are car-ried out in accordance with the DIN VVDE V 0185 series of standards. In its VdS2010 directive “Risk-orientated lightningand surge protection“, the VdS recom-mends that a lightning protection systemType II to be installed for wind turbines.This can control lightning strokes withcurrents measuring up to 150,000 A. Thisrecommendation results from the assess-ment of the risk of damage from a light-ning stroke for structures, as described inDIN V VDE V 0185-2.

Principle of an external lightning protec-tion system for wind turbinesThe external lightning protection systemcomprises air-termination systems,down-conductor systems and an earthtermination system and protects againstmechanical destruction and fire. Light-ning strokes to wind turbines usuallyaffect the rotor blades. Hence, receptors,for example, are integrated to determinedefined points of strike (Fig. 5.1.10.1).

In order to allow the coupled lightningcurrents to flow to earth in a controlledway, the receptors in the rotor blades areconnected to the hub with a metal inter-connecting conductor (often flat lineSt/tZn 30 x 3.5 mm). Carbon fibre brushesor air spark gaps then, in turn, bridge theball-bearings in the head of the nacellein order to avoid the welding of therevolving parts of the structure.In order to protect structures on thenacelle, such as anemometers in theevent of a lightning stroke, air-termina-tion rods or “air-termination cages“ areinstalled (Fig. 5.1.10.2).

The metal tower or, in case of a pre-stressed concrete version, the down-con-ductor systems embedded in the con-crete (round conductor St/tZn Ø8...10 mmor tape conductor St/tZn 30 x 3.5 mm) isused as the down-conductor system. Thewind turbine is earthed by a foundationearthing electrode in the base of thetower and the meshed connection withthe foundation earthing electrode of theoperation building. This creates an“equipotential surface“ which prevents

potential differences in the event of alightning stroke.

5.1.11 Wind load stresses on light-ning protection air-termina-tion rods

Roofs are being used more and more asareas for technical installations.Especially when extending the technicalequipment in the structure, extensiveinstallations are being sited more thanever on the roofs of larger office blocksand industrial structures. It is essential toprotect roof-mounted structures such asair conditioning and cooling systems,transmitters for cell sites on host build-ings, lamps, flue gas vents and otherapparatus connected to the electrical lowvoltage system (Fig. 5.1.11.1).

In accordance with the relevant lightningprotection standards contained in theDIN V VDE V 0185 series, these roof-mounted structures can be protectedfrom direct lightning strokes with isol-ated air-termination systems. Thisrequires an isolation of both the air-ter-mination systems, such as air-terminationrods, air-termination tips or air-termina-tion meshes, and the down-conductorsystems, i. e. to be installed with suffi-cient separation distance from the roof-mounted structures within the zone ofprotection. The construction of an isol-ated lightning protection system createsa zone of protection in which direct light-ning strokes cannot occur. It also pre-vents partial lightning currents fromentering the low voltage system andhence the structure. This is important asthe entering of partial lightning currents


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receptor

wire meshwork

Fig. 5.1.10.1 WT with integrated receptors in therotor blades

Fig. 5.1.10.2 Lightning protection for wind speedindicators at WT

Fig. 5.1.11.1 Protection against direct lightningstrokes by self-supporting air-termina-tion rods

into the building can affect or destroysensitive electrical/electronic installa-tions.Extended roof-mounted structures arealso equipped with a system of isolatedair-termination systems. These are con-nected with each other and also with theearth-termination system. Among otherthings the magnitude of the zone of pro-tection created depends on the numberand the height of the air-termination sys-tems installed.A single air-termination rod is sufficientto provide the protection required bysmaller roof-mounted structures. Theprocedure involves the application of therolling sphere method in accordancewith DIN V VDE V 0185-3 (Fig. 5.1.11.2).With the rolling sphere method, a rollingsphere whose radius depends on the typeof lightning protection system chosen isrolled in all possible directions on andover the structure to be protected. Dur-ing this procedure, the rolling spheremust touch the ground and/or the air-termination system only.This method produces a protection vol-ume where direct lightning strokes arenot possible.To achieve the largest possible zone ofprotection, and also to be able to protectlarger roof-mounted structures againstdirect lightning strokes, the individualair-termination rods should ideally beerected with a corresponding height.This requires to prevent self-supporting

air-termination rods from tilting andbreaking by a suitably designed base andsupplementary guys (Fig. 5.1.11.3).The requirement for the self-supportingair-termination rods to be built as high aspossible must be balanced against thehigher stress exerted by the active windloads. A 40% increase in wind speed, forexample, doubles the active tiltingmoment. At the same time, from theapplication point of view, users demanda lightweight system of “self-supportingair-termination rods“, which are easier totransport and install. To ensure that it issafe to use air-termination rods on roofs,their mechanical stability must beproven.

Stress caused by wind loadsSince self-supporting air-terminationrods are installed at exposed sites (e. g.on roofs), mechanical stresses arisewhich, owing to the comparable locationand the upcoming wind speeds, corre-spond to the stresses suffered by antennaframes. Self-supporting air-terminationrods must therefore basically meet thesame requirements concerning theirmechanical stability as set out in DIN4131 for antenna frames.DIN 4131 divides Germany up into 4 windzones with zone-dependent wind speeds(Fig. 5.1.11.4).When calculating the prospective actualwind load stresses, apart from the zone-dependent wind load, the height of the

structure and the local conditions (struc-ture standing alone in open terrain orembedded in other buildings) must alsobe included. From Fig. 5.1.11.4 it can beseen that around 95% of Germany’s sur-face area lies within Wind Zones I and II.Air-termination rods are therefore gen-erally designed for Wind Zone II. The useof self-supporting air-termination rods inWind Zone III and Wind Zone IV must beassessed for each individual case takingthe arising stresses into account.According to DIN 4131 a constant dynam-ic pressure over the height of a structurecan be expected for structures up to aheight of 50 m. For the calculations, themaximum height of the structure wasconsidered 40 m, so that a total height(height of the structure plus length ofthe air-termination rods) is kept belowthe 50 m mark. When designing self-supporting air-termination rods, the following require-ments must be met for the wind loadstress:

⇒ Tilt resistance of the air-terminationrods

⇒ Fracture resistance of the rods

⇒ Maintaining the required separationdistance to the object to be protect-ed even under wind loads (preven-tion of intolerable deflections)


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Fig. 5.1.11.2 Procedure for installation of air-termination systems according to DIN V VDE V 0185-3

down conductor

earth-termination system

mesh size M


α2

h 1h 2

air-termination rodprotective angle

R

rolling sphere

I 20 m 5 x 5 m

II 30 m 10 x 10 m

III 45 m 15 x 15 m

IV 60 m 20 x 20 m

Typeof LPS

Radius of therolling sphere R

Mesh size MMax. height of the building

Fig. 5.1.11.3 Self-supporting air-termination rodwith variable tripod

air-termination rodwith air-termination tip

bracing

variabletripod

Determination of the tilt resistanceThe dynamic pressure arising (dependson the wind speed), the resistance coeffi-cient cw and the contact surface of thewind on the air-termination rod, gener-ate a uniform load q‘ on the surfacewhich generates a corresponding tiltingmoment MT on the self-supporting air-termination rod. To ensure that the self-supporting air-termination rod is stable,the tilting moment MT must be opposedby a load torque MO, which is generatedby the post. The magnitude of the loadtorque MO depends on the standingweight and the radius of the post. If thetilting moment is greater than the loadtorque, the wind load pushes the air-ter-mination rod over.The proof of the stability of self-support-ing air-termination rods is also obtainedfrom static calculations. Besides themechanical characteristics of the mater-ials used, the following information isincluded in the calculation:

⇒ Wind contact surface of the air-ter-mination rod: determined by lengthand diameter of the individual sec-tions of the air-termination rod.

⇒ Wind contact surface of the guy:very high self-supporting air-termi-nation rods are anchored with 3cables mounted equidistantly

around the circumference. The windcontact surface of these cables corre-sponds to the area projected bythese cables onto a plane in a rightangle to the direction of the wind,i. e. the cable lengths are shortenedaccordingly when considered in thecalculation.

⇒ Weight of the air-termination rodand the guy lines: the dead weightof the air-termination rod and theguy lines is taken into account in thecalculation of the load torque.

⇒ Weight of the post: the post is a tri-pod weighted down with concreteblocks. The weight of this post ismade up of the dead weight of thetripod and the individual weights ofthe concrete blocks used.

⇒ Tilting lever of the post: the tiltinglever denotes the shortest distancebetween the centre of the tripodand the line or point around whichthe whole system would tilt.

The proof of stability is obtained by com-paring the following moments:

⇒ Tilting moment formed from thewind-load-dependent force on theair-termination rod or the guy lines

and the lever arm of the air-termina-tion rod.

⇒ Load torque formed from theweight of the post, the weight of theair-termination rod and the guylines, and the length of the tilt leverthrough the tripod.

Stability is achieved when the ratio ofload torque to the tilting momentassumes a value >1. Basically: the greaterthe ratio of load torque to tiltingmoment, the greater the stability.The required stability can be achieved inthe following ways:

⇒ In order to keep the wind contactsurface of the air-termination rodsmall, the cross sections used have tobe as small as possible. The load onthe air-termination rod is reduced,but, at the same time, the mechani-cal strength of the air-terminationrod decreases (risk of breaking). It istherefore crucial to make a compro-mise between a smallest possiblecross section to reduce the wind loadand a largest possible cross section toachieve the required strength.

⇒ The stability can be increased byusing larger base weights and/orlarger post radii. This often conflictswith the limited areas for erectionand the general requirement for lowweight and easy transport.

ImplementationIn order to provide the smallest possiblewind contact surface, the cross sectionsof the air-termination rods were opti-mised in accordance with the results ofthe calculation. For ease of transporta-tion and installation, the air-terminationrod comprises an aluminium tube (in sec-tions, if so desired) and an aluminium air-termination rod. The post to hold the air-termination rod is available in two ver-sions. One fixed version for lower rodheights and an adjustable post versionfor higher rod heights. With this version,the radius of the post is adjusted to theheight of the air-termination rod to min-imise the space required.

Determination of the fracture resistanceNot only the stability of the air-termina-tion rod must be proven, but also thefracture resistance, since the occuringwind load exerts bending stresses on theself-supporting air-termination rod. Thebending stress in such cases must notexceed the max. permissible stress. Thebending stress occuring is higher forlonger air-termination rods. The air-ter-mination rods must be designed to


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Fig. 5.1.11.4 Division of Germany into wind load zones and corresponding values of dynamic pressure andmax. wind velocityRef.: DIN 4131: 1991-11. Steel radio towers and masts, Berlin: Beuth-Verlag GmbH

München

Augsburg

Regensburg

Nürnberg

Würzburg

Stuttgart

Freiburg

Saarbrücken Mannheim

FrankfurtWiesbaden

Köln

Düsseldorf

Bonn

EssenDortmund

Erfurt Chemnitz

DresdenLeipzig

Halle

Magedburg

Berlin

PotsdamHannover

Bremen

HamburgSchwerin

RostockKiel

zone IV

zone III zone II

zone IZone

I

II

III

IV

Dynamic pressureq [kN/m2]

0.8

1.05

1.4

1.7

Wind velocityv [km/h]

126.7

145.1

161.5

184.7

Windstrength

12 - 17

ensure that wind loads as can arise inWind Zone II cannot cause permanentdeformation of the rods.Since both the exact geometry of the air-termination rod and the non-linear per-formance of the materials used must betaken into account, the proof of the frac-ture resistance of self-supporting air-ter-mination rods is obtained using an FEMcalculation model. The finite elementsmethod, FEM for short, is a numericalmethod for calculation of stresses anddeformations of complex geometricalstructures. The structure under examina-tion is broken down into so-called “finiteelements“ using imaginary surfaces andlines which are interconnected via nodes.The calculation requires the followinginformation:

⇒ FEM calculation model:

The FEM calculation model corres-ponds to the simplified geometry ofthe self-supporting air-terminationrod.

⇒ Material characteristics:

The performance of the material isrepresented by the details of cross-sectional values, modulus of elastici-ty, density and lateral contraction.

⇒ Loads:

The wind load is applied to the geo-metric model as a pressure load.

The fracture resistance is determined bycomparing the permissible bending stress(material parameter) and the max. bend-ing stress which can occur (calculatedfrom the bending moment and the effec-tive cross section at the point of maxi-mum stress).Fracture resistance is achieved if the ratioof permissible to actual bending stress is> 1. Basically, the same principle alsoapplies here: the greater the ratio of per-missible to actual bending stress, thegreater the fracture resistance.Using the FEM calculation model, theactual bending moments for two air-ter-mination rods (length = 8.5 m) were cal-culated as a function of their height withand without guys (Fig. 5.1.11.5). Thisclearly illustrates the effect of a possibleguy on the course of the moments.Whereas the max. bending moment ofthe air-termination rod without a guy inthe fixed-end point is around 1270 Nm,the guy reduces the bending moment toaround 270 Nm. This guy cable makes itpossible to reduce the stresses in the air-termination rod to such an extent that,for the max. expected wind loads, thestrength of the materials used is notexceeded and the air-termination rod isnot destroyed.

ImplementationGuy cables create an additional “bearingpoint“ which significantly reduces thebending stresses occuring in the air-ter-mination rod. Without supplementaryguys, the air-termination rods would notcope with the stresses of Wind Zone II.Therefore, air-termination rods higherthan 6 m are equipped with guy cables.

In addition to the bending moments, theFEM calculation also provides the tensileforces occuring in the guy cables, whosestrength must also be proven.

Determination of the wind-load-dependent deflection of the air-termina-tion rodA further important value calculatedwith the FEM model is the deflection ofthe tip of the air-termination rod. Windloads cause the air-termination rods tobend. The bending of the rod results in achange to the zone of protection.Objects to be protected are no longersituated in the zone of protection and/orproximities can no longer be maintained.The application of the calculation modelon a self-supporting air-termination rodwithout and with guys produces the fol-lowing results (Figs. 5.1.11.6 and5.1.11.7).


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1200

1000

800

600

400

200

00 2 4 6 8

Height of air-termination rod [m]

Bendingmoment

[Nm]

air-termination rodwithout guy cables(length = 8.5 m)

Fig. 5.1.11.6 FEM model of a self-supporting air-ter-mination rod without guy cables (length = 8.5 m)

200

150

100

50

0

-50

-100

-1500 2 4 6 8

Height of air-termination rod [m]

Bendingmoment

[Nm]

air-termination rodwith guy cables(length = 8.5 m)

Fig. 5.1.11.5 Comparison of bending moment cour-ses at self-supporting air-terminationrods with and without guy cables (length = 8.5 m)

Fig. 5.1.11.7 FEM model of a self-supporting air-ter-mination rod with guy cables(length = 8.5 m)

For the example chosen, the calculationgives a displacement of the tip of the air-termination rod with guy of around390 mm. Without guy there would be adeflection of around 3740 mm, a theoret-ical value which exceeds the breakingpoint of the air-termination rod underconsideration.

ImplementationAbove a certain rod height, supplemen-tary guys reduce this deflection signifi-cantly. Furthermore, this also reduces thebending load on the rod.

ConclusionTilting resistance, fracture resistance anddeflection are the decisive factors whendesigning air-termination rods. Base andair-termination rod must be coordinatedto ensure that the loads occuring as aresult of the wind speeds of Zone II donot cause a tilting of the rod, nor dam-age it.It must still be borne in mind that largedeflections of the air-termination rod reduce the separation distance and thusintolerable proximities can arise. Higherair-termination rods require a supple-mentary guy to prevent such intolerabledeflections of the tips of the air-termina-tion rods.The measures described ensure that self-supporting air-termination rods can copewith Zone II wind speeds according toDIN 4131.

5.2 Down-conductor systemThe down-conductor system is the elec-trically conductive connection betweenthe air-termination system and the earth-termination system. The function ofdown-conductor systems is to conductthe intercepted lightning current to theearth-termination system without intol-erable temperature rises, for example, todamage the structure.To avoid damage caused during thelightning current discharge to the earth-termination system, the down-conductorsystems must be mounted to ensure thatfrom the point of strike to the earth,

⇒ Several parallel current paths exist,

⇒ The length of the current paths iskept as short as possible (straight,vertical, no loops),

⇒ The connections to conductive com-ponents of the structure are madewherever required (interval < s; s = separation distance).

5.2.1 Determination of the numberof down conductors

The number of down conductorsdepends on the perimeter of the externaledges of the roof (perimeter of the pro-jection on the ground surface).The down conductors must be arrangedto ensure that, starting at the corners ofthe structure, they are distributed as uni-formly as possible to the perimeter.Depending on the structural features(e. g. gates, precast components), the dis-tances between the various down con-ductors can be different. These possiblydifferent distances, e. g. from 12 m to18 m for a lightning protection systemType III (typically 15 m) must also be tak-en into account when calculating theseparation distance. In each case, theremust be at least the total number ofdown conductor required for the respec-tive type of lightning protection system.The DIN V VDE V 0185-3 standard givestypical distances between down conduc-tors and ring conductors for each type oflightning protection system (Table5.2.1.1).

The exact number of down conductorscan only be determined by calculatingthe separation distance s. If the calculat-ed separation distance cannot be main-tained for the intended number of downconductors of a structure, then one wayof meeting this requirement is toincrease the number of down conduc-tors. The parallel current paths improvethe current splitting coefficient kc. Thismeasure reduces the current in bothdown conductors, and the required sepa-ration distance can be maintained.Natural components of the structure(e. g. reinforced concrete supports, steelskeleton) can also be used as supplemen-tary down conductors if continuous elec-trical conductivity can be ensured.By interconnecting the down conductorsat ground level (base conductor) andusing ring conductors for higher struc-tures, it is possible to symmetrise the dis-tribution of the lightning current which,in turn, reduces the separation distance s.The latest DIN V VDE V 0185 series ofstandards attaches great significance to

the separation distance. The measuresspecified can change the separation dis-tance positively for structures and thusthe lightning current can be safely dis-charged.If these measures are not sufficient tomaintain the required separation dis-tance, it is also possible to use a new typeof high voltage-resistant insulated con-ductors (HVI). These are described inChapter 5.2.4.Chapter 5.6 describes how the exact sep-aration distance can be determined.

5.2.2 Down-conductor system for anon-isolated lightning protec-tion system

The down-conductor systems are primari-ly mounted directly onto the structure(with no distance). The criterion forinstalling them directly on the structure isthe temperature rise in the event oflightning striking the lightning protec-tion system.If the wall is made of flame-resistantmaterial or material with a normal levelof flammability, the down-conductor sys-tems may be installed directly on or inthe wall.Owing to the specifications in the build-ing regulations of the German federalstates, highly flammable materials aregenerally not used. This means that thedown-conductor systems can usually bemounted directly on the structure.Wood with a bulk density greater than400 kg/m2 and a thickness greater than2 mm is considered to have a normal levelof flammability. Hence the down-con-ductor system can be mounted on wood-en poles, for example .If the wall is made of highly flammablematerial, the down conductors can beinstalled directly on the surface of thewall, provided that the temperature risewhen lightning currents flow is not haz-ardous.The maximum temperature rises ∆ T in Kof the various conductors for each typeof lightning protection system are statedin Table 5.2.2.1. These values mean that,generally, it is even permissible to installdown conductors underneath heat insu-lation because these temperature risespresent no fire risk to the insulationmaterials.This ensures that the fire retardationmeasure is also provided.When installing the down-conductor sys-tem in or underneath heat insulation,the temperature rise (on the surface) isreduced if an additional PVC sheath isused. Aluminium wire sheathed in PVCcan also be used.


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Type of LPS Typical distance

I 10 m

II 10 m

III 15 m

IV 20 m

Table 5.2.1.1 Distance between down conductorsaccording to DIN V VDE V 0185-3


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If the wall is made of highly flammablematerial, and the temperature rise of thedown-conductor systems presents a haz-ard, then the down conductors must bemounted to ensure that the distancebetween the down-conductor systemsand the wall is greater than 0.1 m. Themounting elements may touch the wall.The erector of the structure must statewhether the wall, where a down-con-ductor system is to be installed, is madeof flammable material.

The precise definition of the terms flame-resistant, normal level of flammabilityand highly flammable can be taken fromAnnex E of DIN V VDE V 0185-3.

5.2.2.1 Installation of down-conductor systems

The down conductors must be arrangedto be the direct continuation of the air-termination conductors. They must beinstalled straight and vertically so as torepresent the shortest possible directconnection to the earth.Loops, e. g. overprojecting eaves or struc-tures, must be avoided. If this is not pos-sible, the distance measured where twopoints of a down-conductor system areclosest, and the length l of the down-con-ductor system between these points,must fulfil the requirements on separa-tion distance s (Fig. 5.2.2.1.1).

The separation distance s is calculatedusing the total length l = l1 + l2 + l3.

Down-conductor systems must not beinstalled in gutters and downpipes, evenif they are sheathed in an insulatingmaterial. The damp in the gutters wouldbadly corrode the down-conductor sys-tems.If aluminium is used as a down conduct-or, it must not be installed directly (withno distance) on, in or under plaster, mor-tar, concrete, neither should it beinstalled in the ground. If it is equippedwith a PVC sheath, then aluminium canbe installed in mortar, plaster or con-crete, if it is possible to ensure that thesheath will not be mechanically dam-aged, nor will the insulation fracture atlow temperatures.It is recommended to mount down con-ductors to maintain the required separa-tion distance s to all doors and windows(Fig. 5.2.2.1.2).Metal gutters must be connected withthe down conductors at the points wherethey intersect (Fig. 5.2.2.1.3).

The base of metal downpipes must beconnected to the equipotential bondingor the earth-termination system, even ifthe pipe is not used as a down conductor.Since it is connected to the eaves gutter,through which the lightning currentflows, the downpipe also takes a part ofthe lightning current which must be con-ducted into the earth-termination sys-tem. Fig. 5.2.2.1.4 illustrates one possibledesign.

q Aluminium Iron Copper Stainless steel

mm2 Type of lightning protection systemIII+IV II I III+IV II I III+IV II I III+IV II I

16 146 454 * 1120 * * 56 143 309 * * *

5012 28 52 37 96 211 5 12 22 96 460 940

(Ø8mm)

784 9 17 15 34 66 3 5 9 78 174 310

(Ø10mm)

* melting / vaporising

Table 5.2.2.1 Max. temperature rise ∆ T in K of different conductor materials

Fig. 5.2.2.1.1 Loop in the down conductor

Fig. 5.2.2.1.3 Air-termination system with connec-tion to the gutter

downpipes mayonly be used asdown conductor, ifthey are soldered orriveted

the connectionmust be asshort as pos-sible, straightand installedvertically

Fig. 5.2.2.1.2 Down-conductor system

Fig. 5.2.2.1.4 Earthed downpipe

StSt wireØ10 mm

5.2.2.2 Natural components of adown-conductor system

When using natural components of thestructure as a down-conductor system,the number of down conductors to beinstalled separately can be reduced or, insome cases, they can be dispensed withaltogether.

The following parts of a structure can beused as “natural components“ of thedown-conductor system:

⇒ Metal installations, provided thatthe safe connection between thevarious parts is permanent and theirdimensions conform to the minimumrequirements for down conductors.These metal installations may also besheathed in insulating material. Theuse of conduits containing flamma-ble or explosive materials as downconductors is not permitted if theseals in the flanges/couplings arenon-metallic or the flanges /cou-plings of the connected pipes are nototherwise connected so as to be elec-trically conductive.

⇒ The metal skeleton of the structure

If the metal frame of structures witha steel skeleton or the interconnect-ed reinforced steel of the structure isused as a down-conductor system,then ring conductors are notrequired since additional ring con-ductors would not improve the split-ting of the current.

⇒ Safe interconnected reinforcementof the structure

The reinforcement of existing struc-tures cannot be used as a naturalcomponent of the down-conductorsystem unless it can be ensured thatthe reinforcement is safely intercon-nected. Separate external down con-ductors must be installed.

⇒ Precast parts

Precast parts must be designed toprovide terminal connections for thereinforcement. Precast parts musthave an electrically conductive con-nection between all terminal con-nections. The individual componentsmust be interconnected on site dur-ing installation (Fig. 5.2.2.2.1).

Note:In the case of prestressed concrete, atten-tion must be paid to the particular risk ofpossible intolerable mechanical effectsarising from lightning current and result-ing from the connection to the lightningprotection system.For prestressed concrete, connections totensioning rods or cables must only beeffected outside the stressed area. Thepermission of the person responsible forerecting the structure must be givenbefore using tensioning rods or cables asa down conductor.If the reinforcement of existing struc-tures is not safely interconnected, it can-not be used as a down-conductor system.In this case, external down conductorsmust be installed.

Furthermore, façade elements, mount-ing channels and the metal substructuresof façades can be used as a naturaldown-conductor system, provided that

⇒ the dimensions meet the minimumrequirements of down-conductorsystems. For sheet metal, the thick-ness must not be less than 0.5 mm.Their electrical conductivity in verti-cal direction must be ensured. Ifmetal façades are used as a down-conductor system, they must beinterconnected to ensure that theindividual plates are safely intercon-nected with each other by means ofscrews, rivets, or bridging connec-tions. There must be a safe connec-tion capable of carrying currents tothe air-termination system and alsoto the earth-termination system.

⇒ If plates are not interconnected inaccordance with the above require-ment, but the substructure ensures

that they are continuously conduc-tive from the connection on the air-termination system to the connec-tion on the earth-termination sys-tem, then they can be used as adown-conductor system (Figs.5.2.2.2.2 and 5.2.2.2.3).


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expansion joint

expansion joint

Fig. 5.2.2.2.1 Use of natural components - new buil-dings made of ready-mix concrete


Fixed earthing terminalPart No. 478 200

vertical box section

wall fixing

horizontal support

Fig. 5.2.2.2.2 Metal subconstruction, conductivelybridged


Fig. 5.2.2.2.3 Earth connection of a metal façade


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Metal downpipes can be used as naturaldown conductors, as long as they aresafely interconnected (brazed or rivetedjoints) and comply with the minimumwall thickness of the pipe of 0.5 mm.If a downpipe is not safely interconnect-ed, it can serve as a holder for the sup-plementary down conductor. This type ofapplication is illustrated in Fig. 5.2.2.2.4.The connection of the downpipe to theearth-termination system must be capa-ble of carrying lightning currents sincethe conductor is held only along thepipe.

5.2.2.3 Measuring pointsThere must be a measuring point at everyconnection of a down conductor withthe earth-termination system (above thelead-in, if possible).

Measuring points are required to allowthe inspection of the following charac-teristics of the lightning protection sys-tem:

⇒ Connections of the down conductorsvia the air-termination systems tothe next down conductor

⇒ Interconnections of the terminal lugsvia the earth-termination system,e. g. in the case of ring or foundationearthing electrodes (earthing elec-trode Type B)

⇒ Earthing electrode resistance of sin-gle earthing electrodes (earthingelectrode Type A)

Measuring points are not required if thestructural design (e. g. reinforced con-crete structure or steel skeleton) allowsno “electrical“ disconnection of the “nat-ural“ down-conductor system to the

earth-termination system (e. g. founda-tion earthing electrode).

The measuring point may only beopened with the help of a tool for thepurpose of taking measurements, other-wise it must be closed.Each measuring point must be able to beclearly assigned to the design of thelightning protection system. Generally,all measuring points are marked withnumbers (Fig. 5.2.2.3.1).

5.2.2.4 Internal down-conductorsystems

If the edges of the structure (length andwidth) are four times as large as the dis-tance of the down conductor which cor-responds to the type of lightning protec-tion system, then supplementary internaldown conductors must be installed(Fig. 5.2.2.4.1).The grid dimension for the internaldown-conductor systems is around 40 x40 m.

Large structures with flat roofs, such aslarge production halls or also distributioncentres, frequently require internaldown-conductor systems. In such cases,the ducts through the surface of the roofshould be installed by a roofer becausehe is responsible for ensuring that theroof provides protection against rain.The consequences of the partial light-ning currents through internal down-conductor systems within the structuremust be taken into account. The result-ing electromagnetic field in the vicinityof the down conductors must be takeninto consideration when designing theinternal lightning protection system (payattention to inputs to electrical/elec-tronic systems).

5.2.2.5 CourtyardsStructures with enclosed courtyards hav-ing a perimeter greater than 30 m musthave down-conductor systems installedwith the distances shown in Table 5.2.1.1.At least 2 down conductors must beinstalled (Fig. 5.2.2.5.1).

Fig. 5.2.2.3.1 Isolating point with number plate

roofing

heat insulationwood insulation

metal construction

internal downconductor

roof bushing

If the separation distance is too short, the conductive parts of the building constructionhave to be connected to the air-termination system. The effects from the currentshave to be taken into account.

separationdistance s

Fig. 5.2.2.4.1 Air-termination system installed on large roofs - Internal down-conductor system

Fig. 5.2.2.2.4 Down conductor installed along adownpipe

courtyardcircumference > 30 m

15 m7.

5 m

30 m

45 m

Courtyards with circumferences of more than 30 mmust be furnished with min. 2 down conductors.Typical distances according to type of LPS.

metal attic

Fig. 5.2.2.5.1 Down-conductor systems for courtyards

5.2.3 Down conductors of an isol-ated external lightning pro-tection system

If an air-termination system comprisesair-termination rods on isolated masts (orone mast), then this is both air-termina-tion system and down-conductor systemat the same time (Fig. 5.2.3.1).

Each individual mast requires at least onedown conductor. Steel masts or mastswith an interconnected steel reinforce-ment require no supplementary down-conductor system.For optical reasons, a metal flag pole, forexample can also be used as an air-termi-nation system.The separation distance s between theair-termination and down-conductor sys-tems and the structure must be main-tained.If the air-termination system consists ofone or more spanned wires or cables,each end of the cable which the conduc-tors are attached to requires at least onedown conductor (Fig. 5.2.3.2).

If the air-termination system forms anintermeshed network of conductors, i. e.the individual spanned wires or cablesare interconnected to form a mesh(being cross-linked), there must be atleast one down conductor at the end ofeach cable the conductors are attachedto (Fig. 5.2.3.3).

5.2.4 High-voltage resistant, isol-ated down-conductor system– HVI®conductor

A multitude of structures is used in orderto create an exhaustive network of cellsites. Some of these structures have light-ning protection systems. In order todesign and implement the mast infra-structure in accordance with the stan-dards, the actual situation must be takeninto account during the design phasewhile the relevant standards have to bestrictly differentiated. Owing to the dif-ferent protection objectives (protectionagainst lightning or earthing of the cellsite), it is not permissible to mix DIN VVDE V 0185-3 and DIN VDE 0855 Part 300.

For the operator of a mobile phone net-work there are basically three differentsituations:

⇒ Structure has no lightning protectionsystem

⇒ Structure is equipped with a light-ning protection system which is nolonger capable of functioning

⇒ Structure is equipped with a funtion-ing lightning protection system

Structure has no lightning protectionsystemThe cell site is constructed in accordancewith DIN VDE 0855 Part 300. This dealswith the earthing of the cell site. In accor-dance with the concept for protectionagainst surges of the mobile phone net-work operators, supplementary protec-tion against surges is integrated into themeter section.

Structure is equipped with a lightningprotection system which is no longercapable of functioningThe cell site must be connected to theexternal lightning protection system(LPS) as required by the type of lightningprotection system determined. The light-ning current paths required for the cellsite are investigated and assessed. Thisinvolves replacing non-functional com-ponents of the existing installation whichare required to discharge the lightningcurrent, such as air-termination conduct-or, down-conductor system and connec-tion to the earth-termination system.Any observed defects to parts of theinstallation which are not required mustbe notified in writing to the owner of thestructure.

Structure is equipped with a functioninglightning protection systemExperience has shown that most light-ning protection systems are designed

according to lightning protection sys-tems Type III. Regular inspections are pre-scribed for certain structures. It must beplanned to integrate the cell site installa-tion in accordance with the type of light-ning protection system determined. Forinstallations with lightning protectionsystems Type I and II, the surroundings ofthe installation must be recorded photo-graphically to ensure that, if problemssubsequently arise with proximities, thesituation at the time of construction canbe proven. If a cell site is erected on astructure with a functional external light-ning protection system, its erection isgoverned by the latest lightning protec-tion standard (DIN V VDE V 0185). In thiscase, DIN VDE 0855 Part 300 can only beused for the equipotential bonding ofthe antenna cable. Proximities must becalculated as appropriate to the type oflightning protection system. All mechan-ical components used must be able tocope with the prospective partial light-ning currents. For reasons of standardisa-tion, all the steel fixing elements andstructures for holding antennas of manymobile phone network operators mustbe designed for lightning protection sys-tems Type I. The connection should bedone via the shortest route, which is nota problem, however, as the air-termina-tion conductors on flat roofs are usuallydesigned to be meshed. If there is a func-tional lightning protection system on thehost building, this has a higher prioritythan an antenna earthing installation.Because of how it is designed, the type oflightning protection system to be effect-ed must be laid down at the discussionstage of the project:

⇒ If other system components are alsosituated on the roof, it is preferableto install the electrical cable on theexterior side of the structure.

⇒ If other system components aresituated on the roof, and if it isintended to erect a central mast, theinstallation must be equipped withan isolated lightning protection sys-tem.

⇒ If the system technology is locatedwithin the structure, it is preferableto have an isolated lightning protec-tion embedment. Care must be tak-en that the cell site infrastructure isdesigned to be geometrically smallso that the costs of the isolated light-ning protection system are economi-cally viable.

Experience has shown that, in manycases, existing lightning protection sys-tems have old defects which adverselyaffect the effectiveness of the installa-


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Fig. 5.2.3.1 Air-termination masts isolated from thebuilding

mechanical fixing

downconductor

Fig. 5.2.3.3 Air-termination masts spanned withcables with cross connection (meshing)

Fig. 5.2.3.2 Air-termination masts spanned withcables

tion. These defects mean that even if thecell site is correctly “tied-in“ to the exter-nal lightning protection system, damagecan still be caused within the structure.In order to enable a designer of mobilephone networks to erect antenna instal-lations in accordance with the standardseven in difficult situations, the only thingavailable to him used to be the isolatedlightning protection system with hori-zontal distance holders. In such cases,however, the design of the antennainstallation, could really not be consid-ered architecturally aesthetic(Fig. 5.2.4.1).Air-termination systems as shown inFig. 5.2.4.1 are not applicable for loca-tions where the antennas have to bepleasing to look at.

The isolated HVI conductor is an innova-tive solution which provides the installerof lightning protection systems withnovel possibilities for design and for easymaintaining of the separation distance.

5.2.4.1 Installation of a HVI®

isolated down conductorIf no additional measures are provided,impulse voltages > 250 kV cause flash-overs along the surface of insulatingmaterials. This effect is known as creep-ing flashover. Fig. 5.2.4.1.1 shows how acreepage discharge is caused.To prevent creepage discharges, the newHVI conductor has been equipped with aspecially doped special external coating

which enables the high “impulse volt-ages“ caused by the lightning to be“directed“ to a reference potential. Inorder to achieve this, a connectionbetween the special external coating andthe equipotential bonding must be creat-ed at a defined distance (1.40 m – 1.60 mfrom the supply point) (Figs. 5.2.4.1.2 to5.2.4.2.3). There must be no connectionbetween components of the air-termina-tion system and the down conductor.

The coaxial HVI conductor consists of a19 mm2 copper wire, thick-walled high-voltage-resistant insulation, and a specialexternal weatherproof coating.The prefabricated HVI conductor sup-plied by the manufacturer is equippedwith a matched terminal on the supplyside. The earth side is also designed for aterminal. This can be mounted on theHVI conductor on site (delivered: mount-ed on the earth side). This allows the

length of the HVI conductor to be short-ened on site.To avoid low energy flashovers arising asa result of the capacitive displacementcurrents, the HVI conductor can be addi-tionally connected to the equipotentialbonding as the conductor is beinginstalled. These terminals do not have tobe capable of conducting lightning cur-rents since the capacitive displacementcurrents are low in energy and do notlead to dangerous sparking. The HVI con-ductor with its high dielectric strengthcan be assigned an equivalent separationdistance in air of s = 0.75 m.Specifying an equivalent separation dis-tance in air of s = 0.75 m, one obtainsmax. cable lengths as a function of thelightning protection level provided bythe lighting protection system and thecurrent splitting coefficient kc.By improving the current splitting to sev-eral down conductors (reducing cc) it ispossible to further increase the max.cable length for a given lightning protec-tion level for a structure.

5.2.4.2 Installation exampleApplication for cell sitesCell site installations are frequently erect-ed on host structures. There is usually anagreement between the operator of thecell site installation and the owner of thestructure that the erection of the cell siteinstallation must not increase the risk tothe structure. For protection againstlightning, this particularly means that nopartial lightning currents must enter thestructure if there is a lightning stroke tothe frame structure. A partial lightningcurrent within the structure wouldespecially put the electrical and elec-tronic apparatus at risk.Fig 5.2.4.2.1 shows one possible solutionfor the “isolated air-termination system“on the frame structure of an antenna.


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inner conductorinsulation

proximity

Fig. 5.2.4.1.1 Basic development of a creepage discharge at an isolated down conductor without special coating

Fig. 5.2.4.1 Isolated air-termination system withdistance holder

head piece

KS connector

supporting clamp

EB terminal

earth connectionelement

sealingunit range

connection tothe equipotentialbonding

HVI¤ conductor

Fig. 5.2.4.1.2 Components of HVI Conductor

The air-termination tip must be fixed tothe frame structure of the antenna bymeans of an insulating pipe in non conductive material so that it is isolated.The height of the air-termination tip isgoverned by the requirement that thestructure of the frame and any electricaldevices which are part of the cell siteinstallation (BTS) must be arranged in thezone of protection of the air-terminationtip. Structures with several antenna sys-tems must be equipped with multiple“isolated air-termination systems“.Figs.5.2.4.2.2a and b illustrate the instal-lation on an antenna post.

Roof-mounted structuresMetal and electrical roof-mounted struc-tures protrude above roof level and areexposed points for lightning strokes. Therisk of partial lightning currents flowingwithin the structure is also existingbecause of conductive connections withconduits and electrical conductors lead-ing into the structure. To prevent this andto set up the necessary separation dis-tance for the complete structure easily,the air-termination system must beinstalled with a terminal to the isolateddown-conductor system, as shown inFig. 5.2.4.2.3.

If several structures are mounted on theroof then, according to the basic illustra-tion in Fig. 5.2.4.2.3, several isolated air-termination systems must be installed.This must be done to ensure that allstructures protruding above the roofmust be arranged in an area protectedfrom lightning strokes (lightning protec-tion zone 0B).

5.2.4.3 Project example: Trainingand residential building

StructureThe structure in Fig. 5.2.4.3.1 was builtconventionally from the ground floor tothe 6th floor.At a later date, the 7th floor wasattached to the existing roof surface. Theexternal façade of the 7th floor consistsof metal sheets.The media centre is situated on the 3rdfloor, the ground floor is used for admin-istration. All other floors up to the 7thfloor are used for appartments.The roof surface of the 6th and 7th floorswas finished off with a metal attic whosecomponents are interconnected so as tobe non-conductive.The complete structure is 25.80 m high(without attic) up to the roof level (- 7thfloor).Subsequently, five antenna systems formobile phone systems and microwaveswere installed by different operators ofmobile phone networks on the roof sur-face of the 7th floor. The antennas wereerected both in the corners and in themiddle of the roof surface.The cables (coax cables) from the fourantennas in the corners of the roof sur-face were installed in the vicinity of theattic to the south-west corner. From thispoint, the cables are led through a metalcable duct which is connected to the atticof the roof surfaces of the 7th and 6thfloors to the BTS room on the 6 th floor.The cables from the antenna in the mid-dle are also installed by means of a metalcable duct directly to the 2nd BTS roomon the north-east side of the structure tothe 6th floor. This cable duct is also con-nected to the surrounding attics.The structure was equipped with a light-ning protection system. The new installa-tion of the external lightning protectionsystem to protect against damage to thestructure and life hazards was designedin accordance with lightning protectionstandard DIN V VDE V 0185-3.

During the installation of the antennas,the equipotential bonding and earthingmeasures of the system were carried outin accordance with DIN VDE 0855 Part300.


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αα

antenna

s = separation distance

HVI¤ conductor

supporting clampGRP/Al

air-termination rod

sealing unitrange

Fig. 5.2.4.2.1 Isolated air-termination sytem withvoltage-controlled isolated down conductor

air-termination tip

feeding point

HVI¤ conductor

insulating pipe earth connection

Fig. 5.2.4.2.2a Insulating pipe within the antennaarea

earth connectionfeeding point

HVI¤

conductor

insulatingpipe

earthing clamp

Fig. 5.2.4.2.2b Connection to the antenna construc-tion for directing potential

α

metal attic cover in the pro-tective area of the isolated

air-termination system

isolated air-termination system

reinforcement

cable duct

cable duct

foundation earthing electrode

metal earthed roof-mounted structure

EB terminal

separationdistance s

HVI®

conductor I

sealing unit range

Fig. 5.2.4.2.3 Keeping the required separationdistance with voltage-controlled isola-ted down conductor (HVI)

The earthing of the systems, however,was not isolated from the existing exter-nal lightning protection system at theearth-termination system at groundlevel, but directly at the air-terminationsystem.Hence, in the event of a lightning dis-charge, partial lightning currents areconducted directly into the structure viathe coax cable shields. These partial light-ning currents do not only present a lifehazard, they also present a hazard to theexisting technical equipment of thestructure.

New conceptA lightning protection system wasrequired, which prevents partial light-ning currents from being conducteddirectly into the structure via theantenna components (frame structures,cable shields and installation systems). Atthe same time, the required separationdistance s between the frame structuresof the antennas and the air-terminationsystem on the roof surface of the 7thfloor must be realised.This cannot be effected with a lightningprotection system of a conventionaldesign.By installing the HVI conductor, a light-ning protection system was constructedwith an isolated air-termination system.This required the following components:

⇒ Air-termination tips on insulatingpipes in GRP material, secureddirectly to the antenna pole(Fig. 5.2.4.2.2a).

⇒ Down conductor from the air-termination tip by means of an HVI conductor with connection tothe isolated ring conductor(Fig. 5.2.4.3.2).

⇒ Field-controlled feeding point toensure the resistance against creeping flashovers at the input(Figs. 5.2.4.2.2a and 5.2.4.2.2b).

⇒ Isolated ring conductor on insulatingsupports made of GRP, supports ashigh as according to the calculationof the required separation distance

⇒ Down conductors installed separate-ly from the isolated ring conductorvia the respective metal attics andmetal façade to the bare metaldown conductors on the 6th floorwith the required separation dis-tance s to the lower attic(Fig. 5.2.4.3.3).

⇒ Supplementary ring conductor, alldown-conductor systems intercon-nected at a height of approx. 15 mto reduce the required separationdistance s of the interception anddown-conductor system(Figs. 5.2.4.3.4 and 5.2.4.4.1).

The various implementation stagesexplained in detail are summarised inFig. 5.2.4.3.4. It is also important to notethat the proposed design concept wasdiscussed in detail with the system erec-tor in order to avoid mistakes when car-rying out the work.When designing the external lightningprotection system, care was taken thatthe deck on the 6th floor (Fig. 5.2.4.3.1)and the lower attachments (Fig. 5.2.4.3.4)were also arranged in the zone of pro-tection/protective angle of the air-termi-nation system.


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54

3 cable tray1 2

Antennas of the cell site operators (1 - 5)

Fig. 5.2.4.3.1 Total view

isolated ring conductor

cable tray

HVI¤

conductor

Fig. 5.2.4.3.2 Isolated air-termination system andisolated ring conductorRef.: H. Bartels GmbH, Oldenburg,Germany

isolatedring conductor

HVI-Leitung

HVI® conductorconnection toequipotential bonding

Fig. 5.2.4.3.3 Down conductor of isolated ring con-ductor

5.2.4.4 Separation distanceWhen calculating the required separa-tion distance s, not only the height of thestructure but also the heights of the indi-vidual antennas with the isolated air-ter-mination system had to be taken intoconsideration.Each of the four corner antennas pro-trudes 3.6 m above the surface of theroof. The antenna in the middle pro-trudes 6.6 m above the roof surface.

Considering the height of the structure,result the following total heights to betaken into account when calculating theinstallation:

⇒ 4 corner antennas to the base of theair-termination tip + 29.40 m

⇒ 1 antenna in the middle of the roofsurface to the base of the air-termi-nation tip + 32.40 m

⇒ Three further, isolated separate air-termination rods on the west side ofthe roof surface and two isolated air-termination masts on the balcony6th floor, south side, realise the zoneof protection of the complete roofsurface.

A special cable, DEHNconductor, TypeHVI, was used as the isolated down con-ductor, allowing an equivalent separa-tion distance of s = 0.75 m (air) / 1.5 m(solid building materials) to be main-tained.The calculation of the required separa-tion distances was done as shown inFig. 5.2.4.4.1 for three partial areas :

1. Partial section with height + 32.4 mand height + 29.4 m (antennas) to +

27.3 m (isolated ring conductor) onthe roof.

2. Partial section of + 27.3 m to + 15.0 m(isolated ring conductor on roof upto lower supplementary ring con-ductor).

3. Partial section of + 15.0 to ± 0 m(lower ring conductor to groundlevel).

The complete down-conductor systemcomprises six down conductors from theisolated ring conductor at a height of +27.3 m to the supplementary ring con-ductor at ground level + 15.0 m. The ringconductor at ground + 15.0 m is connect-ed with the earthing ring conductor viathe six down conductors of the residen-tial structure and four further down con-ductors on attached parts of the struc-ture.This produces a different splitting of thecurrent in the individual partial areaswhich had to be taken into considerationfor the design of the lightning protectionsystem.The equipotential bonding required andthe earthing of the antenna componentson the roof surface (including the cableducts, metal façades and the attics onboth roof levels) was done using twosupplementary earthing cables NYY1x25 mm2 connected to the equipoten-tial bonding of the individual BTS sta-tions.The erection of this isolated air-termina-tion system on the surface of the roofand on the antenna systems, as well asthe isolated down conductors aroundmetal parts of the structure, prevent par-tial lightning currents from entering thestructure.


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air-termination tip

HVI¤ conductor

isolated ring conductor

bare downcondductorcable duct

attic

ring conductor

bare downconductor

Fig. 5.2.4.3.4 Total view on a new installed external lightning protection system

ring conductor

EB c

ondu

ctor

dow

n co

nduc

tor

kc1

kc2

kc3

L 1L 2

L 3 1st floor

2nd floor

3rd floor

4th floor

5th floor

7th floor

ground floor

6th floor

Fig. 5.2.4.4.1 Calculation of the required separation distance

5.3 Materials and minimumdimensions for air-termi-nation conductors anddown conductors

Table 5.3.1 gives the minimum cross sec-tions, form and material of air-termina-tion systems.

These requirements arise from the elec-trical conductivity of the materials to car-ry lightning currents (temperature rise)and the mechanical stresses when in use.

When using a round conductor Ø8 mm asan air-termination tip, the max. freeheight permitted is 0.5 m. The heightlimit for a round conductor Ø10 mm is1 m in free length.

Note:According to DIN V VDE V 0185-3Clause 1, Table 9, the minimum cross sec-tion for an interconnecting conductorbetween two equipotential bondingbars is 16 mm2 Cu.Tests with a PVC-isolated copper con-ductor and short strokes of 100 kA(10/350 µs) determined a temperaturerise of around 56 K. Thus, a cable NYY1 x 16mm2 Cu can be used as a down con-ductor or as a surface and undergroundinterconnecting cable, for example.


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Material Form Min. cross- Remarkssectionmm2

Copper tape 50 min. thickness 2 mmround 50 Ø8 mmcable 50 min. diameter per wire 1.7 mmround c, d 200 Ø16 mm

Tin-coated tape 50 min. thickness 2 mmcopper a round 50 Ø8 mm

cable 50 min. diameter per wire 1.7 mm

Aluminium tape 70 min. thickness 3 mmround 50 Ø8 mmcable 50 min. diameter per wire 1.7 mm

Aluminium tape 50 min. thickness 2.5 mmalloy round 50 Ø8 mm

cable 50 min. diameter per wire 1.7 mmround c 200 Ø16 mm

Hot-dip tape 50 min. thickness 2.5 mmgalvanised round 50 Ø8 mmsteel b cable 50 min. diameter per wire 1.7 mm

round c, d 200 Ø16 mm

Stainless tape f 60 min. thickness 2 mmsteel e tape 105 min. thickness 3 mm

round f 50 Ø8 mmcable 70 min. diameter per wire 1.7 mmround c 200 Ø16 mmround d 78 Ø10 mm

a Tin-coated or galvanised, mean value 2 µm.b The zinc coating should be smooth, continuous and free of residual flux,

mean value 50 µm.c For air-termination rods only. For applications where mechanical loads, like

wind loads, are not critical, a max. 1 m long rod can be used, which is madeof 10 mm round material.

d For lead-in earthing rods only.

e Chromium ≥ 16 %, nickel ≥ 8 %, carbon max. 0.03 %f For stainless steel in concrete and/or in direct contact with flammable mate-

rial, the min. cross section for round material has to be increased to 75 mm2

(Ø10 mm) for round material and to 75 mm2 (thickness 3 mm) for flat mate-rial

Table 5.3.1 Material, form and min. cross sections of air-termination conductors, air-termination rods anddown conductors

5.4 Assembly dimensionsfor air-termination and down-conductor systems

The following dimensions (Fig. 5.4.1)have been tried and tested in practiceand are primarily determined by themechanical forces acting on the compo-nents of the external lightning protec-tion system.These mechanical forces arise not somuch as a result of the electrodynamicforces generated by the lightning cur-rents, but more as a result of the com-pression forces and the tensile forces,e. g. due to temperature-dependentchanges in length, wind loads or theweight of snow.The information concerning the max. dis-tances of 1.2 m between the conductorholders primarily relates to St/tZn (rela-tively rigid). For using aluminium, dis-tances of 1 m have become the norm inpractice.

DIN V VDE V 0185-3 gives the followingassembly dimensions for an externallightning protection system (Fig. 5.4.2).

Fig. 5.4.3 illustrates the application on aflat roof.

If possible, the separation distances towindows, doors and other openingsshould be maintained when installingdown conductors.Further important assembly dimensionsare:Installation of surface earthing elec-trodes (e. g. ring earthing electrodes)around the structure at a depth of

> 0.5 m and a distance of approx. 1 mfrom the structure (Fig. 5.4.4).

When driving in several earth rods nextto each other (necessitated by groundconditions), the earth rods should be sep-arated by at least the pile depth. Theindividual earth rods must be intercon-nected.For the earth entries or terminals on thefoundation earthing electrode (ringearthing electrodes), corrosion protec-tion must be considered. Measures suchas anticorrosive bands or wires with PVCsheath at a min. of 0.3 m above andbelow the turf (earth entry) must beemployed (Fig. 5.4.5) for protection.An optically acceptable and corrosion-free connection possibility is provided bya stainless steel fixed earthing terminalset to be laid in concrete.Moreover, there must also be corrosionprotection for the terminal lug forequipotential bonding inside the build-ing in damp and wet rooms.


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Fig. 5.4.1 Detail examples of an external lightning protection system at a building with an inclined tiled roof

max. 1.2 m 0.15

m

max.

1.2 m

0.4 m

0.2 m

1.2

mm

ax.

0.5 m

0.3

m1.

5 m

0.5

m

Fig. 5.4.2 Air-termination rod for chimneys

e = 0.2 m appropriate distance

Fig. 5.4.3 Application on a flat roof

building

0.5

m

1 m

Fig. 5.4.4 Dimensions for ring earthing electrodes

0.3 m

0.3 m

corrosion protection

Fig. 5.4.5 Points threatened by corrosion

The material combinations below (withinair-termination systems, down conduc-tors and with parts of the structure) havebeen tried and tested, provided that noparticularly corrosive environmental con-ditions must be taken into consideration.These are values obtained from experi-ence (Table 5.4.1).

5.4.1 Change in length of metalwires

In practice, the temperature-dependentchanges in length of air-termination anddown conductors are often underesti-mated.The older regulations and stipulationsrecommended an expansion piece aboutevery 20 m as a general rule in manycases. This stipulation was based on theuse of steel wires, which used to be theusual and sole material employed. Thehigher values for the coefficients of lin-ear expansion of stainless steel, copperand especially aluminium materials werenot taken into account.In the course of the year, temperaturechanges of 100 K must be expected onand around the roof. The resultingchanges in length for different metalwire materials are shown in Table 5.4.1.1.It is noticeable that, for steel and alu-minium, the temperature-dependentchanges in length differ by a factor of 2.The stipulations governing the use ofexpansion parts in practice are thus asshown in Table 5.4.1.2.

When using expansion pieces, care mustbe taken that they provide flexiblelength equalisation. It is not sufficient tobend the metal wires into an S shapesince these “expansion pieces”, hand-made on site, are not sufficiently flexible.When connecting air-termination sys-tems, for example to metal attics sur-rounding the edges of roofs, care shouldbe taken that there is a flexible connec-tion to suitable components or measures.If this flexible connection is not made,there is a risk that the metal attic coverwill be damaged by the temperature-dependent change in length.

To compensate for the temperature-dependent changes in length of the air-termination conductors, expansionpieces must be used to equalise theexpansion (Fig. 5.4.1.1).


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Steel Alu- Copper StSt Titanium Tin(tZn) minium

Steel (tZn) yes yes no yes yes yes

Aluminium yes yes no yes yes yes

Copper no no yes yes no yes

StSt yes yes yes yes yes yes

Titanium yes yes no yes yes yes

Tin yes yes yes yes yes yes

Table 5.4.1 Material combinations

Table 5.4.1.1 Calculation of the temperature-related change in length ∆L of metal wires in lightning protec-tion

Assumed change in temperature on the roof: ∆T = 100 K

Steel ∆L = 11 • 10-6 • 100 cm • 100 = 0.11 cm = 1.1 mm/m

Stainless steel ∆L = 16 • 10-6 • 100 cm • 100 = 0.16 cm = 1.6 mm/m

Copper ∆L = 17 • 10-6 • 100 cm • 100 = 0.17 cm = 1.7 mm/m

Aluminium ∆L = 24 • 10-6 • 100 cm • 100 = 0.24 cm ≈ 2.4 mm/m

Material Surface under the fixing of the Distanceair-termination system or down conductor between the

expansion soft, hard, pieces

e.g. flat roof with e.g. pantiles in mbitumen- or or

synthetic roof sheetings brickwork

Steel X ≈ 15X ≤ 20

StSt / X ≈ 15Copper X ≤ 15

Aluminium X X ≤ 10

Use of expansion pieces, if no other length compensation is provided

Table 5.4.1.2 Expansion pieces in lightning protection - Recommended application

Fig. 5.4.1.1 Air-termination system - Compensationof expansion with bridging braid

Calculation formula:

∆ ∆L L T= α i i

Material Coefficient of linearexpansion α

1 1⎯⎯ ⎯106 K

Steel 11

Stainless steel 16

Copper 17

Aluminium 24

5.4.2 External lightning protectionsystem for a residential house

Fig. 5.4.2.1 illustrates the design of theexternal lightning protection system fora residential house with attachedgarage.Fig. 5.4.2.1 and Table 5.4.2.1 show exam-ples of the components in use today. No account is taken of the measuresrequired for an internal lightning protec-tion system such as lightning equipoten-tial bonding and surge protection (seealso Chapter 6).

Particular attention is drawn to DEHN’sDEHNsnap and DEHNgrip programme ofholders.The DEHNsnap generation of syntheticholders (Fig. 5.4.2.2) is suitable as a basiccomponent (roof and wall). The cap sim-ply snaps in to fix the conductor in theholder while still being loosely guided.The special snap-in technique exerts nomechanical load on the fastening.DEHNgrip (Fig. 5.4.2.2) is a screwlessstainless steel system of holders whichwas put into the programme to supple-ment the DEHNsnap system of syntheticholders.

This screwless system of holders can alsobe used as both a roof and a wall con-ductor holder for Ø8 mm conductors.Simply press in the conductors and theconductor is fixed in DEHNgrip (Fig. 5.4.2.2).


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EB

Fig. 5.4.2.1 External lightning protection of a residential building

4

5

3

11

7

2

6 8

1213

10

1

15

9

14

16

20

21

19

1718

base part

cap

DEHNgripconductor holder

DEHNsnapconductor holder

Fig. 5.4.2.2 DEHNsnap und DEHNgrip conductor holders

Pos. in Part description Part No.Fig. 5.4.2.1

Bridging bracket made of aluminium 377 006

Bridging braid made of aluminium 377 015

Lead-in earthing rod Ø16 mm 480 150complete 480 175

Rod holder with flange 275 116275 260

Parallel connector 305 000306 020

Cross unit 319 201SV clamps made of St/tZn 308 220SV clamps made of StSt 308 229

Rod holder with cleat and flange 275 260for heat insulation 273 730

Number plate for 480 006marking isolating points 480 005

Air-termination rod with forged tab 100 075with rounded ends 483 075

Rod clamp 380 020

Rod holder with tip 262 130

Earth rod St/tZn 620 150sectional unit with bolt 625 150and hole 620 151

625 151

Impact tip for deep-driven 620 001earth rods 625 001

Connecting clamp for earth rods 620 011unilateral 625 011

for earth rods 620 015625 015

21

20

19

18

17

16

15

14

13

12

11


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Pos. in Part description Part No.Fig. 5.4.2.1

Round conductor Ø8 mm – 840 008DEHNALU, medium-hard, 840 018soft-twistable

Steel strip 30 x 3.5 mm – St/tZn 810 335

Round conductor Ø10 mm – 860 010StSt V4A

Roof conductor holders 202 020for ridge and hip tiles 204 109

204 249204 269206 109206 239

Roof conductor holders 204 149for conductors within 204 179roof surfaces 202 010

202 050202 080206 209206 309

DEHNsnap 204 006DEHNgrip 207 009conductor holder

with cleat and flange 275 160for heat insulation 273 740

Gutter clamp for beads 339 050made of stainless steel 339 059

Single-screw gutter glamp 339 100made of stainless steel 339 109

MV clamp made of Al 390 050MV clamp made of StSt 390 059

Gutter board clamp 343 000

Downpipe clampadjustable for Ø60-150 mm 423 020for any cross sections 423 200

KS connector for connecting conductors 301 000made of StSt 301 009

MV clamp 390 05110

9

8

7

6

5

4

3

2

1

Table 5.4.2.1 Components for external lightning protection of a residential builiding

5.4.3 Application tips for mountingroof conductor holders

Ridge and hip tiles:Adjust roof conductor holders withadjusting screw to suit the dimension ofthe ridge tile (Fig. 5.4.3.1).

The conductor leading can, in addition,be gradually adjusted by means of con-ductor holders from the top centre to thebottom side.(conductor holder can be loosened byeither turning the holder or opening thefixing screw.)

SPANNsnap roof conductor holder withDEHNsnap synthetic conductor holder orDEHNgrip stainless steel conductor hold-er (Fig. 5.4.3.2).

Permanent tension due to stainless steeltension spring. Universal tension rangefrom 180-280 mm with laterallyadjustable conductor leading for Rd8 mm conductors.

FIRSTsnap conductor holder with DEHN-snap synthetic conductor holder for put-ting on existing ridge clamps for dryridges.

For dry ridges, the DEHNsnap conductorholder (1) (Fig. 5.4.3.3) is put on the ridgeclamp already on the structure (2) andtightened manually (only turn DEHN-snap).

Grooved pantiles:The roof conductor holder with pre-formed struts is used for the roof sur-faces. The conductor holder is bent byhand before being hooked into the bat-tens. Additionally, it can also be securedwith nails (Fig. 5.4.3.4).

Smooth tiles (Fig. 5.4.3.5):

Slate roofs:When using it on slate roofs, the internalhook system is bent (Fig. 5.4.3.6) orequipped with a supplementary clamp(Part No. 204 089).


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Fig. 5.4.3.1 Conductor holder with DEHNsnap forridge tiles

Fig. 5.4.3.2 SPANNsnap with plastic DEHNsnap con-ductor holder

Fig. 5.4.3.3 FIRSTsnap for mounting on existing rid-ge clamps

angled by hand

Fig. 5.4.3.4 Roof conductor holder with preformedbrace - Used on grooved pantiles

Fig. 5.4.3.5 Roof conductor holder with preformedbrace - Used on smooth tiles,e.g. plain tiles

angle the inner latchingfor use on slate roofs

Fig. 5.4.3.6 Roof conductor holder with preformedbrace - Used on slate roofs

Grooved tiles:FLEXIsnap roof conductor holder forgrooved tiles, for direct fitting on thegroove (Fig. 5.4.3.7).

The flexible stainless steel strut is pushedbetween the grooved tiles.By pressing on the top grooved tile, thestainless steel strut is deformed andadapts itself to the shape of the groove.Thus it is fixed tightly under the tile.This application with an aluminium strutmakes it easy to adapt to the shape ofthe groove.A notch is provided for an eventuallyexisting window hook.The strut of the holder can also be naileddown (holes in the strut).

Roof conductor holders with preformedstrut, for hooking into the bottomgroove for pantile roofs (Fig. 5.4.3.8).

Flat tiles or slabs:DEHNsnap conductor holder (1)(Fig. 5.4.3.9) and its clamping device (2) ispushed in between the flat tiles (3) (e. g.plain tile) or slabs and tightened manual-ly (only turn DEHNsnap).

Overlapped constructions:In case of overlapped constructions (3)(e. g. slabs and natural slates), DEHNsnapconductor holder (1) (Fig. 5.4.3.10) withclamping terminals (2) is pushed on fromthe side and secured with a screw driverwhen the holder is open.For slabs laid on a slant, DEHNsnap canalso be turned to allow a plumb conduct-or leading.


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insert the holderunderneath

lift tile

press tileon it

Fig. 5.4.3.7 Conductor holder for direct fitting on theseams

insert the holderunderneath

lift tile

press tileon it

Fig. 5.4.3.8 Roof conductor holder for hanging intothe bottom seam of pantile roofs

Fig. 5.4.3.9 ZIEGELsnap, for fixing between flat tilesor plates

Fig. 5.4.3.10 PLATTENsnap roof conductor holderfor overlapped constructions

5.5 Earth-termination sys-tems

A detailed explanation of the terms usedin earth-termination technology is con-tained in DIN V VDE V 0185-3 "Lightningprotection – physical damage to struc-tures and life hazards", DIN VDE 0101"Power supply systems for nominal acvoltages above 1 kV", DIN VDE 0100"Erection of power supply systems withnominal voltages up to 1000 V" (Part 200and Part 540). Below, we repeat only theterminology which is required to under-stand the following designs.

TerminologyEarthis the conductive ground whose electricalpotential at each point is set equal tozero as agreed. The word "earth" is alsothe designation for both the earth as aplace as well as earth as a material, e. g.the type of soil: humus, loam, sand,gravel and rock.

Reference earth(neutral earth) is the part of the earth,especially the surface of the earth out-side the sphere of influence of anearthing electrode or an earth-termina-tion system, in which, between two arbi-trary points, no perceptible voltages aris-ing from the earthing current occur(Fig. 5.5.1).

Earthing electrodeis a conductive component or severalconductive components in electrical con-tact with the earth and forming an elec-

trical connection with it (includes alsofoundation earthing electrodes).

Earth-termination systemis a localised entirety of interconnectedconductive earthing electrodes or metalcomponents acting as such, (e. g. rein-forcements of concrete foundations,cable metal sheaths in contact with theearth, etc.).

Earthing conductoris a conductor connecting a system com-ponent to be earthed to an earthingelectrode and which is installed abovethe ground or insulated in the ground.

Lightning protection earthingis the earthing installation of a lightningprotection system to discharge lightningcurrents into the earth.

Below some types of earthing electrodesand their classification are describedaccording to location, form and profile.

Classification according to location

Surface earthing electrodeis an earthing electrode generally drivenin at a shallow depth down to 1 m. It canconsist of round material or flat stripsand be designed as a star-type, ring ormeshed earthing electrode or a combina-tion thereof.

Earth rodis an earth rod generally driven in plumbdown to greater depths. It can consist ofround material or material with anotherprofile, for example.

Classification according to form andprofileOne distinguishes between:flat strip earthing electrodes, cruciformearthing electrodes and earth rods.

Natural earthing electrodeis a metal component in contact with theearth or with water either directly or viaconcrete, whose original function is notas an earthing electrode but which actsas an earthing electrode (reinforcementsof concrete foundations, conduits, etc.).

Foundation earthing electrodeis a conductor embedded in concretewhich is in contact with the earth over awide area.

Control earthing electrodeis an earthing electrode whose form andarrangement serves more to control thepotential than to maintain a certainearthing electrode resistance.

Types of resistance

Specific earth resistanceρE is the specific electrical resistance ofthe earth. It is given in Ωm and repre-sents the resistance between two oppo-site sides of a cube of earth with edges of1 m in length.

Earthing electrode resistanceRA of an earthing electrode is the resist-ance of the earth between the earthingelectrode and reference earth. RA is prac-tically a resistance.

Impulse earthing resistanceRst is the resistance as lightning currentstraverse from one point of an earth-ter-mination system to the reference earth.

Voltages at current carrying earth-termi-nation systems, control of potential

Earth potentialUE is the voltage arising between anearth-termination system and referenceearth (Fig. 5.5.1).

Potential of the earth’s surfaceϕ is the voltage between one point of theearth’s surface and reference earth (Fig. 5.5.1).

Shock hazard voltageUB is the part of the potential of theearth’s surface which can be bridged byhumans (Fig. 5.5.1), the current path viathe human body running from hand tofoot (horizontal distance from touchablepart around 1 m) or from one hand to theother.


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1 m

UB2

ϕFE

US

FE

ϕ

UB1 ϕFE + SE U

E

UE Earth potentialUB Shock hazard voltageUB1 Shock hazard voltage without poten-

tial control (at the foundation earthingelectrode)

UB2 Shock hazard voltage with potentialcontrol (foundation and controlearthing electrode)

US Step voltageϕ Earth surface potentialFE Foundation earthing

electrodeCE Control earthing electrode

(ring earthing electrode)

reference earth

CE

Fig. 5.5.1 Earth surface potential and voltages at a foundation earthing electrode FE and control earthingelectrode CE flown through by currents


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Step voltageUS is the part of the potential of theearth’s surface which can be bridged byhumans taking one step 1 m long, thecurrent path via the human body run-ning from one foot to the other(Fig. 5.5.1).

Potential controlis the effect of the earthing electrodes onthe earth potential, particularly thepotential of the earth’s surface(Fig. 5.5.1).

Equipotential bondingfor lightning protection systems is theconnection of metal installations andelectrical systems to the lightning protec-tion system via conductors, lightning cur-rent arresters or isolating spark gaps.

earthing electrode resistance / Specificearth resistance

earthing electrode resistance RAThe conduction of the lightning currentvia the earthing electrode into theground does not happen at one pointbut rather energises a particular areaaround the earthing electrode.The type of earthing electrode and theway it is installed must now be chosen toensure that the voltages affecting thesurface of the earth (shock hazard andstep voltages) do not assume hazardousvalues.The earthing electrode resistance RP ofan earthing electrode can best beexplained with the help of a metalsphere buried in the ground.If the sphere is buried deep enough, thecurrent discharges radially to be equallydistributed over the surface of thesphere. Fig. 5.5.2a illustrates this case; asa comparison, Fig. 5.5.2b illustrates thecase of a sphere buried just under theearth’s surface.The concentric circles around the surfaceof the sphere represent surfaces of equalvoltage. The earthing electrode resist-ance RA is composed of the partial resist-ances of individual layers of the sphereconnected in series.

The resistance of such a layer of thesphere is calculated using

where ρE is the specific earth resistance ofthe ground, assuming it is homogeneous,

l the thickness of an imaginary layerof the sphere

and

q the medial surface of this layer of thesphere

To illustrate this, we assume a metalsphere 20 cm in diameter buried at adepth of 3 m at a specific earth resistanceof 200 Ωm.

If now the increase in earthing electroderesistance for the different layers of thesphere is calculated, then as a function ofthe distance from the centre of thesphere, a curve as shown in Fig. 5.5.3 isobtained.The earthing electrode resistance RA forthe spherical earthing electrode is calcu-lated using:

ρE Specific earth resistance in Ωm

t Burial depth in cm

rK Radius of the spherical earthing elec-trode in cm

This formula gives a earthing electroderesistance of RA = 161 Ω for the sphericalearthing electrode.

The trace of the curve in Fig. 5.5.3 showsthat the largest fraction of the totalearthing electrode resistance occurs inthe immediate vicinity of the earthingelectrode. Thus, for example, at a dis-tance of 5 m from the centre of thesphere, 90 % of the total earthing elec-

trode resistance RA has already beenachieved.

Specific earth resistance ρEThe specific earth resistance ρE, whichdetermines the magnitude of theearthing electrode resistance RA of anearthing electrode, is a function of thecomposition of the soil, the amount ofmoisture in the soil and the temperature.It can fluctuate between wide limits.

Values for various types of soilFig. 5.5.4 gives the fluctuation ranges ofthe specific earth resistance ρE for varioustypes of soil.

Seasonal fluctuationsExtensive measurements (literature) haveshown that the specific earth resistancevaries greatly according to the burialdepth of the earthing electrode. Owingto the negative temperature coefficientof the ground (α = 0.02 ... 0.004), the spe-cific earth resistances attain a maximumin winter and a minimum in summer. It istherefore advisable to convert the meas-ured values obtained from earthing elec-trodes to the maximum prospective val-ues, since even under unfavourable con-ditions (very low temperatures), permissi-ble values must not be exceeded. Thecurve of the specific earth resistance ρE as

Rr

r

AE

K

K

=+ρ

π

ti

ii

100

2

12

2

RI

qE= ρ i

equipotential lines

a) Spherical earthingelectrode deep inthe ground

b) Spherical earthingelectrode close to theearth surface

Fig. 5.5.2 Current distribution from the spherical earthing electrode

1 2 3 4 5

160

140

120

100

80

60

40

20

RA = 161 Ω

Eart

hing

ele

ctro

de re

sist

ance

RA

(Ω)

approx. 90%

Distance x (m)

Fig. 5.5.3 Earthing electrode resistance RA of a sphe-rical earthing electrode with Ø20 cm, 3 mdeep, at ρE = 200 Ωm as a function of thedistance x from the centre of the sphere

0.1 1 10 100 1000 10000 ρE in Ωm

Concrete

Boggy soil, turf

Farmland, loam

Humid sandy soil

Dry sandy soil

Rocky soil

Gravel

Lime

River and lake water

Sea water

Fig. 5.5.4 Specific earth resistance ρE of different ground types

a function of the season (ground temper-ature) can be represented to a very goodapproximation by a sine curve having itsmaximum around the middle of Februaryand its minimum around the middle ofAugust. Investigations have furthershown that, for earthing electrodesburied not deeper than around 1.5 m,the maximum deviation of the specificearth resistance from the average isaround ± 30 % (Fig. 5.5.5).

For earthing electrodes buried deeper(particularly for earth rods), the fluctua-tion is merely ± 10 %. From the sine-shaped curve of the specific earth resist-ance in Fig. 5.5.5, the earthing electroderesistance RA of an earth-termination sys-tem measured on a particular day can beconverted to the maximum prospectivevalue.

MeasurementThe specific earth resistance ρE is deter-mined using an earthing measuringbridge with 4 clamps which operatesaccording to the null method.Fig. 5.5.6 illustrates the measuringarrangement of this measuring methodnamed after WENNER. The measurementis carried out from a fixed central pointM which is retained for all subsequentmeasurements. Four measuring probes(earthing spikes 30 ... 50 cm long) aredriven into the soil along a line a – a'pegged out in the ground. From the

measured resistance R one can determinethe specific earth resistance ρE of theground:

R measured resistance in Ωe probe distance in m

ρE average specific earth resistance inΩm down to a depth correspondingto the probe distance e

By increasing the probe distance e andre-tuning the earthing measuring bridge,the curve of the specific earth resistancecan be determined ρE as a function of thedepth.

Calculation of earthing electrode resist-ances

Table 5.5.1 gives the formulae for calcu-lating the earthing electrode resistancesof the most common types of earthingelectrode. In practice, these approximateformulae are quite sufficient. The preciseformulae for the calculations must betaken from the following sections.

Straight surface earthing electrodeSurface earthing electrodes are generallyembedded horizontally in the ground ata depth of 0.5...1 m. Since the layer of soilcovering the earthing electrode dries outin summer and freezes in winter, theearthing electrode resistance RA of such asurface earthing electrode is calculatedas if it lays on the surface of the ground:

RA earthing electrode resistance of astretched surface earthing electrodein Ω


l Length of the surface earthing elec-trode in m

d Half the width of steel strip in m ordiameter of the round wire in m

RI

l

dAE=

ρπ

In

ii

i2

ρ πE e R= 2 i i


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302010

0102030

burial depth < 1.5 m+ ρE in %

burial depth > 1.5 m

− ρE in %

June July Aug. Sept. Oct. Nov.

Jan. Feb. March April May Dec.

Fig. 5.5.5 Specific earth resistance ρE as a functionof the seasons without influencing of rain-fall (burial depth of the earthing electrode< 1.5 m)

e e e

a M a’

measuringdevice

Fig. 5.5.6 Determination of the specific earth resi-stance ρE with a four-terminal measuringbridge acc. to the WENNER method

Earthing electrode resistance Rough estimate Auxiliary

Surface earthing electrode ⎯(star-type earthing electrode)

earth rod ⎯(earth rod)

Ring earthing electrode

Meshed earthing electrode

Earth plate ⎯

Hemispherical earthing electrode

RA Earthing electrode resistance (Ω)

ρE Specific earth resistance (Ωm)

I Length of the earthing electrode (m)

D Diameter of a ring earthing electrode, of the area of the equivalent circuitor of a hemispherical earthing electrode

A Area (m2) of the enclosed area of a ring or meshed earthing electrode

a Edge length (m) of a square earth plate, for rectangular plates value:, while b and c are the two sides of the rectangle

V Content (m3) of a single foundation element

b ci

D = 1 57. V3iRAE=

ρπ Di

RAE=

ρ4.5 ai

D = 1 13. A2iRAE=

ρ2 Di

D = 1 13. A2iRAE=

2

3 D

iiρ

RlA

E= ρ

RlA

E=2 i ρ

Table 5.5.1 Formulae for calculating the earthing electrode resistance RA for different earthing electrodes


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The earthing electrode resistance RA as afunction of the length of the earthingelectrode can be taken from Fig. 5.5.7.

Fig. 5.5.8 shows the transverse and longi-tudinal earthing potential UE for an 8 mlong flat strip earthing electrode.The effect of the burial depth on theearthing potential can be clearly seen.

Fig. 5.5.9 illustrates the step voltage US asa function of the burial depth.

In practice, the calculation is done usingthe approximate formula in Table 5.5.1:

Earth rodThe earthing electrode resistance RA of aearth rod is calculated using:

RA Earthing electrode resistance in ΩρE Specific earth resistance in Ωm

l Length of the earth rod in m

d Diameter of the earthing rod in m

As an approximation, the earthing elec-trode resistance RA can be calculatedusing the approximate formula given inTable 5.5.1:

Fig. 5.5.10 shows the earthing electroderesistance RA as a function of the rodlength l and the specific earth resistanceρE.

Combination of earthing electrodes

The earthing electrode resistances calcu-lated using the formulae and the meas-urement results given in the diagramsapply to low frequency dc current and accurrent provided that the expansion ofthe earthing electrode is relatively small(a few hundred metres). For longerlengths, e. g. for surface earthing elec-trodes, the ac current also has an induc-tive part. Furthermore, the calculated earthingelectrode resistances do not apply tolightning currents. This is where theinductive part plays a role, which canlead to higher values of the impulseearthing resistance for larger expansionof the earth-termination system. Increasing the length of the surfaceearthing electrodes or earth rods above30 m reduces the impulse earthing elec-trode resistance by only an insignificantamount. It is therefore expedient to com-bine several shorter earthing electrodes.In such cases, because of their interac-tion, care must be taken that the actualtotal earthing electrode resistance isgreater than the value calculated fromthe individual resistances connected inparallel.

Star-type earthing electrodesStar-type earthing electrodes in the formof cruciform surface earthing electrodesare important when relatively lowearthing electrode resista nces shall becreated in poorly conducting ground atan affordable price.The earthing electrode resistance RA of acruciform surface earthing electrodewhose sides are at 90° to each other iscalculated using:

RA Earthing electrode resistance of thecruciform surface earthing electrodein Ω


l Side length in m

d Half a bandwidth in m or diameterof the round wire in m

As a rough approximation, for longerlengths of the star arrangement (l > 10 m), the earthing electrode resist-ance RA can be determined using thetotal length of the star obtained fromthe equations in Table 5.5.1.

Rl

l

dAE= +⎛

⎝⎜⎞⎠⎟

ρπ4

22 5

ii

iln .

RlAE=

ρ

Rl

l

dAE=

ρπ2

2

ln

ii

i

RlA

E=2 i ρ

UE

100

80

60

40

20

a

UE

100

80

60

40

20

a

a

t

a

t

V

V

100 cm

t = 0 cm50 cm

t = 0 cm

50 cm100 cm

LONGITUDINAL DIRECTION

TRANSVERSE DIRECTION

Eart

h po

tent

ial U

E (%

)Ea

rth

pote

ntia

l UE (

%)

Distance a (m) from earthing electrode

Distance a (m) from earthing electrode

Fig. 5.5.8 Earth potential UE between supply con-ductor and earth surface as a function ofthe distance from the earthing electrode,at an earth strip (8 m long) in differentdepths

10080604020

0.5 1 1.5 2m

%

Max

. ste

p vo

ltage

in %

of th

e to

tal v

olta

ge

Burial depth

2 4 6 8 10 12 14 16 18 20

100

80

60

40

20ρE = 100 Ωm

Earthing electrode resistance RA

Drive-in depth l of the earth rod

ρE = 500 Ωm

ρE = 200 Ωm

Fig. 5.5.9 Max. step voltage US as a function of theburial depth for a stretched earth strip

Fig. 5.5.10 Earthing electrode resistance RA of earthrods as a function of their length l at dif-ferent specific earth resistances ρE

50 100

100

50

ρE = 100 Ωm

ρE = 200 Ωm

ρE = 500 Ωm

Earthing electrode resistance RA (Ω)

Length I of the stretched surfaceearthing electrode (m)

Fig. 5.5.7 Earthing electrode resistance RA as a func-tion of length I of the surface earthingelectrode at different specific earth resi-stance ρE

Fig. 5.5.11 shows the curve of theearthing electrode resistance RA of cruci-form surface earthing electrodes as afunction of the burial depth;

Fig. 5.5.12 shows the curve of the earthing voltage.

For star-type earthing electrodes, theangle between the individual armsshould be greater than 60°.According to Fig. 5.5.12 the earthingelectrode resistance of a meshedearthing electrode is given by the formu-la:

Where D is the diameter of the analo-gous circle having the same area as themeshed earthing electrode, which isdetermined as follows:For rectangular or polygonal dimensionsof the meshed earthing electrode:

A Area of the meshed earthing elec-trode

For square dimensions (edge length b):

Fig. 5.5.13 illustrates the curve of theimpulse earthing electrode resistance ofsurface earthing electrodes with singleand multiple stars for square-wave volt-ages. As can be seen from this diagram, for a

given length, it is more expedient toinstall a radial earthing electrode thanone single arm.

Foundation earthing electrodeThe earthing electrode resistance of ametal conductor in a concrete founda-tion can be calculated as an approxi-mation using the formula for hemispher-ical earthing electrodes:

Where D is the diameter of the analo-gous hemisphere having the same vol-ume as the foundation

V Volume of the foundation

When calculating the earthing electroderesistance, one must be aware that thefoundation earthing electrode can onlybe effective if the concrete body has alarge contact area with the surroundingground. Water repellent, isolating shield-ing significantly increases the earthearthing electrode resistance.

Earth rods connected in parallelTo keep the interactions within accept-able limits, the distances between theindividual earthing electrodes and earthrods connected in parallel should not beless than the pile depth, if possible.If the individual earthing electrodes arearranged roughly in a circle and if they allhave about the same length, then theearthing electrode resistance can be cal-culated as follows:

Where RA' is the average earthing elec-trode resistance of the individualearthing electrode. The reduction factorp as a function of the length of theearthing electrode, the distance of theindividual earthing electrodes and thenumber of earthing electrodes can betaken from Fig. 5.5.14.

RR

pAA= '

D V= 1 57. i

RDA

E=ρ

π i

D b= 1 1. i

DA

= i 4

π

RDA

E=ρ

2 i


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l

Earthing electrode resistance RA (Ω)

Burial depth (m)

l = Side length

ρE = 200 Ωm

l = 10 m

l = 25 m

%

14

12

10

8

6

4

2

0.5 1 1.5

Fig. 5.5.11 Earthing electrode resistance RA of crossed surface earthing electrodes (90°)as a function of the burial depth

%

100

80

60

40

20

10 20 30 m

45°

Voltage

Distance from the centre of the intersection

direction ofmeasurement II

direct

ion of

measu

remen

t I

Side length 25 m

Fig. 5.5.12 Earth potential UE between the supplyconductor of the earthing electrode andearth surface of crossed surface earthingelectrodes (90°) as a function of thedistance from the cross centre point(burial depth 0.5 m)

0 1 2 3 4 5 6

Ω160

140

120

100

80

60

40

20

0

Impu

lse

eart

h re

sist

ance

Rst

Time µs

n = 12

3

4

RA = 10 Ω

l

n = 4Z = 150 ΩRA = 10 Ωn = 1 ... 4n · l = 300 m

Z Surge impedance of the earth conductorRA Earthing electrode resistancen Quantity of the parallel connected earthingl Mean length of the earthing electrodes

Fig. 5.5.13 Impulse earth resistance Rst of single ormultiple star-type earthing electrodeswith equal length

al

n = 20

10

5

3

2

p Reduction factorn Quantity of the parallel connected earthing

electrodesa Mean distance of the earthing electrodesl Mean length of the earthing electrodes

0.5 1 2 5 10

20

10

5

3

2

1

p

Fig. 5.5.14 Reduction factor p for calculating thetotal earthing electrode resistance RA ofearth rods connected in parallel

Combination of flat strip earthingelectrodes and earth rodsIf sufficient earthing electrode resistanceis provided by earth rods, for examplefrom deep water carrying layers in sandysoil, then the earth rod shall be as close aspossible to the object to be protected. Ifa long feed is required, it is expedient toinstall a radial multiple star-type earthingelectrode in parallel to this in order toreduce the resistance as the current rises.

As an approximation, the earthing elec-trode resistance of a flat strip earthingelectrode with earth rod can be calculat-ed as if the flat strip earthing electrodewere extended by the drive-in depth ofthe earth rod.

Ring earthing electrodeFor circular ring earthing electrodes withlarge diameters (D > 30 m), the earthingelectrode resistance is calculated as anapproximation using the formula for theflat strip earthing electrode (where thecircumference π • D is used for the lengthof the earthing electrode):

For non-circular ring earthing electrodes,the earthing electrode resistance is calcu-lated by using the diameter D of an anal-ogous circle with the same area:

A Area enclosed by the ring earthingelectrode

ImplementationAccording to the DIN VDE standards,each installation to be protected musthave its own earth-termination systemwhich must be fully functional in itselfwithout requiring metal water pipes orearthed conductors of the electricalinstallation.The magnitude of the earthing electroderesistance RA is of only secondary impor-tance for protecting a structure or instal-lation against physical damage. It isimportant that the equipotential bond-ing at ground level is carried out system-

atically and the lightning current is safelydistributed in the ground.

The lightning current i raises the struc-ture to be protected to the earthingpotential UE

with respect to the reference earth.

The potential of the earth’s surfacedecreases with increasing distance fromthe earthing electrode (Fig. 5.5.1).The inductive voltage drop across theearthing electrode during the lightningcurrent rise must only be taken intoaccount for extended earth-terminationsystems (e. g. as required for long surfaceearthing electrodes in poorly conductingsoils with bedrock). In general, theearthing electrode resistance is deter-mined only by the ohmic part.

If isolated conductors are led into thestructure, the earthing potential UE hasits full value with respect to the conduct-or.In order to avoid the risk of puncturesand flashovers here, such conductors areconnected via isolating spark gaps orwith live conductors via surge protectivedevices (see DEHN main catalogue forSurge Protection) to the earth-termina-tion system as part of the lightningequipotential bonding.

In order to keep contact and step volt-ages as low as possible, the magnitude ofthe earthing electrode resistance must belimited.The earth-termination system can bedesigned as a foundation earthing elec-trode, a ring earthing electrode and, forstructures with large surface areas, as ameshed earthing electrode and, in spe-cial cases, also as an individual earthingelectrode.Foundation earthing electrodes must bedesigned in accordance with DIN 18014.The foundation earthing electrode mustbe designed as a closed ring andarranged in the foundations of the exter-nal walls of the structure, or in the foun-dation slab, in accordance with DIN18014. For larger structures, the founda-tion earthing electrode should containinterconnections to prevent an exceed-ing of the max. mesh size 20 m x 20 m.The foundation earthing electrode mustbe arranged to be enclosed by concreteon all sides. For steel strips in non-rein-forced concrete, the earthing electrodemust be installed on edge.

In the service entrance room, a connec-tion must be established between foun-dation earthing electrode and equipo-tential bonding bar. According to DIN VVDE V 0185-3, a foundation earthingelectrode must be equipped with termi-nal lugs for connection of the down-con-ductor systems of the external lightningprotection system to the earth-termina-tion system.Due to the risk of corrosion at the pointwhere a terminal lug comes out of theconcrete, supplementary corrosion pro-tection should be considered (with PVCsheath or by using stainless steel withMaterial No. 1.4571).The reinforcement of plate and stripfoundations can be used as a foundationearthing electrode if the required termi-nal lugs are connected to the reinforce-ment and the reinforcements are inter-connected via the joints.Surface earthing electrodes must beinstalled in a depth of at least 0.5 m.

The impulse earthing resistance ofearthing electrodes is a function of themaximum value of the lightning currentand of the specific earth resistance. Seealso Fig. 5.5.13. The effective length ofthe earthing electrode for the lightningcurrent is calculated as an approximationas follows:

Surface earthing electrode:

earth rod:

Ieff Effective length of the earthing elec-trode in m

î Peak value of the lightning currentin kA


The impulse earthing resistance Rst canbe calculated using the formulae in(Table 5.5.1), where the effective lengthof the earthing electrode Ieff is used forthe length I.

Surface earthing electrodes are alwaysadvantageous when the upper soil layershave less specific resistance than the sub-soil.If the ground is relatively homogeneous(i.e. if the specific earth resistance at thesurface is roughly the same as it is deepdown) then, for a given earthing elec-trode resistance, the construction costs ofsurface earthing electrodes and earthrods are roughly the same.

I îeff E= 0 2. i ρ

I îeff E= 0 28. i ρ

U i R Ldi

dtE A= + i i i1

2

DA

= i 4

π

RDA

E=2

3

iiρ

RD

D

dAE=

ρπ

π2

2

ii

iln

RI IA

E

flat strip earth rod

≈+ρ


5555

According to Fig. 5.5.15, a earth rod musthave only around half the length of asurface earthing electrode.If the conductivity of the ground is betterdeep down than it is on the surface, e. g.

because of ground water, then an earthrod is generally more cost-effective thanthe surface earthing electrode.The issue of whether earth rods or sur-face earthing electrodes are more cost-effective in a particular case, can oftenonly be decided by measuring the specif-ic earth resistance as a function of thedepth.Since earth rods are easy to assemble andachieve excellent constant earthing elec-trode resistances without the need to diga trench and without damaging theground, these earthing electrodes arealso suitable for improving existingearth-termination systems.

5.5.1 Earth-termination systems inaccordance with DIN V VDE V0185-3

Earth-termination systems are the con-tinuation of air-termination and down-conductor systems to discharge the light-ning current into the earth. Further func-tions of the earth-termination system areto create equipotential bondingbetween the down conductors and apotential control in the vicinity of thewalls of the structure.It must be borne in mind that a commonearth-termination system for the variouselectrical systems (lightning protection,low voltage systems and telecommunica-tions systems) is preferable. This earth-termination system must be connected tothe equipotential bonding (MEB – mainequipotential bonding bar).Since DIN V VDE V 0185-3 assumes a sys-tematic lightning equipotential bonding,no particular value is required for theearth earthing electrode resistance. Gen-

erally, however, a low earth resistance(less than 10 Ω, measured with low fre-quency) is recommended.The standard classifies earthing electrodearrangements into Type A and Type B.

For both Type A and B earthing electrodearrangements, the minimum earthingelectrode length I1 of the earthing con-ductor is a function of the type of light-ning protection system (Fig. 5.5.1.1)The exact specific earth resistance canonly be determined by on-site measure-ments using the “WENNER method”(four-conductor measurement).Earthing electrode Type A

Earthing electrode arrangement Type Adescribes individually arranged horizon-tal star-type earthing electrodes (surfaceearthing electrodes) or vertical earthingelectrodes (earth rods), each of whichmust be connected to a down-conductorsystem.There must be at least 2 earthing elec-trodes Type A.Lightning protection systems Type III andIV require a minimum length of 5 m forearthing electrodes. For lightning protec-tion systems, Type I and II the length ofthe earthing electrode is determined as afunction of the specific ground resist-ance. The minimum length for earthingelectrodes I1 can be taken fromFig. 5.5.1.1.Minimum length of each earthing elec-trode is:

I1 x 0.5 for vertical or slanted earthingelectrodes

I1 for star-type earthing electrodes

The values determined apply to eachindividual earthing electrode.

For combinations of the various earthingelectrodes (vertical and horizontal) theequivalent total length should be takeninto account.The minimum length for the earthingelectrode can be disregarded if an earth

earthing electrode resistance of less than10 Ω is achieved.

earth rods are generally driven in verti-cally down to greater depths into naturalsoil which is generally initially encoun-tered below the foundations. Earthingelectrode lengths of 9 m have proved tobe advantageous. earth rods provide theadvantage of lying at greater depths insoil layers whose specific resistance isgenerally lower than in the areas closerto the surface.In frosty conditions, it is recommended toconsider the first 50 cm of a verticalearthing electrode as ineffective.

Earthing electrodes Type A do not fulfilthe equipotential bonding requirementsbetween the down conductors and thepotential control.

Earthing electrodes Type BEarthing electrodes of the Type Barrangement are ring earthing elec-trodes around the structure to be pro-tected, or foundation earthing elec-trodes. The requirements on theseearthing electrodes are described in DIN18014.If it is not possible to have a closed ringoutside around the structure, the ringmust be completed using conductorsinside the structure. Conduits or othermetal components which are perma-nently electrically conductive can also beused for this purpose. At least 80% of thelength of the earthing electrode must bein contact with the earth to ensure that,when calculating the separation dis-tance, the earthing electrode Type B canbe used as the base.The minimum lengths of the earthingelectrodes corresponding to the Type Barrangement are a function of the typeof lightning protection system. For light-ning protection systems Type I and II, theminimum length for earthing electrodesis also determined as a function of the


5555

0 5 101520 30 40 50 60 70 80 90 100

90

80

70

60

50

40

30

20151050

Length of the earthing electrode l (m)

surface earthing electrodeearth rod

ρE = 400 Ωm

ρE = 100 Ωm

Eart

hing

ele

ctro

de re

sist

ance

RA

(Ω)

Fig. 5.5.15 Earthing electrode resistance RA of surfaceand earth rods as a function of the length ofthe earthing electrode I

80

70

60

50

40

30

20

10

00 500 1000 1500 2000 2500 3000

l1 (m)

ρE (Ωm)

type of LPS III-IV

type of LP

S I

type of LPS II

Fig. 5.5.1.1 Min. lengths of earthing electrodes

r

area A1 to beconsidered

circular area A2,mean radius r

A = A1 = A2

r =

r l1

Aπ

With respect to ringor foundationearthing electrodes,the mean radius r ofthe area enclosed bythe earthing electro-de must not be shor-ter than l1.

Fig. 5.5.1.2 Earthing electrode Type B - Determinationof the mean radius - example calculation

specific ground resistance (see alsoFig. 5.5.4).For earthing electrodes Type B, the aver-age radius r of the area enclosed by theearthing electrode must be not less thanthe given minimum length l1.To determine the average radius r, thearea under consideration is transferredinto an equivalent circular area and theradius is determined as shown inFigs.5.5.1.2 and 5.5.1.3.

Below a calculation example:

If the required value of l1 is greater thanthe value r corresponding to the struc-ture, supplementary star-type earthingelectrodes or vertical earthing electrodes(or slanted earthing electrodes) must beadded, their respective lengths lr (ra-dial/horizontal) and lv (vertical) being giv-en by the following equations:

The number of supplementary earthingelectrodes must not be less than thenumber of down conductors, but a mini-mum of 2. These supplementary earthingelectrodes shall be connected to the ringearthing electrode so as to be equidis-tant around the circumference.

If supplementary earthing electrodeshave to be connected to the foundationearthing electrode, care must be takenwith the materials of the earthing elec-trode and the connection to the founda-tion earthing electrode. It is preferable to

use stainless steel with Material No.1.4571 (Fig. 5.5.2.1).The following systems can make addi-tional demands on the earth-terminationsystem, for example:

⇒ Electrical systems – conditions of dis-connection from supply with respectto the type of network (TN, TT, IT sys-tems) in accordance with VDE 0100Part 410

⇒ Equipotential bonding in accor-dance with VDE 0100 Part 540

⇒ Electronic systems – data informa-tion technology

⇒ Antenna earthing installation inaccordance with VDE 0855

⇒ Electromagnetic compatibility

⇒ Substation in or near the structure inaccordance with VDE 0101 and 0141

5.5.2 Earth-termination systems,foundation earthing elec-trodes and foundationearthing electrodes for spe-cial structural measures

Foundation earthing electrodes –Earthing electrodes Type BDIN 18014 "Foundation earth electrode"specifies the requirements on foundationearthing electrodes.Many national and international stan-dards specify foundation earthing elec-trodes as a preferred earthing electrodebecause, when professionally installed, itis enclosed in concrete on all sides andhence corrosion-resistant. The hygro-scopic characteristics of concrete general-ly produce a sufficiently low earthearthing electrode resistance.The foundation earthing electrode mustbe installed as a closed ring in the stripfoundation or the bedplate (Fig. 5.5.2.1)and thus also acts primarily as theequipotential bonding. The division into

meshes ≤ 20 mx20 m and the terminallugs to the outside required to connectthe down conductors of the externallightning protection system, and to theinside for equipotential bonding, mustbe considered (Fig. 5.5.2.2).According to DIN 18014, the installationof the foundation earthing electrode isan electrical engineering measure to becarried out or monitored by a recognisedspecialist electrical engineer.The question of how to install the foun-dation earthing electrode must be de-cided according to the measure requiredto ensure that the foundation earthingelectrode is enclosed on all sides as theconcrete is being poured in.

Installation in non-reinforced concreteNon-reinforced foundations, e. g. stripfoundations of residential structures(Fig. 5.5.2.3), spacers requires the use of.Only by using the spacers at distances ofapprox. 2 m, is it possible to ensure thatthe foundation earthing electrode is"lifted up" and can be enclosed on allsides by concrete.

Installation in reinforced concreteWhen using steel mats, reinforcementcages or reinforcement irons in founda-tions, it is not only possible to connectthe foundation earthing electrode tothese natural iron components. Further-more, this should be done. The function

ll r

v =−1

2

l l rr = −1


5555

Fig. 5.5.1.3 Earthing electrode Type B - Determination of the mean radius

12 m

12 m

5 m

5 m

7 m

7 m

r

area A1to be considered

Example:Residential building.

Sk III. l1 = 5 m

A1 = 109 m2

r =

r = 5.89 m

No further earthingelectrodes required!

109 m2

3.14circular area A2mean radius r

A = A1 = A2

r =

r l1

Aπ

Terminal lugmin. 1.5 m long, noticeably marked− steel strip 30 x 3.5 mm− StSt round steel bar 10 mm− round steel bar 10 mm with PVC coating− fixed earthing point

Foundationearthing electrode− steel strip 30 x 3.5 mm− round steel bar 10 mm

Fig. 5.5.2.1 Foundation earthing electrode with ter-minal lug

20 m

≤ 20

m

Recommendation:Several terminal lugs e.g.in every technical centre

terminal lug

additional terminal conductorfor forming meshes ≤ 20 m x 20 m

Fig. 5.5.2.2 Mesh of a foundation earthing electrode

Fig. 5.5.2.3 Foundation earthing electrode

of the foundation earthing electrode isthus made even more favourable. Thereis no need to use spacers. The modernmethods of laying concrete and thenvibrating it, ensure that the concrete also“flows” under the foundation earthingelectrode enclosing it on all sides.Fig. 5.5.2.4 illustrates one possible appli-cation for the horizontal installation of aflat strip as a foundation earthing elec-trode. The intersections of the founda-tion earthing electrode must be connect-ed so as to be capable of carrying cur-rents. Galvanised steel is sufficient asmaterial of the foundation earthing elec-trode.

Terminal lugs to the outside into theground must have supplementary corro-sion protection at the outlet point. Suit-able materials are, for example, plasticsheathed steel wire (owing to the risk offracture of the plastic sheath at low tem-peratures, special care must be takenduring the installation), high-alloy stain-less steel, Material No. 1.4571, or fixedearthing terminals.If professionally installed, the earthingelectrode is enclosed on all sides by con-crete and hence corrosion-resistant.When designing the foundationearthing electrode, meshes no biggerthan 20 m x 20 m must be created. Thismesh size bears no relation to the type oflightning protection system of the exter-nal lightning protection system.Modern building techniques employ vari-ous types of foundations in a wide vari-ety of designs and sealing versions.The thermal insulation regulations havealso influenced the design of the stripfoundations and foundation slabs.For foundation earthing electrodesinstalled in new structures in accordancewith DIN 18014, the insulation affectstheir installation and arrangement.

Perimeter / Base insulationThe magnitude of the specific resistanceof the perimeter insulating plates is adecisive factor when considering theeffect of perimeter insulation on theearthing electrode resistance of founda-tion earthing electrodes in conventionalarrangements in the foundation (stripfoundation, foundation slab). Thus, for apolyurethane rigid foam with bulk densi-ty 30 kg/m2, for example , a specific resist-ance of 5.4 • 1012Ωm is given. In contrast,the specific resistance of concrete liesbetween 150 Ωm and 500 Ωm. This aloneshows that, in the case of continuousperimeter insulation, a conventionalfoundation earthing electrode arrangedin the foundations has practically no

effect. The perimeter insulation also actsas an electrical insulator.The diagrams below illustrate the variousways of insulating the foundations andwalls for structures with perimeter andbase insulation.The arrangement of the foundationearthing electrodes for each design isshown in Figs.5.5.2.5 to 5.5.2.7 .The exact arrangement of the earthingelectrode in the strip foundation withinsulated sides towards the outside andthe bedplate is not important(Fig. 5.5.2.6).


5555Fig. 5.5.2.4 Foundation earthing electrode in use

granular sub-grade course

foundation slab

concrete

basement floor

drainage

moisture barrierinsulation

soil

perimeter /base insulation


terminal lug

Distance holderPart No. 290 001

MV TerminalPart No. 390 050

Cross unitPart No. 318 201

Fixed earthing terminal for EBPart No. 478 800

Ref.: VDE series 35

Fig. 5.5.2.5 Arrangement of a foundation earthing electrode in a strip foundation (insulated basement wall)


foundation slab

concrete

basement floor

drainage


soil



terminal lug

Distance holderPart No. 290 001




Ref.: VDE series 35

insulating layer

Fig. 5.5.2.6 Arrangement of a foundation earthing electrode in a strip foundation

If the foundation slab is completely insu-lated, the earthing electrode must beinstalled below the bedplate. MaterialV4A (Material No. 1.4571) should be used(Fig. 5.5.2.7).

It is efficient to install fixed earthing ter-minals, especially for reinforced struc-tures. In such cases, care must be takenthat the installation during the construc-tion phase is carried out professionally(Fig. 5.5.2.8).

Black, white tankIn structures erected in regions with ahigh groundwater table, or in locations,e. g. on hillsides, with “pressing” water,the cellars are equipped with specialmeasures to prevent moisture penetrat-ing. The outer walls surrounded by earth,and the foundation slab are sealedagainst the penetration of water to

ensure that no troublesome moisture canform on the inside of the wall.Modern building techniques apply bothabove mentioned processes for sealingagainst penetrating water.One particular issue in this context iswhether the efficiency of a foundationearthing electrode is still provided formaintaining the measures to protect

against life hazards in accordance withDIN VDE 0100 Part 410, and as a lightningprotection earthing electrode in accor-dance with DIN V VDE V 0185.

Foundation earthing electrodes forstructures with white tankThe name "white tank" is used to expressthe opposite of "black tank": a "whitetank" receives no additional treatmenton the side facing the earth, hence it is"white".The "white tank" is manufactured from aspecial type of concrete. The concretebody is waterproof, which, however,does not mean that the concrete cannotabsorb any water. The concrete tankbeing waterproof means that, if wateracts upon one side of it over a long peri-od of time, it does not penetrate the con-crete of the tank. On the side of the tankaway from the water, no water leaksthrough nor become any damp patchesevident.If the concrete is manufactured correctlyand the "white tank" is 10 - 40 cm thick,the maximum permissible value ofwater/concrete is 0.6 (W/C < 0.6). Thepenetration depth of the water for thisconcrete is then a maximum of 5 cm.If a closed round or steel strip ring is laidin the lowest layer of the concrete plateas a foundation earthing electrode, a suf-ficient effect of the earthing electrodecan be expected.


5555

granular sub-grad course

concrete

basement floor


soil


ring earthing electrodeMat No. 1.4571

terminal lugMat No. 1.4571 MV Terminal

Part No. 390 050



Ref.: VDE series 35

foundation slab

reinforcement

Fig. 5.5.2.7 Arrangement of a foundation earthing electrode in case of a closed floor slab (fully insulated)

Fig. 5.5.2.8 Fixed earthing point


concrete

basement floor

drainage


soil

foundationearthing electrode

terminal lug




Ref.: VDE series 35

foil

foundation plate

reinforcement

Fig. 5.5.2.9 Arrangement of a foundation earthing electrode in case of a closed floor slab “white tank”

If a specific value of the earth earthingelectrode resistance is required in orderto maintain the protection against elec-trical shock, e. g. in the TT system (auto-matic disconnection by means of RCDs orfuses), this must be proved via corres-ponding earthing measurements.

If the requirements on the earthing elec-trode resistance to protect against lifehazards (automatic disconnection of sup-ply, shock hazard voltage) are not met,supplementary earthing electrodes (star-type earthing electrodes, earth rods, ringearthing electrodes) must be installed.Fig. 5.5.2.9 illustrates the arrangement ofthe foundation earthing electrode in awhite tank.

Earthing electrodes for structures withblack tankThe name "black tank" derives from themulti-layered strips of black bitumenapplied to the sections of the structurewhich are outside in the ground. Thebody of the structure is coated with bitu-men/tar which is then covered by gener-ally up to 3 layers of bitumen strips.A ring conductor set into the foundationslab above the seal can act as the poten-tial control in the structure. Due to thehigh-impedance insulation to the out-side, however, the earthing electrode isineffective.In order to comply with the earthingrequirements stipulated in the variousstandards, an earthing electrode, e.g. aring earthing electrode, must be installedexternally around the structure or belowall seals in the granular sub-grade course.Wherever possible, the external earthingelectrode should be led into the structureabove the seal of the structure(Fig. 5.5.2.10), in order to ensure thetightness of the tank also in the longterm. A waterproof penetration of the"black tank" is only possible using a spe-cial bushing between earthing electrodeand building (Fig. 5.5.2.11).

Fibre concrete foundation slabsFibre concrete is a type of concrete whichforms a heavy-duty concrete slab withsteel fibres added to the liquid concretebefore hardening.The steel fibres are approx. 6 cm long andhave a diameter of 1 –2 mm. The steelfibres are slightly wavy and are admixedequally to the liquid concrete. The pro-portion of steel fibres is around 20 –30 kg/m3 concrete.The admixture gives the concrete slabboth a high compression strength andalso a high tensile strength and, com-pared to a conventional concrete slab

with reinforcement, it also provides aconsiderably higher elasticity.The liquid concrete is discharged on site.This allows to create large areas with asmooth surface and no joints.It is used for bedplates in the founda-tions of large halls, for example.Fibre concrete has no reinforcement. Thisrequires a supplementary ring conductoror a meshed network to be constructedfor installing earthing measures. The

earthing conductor can be set in the con-crete and, if it is made of galvanisedmaterial, it must be enclosed on all sides.This is very difficult to do on site.It is therefore recommended to install acorrosion-resistant high-alloy stainlesssteel, Material No. 1.4571, below the sub-sequent concrete bedplate. The corres-ponding terminal lugs have to be consid-ered.


5555


concrete

soil

ring earthing electrode

Ref.: VDE series 35

foundation plate

Max. groundwater level

tank seal

terminal lug min. 150 cmlead-in above ground water levele.g. StSt (Mat. No. 1.4571)

soil

Fig. 5.5.2.10 Arrangement of the earthing electrode outside of the tank seal “black tank”


concrete

soil

ring earthing electrode

Ref.: VDE series 35

foundation plate

Max. groundwater level

tank seal

tank seal

Bushing between earthing electrode and buildingPart No. 478 600

soil

Fig. 5.5.2.11 Arrangement of the earthing electrode outside of the tank seal “black tank”

Note:A specialist must install the earthing con-ductors and connecting components inconcrete. If this is not possible, the build-ing contractor can undertake the workonly if it is supervised by a specialist.

5.5.3 Earth rod – Earthing electrode Type B

DIN 18014 stipulates that all new struc-tures must have foundation earthingelectrodes. The earth-termination systemof existing structures can be designed inthe form of a ring earthing electrode(Fig. 5.5.3.1).This earthing electrode must be installedin a closed ring around the structure or, ifthis is not possible, a connection to closethe ring must be made inside the struc-ture.80 % of the conductors of the earthingelectrode shall be installed so as to be incontact with the earth. If this 80 % can-not be achieved, it has to be checked ifsupplementary earthing electrodes TypeA are required.The requirements on the minimumlength of earthing electrodes accordingto the type of lightning protection sys-tem must be taken into account (seeChapter 5.5.1).When installing the ring earthing elec-trode, care must be taken that it isinstalled at a depth > 0.5 m and a dis-tance of 1 m from the structure.If the earthing electrode is driven in aspreviously described, it reduces the stepvoltage and thus acts as a potential con-trol around the structure.

This earthing electrode should beinstalled in natural soil. Setting it ingravel or ground filled with constructionwaste worsens the earth earthing elec-trode resistance.

When choosing the material of theearthing electrode with regard to corro-sion, the local conditions must be takeninto consideration. It is advantageous touse stainless steel. This earthing elec-trode material does not corrode nor doesit subsequently require the earth-termi-nation system to be refurbished withtime-consuming and expensive measuressuch as removal of paving, tar coatings oreven steps, for installing a new flat strip.In addition, the terminal lugs must beparticularly protected against corrosion.

5.5.4 Earth rods – Earthing elec-trodes Type A

The sectional earth rods, System DEHN,are manufactured from special steel andhot-dip galvanised, or they consist ofhigh-alloy stainless steel with MaterialNo. 1.4571 (the high-alloy stainless steelearthing electrode is used in areasespecially at risk from corrosion). Theparticular feature of these earth rods istheir coupling point, which allows theearth rods to be connected withoutincreasing their diameter.Each rod has a bore at its lower end,while the other end of the rod has acorresponding spigot (Fig. 5.5.4.1).

With earthing electrode Type “S”, thesoft metal insert deforms as it is driveninto the bore, creating an excellent elec-trical and mechanical connection.

With earthing electrode Type “Z”, thehigh coupling quality is achieved with amultiply knurled spigot.

With earthing electrode Type “AZ”, thehigh coupling quality is achieved with amultiply knurled and shouldered spigot.The advantages of the DEHN earth rodsare:

⇒ Special coupling:

no increase in diameter so that theearth rod is in close contact with theground along the whole of its length

⇒ Self-closing when driving in the rods

⇒ Simple to drive in with vibrationhammers (Fig. 5.5.4.2) or mallets

⇒ Constant resistance values areachieved since the earth rods pene-trate through the soil layers whichare unaffected by seasonal changesin moisture and temperature

⇒ High corrosion resistance as a resultof hot-dip galvanising (zinc coating70 µm thick)


5555

EB

Fig. 5.5.3.1 Ring earthing electrode around a residential building

Fig. 5.5.4.2 Driving the earth rod in with a workscaffolding and a vibrating hammer

type S type Z type AZ

Fig. 5.5.4.1 Couplings of DEHN earth rods

⇒ Galvanised earth rods also providehot- galvanised coupling points

⇒ Easy to store and transport sinceindividual rods are 1.5 or 1 m long.

5.5.5 Earthing electrodes in rockyground

In bedrock or stony ground, surfaceearthing electrodes such as ring earthingelectrodes or star-type earthing elec-trodes are often the only way of creatingan earth-termination system.When installing the earthing electrodes,the flat strip or round material is laid onthe stony ground or on the rock. Theearthing electrode should be coveredwith gravel, wet-mix slag aggregate orsimilar.It is advantageous to use stainless steelMaterial No. 1.4571 as earthing electrodematerial. The clamped points should beinstalled with particular care and be pro-tected against corrosion (anticorrosiveband).

5.5.6 Intermeshing of earth-termi-nation systems

An earth-termination system can serve awide variety of purposes.The purpose of protective earthing is tosafely connect electrical installations andequipment to earth potential and to pre-vent life hazards and physical damage toproperty in the event of an electricalfault.The lightning protection earthing systemtakes over the current from the downconductors and discharges it into theground.The functional earthing installationserves to ensure that the electrical andelectronic installations operate safelyand trouble-free.The earth-termination system of a struc-ture must be used for all earthing taskstogether, i. e. the earth-termination sys-tem deals with all earthing tasks. If thiswere not the case, potential differencescould arise between the installationsearthed on different earth-terminationsystems.Previously, a “clean earth” was some-times applied in practice for functionalearthing of the electronic equipment,separately from the lightning protectionand the protective earth. This is extreme-ly disadvantageous and can even be dan-gerous. In the event of lightning effects,great potential differences up to a few100 kV occur in the earth-terminationsystem. This can lead to destruction ofelectronic installations and also to lifehazards. Therefore, DIN V VDE V 0185-3and -4 require continuous equipotentialbonding within a structure.The earthing of the electronic systemscan be constructed to have a radial, cen-tral or intermeshed 2-dimensional designwithin a structure, (Fig. 5.5.6.1). Thisdepends both on the electromagnetic

environment and also on the characteris-tics of the electronic installation. If alarger structure comprises more than onebuilding, and if these are connected byelectrical and electronic conductors, thencombining the individual earthing sys-tems can reduce the (total) earth resist-ance. In addition, the potential differ-ences between the structures are alsoreduced considerably. This diminishesnoticeably the voltage load of the electri-cal and electronic connecting cables. Theinterconnection of the individual earth-termination systems of the structureshould produce a meshed network. Themeshed earthing network should be con-structed to contact the earth-terminationsystems at the points where the verticaldown conductors are also connected. Thesmaller the mesh size of the network ofthe earthing installation, the smaller thepotential differences between the struc-tures in the event of a lightning stroke.This depends on the total area of thestructure. Mesh sizes from 20 m x 20 m upto 40 m x 40 m have proved to be cost-effective. If, for example, high vent stacks(preferred points of strike) are existing,then the connections around this part ofthe plant should be made closer, and, ifpossible, radial with circular interconnec-tions (potential control). When choosingthe material for the conductors of themeshed earthing network, the corrosionand material compatibility must be takeninto account.


5555

workshop stock administration

gate

production

productionproduction

power centre

Fig. 5.5.6.1 Intermeshed earth-termination system of an industrial facility

5.5.7 Corrosion of earthing elec-trodes

5.5.7.1 Earth-termination systemswith particular considerati-on of corrosion

Metals in immediate contact with soil orwater (electrolytes) can be corroded bystray currents, corrosive soils and the for-mation of voltaic cells. It is not possible toprotect earthing electrodes from corro-sion by completely enclosing them, i. e.by separating the metals from the soil,since all the usual sheaths employed untilnow have had a high electrical resistanceand therefore negate the effect of theearthing electrodes.Earthing electrodes made of a uniformmaterial can be threatened by corrosionfrom corrosive soils and the formation ofconcentration cells. The risk of corrosiondepends on the material and the typeand composition of the soil.Corrosion damage due to the formationof voltaic cells is being increasinglyobserved. This cell formation betweendifferent metals with widely differentmetal/electrolyte potentials has beenknown for many years. What is not wide-ly realised, however, is that the reinforce-ments of concrete foundations can alsobecome the cathode of a cell and hencecause corrosion to other installations.With the changes to the way buildingsare constructed – larger reinforced con-crete structures and smaller free metalareas in the ground – anode/cathode sur-face ratio is becoming more and moreunfavourable, and the risk of corrosionof the more base metals is inevitablyincreasing.In many cases it was previously suspectedthat the corrosion was caused by otherinfluences, e. g. ac currents. By means ofextensive measurements it could beproved, however, that ac currents withthe technical frequencies of 16 2/3 and50 Hz at the current densities occurringin practice could not be neglected as areason for the corrosion of bare metalsusually used in the ground nowadays.An electrical isolation of installations act-ing as anodes to prevent this cell forma-tion is only possible in exceptional cases.The aim nowadays is to integrate allearthing electrodes including thosemetal installations connected to theearth in order to achieve equipotentialbonding and hence maximum safetyagainst shock hazard voltages at faults orlightning strokes.In high voltage installations, high volt-age protective earthing electrodes areincreasingly being connected to low volt-age operating earthing electrodes in

accordance with DIN VDE 0101. Further-more, DIN VDE 0100 Part 410 requires theintegration of conduits and other instal-lations into the shock hazard protectivemeasures. Thus, the only way of prevent-ing or at least reducing the risk of corro-sion for earthing electrodes and otherinstallations in contact with them ischoosing suitable materials for theearthing electrodes.DIN VDE 0151 "Material and minimumdimensions of earth electrodes withrespect to corrosion" has been availablesince June 1986 as a white paper. Apartfrom decades of experience in the fieldof earthing technology, the results ofextensive preliminary examinations havealso been embodied in this standard.Many interesting results are availablewhich are important for the earthingelectrodes, including those of lightningprotection systems.The fundamental processes leading tocorrosion are explained below.Practical anticorrosion measuresespecially for lightning protectionearthing electrodes shall be derived fromthis and from the wealth of materialalready acquired by the VDE task forceon "Earthing electrode materials".

Terms used in corrosion protection andcorrosion protection measurements

Corrosionis the reaction of a metal material to itsenvironment which leads to impairmentof the characteristics of the metal mater-ial and/or its environment. The reaction isusually of electrochemical character.

Electrochemical corrosionis corrosion during which electrochemicalprocesses occur. They take place exclu-sively in the presence of an electrolyte.

Electrolyteis an ion-conducting corrosive medium(e. g. soil, water, fused salts).

Electrodeis an electron-conducting material in anelectrolyte. The system of electrode andelectrolyte forms a half-cell.

Anodeis an electrode from which a dc currententers the electrolyte.

Cathodeis an electrode from which a dc currentleaves the electrolyte.

Reference electrodeis a measuring electrode for determiningthe potential of a metal in the elec-trolyte.

Copper sulphate/Electrodeis a reference electrode which can hardlybe polarised, made of copper in satur-ated copper sulphate solution.The copper sulphate electrode is themost common form of reference elec-trode for measuring the potential of sub-terranean metal objects (Fig. 5.5.7.1.1).

Corrosion cellis a voltaic cell with different local partialcurrent densities for dissolving the metal.Anodes and cathodes of the corrosioncell can be formed

⇒ on the material

due to different metals (contact cor-rosion) or different structural com-ponents (selective or intercrystallinecorrosion).

⇒ on the electrolyte

caused by different concentrationsof certain materials having stimula-tory or inhibitory characteristics fordissolving the metal.

PotentialsReference potentialPotential of a reference electrode withrespect to the standard hydrogen elec-trode.

Electropotentialis the electrical potential of a metal or anelectron-conducting solid in an elec-trolyte.


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12

34

5

6

1 Electrolyte copper bar with hole formeasurements

2 Rubber plug3 Ceramic cylinder with porous base4 Glaze5 Saturated Cu/CuSO4 solution6 Cu/CuSO4 crystals

Fig. 5.5.7.1.1 Application example of a non-polaris-able measuring electrode (copper/cop-per sulphate electrode) for tapping apotential within the electrolyte (cross-sectional view)

5.5.7.2 Formation of voltaic cells,corrosion

The corrosion processes can be clearlyexplained with the help of a voltaic cell.If, for example, a metal rod is dipped intoan electrolyte, positively charged ionspass into the electrolyte and conversely,positive ions are absorbed from the elec-trolyte from the metal band. In this con-text one speaks of the “solution pres-sure” of the metal and the "osmotic pres-sure" of the solution. Depending on themagnitude of these two pressures, eithermore of the metal ions from the rod passinto the solution (the rod thereforebecomes negative compared to the solu-tion) or the ions of the electrolyte collectin large numbers on the rod (the rodbecomes positive compared to the elec-trolyte). A voltage is thus createdbetween two metal rods in the elec-trolyte.In practice, the potentials of the metals inthe ground are measured with the helpof a copper sulphate electrode. This con-sists of a copper rod dipped into a satur-ated copper sulphate solution (the refer-ence potential of this reference electroderemains constant).

Consider the case of two rods made ofdifferent metals dipping into the sameelectrolyte. A voltage of a certain magni-tude is now created on each rod in theelectrolyte. A voltmeter can be used tomeasure the voltage between the rods(electrodes); this is the differencebetween the potentials of the individualelectrodes compared with the elec-trolyte.

How does it now come that current flowsin the electrolyte and hence that mater-ial is transported, i.e. corrosion occurs?If, as shown here, the copper and ironelectrodes are connected via an ammeteroutside the electrolyte, for example, thefollowing (Fig 5.5.7.2.1) is ascertained: inthe outer circuit, the current i flows from+ to –, i.e. from the “nobler” copper elec-trode according to Table 5.5.7.2.1 to theiron electrode.In the electrolyte, on the other hand, thecurrent i must therefore flow from the"more negative" iron electrode to thecopper electrode to close the circuit. As ageneralisation, this means that the morenegative pole passes positive ions to theelectrolyte and hence becomes the

anode of the voltaic cell, i.e. it dissolves.The dissolution of the metal occurs atthose points where the current entersthe electrolyte.A corrosion current can also arise from aconcentration cell (Fig 5.5.7.2.2). In thiscase, two electrodes made of the samemetal dip into different electrolytes. Theelectrode in electrolyte II with the higherconcentration of metal ions becomeselectrically more positive than the other.Connecting the two electrodes enablesthe current i to flow and the electrode,which is electrochemically more nega-tive, dissolves.


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electrolyte

i

ielectrode II

Cuelectrode I

Fe

Fig. 5.5.7.2.1 Galvanic cell: iron/copper

Definition Symbol(s) Measuring Copper Lead Tin Iron Zincunit

1 Free corrosion UM-Cu/CuSO4V 0 to – 0.1 – 0.5 to – 0.4 to – 0.5 to – 0.9 to

potential in the soil1) – 0.6 – 0.6 2) – 0.8 3) – 1.1 5)

2 Cathodic protective UM-Cu/CuSO4V – 0.2 – 0.65 – 0.65 2) – 0.85 4) – 1.2 5)

potential in the soil1)

3 Electrochemical K = ∆ m kg / (A • year) 10.4 33.9 19.4 9.1 10.7equivalent It

4 Linear corrosion rate Wlin = ∆ s/t mm/year 0.12 0.3 0.27 0.12 0.15at J = 1 mA/dm2

1) Measured to saturated copper/copper sulphate electrode (Cu/Cu SO4).2) Values are verified in presently performed tests. The potential of tin-coated copper depends on the thickness of the

tin coating. Common tin coatings up to now have amounted up to a few µm and are thus between the values of tinand copper in the soil.

3) These values do also apply to lower alloyed types of iron. The potential of steel in concrete (reinforcing iron offoundations) depends considerably on external influences. Measured to a saturated copper/copper sulphate electro-de it generally amounts to – 0.1 to – 0.4 V. In case of metal conductive connections with wide underground installa-tions made of metal with more negative potential, it is cathodically polarised and thus reaches values up to approxi-mately – 0.5 V.

4) In anaerobic soils the protective potential should be – 0.95 V.5) Hot-dip galvanised steel, with a zinc coating according to the above mentioned table, has a closed external pure

zinc layer. The potential of hot-dip galvanised steel in the soil corresponds therefore to approximately the statedvalue of zinc in the soil. In case of a loss of the zinc layer, the potential gets more positive. With its complete corro-sion it can reach the value of steel.The potential of hot-dip galvanised steel in concrete has approximately the same initial values. In the course oftime, the potential can get more positive. Values more positive than approx. – 0.75 V, however, have not beenfound yet. Heavily hot-dip galvanised copper with a zinc layer of min. 70 µm has also a closed external pure zinclayer. The potential of hot-dip galvanised copper in soil corresponds therefore to approx. the stated value of zinc insoil. In case of a thinner zinc layer or a corrosion of the zinc layer, the potential gets more positive. Limit valueshave still not been defined yet.

Table 5.5.7.2.1 Potential values and corrosion rates of common metal materials

A concentration cell of this type can beformed, for example, by two iron elec-trodes, one of which is fixed in concretewhile the other lies in the ground(Fig. 5.5.7.2.3).

Connecting these electrodes, the iron inthe concrete becomes the cathode of theconcentration cell and the one in theground becomes the anode; the latter istherefore destroyed by ion loss.For electrochemical corrosion it is gener-ally the case that, the larger the ions andthe lower their charge, the greater thetransport of metal associated with thecurrent flow i, (i. e. i is proportional tothe atomic mass of the metal).In practice, the calculations are carriedout with currents flowing over a certainperiod of time, e. g. over one year. Table5.5.7.2.1 gives values which express theeffect of the corrosion current (currentdensity) in terms of the quantity of metaldissolved. Corrosion current measure-ments thus make it possible to calculatein advance how many grammes of ametal will be eroded over a specific peri-od.Of more practical interest, however, isthe prediction if, and over which periodof time, corrosion will cause holes or pit-ting in earthing electrodes, steel tanks,pipes etc. So it is important whether theprospective current attack will take placein a diffuse or punctiform way.For the corrosive attack, it is not solelythe magnitude of the corrosion current

which is decisive, but also, in particular,its density, i.e. the current per unit ofarea of the discharge area.It is often not possible to determine thiscurrent density directly. In such cases, thisis managed with potential measure-ments the extent of the available "polar-isation" can be taken from. The polarisa-tion behaviour of electrodes is discussedonly briefly here.Let us consider the case of a galvanisedsteel strip situated in the ground andconnected to the (black) steel reinforce-ment of a concrete foundation (Fig5.5.7.2.4). According to our measure-ments, the following potential differ-ences occur here with respect to the cop-per sulphate electrode:

steel, (bare) in concrete: – 200 mV

steel, galvanised, in sand: – 800 mV

Thus there is a potential difference of600 mV between these two metals. Ifthey are now connected above ground, acurrent i flows in the outer circuit fromreinforced concrete to the steel in thesand, and in the ground from the steel inthe sand to the steel in the reinforce-ment.The magnitude of the current i is now afunction of the voltage difference, theconductance of the ground and thepolarisation of the two metals.Generally, it is found that the current i inthe ground is generated by changes inthe material.But a change to the material also meansthat the voltage of the individual metalschanges with respect to the ground. Thispotential drift caused by the corrosioncurrent i is called polarisation. Thestrength of the polarisation is directlyproportional to the current density.Polarisation phenomena now occur atthe negative and positive electrodes.However, the current densities at bothelectrodes are mostly different.

For illustration, we consider the follow-ing example:A well-insulated steel gas pipe in theground is connected to copper earthingelectrodes.If the insulated pipe has only a few smallspots where material is missing, there is ahigher current density at these spotsresulting in rapid corrosion of the steel.In contrast, the current density is lowover the much larger area of the copperearthing electrodes where the currententers. Thus the polarisation is greater at themore negative insulated steel conductorthan at the positive copper earthing elec-trodes. The potential of the steel con-ductor is shifted to more positive values.Thus, the potential difference across theelectrodes decreases as well. The magni-tude of the corrosion current is thereforealso a function of the polarisation char-acteristics of the electrodes. The strength of the polarisation can beestimated by measuring the electrodepotentials for a split circuit. The circuit issplit in order to avoid the voltage drop inthe electrolyte. Recording instrumentsare usually used for such measurementssince there is frequently a rapid depolari-sation immediately after the corrosioncurrent is interrupted.If strong polarisation is now measured atthe anode (the more negative electrode),i.e. if there is an obvious shift to morepositive potentials, then there is a highrisk that the anode will corrode.

Let us now return to our corrosion cell -steel (bare) in concrete/steel, galvanisedin the sand (Fig. 5.5.7.2.4). With respectto a distant copper sulphate electrode, itis possible to measure a potential of theinterconnected cells of between – 200and – 800 mV. The exact value dependson the ratio of the anodic to cathodicarea and the polarisability of the elec-trodes. If, for example, the area of the rein-forced concrete foundation is very largecompared to the surface of the gal-vanised steel wire, then a high anodiccurrent density occurs at the latter, sothat it is polarised to almost the potentialof the reinforcement steel and destroyedin a relatively short time.High positive polarisation thus alwaysindicates an increased risk of corrosion. In practice it is, of course, now importantto know the limit above which a positivepotential shifting means an acute risk ofcorrosion. Unfortunately, it is not possi-ble to give a definite value, which appliesin every case; the effects of the soil con-ditions alone are too various. It is, how-


5555

electrolyte I

i

permeable to ions

electrode IIelectrode I

electrolyte II

i

Fig. 5.5.7.2.2 Concentration cell

soil

i

electrode IIFe

electrode IFe

i

concrete

Fig. 5.5.7.2.3 Concentration cell:Iron in soil / Iron in concrete

soil

i

electrode IISt

electrode ISt/tZn

i

concrete

Fig. 5.5.7.2.4 Concentration cell:Galvanised steel in soil / steel (black)in concrete

ever, possible to stipulate fields of poten-tial shifting for natural soils.

Summary:A polarisation below + 20 mV is generallynon-hazardous. Potential shifts exceed-ing + 100 mV are definitely hazardous.Between 20 and 100 mV there will alwaysbe cases where the polarisation causesconsiderable corrosion phenomena.To summarise, one can stipulate:The precondition for the formation ofcorrosion cells (voltaic cells) is always thepresence of metal and electrolyticanodes and cathodes connected to beconductive.

Anodes and cathodes are formed from

⇒ Materials• different metals or different surface

conditions of a metal (contact corro-sion),

• different structural components(selective or intercrystalline corro-sion),

⇒ Electrolytes:different concentration (e. g. salinity,ventilation).

In corrosion cells, the anodic fields alwayshave a more negative metal/electrolytepotential than the cathodic fields.The metal/electrolyte potentials aremeasured using a saturated copper sul-phate electrode mounted in the immedi-ate vicinity of the metal in or on theground. If there is a metal conductiveconnection between anode and cathode,then the potential difference gives rise toa dc current in the electrolyte which pass-es from the anode into the electrolyte bydissolving metal before entering againthe cathode.

The “area rule” is often applied to esti-mate the average anodic current densityIÄ:

UA, UC Anode or cathode potentials in V

ϕK Specific polarisation resistanceof the cathode in Ωm2

AA, AC Anode or cathodesurfaces in m2

The polarisation resistance is the ratio ofthe polarisation voltage and the totalcurrent of a mixed electrode (an elec-trode where more than one electrodereaction takes place).In practice, it is indeed possible to deter-mine the driving cell voltages UA – UC andthe size of the areas AC and AA as anapproximation for estimating the rate ofcorrosion. The values for ϕA (specificpolarisation resistance of the anode) andϕC, however, are not available to a suffi-cient degree of accuracy. They depend onthe electrode materials, the electrolytesand the anodic and cathodic current den-sities.The results of examinations availableuntil now allow the conclusion that ϕA ismuch smaller than ϕC.

To ϕC applies:

steel in the ground approx. 1 Ωm2

copper in the ground approx. 5 Ωm2

steel in concrete approx. 30 Ωm2

From the area rule, however, it is clear,that powerful corrosion phenomenaoccur both on enclosed steel conductorsand tanks with small spots in the sheathwhere material is missing, connected tocopper earthing electrodes, and also onearthing conductors made of galvanisedsteel connected to extended copperearth-termination systems or extremelylarge reinforced concrete foundations.By choosing suitable materials it is possi-ble to avoid or reduce the risk of corro-sion for earthing electrodes. To achieve asatisfactory service life, material mini-mum dimensions must be maintained(Table 5.5.8.1).

5.5.7.3 Choice of earthing electrodematerials

Table 5.5.8.1 is a compilation of theearthing electrode materials and mini-mum dimensions usually used today.

Hot-dip galvanised steelHot-dip galvanised steel is also suitablefor embedding in concrete. Foundationearthing electrodes, earthing electrodesand equipotential bonding conductorsmade of galvanised steel in concrete maybe connected with reinforcement iron.

Steel with copper sheathIn the case of steel with copper sheath,the comments for bare copper apply tothe sheath material. Damage to the cop-per sheath, however, creates a high riskof corrosion for the steel core, hence acomplete closed copper layer mustalways be present.

Bare copperBare copper is very resistant due to itsposition in the electrolytic insulation rat-ing. Moreover, in combination withearthing electrodes or other installationsin the ground made of more “base”materials (e. g. steel), it has additionalcathodic protection, albeit at theexpense of the more "base" metals.

Stainless steelsCertain high-alloy stainless steels accord-ing to DIN 17440 are inert and corrosion-resistant in the ground. The free corro-sion potential of high-alloy stainlesssteels in normally aerated soils is mostlyclose to the value of copper.Stainless steels shall contain at least 16 %chrome, 5 % nickel and 2 % molybde-num.Extensive measurements have shownthat only a high-alloy stainless steel withthe Material No. 1.4571, for example, issufficiently corrosion-resistant in theground.

Other materialsOther materials can be used if they areparticularly corrosion-resistant in certainenvironments or are at least equally asgood as the materials listed in Table5.5.8.1.

IU U A

AÄC A

C

K

A

=−

ϕ in A/m2i


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5.5.7.4 Combination of earthingelectrodes made of differentmaterials

The cell current density resulting fromthe combination of two different metalsinstalled in the earth to be electricallyconductive, leads to the corrosion of themetal acting as the anode (Table5.5.7.4.1). This essentially depends on theratio of the magnitude of the cathodicarea AC to the magnitude of the anodicarea AA.The "Corrosion behaviour of earthingelectrode materials" research project hasfound the following with respect to thechoice of earthing electrode materials,particularly regarding the combinationof different materials:A higher degree of corrosion is only to beexpected if the ratio of the areas is

Generally, it can be assumed that thematerial with the more positive potentialwill become the cathode. The anode of acorrosion cell actually present can berecognised by the fact that it has themore negative potential when openingthe metal conductive connection.Connecting steel installations in theground, the following earthing electrodematerials always behave as cathodes in(covering) soils:

– bare copper,

– tin-coated copper,

– high-alloy stainless steel.

Steel reinforcement of concrete founda-tionsThe steel reinforcement of concretefoundations can have a very positivepotential (similar to copper). Earthingelectrodes and earthing conductors con-nected directly to the reinforcement oflarge reinforced concrete foundations

should therefore be made of stainlesssteel or copper.This also applies particularly to short con-necting cables in the immediate vicinityof the foundations.

Installation of isolating spark gapsAs already explained, it is possible tointerrupt the conductive connectionbetween systems with very differentpotentials installed in the ground byintegrating isolating spark gaps. Normal-ly, then it is no longer possible for corro-sion currents to flow. At upcomingsurges, the isolating spark gap operatesand interconnects the installations forthe duration of the surges. However, iso-lating spark gaps must not be installedfor protective and operating earthingelectrodes, since these earthing elec-trodes must always be connected to theplant.

5.5.7.5 Other anticorrosion meas-ures

Galvanised steel connecting cables fromfoundation earthing electrodes to downconductorsGalvanised steel connecting cables fromfoundation earthing electrodes to downconductors shall be laid in concrete ormasonry up to above the surface of theearth.If the connecting cables are led throughthe ground, galvanised steel must beequipped with concrete or syntheticsheathing or, alternatively, terminal lugswith NYY cable, stainless steel or fixedearthing terminals must be used.Within the masonry, the earth conduc-tors can also be led upwards without cor-rosion protection.

Earth entries made of galvanised steelEarth entries made of galvanised steelmust be protected against corrosion for adistance of at least 0.3 m above andbelow the surface of the earth.

Generally, bitumen coatings are not suffi-cient. Sheathing not absorbing moistureoffers protection, e. g. butyl rubber stripsor heat-shrinkable sleeves.

Underground terminals and connectionsCut surfaces and connection points in theground must be designed to ensure thatthe corrosion resistance of the corrosionprotection layer of the earthing elec-trode material is the same for both. Con-nection points in the ground must there-fore be equipped with a suitable coating,e. g. sheathed with an anticorrosiveband.

Corrosive wasteWhen filling ditches and pits to installearthing electrodes, pieces of slag andcoal must not come into immediate con-tact with the earthing electrode mater-ial; the same applies to constructionwaste.

A

AC

A

> 100


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Material with great area

Material with Galvanised Steel Steel Coppersmall area steel in concrete StSt

Galvanised steel + + –– ––

Steel + + + +

Steel in concrete + + + +

Steel with Cu coating + + + +

Copper / StSt + + + +

Table 5.5.7.4.1 Material combinations of earth-termination systems for different area ratios (AK > 100 x AA)


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5.5.8 Materials and minimumdimensions for earthing elec-trodes

Table 5.5.8.1 illustrates the minimumcross sections, shape and material ofearthing electrodes.

5.6 Electrical isolation ofthe external lightningprotection system – Separation distance

There is a risk of uncontrolled flashoversbetween components of the externallightning protection system and metaland electrical installations within thestructure, if there is insufficient distancebetween the air-termination or down-conductor system on one hand, andmetal and electrical installations withinthe structure to be protected, on theother.

Metal installations such as water and airconditioning pipes and electric powerlines, produce induction loops in thestructure which are induced by impulsevoltages due to the rapidly changingmagnetic lightning field. These impulsevoltages must be prevented from causinguncontrolled flashovers which can alsopossibly cause a fire.Flashovers on electric power lines, forexample, can cause enormous damage tothe installation and the connected con-sumers. Fig. 5.6.1 illustrates the principleof separation distance.The formula for calculating the separa-tion distance is difficult for the practi-tioner to apply.

Min. dimension

Material Form Earth rod Earthing Earth plate NotesØ conductor

Copper cable f 50 mm2 min. wireØ1,7 mm

round f 50 mm2 Ø8 mm

strip 50 mm2 min. thickness2 mm

round 20 mm

pipe 20 mm min. wallthickness 2 mm

plate 500 x 500 mm min. thickness2 mm

grid-type 600 x 600 mm 25 x 2 mmplate cross section

Steel galvanised 20 mm Ø10 mmround a,b

galvanised 25 mm min. wallpipe a,b thickness 2 mm

galvanised 100 mm2 min. thicknessstrip a 3 mm

galvanised 500 x 500 mm min. thicknessplate a 3mm

galvanised 600 x 600 mm 30 x 3 mmgrid-type plate cross section

copper-plated 14 mm min. 250 µm

round c coating with99.9 % copper

bare, round e Ø10 mm

bare or 75 mm2 min. thicknessgalvanised 3 mmstrip d,e

galvanised 100 mm2 min. wire-Øcable d 1.7 mm

Stainless round 20 mm Ø10 mm h

steel g

strip h 100 mm2 min. thickness3 mm

a The zinc coating must be smooth, continuous and free of residual flux,mean value 50µm for round and 70 µm for flat material.

b The material must be formed correspondingly before galvanising.c The copper must be connected unresolvably with the steel.d Only permitted, if embedded completely in concrete.e Only permitted for the part of the foundation in contact with the earth, if

connected safely with the reinforcement every 5 m.f Can also be tin-coated.g Chrome ≥16 %, nickel ≥5 %, molybdenum ≥2 %, carbon ≤0.03 %.h Also permitted as earth entry.

Note: Aluminium and aluminium alloys must not be laid in soil

Table 5.5.8.1 Material, form and min. cross sections of earthing electrodes

L

s

s

soil

EB

MDB

foundationearthing electrode

electrical installation

metal installation

downconductor

s Separation distanceMDB Main distribution board

Fig. 5.6.1 Illustration - Separation distance

The formula is:

where

ki is a function of the type of light-ning protection system chosen,

kc is a function of the geometricarrangement (current splittingcoefficient),

km is a function of the material in thepoint of proximity and

L (m) is the geometric distance measuredfrom the point of the proximity tothe next point of the lightningequipotential bonding level.

The coefficient ki (induction factor) ofthe corresponding type of lightning pro-tection system represents the threat fromthe steepness of the current.The following values are defined for thetypes of lightning protection system:

Factor kc takes into consideration thesplitting of the current in the down-con-ductor system of the external lightningprotection system. The standard givesdifferent formulae for determining kc. Inorder to achieve the separation distanceswhich still can be realised in practice, par-ticularly for higher structures, it is recom-mended to install ring conductors, i. e. tointermesh the down conductors. Thisintermeshing balances the current flow,which reduces the required separationdistance.The material factor km takes into consid-eration the insulating characteristics ofthe surroundings. This calculationassumes the electrical insulating charac-teristics of air to be a factor of 1. All othersolid materials used in the constructionindustry (e. g. masonry, wood, etc.) insu-late only half as well as air.

Further material factors are not given.Deviating values must be proved by tech-nical tests. A factor of 0.7 is specified forthe GRP material (glass fibre-reinforcedplastic) used in the products of the isol-ated air-termination systems from DEHN+ SÖHNE (DEHNiso distance holder,DEHNiso Combi). This factor can be usedfor calculation in the same way as theother material factors.

Length L is not the actual length of thedown conductor but the plumb distance(vertical measurement), measured fromthe point of the “proximity” to the nextequipotential bonding or the next light-ning equipotential bonding level.

Each structure with lightning equipoten-tial bonding has an equipotential surfaceof the foundation earthing electrode orearthing electrode near the surface ofthe earth. This surface is the referenceplane for determining the distance L.

If a lightning equipotential bonding levelis to be created for high structures, thenfor a height of 20 m, for example, thelightning equipotential bonding must becarried out for all electrical and elec-tronic conductors and all metal installa-tions. The lightning equipotential bond-ing must be realised by using surge pro-tective devices Type I.Otherwise, even for high structures, theequipotential surface of the foundationearthing electrode/earthing electrodeshall be used as reference point and basisfor the length L. Higher structures aremaking it more and more difficult tomaintain the required separation dis-tances.

The potential difference between thestructure’s installations and the downconductors is equal to zero near theearth’s surface. The potential differenceincreases with increasing height. This canbe imagined as a cone standing on its tip(Fig. 5.6.2).

Hence, the separation distance to bemaintained is greatest at the tip of thebuilding or on the surface of the roofand becomes less towards the earth-ter-mination system.This requires a multiple calculation of thedistance from the down conductors witha different distance L.

The calculation of the current splittingcoefficient kc is often difficult because ofthe different structures.If a single air-termination rod is erectednext to the structure, for example, thetotal lightning current flows in this oneair-termination conductor and downconductor. Factor kc is therefore equal to1.The lightning current cannot split here.Therefore it is often difficult to maintainthe separation distance. In Fig. 5.6.3, thiscan be achieved by erecting the mast fur-ther away from the structure.

Almost the same situation occurs for air-termination rods e.g. for roof-mountedstructures. Until it reaches the next con-nection of the air-termination rod to theair-termination or down conductor. Thisdefined path carries 100 % (kc = 1) of thelightning current (Fig. 5.6.4).

s kk

kL mi

c

m

= i ( )


5555protective angle

s

I

Fig. 5.6.3 Air-termination mast with kc = 1

s

s

soil

down conductor

earthing electrode

Fig. 5.6.2 Potential difference with increasing height

Type of LPS Coefficient ki

I 0.1

II 0.075

III / IV 0.05

Material Factor km

Air 1

Solid material 0.5

If two air-termination rods or air-termi-nation masts have a cable spannedbetween them, the lightning current cansplit between two paths (Fig. 5.6.5).Owing to the different impedances,however, the splitting is not always 50 %to 50 %, since the lightning flash doesnot always strike the exact centre of thearrangement but can also strike alongthe length of the air-termination system. The most unfavourable case is taken intoaccount by calculating the factor kc in theformula.This calculation assumes an earth-termi-nation system Type B. If single earthingelectrodes Type A are existing, thesemust be interconnected.

h plumb distance, height of the build-ing

c mutual distance of the air-termina-tion rods or air-termination masts

The following example illustrates the cal-culation of the coefficient for a gableroof with two down conductors(Fig. 5.6.6). An earth-termination systemType B (ring or foundation earthing elec-trode) is existing.

The arrangement of the down-conductorsystem shown in Fig. 5.6.6 should nolonger be installed, not even on adetached house either. The current split-ting coefficient is significantly improvedby using two further down conductors,i.e. a total of 4 (Fig. 5.6.7). The followingformula is used in the calculation:

h plumb distance, height up to thegable of the building

c mutual distance of the down con-ductors

n is the total number of down conduc-tors

Result: kc ≈ 0.45

The example of a detached house with alightning protection system Type III (ki = 0.05) and both values of the factor kcdetermined (for 2 and 4 down conduc-tors) is intended to illustrate the calcula-tion of the separation distance s for theridge conductor. The required distance between the ridgeconductor and the electrical conductor,e. g. for the loft lighting, shall be deter-mined (Fig. 5.6.8).The roofing and the roof structure aresituated between the two conductors.The material factor is thus km = 0.5.An earth-termination system Type B(foundation earthing electrode, ringearthing electrode) is taken as given.

Separation distance for 2 down conduc-tors (first example kc = 0.7) height of thestructure 9 m, the electrical conductor isinstalled at a height of 8.5 m (distancefrom ridge conductor 0.5 m).

Result: s = 0.595 m

The actual distance of 0.5 m is not suffi-cient since the required separation dis-tance is 0.595. There is a risk of uncon-trolled flashovers.

s m= 0 050 7

0 58 5.

.

.. ( )

s k L mi= k

k c

m

( )

kc = + +1

2 40 1 0 2

12

93

i. .

kn

c

hc = + +1

20 1 0 2 3. .

kc =+

+=

9 12

2 9 120 7

i.

kh c

h cc =+

+2


5555

s

soil

kc = 1

M

Fig. 5.6.4 Flat roof with air-termination rod and ven-tilation outlet

Fig. 5.6.5 Determination of kc with two masts withoverspanned cable and an earthing elec-trode Type B

c

f

Fig. 5.6.6 Determination of kc for a gable roof with2 down conductors

h

c

Fig. 5.6.7 Gable roof with 4 down conductors

s

L

electrical conductor

lamp

Fig. 5.6.8 Separation distance sProblematic installation of electrical conductors

If the number of down conductors isincreased by 2 (second example kc= 0.46),the separation distance results as follows:

Result: s ≈ 0.39 m

The required separation distance of0.39 m (less than 0.5 m) between the rooflighting and the air-termination systemon the ridge is maintained.

To determine the separation distance atthe height of the eaves gutter (5 m aboveground level), the calculation is as fol-lows:


Consequently, in case of a wall thicknessof 24 cm, an electrical conductor could beinstalled in the inside of the structure,e.g. in a cable duct , without risk ofuncontrolled flashovers.

For structures with flat roofs, the currentsplitting coefficient is calculated as fol-lows, if the down conductors are distrib-uted equally on the perimeter (same dis-tance). In this case, an earthing electrodearrangement Type B is a precondition(Fig. 5.6.9).


c mutual distance of the down con-ductors

n the total number of down conduc-tors

Due to local conditions (e. g gates, sup-port distances) down conductors canoften not be arranged equally(Fig. 5.6.10). In such cases, a correctionfactor must be incorporated into the cal-culation.

n total number of down conductors

cs distance from the next down con-ductor

cd distance from the next down con-ductor on the other side


If electrical structures or domelights arelocated on the flat roof (Fig. 5.6.11), thentwo current splitting coefficients must betaken into account when calculating theseparation distance. For the air-termina-tion rod, kc = 1 to the next air-termina-tion/down conductor.The calculation of the current splittingcoefficient kc for the subsequent courseof the air-termination system and downconductors is performed as explainedabove. For illustration, the separation

distance s for a flat roof with roof-mounted structures is determined below.

Example:Domelights were installed on a structurewith a lightning protection systemType III. They are controlled electrically.

Structure data:

⇒ Length 40 mWidth 30 mHeight 14 m = perimeter 140 m

⇒ Earth-termination system, founda-tion earthing electrode Type B

⇒ Number of earthing electrodes: 11

⇒ Distance of the down conductors: min. 12 mmax. 16 m

⇒ Height of the electrically controlleddomelights: 1.5 m

The calculation of the current splittingcoefficient kc for the structure is:

Result: kc ≈ 0.345

It is not necessary to calculate the factorkc for the air-termination rod kc = 1.

Calculation of the separation distance forthe top edge of the roof of the structure:

The material factor km is set as for solidbuilding material km = 0.5.


Calculation of the separation distance forthe air-termination rod:

The material factor is km = 0.5 because ofthe position of the air-termination rodon the flat roof.

Result: s = 0.15 m

s m= 0 051

0 5.

.( ) 1.5

s m= 0 050 345

0 5.

.

.( ) 14

kc = + +1

2 110 1 0 2

12

14

16

123 6

i. .

kn

c

h

c

ccs d

s

= + +1

20 1 0 2 3 6. .

kn

c

hc = + +1

20 1 0 2 3. .

s m= 0 050 45

0 5.

.

.( ) 5

s m= 0 050 45

0 58 5.

.

.. ( )


5555

Fig. 5.6.10 Values of coefficient kc in case of asym-metric arrangement of the down con-ductors

s

km = 0.5

Fig. 5.6.11 Material factors of an air-terminationrod on a flat roof

Fig. 5.6.9 Values of coefficient kc in case of a meshed network of air-termination con-ductors and an earthing Type B

This calculated separation distancewould be correct if the air-terminationrod were erected on the surface of theearth (lightning equipotential bondinglevel).In order to obtain the separation dis-tance completely and correctly, the sepa-ration distance of the structure must beadded.

Stot = sstructure + sair-termination rod

= 0.48 m + 0.15 m

Stot = 0.63 m

This calculation states that a separationdistance of 0.63 m must be maintained atthe uppermost point of the domelight.This separation distance was determinedusing the material factor 0.5 for solidmaterials.Erecting the air-termination rod with aconcrete base, the “full insulating char-acteristics” of the air are not available atthe foot of the air-termination rod(Fig. 5.6.11).

If lightning equipotential bonding levelsare created for high structures at differ-

ent heights by integrating all metalinstallations and all electrical and elec-tronic conductors by means of lightningcurrent arresters (SPD Type I), then thefollowing calculation can be carried out.This involves calculating distances to con-ductors installed on only one lightningequipotential bonding level, and also tothose installed over several levels.This assumes an earth-termination sys-tem in form of a foundation or ringearthing electrode (Type B) (Fig. 5.6.12).

As previously explained, supplementaryring conductors can be installed aroundthe structure (truss) to balance the light-ning current. This has a positive effect onthe separation distance. Fig. 5.6.13 illus-trates the principle of ring conductorsaround the structure, without installing alightning equipotential bonding level byusing lightning current arresters at theheight of the ring conductors.


5555h 1

h 2h 3

h 4h n

I a

I gI f

I bI c

I d

cs cd

da

db

dc

dd

df

dg

(A)

Fig. 5.6.12 Values of coefficient kc in case of anintermeshed network of air-termination,ring conductors interconnecting thedown conductors and an earthing Type B

12n

csL

3 cdcs

6

1n

1n

ring conductor

dow

n co

nduc

tor

kc1

kc2

kc3

L 1L 2

L 3

kc1 = + 0.1 + 0.2 · ·

kc2 = + 0.1

kc3 = + 0.011st floor

2nd flor

3rd floor

4th floor

5th floor

6th floor

7th floor

ground floor

Fig. 5.6.13 Principle of ring conductors installed around a building

The individual segments are assigned dif-ferent current splitting coefficients kc. Ifthe separation distance for a roof-mounted structure shall now be deter-mined, the total length from the equipo-tential surface of the earthing electrodeto the uppermost tip of the roof-mount-ed structure must be used as the base(sum of the partial lengths). If the totalseparation distance sg is to be deter-mined, the following formula must beused for the calculation:

With this design of supplementary ringconductors around the structure, it is stillthe case that no partial lightning cur-rents whatsoever are conducted into thestructure.Even if the numerous down conductorsand supplementary ring conductors donot allow a maintaining of the separa-tion distance for the complete installa-tion, it is possible to define the upperedge of the structure as the lightningequipotential bonding surface (+/– 0).This roof-level lightning equipotentialbonding surface is generally implement-ed for extremely high structures where itis physically impossible to maintain theseparation distance.

This requires the integration of all metalinstallations and all electrical and elec-tronic conductors into the equipotentialbonding by means of lightning currentarresters (SPD Type I). This equipotentialbonding is also directly connected to theexternal lightning protection system.These previously described measuresallow to set the separation distances onthe upper edge of the structure to 0. Thedisadvantage of this type of design isthat all conductors, metal installations,e. g. reinforcements, lift rails and thedown conductors as well, carry lightningcurrents. The effect of these currents onelectrical and electronic systems must betaken into account when designing theinternal lightning protection system(surge protection).It is advantageous to split the lightningcurrent over a large area.

5.7 Step and contact voltages

DIN V VDE V 0185-3 draws attention tothe fact that, in special cases, contact orstep voltages outside a structure in thevicinity of the down conductors can pre-sent a life hazard even though the light-ning protection system was designedaccording to the latest standards.Special cases are, for example, theentrances or canopies of structures fre-quented by large numbers of peoplesuch as theatres, cinemas, shopping cen-tres, where bare down conductors andearthing electrodes are present in theimmediate vicinity.

Structures which are particularly exposed(at risk of lightning strokes) and freelyaccessible to members of the public mayalso be required to have measures pre-venting intolerably high step and contactvoltages.These measures (e. g. potential control)are primarily applied to steeples, obser-vation towers, mountain huts, floodlightmasts in sports grounds and bridges.Gatherings of people can vary from placeto place (e. g. in shopping centreentrances or in the staircase of observa-tion towers). Measures to reduce stepand contact voltages are therefore onlyrequired in the areas particularly at risk.Possible measures are potential control,isolation of the site or the additionalmeasures described below. The individ-ual measures can also be combined witheach other.

Definition of contact voltages

Contact voltage is a voltage acting upona person between his position on theearth and when touching the down con-ductor.The current path leads from the hand viathe body to the feet (Fig. 5.7.1).

For a structure built with a steel skeletonor reinforced concrete, there is no risk ofintolerably high contact voltages provid-ed that the reinforcement is safely inter-connected or the down conductors areinstalled in concrete. Moreover, the contact voltage can be dis-regarded for metal façades if they areintegrated into the equipotential bond-ing and/or used as natural componentsof the down conductor.

If a reinforced concrete with a safe tie-inof the reinforcement to the foundationearthing electrode is already presentunder the surface of the earth in theareas outside the structure which are atrisk, then this measure already improvesthe curve of the gradient area and acts asa potential control. Hence step voltagescan be left out of the considerations.The following measures can reduce therisk of someone being injured by touch-ing the down conductor:

⇒ the probability of people accummu-lating can be reduced with informa-tion or prohibition signs; barriers canalso be used.

⇒ the position of the down conductorscan be changed, e. g. not in theentrance of the structure

sk

kk l k l k lg

i

mc c c= + +( )1 1 2 2 3 3i i i


5555

1 m

ϕFE

US

FE

ϕ

ϕFE + SE UE

Ut

UE Earth potentialUt Touch voltage

US Step voltageϕ Potential of earth surfaceFE Foundation earthing electrode

reference earth

Fig. 5.7.1 Illustration of touch voltage and step voltage

⇒ the down conductor is sheathed ininsulating material (min. 3 mm cross-linked polyethylene with an impulsewithstand voltage of 100 kV1.2/50 µs)

⇒ The specific resistance of the surfacelayer of the earth at a distance of upto 3 m around the down conductormust be not less than 5000 Ωm.

A layer of asphalt with a thickness of5 cm generally meets this require-ment

⇒ Compression of the meshed networkof the earth-termination system bymeans of potential control

NoteA downpipe, even if it is not defined as adown conductor, can present a hazard topersons touching it. In such a case, onepossibility is to replace the metal pipewith a PVC one (height: 3 m; zone 0c).

Definition of step voltagesStep voltage is a part of the earthingpotential which can be bridged by a per-son taking a step over 1 m. The currentpath runs via the human body from onefoot to the other (Fig. 5.7.1).

The step voltage is a function of the formof the gradient area.As is evident from the illustration, thestep voltage decreases as the distancefrom the structure increases. The risk topersons therefore decreases the moreafter they are from the structure.

The following measures can be taken toreduce the step voltage:

⇒ Persons can be prevented fromaccessing the hazardous areas (e. g.by barriers or fences)

⇒ Reducing the mesh size of theearthing installation network –Potential control

⇒ The specific resistance of the surfacelayer of the earth at a distance of upto 3 m around the down-conductorsystem must be not less than5000 Ωm.

A layer of asphalt with a thickness of5 cm generally meets this require-ment

If large numbers of people frequentlycongregate in a hazardous area near tothe structure to be protected, then apotential control must be provided toprotect them.

The potential control is sufficient if theresistance gradient on the surface of the

earth in the field to be protected doesnot exceed 1 Ω/m.

To achieve this, an existing foundationearthing electrode should be supple-mented by a ring earthing electrodeinstalled at a distance of 1 m and a depthof 0.5 m. If the structure already has anearth-termination system in form of aring earthing electrode, this is already“the first ring” of the potential control.Additional ring earthing electrodesshould be installed at a distance of 3 mfrom the first one and the subsequentones. The depth of the ring earthingelectrode shall be increased (in steps of0.5 m) the more after it is from the struc-ture (see Table 5.7.1).

If a potential control is implemented fora structure, it must be installed as follows(Fig. 5.7.2):The down conductors must be connectedto all the rings of the potential control. The individual rings must be connectedat least twice, however.

If ring earthing electrodes (controlearthing electrodes) cannot be designedto be circular, their ends must be con-nected to the other ends of the ringearthing electrodes. There should be atleast two connections within the individ-ual rings (Fig. 5.7.5).


5555

Distance from Depththe building

1st Ring 1 m 0.5 m

2nd Ring 4 m 1.0 m

3rd Ring 7 m 1.5 m

4th Ring 10 m 2.0 m

Table 5.7.1 Ring distances and depths of thepotential control

symbolic course

refe

renc

e e

arth

0.5

m

1 m

1.5

m

1 m 3 m 3 m

2 m

3 m

Fig. 5.7.2 Potential control - Illustration and symbolic course of the gradient area

When choosing the materials for the ringearthing electrodes, attention must bepaid to the possible corrosion load(Chapter 5.5.7). Stainless steel V4A (Material No. 1.4571)has proved to be a good choice for tak-ing the formation of voltaic cellsbetween foundation and ring earthingelectrodes into account.Ring earthing electrodes can be designedas round wires Ø10 mm or as flat strips30 x 3.5 mm.


5555

Fig. 5.7.3 Possible potential control in entrance areaof the building

mast

1m 3m 3m

clamped points

Fig. 5.7.4 Potential control performance for a floodlight or cell site mast

mast conn

ectio

n to

e.g

. exi

stin

g fo

unda

tion

(rei

nfor

ced

conc

rete

)

1m3m3m

Fig. 5.7.5 Connection control at the ring / foun-dation earthing electrode

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Lightning Protection

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