1

TP1121-1May 1997

A definitive guide to earthing and bondingin hazardous areas

by LC Towle BSc CEng MIMechE MIEE MInstMCTechnology Director

The MTL Instruments Group plc

1 Introduction

This document details what has proved to beacceptable practice for earthing and bondingof electrical apparatus used in hazardous areas.The subject is not complex, but partiallybecause it is relevant to more than one area ofelectrical expertise a systematic approach tothe subject is desirable. There are numerouscodes of practice which specify how earthingand bonding should be carried out, but thefundamental requirements are independent ofthe geographic location of the installation andhence there should be no significant differencein requirements. This document predominantlydescribes what is acceptable practice in theUnited Kingdom and Europe. If a national codeof practice exists and differs fundamentally fromthis document then it should be questioned. It maybe considered expedient to comply with such acode but it is important to be assured that doingso results in a safe installation.

Some parts of this note state what are well-known basic principles to practising electricaland instrument engineers. They are restatedprimarily for the sake of completeness, andease of reference .

2 Definitions

One of the major causes of difficulty is thatthe terms bonding and earthing are usedinterchangeably. In this document the termsare defined as follows.

Earthing is the provision of a specific returnpath for fault currents so as to operateprotective devices in a very short time.

Bonding is the interconnection of twoadjacent pieces of conducting material so asto prevent a potential difference between themwhich would be a hazard to people or becapable of causing an ignition.

Occasionally a system is referenced to theground on which it stands by using mats ofcopper or rods driven into the ground. For thepurpose of differentiating this process fromthat of earthing and bonding in this document,this process will be referred to as grounding.

3 The reasons for earthing andbonding

The basic reasons for earthing and bonding arequite simply:

a) To provide a dedicated reliable low-impedance return path for fault currents so thatthe fault can be detected and the source ofpower removed as quickly as possible.

b) To prevent potential differences whichwould create a possible electrocution hazardto personnel or produce sparking capable ofcausing ignition.

c) To minimise the effect of lightning strikeseither directly on the installation or adjacentto it.

d) To control or prevent the build-up ofelectrostatic discharges.

e) To minimise the effect of electricalinterference and provide a signal reference forinstrumentation systems.

f) To satisfy segregation and define fault-pathrequirements necessary to ensure the safetyof explosion-proof apparatus.

It is desirable to remove fault currents asquickly as possible (less than a second) so asto prevent the dissipation at the point of faultfrom causing a fire or explosion. The majorityof gases require a temperature in excess of200°C to spontaneously ignite and a similartemperature can cause fires and will destroyconventional insulation.

It is interesting that the potential differencewhich is not acceptable from the electrocutionrequirement is not significantly different fromthat required to ignite gases. The sensitivity ofthe human body to electricity is quite complexsince it is both frequency and time dependent.There are many excellent references on thesubject, one of which is ‘Touch Voltages inElectrical Installations’ by BD Jenkins. Asimplified analysis is represented by figure 1.This suggests that limiting the current betweenthe body’s extremities to 5mA can be achievedby restricting the available voltage to 25V rmsover a separation of perhaps 2.5m.

Similarly the familiar ignition curves from theCENELEC apparatus standard, figure 2, suggestthat a voltage in excess of 10V is necessary tocreate a spark capable of causing ignition. Therequirements for spark prevention andelectrocution are not therefore significantlydifferent.

Figure 1 Sensitivity of human body to electrical currents

Minimum igniting currentsapplicable to electrical

apparatus with cadmium,zinc, magnesium

or aluminium

5A

2A

1A

500mA

200mA

100mA

50mA

20mA

10mA10V 20V 50V 100V 200V 500V

IIIAIIB

IIC

Figure 2 Minimum igniting current curves

The requirements of both bonding andearthing from an electrical viewpoint are notsignificantly more onerous on a hazardousplant than those of a conventional plant. Theconsequences of a failure may be moredangerous on a hazardous plant andconsequently additional precautions toincrease the reliability of the bonding andearthing are usually taken.

The following sections examine each of thefundamental requirements in more detail.

500Ω

500Ω

500Ω

500Ω

25V for 5mA shock

5–10VThreshold

Sensitivity between hands

Threshold of sensation 1mADisturbing shocks 5mAFreezing current (woman) 6–25mA

(man) 9–30mAVentricular fibrillation 1–5ACardiac arrest 10A

Note: The heart muscles only require 30mA to paralyse them.

Data at 60 Hz

2

4 Earthing

The primary purpose of earthing is to providea well-defined reliable return path for any faultcurrent which may develop. The concept isbest illustrated by considering the self-contained situation illustrated in figure 3 wherethe electricity is generated locally.

The fault current is returned to its source andnot to ground. The return path has to providea possible path wherever the fault developsand hence is usually connected to the metallicstructure at any convenient point along itspath.

Ideally a fault should cause sufficient currentto flow to operate the protective devices in arelatively short time. A fault which hassignificant impedance would perhaps notallow enough current to flow so as to operatethe protective fuses, and the resultant heatcould create a hazard. In almost all installationsproviding power to hazardous areas it is notusual to rely on fuses for adequate electricalprotection. A combination of earth leakage andout-of-balance current monitoring is almostalways used.

During the time that the protective networktakes to operate, the plant may be transientlyat risk and consequently it is important toreduce this time as far as possible. This transientrisk has always been accepted in Zone 1 and 2locations but is possibly not acceptable in thecontinuously hazardous location of Zone 0.Where explosion-proof equipment must beused in Zone 0 then the difficult problem ofminimising the transient fault current must beaddressed.

5 Bonding

Bonding is the process which ensures thatadjacent conducting materials are reliablyconnected together. An effective bond providesa path for structural currents and ensures thatthe interconnected objects are at the samepotential.

Figure 4 illustrates how all the equipment isbonded to the structure, thus ensuring that noappreciable voltage difference which could bedetrimental to personnel safety is created. Thebond is effectively in parallel with the humanand the fault current is divided between them.An effective bond will have a resistance of20mΩ and therefore would need a fault currentof 1250A to generate the 25V which is themaximum desirable to ensure personnel safety.Since there are invariably a number of parallelplant bonds, the probability of such asignificant current flowing through a particularbond is very small.

When both bonding and earthing are complete,then fortunately they reinforce one another.The bond provides an alternative return pathand the earthing conductor duplicates thefunction of the bond. The system becomes asillustrated in figure 5, creating an effectiveinterconnected web of return path andbonding. This has the merit that safety is nolonger dependent on a single conductor orconnection. The resultant equipotential planeis not significantly different from that created

Generator

Earth return path

Protectivedevices

Distributionsystem

Motor

FAULT

Figure 3 Power fault return path

Motor

Distributionsystem

Generator

Structure

Bondingconductors

FAULT

Earth return path

Motor

Distributionsystem

Generator

Structure

Bondingconductors

FAULT

Figure 4 Bonding of electrical equipment

Figure 5 Combined effect of earthing and bonding

3

by the German practice of systematic

interconnection of all system structures and

deliberate earth mats. The merit of the German

system is that adequate provision is made for

easy connection to the equipotential plane

which makes a clear statement of the desirable

practice so much easier. If a plant is being

constructed on a clear site then serious

consideration to adopting the German

techniques should be given.

A side-effect of having an effective

equipotential plane is that its inductance is

quite low which has a beneficial effect in

reducing problems associated with high-

frequency transmission and the fast rise-time

transients of lightning but does not significantly

affect mains frequency currents.

6 Grounding

The primary reasons for connecting electrical

circuits and structures to the earth mats which

attempt to make a connection to the surface

of the planet are to provide a return path for

the electrical supply to a plant, and to minimise

the effect of lightning strikes.

There are a number of ways in which electrical

supplies on plants are derived but a very

common system is that illustrated in figure 6.

The electrical power for the installation is fed

at some relatively high distribution voltage

from the grid system and connected at the

plant by a distribution transformer which

provides a 440V 3-phase star-connected system.

The centre point of the star is the system

neutral and is connected to a specially

constructed mat which connects to the ground.

This connection provides a return path for any

fault current which is derived from the grid

distribution system. The return path is not well

defined. The route may be via the pylon earths,

the protective conductor and any other

electrical conductor which happens to be

convenient. The impedance of this path is not

too critical since the high distribution voltage

will drive a detectable current through a

relatively high resistance connection.

A secondary effect of referencing the neutral to

ground is to provide a parallel return path

through the ground for any fault currents which

would normally flow through the structure. It

is not usual to rely on such paths for electrical

protection in hazardous areas since they are not

well defined. If for some reason part of a plant

is not adequately bonded, then if it makes some

connection to the ground it becomes partially

protected. This is not a satisfactory state of affairs

but is preferable to having no interconnection.

In general, with the possible exception of

lightning protection which is discussed in the

next section, the connection to ground is not

important in discussing electrical protection on

hazardous plants.

7 Lightning

This enthralling subject is worthy of

considerable discussion and a much fuller

picture can be derived from reading the

application notes produced by Telematic Ltd

and listed in the references at the end of this

document. A brief analysis follows which

Generation

Protectiveconductor

Local sitetransformer

Fault path

Figure 6 Distribution system return path

attempts to show the interaction between

lightning bonding and grounding and other

related bonds.

The primary cause of the problem is the down-

strokes between the electrostatic charge

generated in the lower part of thunder clouds

(usually cumulo-nimbus) and the corresponding

induced charge in the ground as illustrated in

figure 7.

The magnitude of the current and its rate of

rise are both important: a typical strike is 100kA,

rising to its maximum in 10µs. If this current

strikes a vertical structure such as a storage tank

as illustrated in figure 8, and if the tank

inductance is of the order of 0.1µH/m, then the

voltage gradient in the structure is 1kV/m.

Cloud

100kA10µsrisetime

Return strike along ionisedpath, usually 2 to 3 times

Figure 7 Simplified lightning strike

4

100kA10µs

10m0.1µH/m(10kV)

30kV

Potential equalisingnetwork

0.1µH/m10kA500m(50kV)

30kV

Computer 0V

Power 0V

Figure 8 Typical potential differences caused by a lightning strike

Side flashes are associated with voltages of 5kV

or more and plant must be bonded to adjacent

structures at intervals of less than 5 metres.

Conventionally the low frequency or dcresistance of lightning conductors is measuredbut in practice, with rapidly rising currentdischarges, it is the inductance of the structureor conductor which matters.

The usual practice is to provide tall structuressuch as fractionating columns with a goodconnection to ground (usually inherent in theconstruction of the column) and assume thatthe major portion of the lightning strike (90%)disappears into the ground. This does notappear to be always the case since quitefrequently the ground has a very highresistance and the current dispersion is noteasy to predict, but follows different paths witha magnitude determined by the relativeimpedances of available circuits. The simplifiedmodel usually chosen is as illustrated in figure8 and although the currents and consequentlythe voltage gradients are smaller in the horizontalplane, the cross-bonding must be maintained toavoid significant potential differences.

The susceptibility of a plant to lightning strikesis primarily decided by its location. Transientlya plant has currents and voltages capable ofcausing ignition during a lightning strike.Where the probability of a flammable mixtureof gases is high, i.e. in a Zone 0, then particularcare to maintain a Faraday-cage type ofprotection is desirable. In other zones,precautions to avoid side flashes are necessaryand the transient risk accepted.

Transient protection of instrumentation andother sensitive services is necessary from bothan operational and safety viewpoint and this isdiscussed later in this document. The level ofprecaution to be taken is a balancing of thelikelihood of a lightning strike and the possibleconsequences of equipment failure measuredagainst the cost of installing surge protection.

8 Static electricityThe avoidance of potential differences createdby static electricity which could result inignition capable sparks is a necessaryrequirement of a hazardous plant. Thepredominant hazard is not from electricalequipment but from materials being handledon or used in the construction of the plant.

Static is invariably generated by chargeseparation occurring as a result of intermittentcontact between non-conducting materials.Such separation can frequently be avoided byusing materials which are partially conducting.Some commonly used materials such aspetroleum frequently contain antistaticadditives. Non conductive fluids or powdersin motion are a frequent cause of static, whichis more easily generated as the velocity ofmovement is increased. As eddies andturbulence increase there is a marked increasein static generated. Anything which generatesdiscontinuities in the flow – such as filters,

control valves or sudden changes in pipe cross-section – is detrimental.The removal of static is usually accomplishedby providing a return path which recombinesthe separated charges. The requirement isusually met by bonding together all theelectrically conducting parts of an installation.

Figure 9 illustrates the bonding systemnecessary for filling road tankers where staticproblems can exist due to vehicle movementand the transfer of hydrocarbons. There areproblems associated with making the initialconnection without creating an incendivespark, and also providing a monitoring systemwhich cannot readily be bypassed.

The General Requirements of the CENELECstandards for electrical apparatus require thatouter enclosures which are plastic should haveantistatic properties and where thisrequirement cannot be met then they shouldbe labelled so as to avoid the generation ofstatic when they are cleaned or subject to

FLAMMABLELIQUID

Bond

Gantry

Figure 9 Static bond requirement for a road tanker

5

Parasiticcapacitance

Motor

Transientcurrent

Armour

Structure

Transformer

Figure 10 Structural currents

friction. In many locations the presence ofhigh humidity and conducting dirt and saltencrustation makes the creation of staticextremely unlikely. In some clean and drylocations, particularly where insulatingpowder is available, then static is a real riskand adequate precautions must be taken.

Except where special mechanisms exist, it isdifficult to draw a heavy spark from aninsulating surface since the amount of chargewhich is extracted by a point approach islimited. A much more dangerous situation iscreated by having a conductive piece of metal,which is not bonded to the adjacentconductive surfaces, mounted on the insulatingmaterial. This piece of metal (e.g. a metallic foillabel) can become charged and the resultantcapacitor discharged by a short circuit to theadjacent surface (a voltage of 5kV storessufficient energy to ignite hydrogen in acapacitance of 1.6pF).

For a more comprehensive treatment of therisks due to static one of the best sources ofinformation are the current British Standards.A CENELEC document is in the course ofpreparation and is worth studying if you haveaccess to such material. It will eventually bepublished as an EN.

9 Interference avoidance

This section concentrates predominantly onlow-frequency interference since this is theprincipal source of problems in the processcontrol field. Recently there has been anincreased awareness of the interferenceaspects of electrical equipment as a result ofthe European Community Directive on thesubject and the emergence of numerousrelated IEC standards. In the long term this mayresult in equipment which radiates lessinterference and is less susceptible to othersources of interference.

The major cause of interference which isusually considered is the effect of magneticcoupling between cables. This does not causemany problems in hazardous plants, sincealmost all electrical power is provided viacables which carry current to and from theload and hence generate only a limitedmagnetic field.

Invariably the power cables are armouredcable and the ferrous armour creates aneffective magnetic screen. In theory aneffective electromagnetic screen operates byallowing the magnetic field to generate acurrent in the screen which generates its ownmagnetic field which almost cancels theinitiating magnetic field. For this process tofunction there is no necessity for the screento be bonded. It is however normal practiceto do so in order to utilise the magnetic screento provide capacitive screening and also todetermine its electrostatic potential.

In almost all structures there is a significantcurrent caused by the parasitic capacitanceswhich inevitably occur in all electricalapparatus. The problem can best be illustratedby considering the capacitive current fromapparatus such as high voltage 3-phaseelectric motors. A typical motor will have a

significant capacitance between its windingsand structure as shown in figure 10 and thecurrent which flows through this capacitancewill be of the order of 100mA in normaloperation, with a significant third harmoniccontent. The majority of petrochemicalinstallations have currents of 300 to 400mAcirculating through them. When the motor isswitched on, this capacitance has to becomecharged and the current to achieve this isquite high for a very short time (6.6kA for10µs). In general, the currents arising innormal operation do not cause a majorinterference problem but the high transientcurrents generated by switch-on can causesignificant problems.

Commonly, sensitive circuits are bonded to thestructure at one point so as to avoid some ofthis structural current circulating through thesensitive circuit. The general principle istherefore to bond sensitive circuits to theequipotential plane at one point only and tochoose the point carefully so that the currentreturn paths do not share a common path withthe structural currents. The principle to befollowed is illustrated in figure 11.

The transient current to charge the motorcapacitance flows through the structural bondback to the neutral star point. If the 0V of thecomputer is returned to the neutral star pointY then the common mode voltage generatedby the surge current is across the relativelysmall impedance XY and is possibly acceptable.However if the computer 0V is connected tothe structure at the point Z then the commonmode voltage is related to the large impedanceZX and hence usually causes difficulties. Theproblem associated with multiple bonds to thestructure is illustrated by the thermocouplewhich is in contact with the motor. If theamplifier does not have isolation between theinput and the computer 0V, a proportion ofthe transient structural current will f lowthrough the parallel path created. Since thecurrent flows through the sensitive inputcircuit of the amplifier it will cause ameasurement problem or could possiblydamage the circuitry.

Figure 11 Reducing interference from structural currents

StructureZ

0V

Computer

X

Y

Transientcurrent

Motor

The problems of multiple earthing should not beexaggerated and are predominantly operational. Aconductor in parallel with a well-bonded structurewill carry only a part of the structural current(tens of milliamps continuous). It is not likelyto become hot or generate an incendive sparkwhen broken because it will have a lowinductance. Multiple earthing of intrinsicallysafe circuits is not permitted because of the ill-defined nature of the resultant circuits and onlypartially because of concern that such a circuitcould be hazardous.

The primary cause of low frequencyinterference within electrical equipment is alsostray capacitive currents. It is necessary in allelectronic equipment to provide a well-definedreturn path for any unwanted currents whichare induced into the circuit. The predominantproblem is created by capacitance between theprimary and secondary of mains transformersand is illustrated in figure 12.

In modern circuits using switch-mode powersupplies the inter-winding capacitance is muchsmaller but the frequency is higher, hence theproblem is still significant. The currents arerelatively small (250µA) and provided a well-defined return path is available through the lesssensitive parts of the circuitry via the link XY,they cause no problem. If however this link isomitted then this current may follow the pathindicated, through the sensitive input circuit,

Figure 12 Transformer-coupled interference

field wiring and capacitance to the wiringscreen, creating an interference problem.Because this problem is increased by theconnection of the field wiring it is frequentlymis-diagnosed as being caused by pick-up inthe field wiring. The cure is however to securelyconnect the 0V rail of the computer to theneutral star point, thus providing a return pathfor these currents and also providing a well-defined electrically quiet reference potential forthe computer.

There is usually a small current induced in thefield wiring. A possible form is illustrated infigure 13. If screened cable is used then theunwanted current can be returned to theneutral star point without passing through anysensitive part of the circuit. If unscreened cableis used then the unwanted current impingesupon the field wiring and finds it way back tothe neutral star point via the sensitive inputcircuitry of the computer, which creates aninterference problem.

The interference current is small and hence thereturn path does not need to be of lowresistance. However the earthing lead isnormally made robust for mechanical reliabilityreasons. If screened cable is used for safetyreasons then the screen and its earthing cablehave to be sufficiently electrically robust tocarry the possible fault current for sufficienttime to ensure that the fault is cleared. In

Figure 13 Field wiring interference

practice, the use of a robust cable (10mm2) doesnot increase the installation cost appreciablyand avoids the need to deeply consider all thepossible implications.

10 Explosion-proof equipment

All explosion-proof equipment relies for itssafety integrity on being adequately electricallyprotected so that electrical overloads do notgenerate excessive heat or incendive sparks.This document concentrates on intrinsic safety,where there tends to be more emphasis on therequirements of electrical protection andearthing. This is largely because of historicalbackground to the development of thetechnique but is justified to some extentbecause of the use of intrinsically safeequipment in the more hazardous location ofZone 0. The possibility of working on circuitswithout isolating them and the requirementthat some monitoring equipment has to remainfunctional in the presence of major gas releasesor catastrophic circumstances also leads tofurther concern.

The earthing of shunt diode safety barriersillustrates the fundamental requirements verywell and hence is discussed in considerabledetail. The shunt diode safety barrier wasintroduced to remove the necessity to certifycomplex safe-area equipment. It is designed topermit the normal operation of the circuit, andif a fault occurs within the safe-area equipmentit should prevent the passage of a level ofenergy which can cause ignition or a level ofpower which can cause excessive temperaturerise. A more detailed description of theoperation of shunt diode barriers and alsogalvanic isolators is given in TP1113 ‘Shuntdiode safety barriers and galvanic isolators – acritical comparison’.

The protection technique of shunt diodebarriers is illustrated by figure 14. A fault currentderived from the mains phase voltage invadesthe safe-area side of the barrier. Part of thecurrent may flow through the fuse of thebarrier, rupturing it; but a significant part of itwould flow through the 0V rail of the barriersystem, being limited in magnitude only by theimpedance of the fault circuit, and its durationdetermined by the fuse or other fault-currentlimitation protecting the phase providing thefault current. The return path provided to theneutral star point ensures that the fault currentdoes not enter the hazardous area, bypresenting a lower-impedance path alongwhich the current prefers to flow.

Barrier-protected intrinsically safe circuits areearthed at the barrier busbar only, andelsewhere are insulated from the plantstructure. The field mounted instrumentillustrated in figure 14 would normally have itsenclosure bonded to the structure and itsinternal electronics isolated from the case anddirectly connected to the barrier. (The level ofisolation required is to be capable ofwithstanding a 500V test. It is howeveradvisable to avoid doing a 500V test on aninstallation where there is any possibility of aflammable gas being present).

6

At 50Hzi=V.2πfc=250µA

3000pFParasiticcapacitance

Mains supply

L

N

E

Instrumentamplifier

DC powersupply

circuitry

i

i

X

Y

Cablecapacitance

300pF = 10MΩ @ 50Hzi = 25µA

250V 50Hz L2

L1

i

Cable screen

DC powersupply

circuitry

N

i

2/9

id

ih

Figure 14 Safe-area barrier fault current

During the short time that the fault current isflowing, a voltage drop occurs in the returnpath between the points X1 and X. This voltagedifference is transferred to the field mountedinstrument since the enclosure is bonded tothe structure at the same potential as the pointX and the internal electronics directlyconnected to the point X1. Since this voltagedifference occurs in the hazardous area it isdesirable that it should be less than 10V sothat the probability of an incendive spark isacceptably low. If the fault current from the240V supply is 100A (a fault circuit impedanceof 2.4Ω) then the impedance between X andX1 should be less than 0.1Ω. It is important torecognise that this is the resistance of theconductor between the barrier busbar and theneutral star point. The resistance of the

connection to ground is not important sincethe fault current does not return to ground, itreturns to the neutral star point. Various codesof practice suggest that a value of 1Ω isacceptable but this is possibly too high.The lower value is usually readily achievable,since the barrier earth connection is alwaysquite short and is a robust cable to ensure itsmechanical integrity. For example, a 10mm2

copper conductor has a resistance of 2.8mΩ/m. Hence a 25m cable would have a resistanceof 70mΩ and so would satisfy the requirement.

It is interesting that a mains supply fault inthe hazardous area, as illustrated in figure 15,produces a similar potential difference createdby the fault current flowing through the plantstructure. The multiple return paths and cross

bonding must create a low resistance returnpath of the same order as the barrier earthconductor so as to avoid significant voltagedifferences.The use of galvanic isolators as interfaceschanges the earthing requirements from beinga primary contributor to the method ofprotection, to a secondary one. Figure 16shows the fault being removed in a relatively shorttime by having a well-defined return path on thesafe-area side of the isolator. Voltage elevation ofthe safe-area side of the isolator is not transferredto the hazardous area, but a prolonged mains faultwould damage the components on the safe-areaside, or more probably damage the computer inputcircuit. In these circumstances the earth return isnot vital to safety and is primarily essential foroperational and electrical protection reasons.

Instrumentsystem

X1

Barrierbusbar

Field mountedinstrument

Isolatedinternal

components

L

N

E

X

Plant bond

Hazardous area Safe area

0V

Figure 15 Hazardous-area barrier fault current

7

Instrumentsystem

L

N

E

X

Plant bond

Field mountedinstrument

Isolatedinternal

components

Hazardous area Safe area

X1

BarrierBusbar

8

11 Combined earthing forinterference avoidance and intrinsicsafety purposes

In almost all circumstances the 0V rail of thecomputer and the barrier busbar are linked bythe method of measurement and hence theearth returns are combined.

Fig. 17 reiterates the normal practice ofreturning the computer 0V and the screens ofthe field wiring separately from the structuraland power system to the neutral star point. Thissystem ensures a defined fault return path forany power faults or induced interferencecurrents and prevents the power systemcurrents generating a common mode voltageon the 0V rail of the computer.

The introduction of the shunt diode barrierdoes not appreciably change the circumstances,as illustrated by figure 18 . The earth returnsare combined by linking the computer 0V railand screens to the barrier busbar, and the earthreturn path should meet the resistancerequirement of at least 1Ω but preferably 0.1Ω.

When isolators are used, the barrier busbar isomitted and the screens of the field wiring areconnected to the 0V rail of the system asindicated in figure 19.

In these circumstances the earth return fromthe 0V rail has to be of the same standard as forshunt diode safety barriers since the transientpotential differences developed across it appearas voltage differences between the cable screensand the structure within the hazardous area.

Protection against transients caused bylightning and power surges also impinges uponearthing and bonding requirements. A simplifiedview is that surge protection devices (SPDs)act in much the same way as shunt diode safetybarriers in that they protect equipment by

Instrumentsystem

Field mountedinstrument

Isolatedinternal

components

L

N

E

Plant bond

Hazardous area Safe area

0V

Isolator

Figure 17 Earth return system for non-hazardous plant

Figure 16 Safe-area isolator fault current

Shunt diode barrier Computer

24V

0V

Plant bond

Figure 18 Earth-return system for plant using shunt diode barrier

Computer

24V

9

24V

Isolator

0V

Plant bond

Figure 19 Earth-return system for plant using isolators

providing an alternative path for the surgecurrent, and limit the differential and commonmode voltages applied to the apparatus. Thesubject is dealt with comprehensively in theTelematic Ltd application notes listed in thebibliography and these should be studied iflightning surge damage is considered to be asignificant problem. The solution normallyadopted is to use the lightning surgesuppression earth return to establish the 0V ofthe system as illustrated in figure 20. The safe-area and barrier circuits are bonded to the surgesuppressor to prevent any differential voltagesbeing established. In these circumstances thefast rising edge of the surge current may causesome common mode problems if the return pathhas significant inductance, and hence the length ofthe grounding conductor should be minimised.

12 Practical consideration

The conventional installation becomes asillustrated in figure 21 with the barrier busbarand computer 0V insulated from the structure andreturned separately to the neutral star point bond.

It is normally a requirement that the barrierbusbar return path be periodically checked. Thischeck is much easier to do if the connection isduplicated as shown in figures 21 and 22. Ifthis is done then an accurate resistancemeasurement can be made by disconnectingone lead and inserting a low-voltage meter inseries with the resultant loop. (Note that theloop resistance is four times the parallelresistance of the normal installation). A recordof this measurement should be maintained andany instability in the readings investigated. Agreat advantage is that such a check can be donewithout a major disturbance of plant function.

The two connecting wires need to be routedclose to one another but should form aneffective ring main as shown in figure 22.Concern is frequently expressed about the resultantloop generating a magnetic pick-up problem, but Figure 20 Earth-return system for plant using surge protection devices and shunt diode barrier

Figure 21 Conventional shunt diode barrier installation

Instrumentsystem

L

N

E

X

Plant bond

Field mountedinstrument

Isolatedinternal

components

Busbarbond

Junctionbox

Barrierbusbar

0V

Hazardous area Safe area

Surge protectiondevice

ComputerShunt diodebarrier

Plant bond

24V

0V

SPD

10

HA

ZA

RD

OU

S A

RE

A W

IRIN

G

SA

FE

AR

EA

WIR

ING

HA

ZA

RD

OU

S A

RE

A W

IRIN

G

SA

FE

AR

EA

WIR

ING Instrument

system

0V

Resistance meterchecks complete loop

considerable experience suggests that this is not apractical problem. The earth return lead needs tobe identified and a common practice is to bind thetwo leads together with blue insulating tape atfrequent intervals so as to distinguish them fromother similar conductors.

With this type of installation it is worthwhile tomeasure the resistance between the busbar and thecabinet, since this is an indication of theeffectiveness of the structural bond. If facilities areavailable a note of the voltage waveform existingbetween the structure and busbar should be made.This waveform is frequently an indicator ofdeterioration in the structural bond or theintroduction of interference-generating equipment.Frequently a knowledge of this waveform is usefulin diagnosing problems.

Some care has to be taken that the terminals usedfor earth connections should be of high quality andvibration-proof. The best practical solution is to usethe terminals that are suitable for increased safety(Ex e) installations. It is permissible to carry criticalearth connections via plugs and sockets, but threestrategically placed pins must be used and someprecautions taken to prevent disconnectionwithout first removing power from the protectedinstallation.

13 Screens and armour

The basic principle to be observed is that screensare bonded to the equipotential plane at onepoint only and elsewhere are to be adequatelyinsulated. The usual practice is to bond the screenat the safe area and frequently at the barrierbusbar as illustrated in section 11. Bonding thescreen at one other point is not howeverprohibited and appendix 1 explores some otherpossible variations of cable construction and theuse of screens.

When screens are used to guard against pick-upfrom high frequencies, they are usually earthedat a number of points so as to prevent the screenpresenting a tuned aerial to the high frequency.For intrinsically safe circuits with this problemthe acceptable solution is to include 1000pFcapacitors to ground at convenient points suchas junction boxes. These effectively detune thescreen but do not provide a path for the lowfrequency currents which can cause interference

Figure 22 Testing of shunt diode return path

problems if they are permitted to flow in the screen.Since a screen and its enclosed cable areeffectively coaxial, the effect of currents in thescreen is not as great as might initially beexpected.

Screens have frequently to be terminated injunction boxes without bonding them to thestructure. The preferred technique should be tomake off the screen into a suitable ferrule anduse a terminal block to ensure that it remainssecure and isolated. This also provides a useful0V facility within the junction box whichsimplifies some aspects of fault finding. Othertechniques are permissible but are usually lesssatisfactory.

Unused cores within cables are treated in thesame way as screens, being usually connectedto the safe-area 0V system and insulated at thefield end. These cores should always beterminated in a terminal so that if they are usedat some future time they can readily beconnected.

The capacitance of a cable, which is used whencalculating the energy stored in a cable forintrinsic safety purposes, is considerably affectedby the presence of a screen. It is important thatthe higher value associated with a screened cablebe used in making the safety assessment.

Where armoured cable is used for intrinsicallysafe systems, then it is acceptable practice toregard the armour as primarily for mechanicalprotection. It therefore becomes part of the plantstructure and hence can be multiple-bonded.Bonding is usually achieved by usingconventional glands which connect the armourto the structure whenever they are used. Thisdoes mean that the armour will carry a part ofthe current which flows in the structure.

14 The use of separate earths

There is a strong body of opinion whichadvocates the use of separate groundmats forinstrument systems, computer 0V, power systemsand lightning. This separate-earth theorygenerates numerous expensive groundingsystems which have to be isolated from oneanother or interconnected by zero impedancedepending on which problem has to be solved.

The problem with trying to refute this strangearrangement is that in some circumstances itappears to have beneficial results.

The system which is frequently advocated is theconnecting together of all the sensitive 0Vconnections of instrument systems andconnecting them to a separate earth rod asillustrated in figure 23. The capacitiveinterference currents discussed in section 9 thenflow down the separate rod and at some pointtransfer into the power system earth mat andreturn to the neutral star point via theinterconnecting cable. The discipline ofconnecting together the sensitive 0Vconnections and separating them from thestructure is beneficial from an interferenceviewpoint and the impedance of the return pathis not critical since the currents are small. Thesystem however functions because the currentsare returned to the neutral star point and thiscan be achieved much more reliably andeconomically by returning the 0V of the systemto the neutral star point at the point X, asadvocated elsewhere.

In an instrumentation system without barriers apower fault to the 0V system has to be clearedby the current which passes between the twoearth mats. The indeterminate resistancebetween these mats may not be low enough forthe protective system to operate. When a systemuses shunt diode safety barriers (as in figure 23)then the resistance of the return path cannotachieve the 1Ω level demanded by most codesof practice and certainly will not approach thedesirable level of 0.1Ω. The separate earth-rodsystem is therefore generally not adequate withpower faults, and is not acceptable for anyhazardous-area installation.

There is a much greater problem if there is asignificant probability of the installation beingstruck by lightning. If the lightning andinstrumentation earths are not cross-bonded,then the possible series-mode potential appliedto the instrumentation system can readily bedemonstrated to be hundreds of kilovolts for arelatively modest lightning strike. Some earthingsystems use low resistance high frequencychokes between earths but their use withoutsome voltage limiting device across them isdifficult to understand.

The use of separate earth rods is not justified inprocess control installations. If they are insistedupon by the computer or instrumentation‘expert’ then the obvious question to ask is howdo computers operate on aeroplanes and shipswhere a single ground connection is not possibleand two separate connections to ground createproblems for even the most vivid imagination?The fundamental requirements are therefore forbonding to prevent voltage differences, and forwell-defined current return paths forinterference rejection and electrical protection.

15 Location with remote neutral starpoint

There are situations where the neutral star point issome considerable distance away from the pointat which the barriers are installed and in thesecircumstances there could possibly be a largevoltage between the plant bond potential and

11

Figure 23 Use of separate earth rod

the neutral star point. A typical installation mightbe a remote tank farm with only a limitedrequirement for power. Such a location wouldusually have a local distribution centre whichwould have a local earth mat which sets thepotential of the local earth plant as illustratedin figure 24. The supply to the distributioncentre has a fault return path to the neutral starpoint, provided by a specific conductor, thecable armour, and supplemented by an ill-defined path between the earth mats. Even withsensitive overload protection a modest faultcurrent would generate a significant fault

voltage between the neutral star point at thedistribution transformer and the plant referencepotential at the local distribution centre. Thispotential difference is evenly distributed overa long distance and providing that the returnpath is sufficiently robust to avoid local heating,does not cause any problem.

The barrier busbar should in thesecircumstances be connected to the localdistribution centre busbar, and not connectedby an isolated lead to the distant neutral starpoint. This local connection ensures that no

Figure 24 Location with remote neutral star point

large voltage develops between the plantstructure and the instrumentation circuits. The0.1Ω requirement applies to the connectionbetween the barrier busbar and the localdistribution centre ground. The return path tothe neutral star point is still necessary forelectrical protection reasons, but in thesecircumstances voltage drop across the returnpath does not affect safety. The final installationtherefore complies with the two basicrequirements of minimising the voltagedifferences within the hazardous area and havinga secure return path for any fault current.

Instrumentsystem

X1

Barrierbusbar

Field mountedinstrument

Isolatedinternal

components

L

N

E

X

Plant bond

Hazardous area Safe area

0V

Instrumentsystem

L

E

Plant bond

Field mountedinstrument

Isolatedinternal

components

Junctionbox

0V

X

X1

Hazardous area Safe area

Return path providedby cable armour supplemented

as necessary

Large separationLocal

distributioncentre

Data highway toremote data collection

system

N

Isolation plussurge protection

Current protection

12

Similar circumstances occur in numerous othersituations. For example, when associated safe-area apparatus is mounted within flameproofenclosures then the circuits are usually bondedto the enclosure for optimum safety of theinstallation. In all circumstances, if aninstallation requires a long return pathconductor from the barrier busbar, then it isbeing connected at the wrong point. Theinstallation should be redesigned so as tominimise the fault difference voltage in thehazardous area.

16 PME (protective multipleearth) installation

In this type of distribution system the neutralreturn conductor and the protective bondingconductor are combined, largely to reduce the costof the installation. Figure 25 gives a much-simplifieddiagram of a fairly complex and ill-defined situation.

The neutral is connected to the highlyconductive strata in the ground via an earth rodat the distribution transformer, and also at eachuser installation, as illustrated. If all theconnections are good (1 to 2Ω) and the systemis working as designed, each user provides aload as indicated, the return current flowspartially in the ground and partially in theneutral, and the system is reasonably safe.There are many possible faults, but consider abreak in the neutral at the point indicated. Theloads and earth resistances in parallel areequivalent to 3.3Ω; the conductive soil rises to58V with respect to the transformer earth rodand the garage earth rises to 77V. This in itself

Figure 25 Illustrative of a PME system

is not dangerous, unless by some unsuspectedroute, e.g. the traditional wire fence on woodensupports, the reference potential can betransferred to the garage forecourt.

There are other possibly more frighteningdangers if – coincident with a neutral fault –the other installations are not adequatelyearthed. In these circumstances all the faultcurrent flows via the garage earth connection,probably the immersed storage tanks. Thewhole situation is so ill-defined that a safetyanalysis is very difficult and the very lowprobabilities of intrinsic safety fault countsbecome almost insignificant.

Because of the increased probability of mainssurges on PME installations then considerationshould be given to fitting a mains surgesuppressor on all such installations.

On these installations safety must rely implicitly onthe high-current capability, low-resistance bondingof the installation. The intrinsically safe systemshould be connected at one point to the PME systemwhere the incoming neutral is connected to thelocal ground. Connected in this way, the intrinsicallysafe system does not modify the risk on theparticular site. If instrument signals are transmittedfrom a PME site, then the outgoing signals shouldbe isolated so that the site voltage distributionis not affected by the signal leads. Someconsideration of the need to fit surgesuppression should be given. This isolationshould preferably be in a safe area on the site.

There are a few installations, usually whenthe electrical power requirements are lowand the installation is remote, when fittinga power isolation transformer creates amuch safer system from a PME supply.Figure 26 shows a gas pipeline outstationsystem which demonstrates this principle.

The PME earth connection must provide areasonable connection back to the supplysystem or it does not comply with PMEinstallation requirements. If possible it shouldbe positioned a short distance away from theinstrumentation point.

The PME supply on this type of installation willusually require to be fitted with some form ofsurge suppression.

The section of the pipeline being monitoredshould normally be isolated and an effectivebypass should be connected to provide a pathfor any currents passing along the pipeline. Theisolated section then becomes the referencepotential for the hazardous area.

The section of the pipeline is frequently connectedto an earth mat as shown, which in practicecontributes very little and may confuse the cathodicprotection being applied to the remainder of thepipeline. The barrier 0V is then referenced to theisolated pipe section and also returned to the centretap of the isolation transformer so as to provide areturn path for any fault current which flows.

Highly conductive soil layer

58V

2Ω

1Ω

2Ω

1Ω

Neutralbreak

N0V

250V

10Ω

20Ω

77V

10Ω

1Ω

L20ΩSafe area

13

Figure 26 Earthing and bonding of a gas pipe monitoring station

If land lines are used for the telemetry system thensome form of isolation is normally provided in thetelemetry output and surge suppression appliedas indicated.

In some locations remote outstations arepowered by solar panels. This simplifies thesituation considerably by removing the PMEsupply and the mains isolating transformer, butthe interconnections are otherwise identical.

In these small tightly-bonded locations thenthere is little point in using isolated interfaces,and the use of barriers reduces the powerconsumption from the restricted battery supply.

17 Isolated intrinsically safecircuits

In some situations there is a historic preferencefor fully isolated circuits in intrinsic safety sincean initial connection to ground wouldapparently have no effect. This is no doubttraceable back to the original bell signaltransformer circuit. It is however possible thatfully isolated circuits could be charged to apotential which would store sufficient energyin their capacitance to ground to make a shortto ground incendive (0.01µF is incendive whencharged to 200V in hydrogen). This apparentlysignificant problem does not create a hazardoussituation in practical installations, possiblybecause the majority of ‘floating’ circuits areheld at or near to ground potential by straycapacitance and/or leakage resistances. All thecodes of practice known to the author permit

fully-floating intrinsically safe circuits and hencethis hazard is largely ignored.

Fortunately the majority of ‘fully isolated’circuits such as fire detection circuits areconnected to ground via a high value resistorconnected for earth leakage detectionpurposes. Quite a high value of resistance(100kΩ) will serve to prevent a circuit beingcharged in normal circumstances. The referencepotential in these circumstances should bechosen so as not to be subject to high voltageinvasion.

The use of earth leakage detection foranticipating field wiring faults, and also formonitoring the performance of circuits wherehigh operational reliability is very important, isa well established technique.

18 Intermediate supplies

Usually there are some questions as to whetherthe supplies connected to the safe-areaterminals of interfaces should be earthed or not.In almost all circumstances earthing or bondingone side of a power supply is advisable since itdefines a path for capacitive interferencecurrents and makes the analysis of whathappens under fault conditions much easier.With all floating systems, the analysis of possiblesneak paths caused by multiple earthing is analmost impossible task.

Battery supplies form one of the more commonsources of intermediate power, usually as trickle

charged back-up for mains supply failure. Over-heating and emission of oxygen and hydrogencreate significant problems and if possible it isadvisable to avoid locating batteries in ahazardous area. The need for adequateventilation and good installation andmaintenance requires attention in any type ofinstallation, so as to avoid the hazards associatedwith standby batteries. Floating batteries,particularly where the battery output is takento several circuits via extensive field wiring,usually create numerous sneak path possibilitiesand hence are best avoided. If floating batterysystems must be used then the use of shuntdiode safety barriers is possible but verydifficult. The use of isolators with three-portisolation is the preferred option.

19. Ships

In general the primary source of power in shipsis generated as a floating system with earthleakage monitoring in order to give maximumavailability of the critical systems under faultand emergency conditions. However, from anexplosion safety viewpoint the hull of the ship isthe reference potential and the safety requirementis met by preventing significant voltage differencesbetween the hull and the intrinsically safe circuits.

The usual solution is as illustrated in figure27 where a mains power supply (usually110V) is developed from the floating supplyand referenced to the hull. The installationcan then utilise the code of practice relatingto land based installations.

Shunt diodesafety barriers

Instrumenttelemetrypackage

Surge diverter

Telemetrysignal

Isolationtransformer

Hazardous area

Pipeline bond

Insulating flange

Transducers

Instrument ground

PME ground

Insulatingflange

14

Figure 27 Typical earthing and bonding system for an instrument installation on a ship

Hazardous area Safe areaArmourconnectionvia gland

24V dcinstrument

Typicalbarriers

Mainsinstrument

Chassis bondingconnections to

cabinet

Metal cabinet

Fieldinstrumentsystems

electronics

Out-of-balancecurrent detection

Fromisolated

distributionsystem

Intrinsically safe earth bond (<1Ω) between these busbars

Power distribution system+

–

Fieldinstrumentsystems

electronics

Armour24V dcsupply

Local isolatingtransformer

Ships’ referenceprotection

Ships hull

Barrier0V

Interface cabinet System cabinets

Screen

Armour

Plant bond(structure + conductors + soil)

0V Powerbusbar0V Clean busbar

Electricaldistribution

busbar

Motor

Plant Control room Sub stationtransformer

Computer 0V

Power systemground

Lightning earth

Luminaire

Figure 28 Summary of conventional earthing and bonding system

TP1117, whimsical ly ent i t led ‘Under -cur rent s in Mar ine I S Ea r th ing ’ , i snever theless a more comprehensiveaccount of the use of intrinsically safeequipment on ships.

20 Offshore installations

Where the generation of power is on theoffshore installation then the possiblepower fault paths are well defined. Usuallymul t ip le genera tor s feed in to adistribution system which feeds a 440V 3-phase transformer which has its neutralstar point referred to the rig at one point.This point becomes the reference point

of the rig and the installation can proceedusing the same code of practice as anonshore installation.

In prac t ice , on s tee l - s t r uc turedinstallations the impedance of the rig isso low that sensitive earth connectionscan be made at almost any point. On someinstallations there are significant magneticfields and circulating currents in theimmedia te v ic in i ty o f the e lec t r ica lgeneration equipment. Apart from thisarea there are very few problems. Thereare numerous tales of large circulatingcurrents, significant voltages betweendeck plates and high radio frequency

fields, but they are always observed bysomeone else on some other installationand probably do not exist.The problems associated with satelliteplatforms and sub-sea installations aresimilar to those considered in section 15.

15

21 Conclusion

Whenever the earthing and bonding planof an ins ta l l a t ion i s comple te thefollowing checklist should be followed:

1 Are all electrically conducting objectsbonded together?

2 Have return paths for all fault currents beenprovided? Are they secure and robust?

3 Is there a significant risk of an adjacentlightning strike?Are adequate earth mats provided?Are all the earth mats cross-bonded to eachother?Is the sensitive electronic apparatusprotected against surges on both theinstrument and power leads?In particular are all power and signalleads from distant sources protected?

4 Have all the sensitive 0V connections,together with intrinsic safety and surgesuppression earths, been collected togetherat one point?Is there a single connection between thatpoint and the plant bond at a point whichis normally electrically quiet but will carrythe surge current?Does the f low of these fault or surgecurrents generate an acceptable voltagedifference in the hazardous area?

5 Is there an electrostatic problem andhave the necessary precautions been taken?

6 Is there a Zone 0 and has special careappropriate to this most hazardous area beentaken?

A positive response to these prompts increasesthe probability of the plant being safe andoperationally reliable.

Having done all these things, get someone elseto check it.

Figure 28 illustrates the best solution forconventional industrial locations and is theanswer to almost all problems. The morecomplex situations are relatively unusual.

Appendix 1 deals with the possible connectionsof screens in almost all foreseeablecircumstances. It is an interpretation of theBritish Standard Code of Practice by the authorbut it has existed for some time and has beenwidely discussed and agreed in variousappropriate committees. It can therefore beregarded as having considerable status.Figures 1 to 6 illustrate various possiblecombinations of screens within cables, figure7 illustrates the usual installation of anarmoured cable with both internal and overallscreens.

Figure 1: Conventional limit switch installation

The screen is shown connected to the barrier 0 volt rail. This illustrates the usual circumstances for individualcircuits. It is not however essential for safety reasons; the screen could be bonded at the switch and isolatedat the barrier if this is considered operationally desirable.

Note: Although failure between the wires and between the wire and screen does not create anincendive spark, it is usual for cables to conform to the insulation requirements. This does notpreclude the possibility of using bare wires and/or bare screens in exceptional circumstances.

Example 1

Requirements for screens in intrinsically safe cables.

Cable construction

Example 2

Cable construction

Figure 2: Conventional level switch installation

The cable construction is identical to that of the previous application.

The screen is shown bonded to the liquid container and isolated at the interface, which would be the usual circumstances.However, it is permitted to bond the screen at the interface and isolate it at the container.

The following examples illustrate the more common combinations of ‘earthed’ circuits and screens.

Isolatedinterface

Appendix 1

16

Figure 3: Limit switches in common multicore possibly subject to mechanical damage

In this installation the two screens are necessary to prevent an accidental interconnection between the two circuits.The integrity of each screen can be readily established if the screens are insulated from each other.

The two screens should be independently connected to the 0V rail as indicated.

Example 3

Cable construction

Example 4

Cable construction

Figure 4: Limit switches connected via a multicore adequately supported and protected.

Where the screens are not required for safety purposes, and provided that they are all bonded at the same place, thereis no requirement for insulation between the screens. They must however be insulated from thecircuit cores and have an overall cover to prevent inadvertent contact with earthy cable trays, etc.

This does permit the use of multicores made up from separately screened quads etc, commonly used forvibration transducers and similar devices.

Insulation

Insulation

Figure 5: Limit switch and float switch in individually screened circuits bonded at different points

If the two circuits of figure 1 and figure 2 are combined into one multicore, then the two screens are bonded atdifferent points and hence must be isolated from one another. It may be more practical to bond both screens atthe barr ier 0V rai l . Assuming the two screens have to be bonded at the points i l lustrated then the cableconstruction becomes as illustrated.

Example 5

Cable construction

Example 6

Cable construction

Figure 6: Circuits bonded at different points with an overall screen

If the two circuits of figure 1 and figure 2 are combined in one multicore with an overall screen, then that screen may bebonded at any point and insulated to withstand a test of 500V elsewhere.