n° 173 earthing systems worldwide and evolutions E/CT 173, first issued September 1995 Roland Calvas An ENSERG 1964 engineering graduate (from the Ecole Nationale Supérieure d'Electronique et Radioélectricité de Grenoble) and an Institut d'Administration des Entreprises graduate, he joined Merlin Gerin in 1966. During his professional career, he has been sales manager and then marketing manager for protection of persons. He is currently in charge of technical communication for the groupe Schneider. Bernard Lacroix An ESPCI 74 engineering graduate (from the Ecole Supérieure de Physique et Chimie Industrielle de Paris), he then worked 5 years for Jeumont Schneider, where his activities included development of the TGV chopper. After joining Merlin Gerin in 1981, he was then in turn Sales Engineer for UPS and sales manager for protection of persons. Since 1991 he is in charge of prescription for LV Power distribution.
An ENSERG 1964 engineeringgraduate (from the Ecole NationaleSupérieure d'Electronique etRadioélectricité de Grenoble) and anInstitut d'Administration desEntreprises graduate, he joinedMerlin Gerin in 1966.
During his professional career, hehas been sales manager and thenmarketing manager for protection ofpersons. He is currently in charge oftechnical communication for thegroupe Schneider.
Bernard Lacroix
An ESPCI 74 engineering graduate(from the Ecole Supérieure dePhysique et Chimie Industrielle deParis), he then worked 5 years forJeumont Schneider, where hisactivities included development ofthe TGV chopper.
After joining Merlin Gerin in 1981, hewas then in turn Sales Engineer forUPS and sales manager forprotection of persons.Since 1991 he is in charge ofprescription for LV Powerdistribution.
2. Earthing systems worldwide General p. 9Influence of MV earthing systems p. 9
LV earthing systems p. 10
Earthing systems of private p. 11LV networks in some countries
3. Evolutions and choices of earthing Evolution of electrical p. 15systems installations
Earthing systems and disturbances p. 15in electronic systems
Evolution of earthing systems p. 17
Choosing the earthing system p. 18
4. Conclusion p. 21
Appendix 1: IEC 364 standard p. 22
Appendix 2: bibliography p. 24
Following an historical review of theorigins of Earthing Systems, this«Cahier Technique» goes on to provideinformation on the practices in some
countries concerning medium voltage,HV/LV substations, in particular inLV public, industrial and tertiarydistribution.Electrical installations are evolving,electronics are everywhere, thusleading us to look afresh at earthingsystems used in LV and indeed even topredict an evolution which should bringthe TN-S and TT systems closertogether.The criteria for the selection of earthingsystems has changed. We advise thosenot very familar with earthing systems
standardised by IEC 364 to first read«Cahier Technique» n° 172.
Today electrical installation standardsare highly developed and cover allmajor aspects for a safe installation.
In LV, the reference standard isIEC 364 (see appendix no. 1).
Standard makers have paid particularattention to the measures to beimplemented to guarantee protection ofpersonnel and property (part 4 of the
above-mentioned standards).This concern has resulted in thestandardisation of three EarthingSystems.
Before reminding readers of thesethree systems, a concise historicalreview will certainly be of use.
historyElectrical hazard and protection ofpersons
c in the 18th century, the static
electricity produced by friction of certaininsulating bodies formed a «scientific»diversion causing experimenters to jump up.... in drawing rooms.A few dangerous experiments showedthe electrical nature of lightning.And in 1780: by chance an«electrostatic machine» made afrog’s legs move. Galvani observedthe contraction of muscles by electricity.
c in 1880: in order to transmit electricityover several kilometres, DC voltage leftthe 100 V range (required for arc lampoperation) and rose to 1,300 V (1882
exhibition in Munich) (see fig. 1), andthen to 3,000 V (Grenoble-Vizille link in1883).Insulation faults cause leaks andshort-circuits.The 100 V DC voltage can allegedly betouched without risk.
c in 1886: the first distributioninstallation in the USA: 12 A/500 V/ACgenerator and 16 small transformerssupply consumers with 100 V AC forthe first time;
1. review of standardised earthing systems
fig. 1: Mr. Desprez's installation located in Munich Palace during the Munich exposition.
c in 1889: AC and DC current wage warin North America:v Edison defends DC and describes thedangers of AC for personnel. He carriesout tests on dogs and horses,v Westinghouse supports AC.Edison challenges Westinghouse to aduel: each will be subjected to identicalvoltages of 100, 150, 200 V etc. inDC for Edison and in AC for
Westinghouse...: prediction: at 200 V AC,death will ensue for Westinghouse!The duel did not come off... a telegraphoperator climbing on a pole waselectroducted in the very heart of NewYork.
c in 1890: Kremler entered the electricchair and was electrocuted with...AC current!Thus, at the end of the 19th century, itwas obvious to the technico-scientific
community that electric current wasdangerous for man, and that AC wasmore dangerous than DC.
emergence of earthingsystemsThese systems are the result of alengthy evolution guided by thesearch for increased personnel
protection.Between 1880 and 1920,transmission and distribution ofelectrical power took place in«unearthed neutral».Lines are uninsulated, placed out ofreach, supported by insulators; nopoints of the network are deliberatelyearthed.In homes, voltage is 100/110 V AC.Throughout this period, fuses blow andpersons «receive Electric Shocks»
(see fig. 2). However, in view ofdistribution voltage level, few personsare electrocuted.
c in UK, in the last quarter of 19 th
century, electric arc lighting wasdeveloping rapidly. When it wasintroduced into houses, insurancecompanies became concerned aboutdanger of fire due to undersized cables,poor jointing and insulation breakdown.Many insurance companies producedsets of rules to minimise their risks.
In May 1882, the Council of the Societyof Telegraph Engineers and ofElectricians (later to become theInstitution of Electrical Engineers),appointed a committee to consider rulesfor the prevention of fire risks due toelectic light. These rules were notpopular with the insurance companieswho continued to publish their own. TheIEE had yet to become a recognisedauthority on the subject. By the thirdedition of the IEE rules in 1897, therewas still strong opposition frominsurance companies and it was notuntil 1916 that the final oppositioncrumbled and the IEE rules becameuniversally accepted in the UK.
In the first edition of the rules, in 1882,two items were concerned with dangerto people: no one should be exposed tomore than 60 V and the potential
between two points in the same roomshould not exceed 200 V. The earthingof metalwork of appliances working atdomestic voltages was first required inthe eighth edition in 1924, althought itwas soon recognised that an adequateearth was not always easy to obtain.
In 1930, the requirement for an earthleakage trip operating at 30 mA or lesswas introduced (since deleted).
c in France in 1923 a «standard» forelectrical installations makes earthingof frames a «requirement»:v casings of fixed and moving motors,
which may be touched in a non-insulated area, in installations with avoltage greater than 150 V,v fixed and portable electricalhousehold appliances with a powergreater than 4 kW,v electrical bath heater enclosuresinstalled in bathrooms,v metal parts placed in premisessteeped in conductive liquids andwhich, due to insulation faults, mightbecome live.
a
1st fault
nothing happens
a
double fault
the fuse blows
if full fault
a
Earthing of load
frames (1923) to prevent
Electric Shock by
«indirect» contacts
fig. 2: the emergence of the unearthed neutral.
The standard provides absolutely noinformation on earthing conditionsor on the value of earth connection
resistance, and stipulates noprotection device. Although it containsa few rules for fuses these are only forinstallation conditions.In order to prevent fuses blowing on adouble insulation fault, it quicklybecome obvious that indication of thepresence of the first fault was a goodidea.For this reason, the first failsafeinsulation monitor was installed inindustrial installations (see fig. 3). fig. 3: lamp insulation monitor in industry.
If a lamp goes out, there is a faultbetween the corresponding phase andthe earth.Thus the first earthing system came
into existence: the unearthed neutral.The Permanent Insulation Monitor(PIM), with three lamps (in three-phase)was used up to 1955.
In 1951, the first «tube» PIMs,injecting DC, were installed in mines:insulation of phases and neutral wasmonitored.
In 1962 the first transistor PIMs(Vigilohm TA) were produced, and in1972 the first PIMs injecting lowfrequency AC.
In 1927 a decree stipulated theearthing of the transformer neutral in
public distribution in France(U uuuuu 150 V AC).
At this time, production of electricityin France was approximately 350 kWh/ inhabitant/year (in 1900 it was 7); atenth of this production was distributedin LV.
Electricity firms supply a number ofconsumers by transformer. However, inunearthed neutral, two earthing faultsoccurring at two different consumers,do not always cause fuses to blow andthere is a definite risk of «fire» (the«indirect contact» risk exists, but is low
and not known).Application of the 1927 decree thusstipulates more reliable disconnectionof the faulty consumer, therebyensuring a sound network ismaintained.
In 1935, the decree on protection ofworkers and standard C 310, taken upby standard C 11 of 1946, began tomention the risk inherent in insulationfaults. It is at this moment that thecombination of «earthing of loads andautomatic breaking devices» firstappeared. The latter may be fuses,«RCDs» or voltmeter relays of frame/ earth voltage (see fig. 4).
Note that protection devices with athreshold of under 30 A are supposedto guarantee safety!The first residual current connectioncircuit-breakers were manufactured in1954. In addition to protection of
persons and disconnection ofconsumers, they made it possible tocombat illegal connections (currentstealing between phase and earth
when 127 A single-phase moves to220 V two-phase) (a single currentmeasuring winding in the meter).
This is how the earthed neutral cameinto existence in France. However, itwas not until the decree of the 14.11.62on protection of workers and standardC 15-100 (blue) of the 28.11.62 thatfault loop impedance and thus earthconnections were defined accurately,according to fuse rating or RCDthreshold then set by standardC 62-410 at 450 ± 200 mA.Standard C 15-100 of 1962 thus gave
official status to the unearthed neutraland the earthed neutral (measurementB1), as well as to the TN system(measurement B3).It made a clear distinction betweendirect and indirect contact and lists theprimary protection measurements (A)
and how to protect by automaticdisconnecting devices (B), withouthowever giving information onoperating times.
Alongside the standard, the decree ofthe 14.11.62 legalised the unearthedand the earthed neutrals.In 1973 a decision of the Board ofEmployment authorised the TNsystem in France.Between 1962 and 1973 each earthingsystem had its ardent supporters inFrance and elsewhere. The TN systemhas the advantage of a simple principle:the SCPDs de-energise loads (orLV consumers) having an insulationfault.
The TN system (exposed-conductiveparts connected to neutral) is used insome countries in public distribution(not in France): (see fig. 5).As personnel protection against indirectcontact is involved, use of this systemrequires complete mastery of loop
impedances (irrespective of where thefault occurs) to ensure operation of theSCPD which will disconnect the faultypart within the specified time.
Definition of these times by IEC expertsin the nineteen seventies, according toimpedance of the human body andpathophysiological effects, has madeits use possible.
It should be noted that transformationof an insulation fault into a short-circuitincreases risk of damage to equipmentand of fire. With this in mind, let usremember that protection is based onthe rapid evolution of an insulation faultto a full fault between phase andneutral.
earthing systems of IEC 364The three earthing systemsinternationally standardised arecurrently taken over in many nationalstandards.These three systems have beenstudied in detail in «Cahier Technique»n° 172 and, for each of them, thehazards and associated protectionswitchgear have been presented. Weshall however briefly review theirprotection principle.
The TN system
(see fig. 6)c the transformer neutral is earthed;c the frames of the electrical loads areconnected to the neutral.The insulation fault turns into ashort-circuit and the faulty part isdisconnected by Short-CircuitProtection Devices (SCPD).The fault voltage (deep earth/frame),known as «indirect contact» is ≈ Uo/2if the impedance of the «outgoing»circuit is equal to that of the «return»one. When it exceeds the conventionallimit voltage (U
L), which is normally
50 V, it requires disconnection, which
must be especially quick when Ud islarger than U
L.
The TT system(see fig. 7)c the transformer neutral is earthed;c the frames of the electrical loads arealso connected to an earth connection.
The insulation fault current is limited bythe impedance of the earth connectionsand the faulty part is disconnected by aResidual Current Device (RCD).
The fault voltage is:
Uc = UoRA
RB + RA
greater than
voltage UL, the RCD comes into action
as soon as Id u UL
RA
.
The IT systemc the transformer neutral is not earthed.It is theoretically unearthed, but in factis connected to the earth by the straycapacities of the network and/or by ahigh impedance ≈ 1,500 Ω (impedance-
earthed neutral).
c the frames of the electrical loads areconnected to the earth. If an insulationfault occurs, a small current is developeddue to the network’s stray capacities(see first diagram, fig. 8).
The voltage developed in the earthconnection of the frames (a few volts atthe most) does not present a risk.
If a second fault occurs (see seconddiagram, fig. 8) and the first one has notyet been eliminated, a short-circuitappears and the SCPDs must provide thenecessary protection.
The frames of the relevant loads arebrought to the potential developedby the fault current in their protectiveconductor (PE).
generalIn all industrialised countries,LV networks and loads are earthed forsafety reasons to guarantee protectionagainst electric current for persons.The objectives are always the same:c fixing the potential of live conductorswith respect to the earth in normaloperation;c limiting voltage between the framesof electrical equipment and the earth
should an insulation fault occur;c implementing protection deviceswhich remove the risk of ElectricShocks or electrocution of personne;c limiting rises in potential due toMV faults.
influence of MV earthingsystemsWhile the first three objectives listedabove fall into the range of LV earthingsystems, the fourth has considerablerepercussions on safety of personnel
and property in LV. Thus, atMV/LV substation level, a MV phase/ frame fault or a fault between MV andLV windings may present a risk forequipment and users of theLV network.
In public and industrial MV, except incertain special cases, the neutral is notdistributed and there is no protectiveconductor (PE) between substations orbetween the MV load and substation.A phase/earth fault thus results in asingle-phase short-circuit currentlimited by earth connection resistanceand the presence of limitationimpedances, if any (zero sequencegenerator).
The current tendency, in variouscountries, is to limit the zero sequencefault currents of MV networks, thusallowing:
2. earthing systems worldwide
c increased continuity of service(availability of electrical power) byauthorising automatic reconnection ona transient fault,c connection or not of the frames of theMV/LV substation and those of theLV neutral to avoid risk for LV usersand equipment.IEC 364-4-442 states that the earthingsystem in a MV/LV substation must besuch that the LV installation is not
subjected to an earthing voltage of:c Uo + 250 V: more than 5 s;c Uo + 1,200 V: less than 5 s(Uo 3 in IT). This means that thevarious devices connected to theLV network must be able to withstandthis constraint (see fig. 9a).
The same standard states that ifRp > 1 Ω, the voltage Rp Ih
MTmust be
eliminated, for example:c in under 500 ms for 100 V;c in under 100 ms for 500 V.
If this is not so, Rp and RN
must beseparate whatever the LV earthingsystem. This rule, not always compliedwith in certain countries, often leads tothe separation of the two earthconnections (for MV networks with a
high zero sequence fault current). If allthe earth connections (substation-neutral-applications) have beengrouped into a single one, a rise inpotential of LV frames may beobserved which can be dangerous(see fig. 9b).
N
MV MV LV
IhMT
RP RB
HV
fig. 9a: if Rp and R B are connected, the fault current causes the potential of the LV network to
rise with respect to the earth.
IhMT
MV
RT (RPBA)
LV
fig. 9b: the LV load frames are raised to the potential I h MT R T .
The table in figure 10 gives a fewexamples for public distributionworldwide. It shows that, in manycountries, the earth connections of the
substation and neutral must beseparate if their resulting value is notless than 1 Ω.Note that the impedance-earthedIT earthing system is the mostcommonly used in MV industrialnetworks. The «zero sequencegenerator» supplies a resistive currentaround twice the capacitive current ofthe network (see «Cahier Technique»n° 62), thus allowing use of a RCD toensure protection by disconnection ofthe faulty feeder.
LV earthing systemsThe MV/LV transformers used aregenerally Dy 11 (delta/star). Howeverthe use of midpoint single-phasedistribution for public distribution in theUSA and Japan should be pointed out(see fig. 11).Most countries apply or deriveinspiration from standard IEC 364which defines the TN, IT and TTearthing systems and the protectionconditions, both for public and privatedistribution.
In public distributionThe most common systems areTT and TN; a few countries, inparticular Norway, use the IT system.The table in figure 12 lists someexamples for public distribution(LV consumers).This table shows that Anglo-Saxoncountries mainly use the TN-C, whereasthe TT is used in the rest of the world.
fig. 11: coupling of the secondary windings
of MV/LV transformers.
a) star
three-phase
b) midpoint
single-phase
fig. 12: public distribution examples worldwide (LV consumers) - LV earthing systems.
country MV earthing frame observationssystem connection
Germany unearthed or compensed connected if Rp < 2 Ω or 5 Ω
10 and 20 kV Id < 60 A Id x RT < 250 V
Australia directly earthed separated Rp < 10 Ω
11 and 12 kV Id= a few kA except if RT < 1 Ω
Belgium limitation impedance separated Rp < 5 Ω
6.3 and 11 kV Id < 500 A d u 15 m
France limitation separated
20 kV impedance except if RT
overhead Id i 300 A < 3 Ω Rp < 30 Ω
underground Id i 1,000 A < 1 Ω Rp < 1 Ω
Great Britain direct or limitation separated Rp < 25 Ω
11 kV impedance except if RT < 1 Ω
Id < 1,000 AItaly unearthed separated Rp < 20 Ω
10-15 and 20 kV Id i 60 A
(more in reality)
Ireland unearthed on 10 kV separated stipulations10 and 38 kV compensated on 38 kV except if RT < 10 Ω on how to
Id < 10 A produce RpJapan unearthed connected
6.6 kV Id < 20 A RT < 65 Ω
Portugal limitation separated except Rp < 20 Ω
10 to 30 kV impedance if RT < 1 Ω
overhead Id i 300 A
underground Id i 1,000 AUSA directly earthed connected the earths of the source
4 to 25 kV or by of the MV/LV substation
low impedance and of the LV neutral
Id = a few kA are connected
fig. 10: public distribution examples - MV earthing systems.
country LV earthing observations
system
Germany TT and TN-C the TN is the most commonly used; RT must be < 2 Ω;
230/400 V earth connection at the consumer's, even in TN
Belgium TT Ru < 100 Ω
230/400 V 30 mA RCD for sockets
Spain TT Ru < 800 Ω with 30 mA RCD at supply end of the
230/400 V installation
France TT Ru < 50 Ω, (100 Ω shortly)
230/400 V 30 mA RCD for sockets
Great Britain TT and TN-C - town areas: TN-S and TN-C (New Est installations:
240/415 V 15 %), the earth connection (< 10 Ω) of the neutral
is provided by the distributor
- rural areas: TTItaly TT RCD with I∆n as a function of Ru (I∆n < 50/Ru).
230/400 V For consumers without earth connection 30 mA RCD
Japan TT Ru < 100 Ω, frequent use of 30 mA RCD,
100/200 V no search for equipotentiality
Norway IT premises in insulating materials and poor earth
230/400 V connections account for this choice.
homes with signalling 30 mA RCD.
tripping of connection circuit breaker if 2 faults.
Portugal TT Ru < 50 Ω (100 Ω as from 1995).
USA TN-C earthing of neutral at LV consumers (all earth
120/240 connections are connected to the source substation).
- as the PE is not mechanicallyconnected to the faulty live conductor(cables on cable path acting as a PE),the electrodynamic forces due to the
high fault current separate the cablefrom its support (American LV networkshave very high prospective faultcurrents). This causes a transient faultwith, as a result, a risk of non-operationof the SCPDs and an increase in firerisk.Note that when the PE is a distributedconductor, earthing of the transformerneutral is sometimes performedthrough a low impedance, in order tolimit the I2t at the fault point (Id i than1,000 A).c protection devices used in TN-S
In addition to the use of SCPDs,remember that to the American way ofthinking the main purpose of the«earth» protection devices used isprotection of property and limitation ofthe fire risk.
In this field, the NEC imposes minimumrequirements, i.e. use of residualcurrent protection devices on LVinstallations when the following3 conditions are met:- neutral directly earthed,- phase-to-neutral voltage greater than150 V and less than 600 V,
- nominal current strength of the deviceat the supply end greater than1,000 V,v implementing the RCDsThis protection can be performed inthree ways:- «Residual Sensing» (detection ofresidual current by vectorial addition ofcurrents in live conductors), (seefig.14). This assembly, known asNicholson’s, requires the installation ofa current transformer on the neutral; inthe USA the neutral is neither switchednor protected.
- «Source Ground Return» (residualcurrent device placed in the neutral-earth link) and usable only at the supplyend of the installation. It allows parallel-connection of sources (see fig. 15),- «Zero sequence» (classical RCD).For low current detection, it can beused at various levels of the installationto form discriminating protection(see fig. 16),
c additional measureLimitation resistance is monitored by anohmic relay:v if resistance is broken: the earthing
system becomes an IT: operation cancontinue, but the RCD will open afeeder on a double fault,v if resistance is short-circuited, theearthing system becomes TN-S andthe first insulation fault causes theBT circuit-breaker to open, unless, ofcourse, the electrical maintenanceservice has acted in time.
Compared with the classical TT andTN-S systems, this earthing system isto be preferred when Uo voltage isgreater than 400 V (as is the case in
mines), since it limits contact voltage.The desire to limit insulation faultcurrents is fairly widespread for avariety of reasons:c high short-circuit power: USA;c uncertain loop impedance: mines,worksites;
c limitation of damage and/or firehazard: process - mines -petrochemistry (note that British
example, an earthing system which ishalf TN-S and half TT (see fig. 18).It uses RCDs for protection.
Characteristics of this earthing system:c the protective conductor isdistributed;c the load frames are connected tothe PE which is earthed atMV/LV substation level;c a resistor placed between thetransformer neutral and the earthconnection limits the insulation faultcurrent to less than 20 A.This system has both advantages anddrawbacks:c advantages:v a low contact voltage despite use of anetwork voltage of 525/900 V,
Uo RPE
RPE + RPh + 27 Ω
v a low fault current, thus considerablyreducing risk of fire and damage tofaulty loads,v discriminating protection by RCD withuse of time discrimination.Note that use of RCD is particularlyadvantageous since the LV networktopology is constantly evolving (loopimpedance!).c drawbacks:In the event of HV/LV disruptive
breakdown in the transformer, there isa risk of rise in potential of the liveLV network conductors compared withthe earth and frames (Ih
MTR): this risk
can be reduced by use of a surgelimiter.Moreover, a residual current deviceplaced on the neutral/earth circuitcauses the MV circuit-breaker to openimmediately if it detects a fault currentgreater than 20 A.
fig. 18: earthing system used in RSA (mines).
Petroleum (BP) produces all itsinstallations worldwide using theimpedance-earthed TN-S (see fig. 18)with a resistance of 3 Ω in LV and
of 30 Ω in 3.2 kV).
In China
China is waking up! However, it haslong been under the technical influenceof the USSR, which is a member of theIEC (Russian is one of the IEC’s officiallanguages together with English andFrench).Consequently, all three earthingsystems are known and are used tovarying degrees.c IT is used when continuity of serviceis vital and there is a real risk for
persons (hospitals);c TT used in public distribution, is alsoused in industry and the tertiary sector,but increasingly less so, perhaps due tothe rare use of time discrimination;
c TN-C, which originated in the USSR,has completely gone out of use;c TN-S is increasingly chosenby Design Institutes for largeprojects.
A clash of technical cultures isinevitable:c electrical engineers have problemswith the harmonics generated by staticconverters. These harmonics causetemperature rises in transformers,destruction of capacitors and abnormalcurrents in the neutral;c electronic engineers place filtersupstream of their products, which donot always withstand overvoltages andlower network insulation;
c lamp manufacturers are unaware ofthe problems caused by energisinginrush currents, harmonics and highfrequencies generated by certainelectronic ballasts;c computer engineers (same applies todesigners of distributed intelligencesystems) are concerned withequipotentiality of frames andconducted and radiated interference.
These specialists sometimes haveproblems understanding one anotherand do not necessarily all have thesame approach. Also, very few of them
are familiar with earthing systems andtheir advantages and drawbacks facedwith the evolution in the techniquesdescribed above.
earthing systems anddisturbances in electronicsystemsElectromagnetic disturbances assumemany different forms, namely:c continuous or occasional;c high or low frequency;c conducted or radiated;
c common or differential mode;c internal or external to the LV network.
Choice of earthing system is not aneutral one as regards:c sensitivity to disturbances;c generation of disturbances;c effects on low current systems.
Readers wishing to improve theirknowledge in this area should study thefollowing «Cahiers Techniques»:c n° 149 - EMC: Electromagneticcompatibility;c n° 141 - Les perturbations électriquesen BT;c n° 177 - Perturbations des systèmesélectroniques et schémas des liaisonsà la terre.
This section will only review the mostimportant aspects, without describing
earthing system behaviour facedwith MV (50 Hz) faults.
Faced with harmonics
The TN-C should be avoided sincerank 3 harmonics and multiples of 3flow in the PEN (added to neutralcurrent) and prevent the latter frombeing used as a potential referencefor communicating electronicsystems (distributed intelligencesystems). Moreover, if the PEN isconnected to metal structures, boththese and the electric cables become
sources of electromagneticdisturbance.
NoteThe TNC-S (TN-S downstream from aTN-C should also be avoided eventhough risks are smaller).
Faced with fault currents
c short-circuits: avoid separating thelive conductors; otherwise the Icccreates an electromagnetic pulse in theresulting loop;c electrical earthing fault: the PE mustfollow the live conductors as closely aspossible, or, better still, be in the same
multi-conductor cable. Otherwise, asabove, the transmitting loop effectappears. The higher the fault current,the greater this effect. The TT earthingsystem will thus be preferred, as theTN and IT (2nd fault) can developcurrents a 1,000 times greater.
3. evolutions and choices ofearthing systems
evolution of electricalinstallationsIn 1960 the tertiary sector had barelystarted to develop: plants, normallylarge in size, were often installed nextto source substations.The main concern of companies wasoperation of processes; boasting acompetent electrical service, some ofthem would be won over by theunearthed neutral.
Little by little, the safety guaranteed bythis system led it to be stipulated intertiary installations wheredependability was of prime importance:e.g. hospitals.
In the 1990’s, electrical power is theuniversal driving force in homes,tertiary and industry.Although public distribution has madeenormous headway in terms ofavailability of electrical power, thisavailability is still not always sufficient,and generator sets and uninterruptiblepower supplies are thus used.
c the housing sector no longer acceptspower cuts;c tertiary is a major computerconsumer;c industry has set up in rural areas, is amajor automation system consumerand is increasingly using staticconverters; for example, motors arecontrolled by a speed controller andfunctionally linked to a PLC.
In all buildings, «intelligent» devices areincreasingly being controlled bytechnical management systems(process - electrical distribution -
building utilities).These digital systems, includingdistributed computing, nowadaysrequire the problem-free joint existenceof high and low currents; in otherwords, electromagnetic compatibility(EMC) is vital.
c protecting sensitive equipment byinstalling a surge limiter (varistor, ZnO
lightning arrester) directly at theirterminals.As regards common mode
overvoltages (lightning), ZnO lightning
arresters should be installed at theorigin of the LV installation with theshortest possible earth connections. In
this case, although the TN and TT
earthing systems may seem more
suitable than IT but overvoltages are
transmitted on LV phases. In actualfact, at the frequencies considered, thephase/neutral impedance of the LVwindings is very high (the phases are
as though they were «unearthed» evenif the neutral is earthed).
Faced with HF disturbances:All the earthing systems are equivalent.Advice for minimising the effects of HFdisturbances:c use the Faraday cage effect forbuildings (metal structures and meshedfloors), or for certain rooms in thebuilding reserved for sensitiveequipment,c separate the frame network(structural and functional frames) fromthe earth network (PE),
c avoid loops which may be formedby the high and low currentcircuits of communicating devices orplace low current links (framesurfaces - ducts/metal screens -accompanying frames) under a«reduction effect»,c avoid running them too close to powercables and make them cross at 90°;c use twisted cables, or, even better,shielded twisted cables.There are still not many standards inthis area and they are often prepared(EMC standards) by electronicengineers. Installation standardIEC 364, sections 444 and 548, shouldprovide increasingly morerecommendations.
fig. 19: equipotentiality of the PE on an insulation fault.
a)
With a single load frame earth connection:
In TT: all the frames are at the same potential, even during a fault; no disturbance of
communictions by bus.
b)
In TN: on an insulation fault, the voltage drop in the PE causes the reference potential of the
communicating devices to vary.
The frames of devices 2, 3..., are at the potential ≈ Uo
installation cost (see the American TNwhere the neutral is not even
protected);
Safety of personnel is guaranteed,but that of property (fire, damage toelectrical equipment) is less so.
Proliferation of low current power
electronics is increasing and will
continue to increase complexity of itsimplementation.Derived from the TT of the nineteen
twenties, the TN was a solutionfor controlling fault current value
and ensuring that all insulationfaults could be eliminated by
a SCPD.It grew up in Ango-Saxon countries
where rigour of installation designersand users is excellent.
The logical evolution is TN-C →
TN-C-S → TN-S → TN-S with fault
current limitation to limit fire hazards,damage to loads and malfunctioningsdue to widespread use of distributed
electronics (see fig. 20).
A survey carried out in Germany in1990 showed that 28 % of electrical(electronic) problems were due to EMC.In terms of protection, the TN system
often uses fuses; already hindered by
an overlong breaking time when limitsafety voltage U
Lis 25 V, they will be
further hindered in the long term if
LV networks with voltages greater than230/400 V are developed. The use of
RCDs (impedance-earthed TN-S)
solves this problem.
Evolution of the IT
The earliest electrical installations(1920) were produced in IT. However,double faults quickly gave this system abad name (failure to master loopimpedances).Standards gave it official status in thesixties in order to meet continuity ofservice requirements of processindustries and safety requirements inmines.
fig. 20: evolution of the TN.
a) TN-C earthing system
PEN
PhMV LV
3
Solution used in the USA (Id of the order of 500 A), in RSA (I∆ ≈ 20 A); limits fire risks, damage
and potential reference problems for distributed electronics.
This earthing system is similar to the TT one.
Ph
N
PEr
MV LV
RCD
3
d) impedance-earthed TN-S
c) TN-S earthing system
N
Ph
PE
MV LV3
Avoids equipotentiality disturbances due to flow of neutral current and 3K harmonics in
the PEN.
Ph
N
PE
MV LV
3
(1)
(2)
(1) new earth connection preferable if the transformer is at a distance (public distribution);
improves local equipotentiality compared with the eath. This solution is used in Germany and is
being experimented in France (in DP).
(2) in France, the C 15-100 stipulates changing to TN-S when cross-section of conductors
Today, the IT system closely resemblesthe TN-S as regards installation (anadditional surge limiter and insulationmonitor).
It is the champion of continuity ofservice and safety on the first fault, ifthis fault is promptly tracked andeliminated.Following widespread use of thedistributed PE throughout theinstallation (as in TN), this system, inwhich the second fault current cannotbe limited, will not really evolve, exceptfor the rapid fault tracking techniques.As the likelihood of a double faultincreases with the number of feedersand size of the installation, its useshould be reserved for parts of the
network and for control and monitoringcircuits with, naturally, use of isolatingtransformers (see fig. 21).On these small circuits, use of theimpedance-earthed IT allows signallingRCDs for fault tracking.
Evolution of the TTTo begin with, electrical distribution inFrance was in single-phase 110 V,followed by two-phase 220 V.Earthing of frames, combined with useof RCDs, aimed at de-energisingconsumers with insulation faults andcheaters. The development of electric
household appliances led to protectpeople against indirect contacts.Protection against indirect contacts byRCD with standardised operating timeswas made official in the nineteensixties.Today, the tendency is (as in TNand IT) to distribute the PE throughoutthe installation and thus to use only oneapplication earth connection(see fig. 22).This tendency should continue with theuse of the LV neutral earth connectiononly (as in TN and IT), but maintainingthe advantage (damage, fire, EMC) of a
small insulation fault current.
choosing the earthingsystemChoice may be determined by normalpractice in the country.Choice of earthing system should beinfluenced by electrical power usersand network operators (electricalservice). Experience shows howeverthat the choice is mainly made by theengineering firms designing theinstallation.
a) at the outset
MV LV
N
PhM3
fig. 21: evolution of the IT.
IT is used mainly on small networks or parts of networks downstream from TN and
TT systems.
b) in 1960
N
Ph
PE
MV LV
CPIlimiter
MV LV
IT
TN-Sor
TT
d) 2000
Becoming more similar to the TN-S (PE distributed, calculation of loop impedances).
NPh
PE
MV LV
CPIlimiter
c) in 1990
Limitation of number of earth connections and interconnection of frames or use of RCD to
For users and operatorsThese both demand absoluteDEPENDABILITY; electrical powershould thus always be available and be
completely risk-free, i.e. «out of sight,out of mind».The elements making up installationdependability:c safety;c availability;c reliability;c maintenability, must therefore beoptimised.In addition, a new requirement,electricity must not disturb thenumerous low current devices.These are the criteria used to make thebest choice according to:c type of building;c the activity it houses;c whether or not an electrical service isavailable.In safety terms, the TT is the best,In availability terms, the IT is the mostsuitable,In maintenability terms, fault trackingis fast in TN (thanks to the SCPD) butrepair time is often long. Conversely,in IT, tracking of the first fault may bemore difficult, but repairs are quickerand less costly.The TT is a good compromise.In reliability terms, the protection
devices used are reliable, but reliabilityof the installation and loads may beaffected:c in TN-C by the fact that the PEN, notprotected, may be damaged byharmonic currents;c in TN-C and TN-S;v by insufficient rigour for extensions,v by use of replacement sources withlow short-circuit power,v by the effects of electrodynamicforces;c in IT, on a double fault, the risksinherent in TN described above alsoexist. However if tracking andelimination of the 1st fault are rapid,installation reliability is excellent.c in TT, by disruptive breakdown byreturn of the loads due to a fault in theHV/LV transformers. However thelikelihood of this fault occurring is smalland preventive solutions are available,e.g. use of surge arresters betweenone of the live conductors and the loadearth connection.In disturbance terms, the TT is to bepreferred to the TN-S whose high faultcurrents may be the source ofdisturbance.
a) at the outset
b) in 1960
N
Ph
PE
HV LV
RCD
RCD RCD
RCD
Multiple RCDs with time discrimination, local equipotentialites and minimum number of earth
connections.
c) in 1990
N
Ph
PE
HV LV
Same use of RCDs. PE distributed as in TN-S and IT.
In some installations, the two earth connections are connected... it is TN-S without impedance
calculation as RCDs are used.
d) 2000
r
HV LV
CDR
To retain the advantage of the small fault current (damage and EMC), an impedance-earthed
TT (r ≈ 12 Ω / Id = 20 A) emerges with a single earth connection. This system requires the use of
a surge limiter if the MV zero sequence current exceeds ≈ 80 A - DDRs are used in the same
The three earthing systems(TN - IT - TT) and their implementationare clearly defined in installationstandards (IEC 364).
Their respective use varies fromcountry to country:
c mainly TN in Anglo-Saxon countries;c TT often used in the other countries;c IT used when safety of persons andproperty, and continuity of service areessential.
All three systems are considered toguarantee personnel protection.Two major changes have had aconsiderable effect on choice ofearthing systems:c search for optimum continuity ofservice;
c proliferation of high current(disturbers) and low current (disturbed)electronic devices, which areincreasingly set up in communicatingsystems.Thus the general tendency for earthingsystems, in both MV and LV, is to limitinsulation fault currents.At present, the fault currents oftraditional LV earthing systems havethe following standard values:c IT (1st fault): Id < 1 A;
c TT: Id ≈ 20 A;c TN: Id ≈ 20 kA;c IT (2nd fault): Id ≈ 20 kA.
Limiting fault currents:c simplifies maintenability of theelectrical installation, thus increasingavailability;
c minimises the fire hazard;c can reduce contact voltage;c and, for sensitive systems, minimisesdisturbance due to electromagneticradiation and common impedance.
Moreover, in view of the proliferation ofcommunicating digital systems(computers, video, automation, TBMetc., it is vital that earthing systemsprovide a potential reference which isnot disturbed by high fault currents and
harmonics.Consequently, future evolution shouldfavour earthing systems generatingfault currents which do not exceed afew dozen amps.TT earthing systems should thereforebe increasingly used.
c 364-1 - Electrical installations ofbuildings(NF C 15-100).
c 364-1 - 1992Part 1: Scope, object andfundamental principles
c 364-2-21 - 1993Part 2: Definitions - Chaper 21 - Guideto general terms
c 364-3 - 1993Part 3: Assessment of general
characteristicsc 364-4Part 4: Protection for safety
v 364-4-41 - 1992Chapter 41 : Protection against ElectricShock
v 364-4-42- 1980Chapter 42: Protection against thermaleffects
v 364-4-43 - 1977Chapter 43: Protection againstovercurrent
v 364-4-45 - 1984Chapter 45: Protection againstundervoltage
v 364-4-46 - 1981Chapter 46: Isolation and switching
v 364-4-47 - 1981Chapter 47: Application of protectivemeasures for safety - Section 470:General - Section 471: Measures ofprotection against Electric Shock
v 364-4-442 - 1993Chapter 44: Protection againstovervoltages - Section 442: Protectionof low-voltage installations againstfaults between high-voltage systems
and earthv 364-4-443 - 1993Chapter 44: Protection againstovervoltages - Section 443: Protectionagainst overvoltages of atmosphericorigin or due to switching
v 364-4-473 - 1977Chapter 47: Application of protectivemeasures for safety - Section 473:Measures of protection againstovercurrent
v 364-4-481 - 1993Chapter 48: Choice of protectivemeasures as a function of externalinfluences - Section 481: Selection ofmeasures for protection against ElectricShock in relation to external influences
v 364-4-482 - 1982
Chapter 48: Choice of protectivemeasures as a function of externalinfluences - Section 482: Protectionagainst fire
c 364-5Part 5: Selection and erection ofelectrical equipment.
v 364-5-51 - 1979Chapter 51: Common rules
v 364-5-51 - 1 - 1982Amendment No. 1364-5-51 - 1979.
v 364-5-51 - 2 - 1993
Amendment No. 2364-5-51 - 1979.
v 364-5-53 - 1986Chapter 53: Switchgear andcontrolgear
v 364-5-53 - 2 - 1992Amendment No. 1364-5-53 - 1986.
v 364-5-54 - 1980Chapter 54: Earthing arrangements andprotective conductors
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