Earthing 173

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n 173

earthing systemsworldwide and

evolutions

E/CT 173, first issued September 1995

Roland Calvas

An ENSERG 1964 engineeringgraduate (from the Ecole NationaleSuprieure d'Electronique etRadiolectricit 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 Suprieure 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.

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Cahier Technique Merlin Gerin n 173 / p.2

Electrocution Electro Shock resulting in death

Electro Shock Application of a voltage between two parts of the body

EMC Electro Magnetic Compatibility

GFLD Ground Fault Location Device

In Operating threshold of a RCD

MT/HVA Medium Voltage: 1 to 35 kV as in CENELEC (circular of the 27.07.92)

Class A High Voltage: 1 to 50 kV as in French decree of the 14.11.88

PIM Permanent Insulation Monitor

RCD Residual Current Device

SCPD Short-Circuit Protection Device (circuit breakers or fuses)

STD Short Time Delay protection (protection against short-circuit overcurrents by circuit-breaker with rapidtrip release)

TBM Technical Building Management

TEM Technical Electrical Power Distribution Management

TPM Technical Process Management (automation of...)

UL Conventional limit voltage (maximum acceptable contact voltage) known as the safety voltage

glossary

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earthing systems worldwide anevolutions

summary

1. Review of standardised History p. 4

Emergence of earthing p. 4systems

Earthing systems of IEC 364 p. 7

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, thisCahier 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 readCahier Technique n 172.

earthing systems

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Electrical power was actually used in1900.

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

cin the 18th century, the static

electricity produced by friction of certaininsulating bodies formed a scientificdiversion causing experimenters tojump up.... in drawing rooms.A few dangerous experiments showedthe electrical nature of lightning.And in 1780: by chance anelectrostatic machine made afrogs legs move. Galvani observedthe contraction of muscles by electricity.

cin 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.

cin 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.

cin 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 inunearthed 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

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(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.

A lamp going out indicates

an insulation fault on

the corresponding phase.

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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 (theindirect 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

a

SCPDorRCD

Ph

N

SCPDorRCD

a

SCPD SCPD

Ph

N

fig. 4: earthing in public distribution.

fig. 5: TN-C system in public distribution.

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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 Techniquen 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 outgoingcircuit is equal to that of the returnone. 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.

RB RA

PE

N

fault

fig. 6: TN-C and TN-S systems.

PE

N

fault

TN-C system

PEN

fault

TN-S system

fig. 7: TT-system.

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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 (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 networks 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).

fig. 8: IT system.

1st fault

double insulation fault

RA

PE

BA

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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 , 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 RBare 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 potentialIhMTRT.

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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 .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 Techniquen 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

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The TN-C requires costlyequipotentiality research:c for the distributor:v in the USA, installation of an

additional conductor throughout MVand LV distribution with earthing every400 m,v in Great Britain, numerous earthconnections are installed on the townpublic LV network neutral, if TN-Cused, thus meaning consumers maynot need their own earth connections,v in Germany, an earth connection ismade for the neutral just upstream fromthe consumers connection point,c for the consumer:as a rule, connection to the protectionconductor of the metal structures of the

building and all the metal ducts.In industrial and tertiaryLV distributionThe 3 earthing systems are used tovarious degrees in all countries:c the TN-C system is particularly usedin Anglo-Saxon countries for carefullydesigned and produced installations(SCPD/loop impedance matching)corresponding to modern blocks of flatswhere all metal parts are connected tothe protective conductor and whereexplosion and fire risks are extremelylow.

It is not currently recommended inpremises equipped with communicatingelectronic systems (computer TBM,TPM and CTM -Centralised TechnicalManagement- networks) as currents inthe neutral and thus in the PE causepotential references to vary. Do notforget that the TN-C can no longer beused when cross-section of liveconductors is < 10 mm2 Cu.c the TN-C system is also used inAnglo-Saxon countries and requires anadditional conductor and careful designand production. However its use is

more flexible and RCDs are used forpersonnel protection (for very longcables), for fire protection and forextensions without loop impedancecalculation.Nonetheless, the insulation faultcurrents which are short-circuitcurrents, may, if the PE is connected,in distribution, to metal structures,generate electromagnetic disturbancespreventing electronic equipment fromworking properly (sum of currents incable not zero and stray currents).

TT system is by far the most commonlyused.Moreover, its simplicity makes it withoutdoubt the system best suited to

developing countries.Following these brief considerations onthe three official earthing systems, weshall now see how they areimplemented in some countries.

earthing systems of privateLV networks in somecountriesIn the USAAll the various earthing systems areused: the TN-S (see fig. 13) is the most

common, but the IT and impedance-earthed IT are used in processfactories.c implementation of the TN-S has twomajor special features:v the neutral is not protected and notswitched, which may present risks forpersons and property:- potential of the neutral compared withthe earth may be high on a faultoriginating at MV level; this presents arisk- rank 3 harmonic currents andmultiples of 3 add up in the neutral andmay cause unacceptable temperaturerisev the protective conductor is oftenmade up by the cable path and themetal tubes conveying the liveconductors:- the impedance of this PE is hard tocontrol. Thus NEC paragraph 230-95(National Electrical Code) considersthat SCPDs do not always guaranteesafety in the event of an insulation fault,

Finally, as the neutral is not protectedin some countries (measure authorisedby standard IEC 364), it may bedamaged by overcurrents, in particular

when loads generating rank 3 harmoniccurrents and multiples are supplied bythe network. In some cases this resultsin neutral cross-section being doubled(seen in the USA).... In the long term,international standards should specifysystematic protection of the neutral andindeed protection (without breaking) ofthe PEN in TN-C.c the IT system requires as much careas the TN-S one. Permanent insulationmonitoring allows fault prediction,currently simplifed by digital systemswhich monitor insulation evolution for

each feeder.This system calls for fault tracking andelimination; thus electrical engineersshould preferably be present on site.In many countries, the unearthedneutral is used whenever continuity ofservice is essential or when human lifeis at stake (e.g. hospitals), however, inUK, TN-S is used in hospitals.c the TT system is the easiest one toimplement; insulation fault currents are1,000 times smaller than in TN or IT(2nd fault), thus accounting for its valueas regards risk of fire, explosion,

material damage and electromagneticdisturbances.Its weak point is the risk of disruptivebreakdown by return on an insulationfault in the substation on the MV side ifthe zero sequence fault current is highand if the substation and neutral framesare connected.

There are no statistics on use ofearthing systems worldwide, but the

a load

N

fig. 13: TN-S earthing system in the USA.

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- 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 theearth 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 asNicholsons, 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),

fig. 14: residual sensing.

R

N

fig. 15: source ground return.

R

N

fig. 16: zero sequence.

Rearthleakagecircuit-breaker

N

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v incoming device settings:

- max threshold: 1,200 A,

- tripping time: must not exceed 1 s for

a fault current of 3,000 A.

NoteThe NEC does not specify a tripping

time at 1,200 A, but it is common

practice to install protection deviceswith lower thresholds and as

instantaneous as possible.

c discrimination of residual current

devices.

The NEC 230 paragraph 95 only

stipulates earth protection for the

incoming device. Naturally, this

protection must also be installed

downstream to prevent the entire

installation being put out of operation inthe event of one earth fault.

Discrimination has then also to be

achieved between the various

protection devices. This problem can

be solved in two ways:

v between the earth protection devices

with discrimination of the:

- time type by a time delay of 0 s to 1 s,- logic type or Zone Selective

Interlocking; recommended in the

USA, it prevents long time delays

(reduction of the passing I2t) and easily

achieves discrimination on 3 levels ormore,

v between the earth and thermal-

magnetic protection

Discrimination will be determined by

comparing the setting threshold of the

upstream earth protection with the

curve I = f(t) of the thermal-magnetic

setting of the downstream protection

device (see fig. 17).Use of fewer earth protection

devices in subdistribution switchboards

leads to higher settings of protection

devices, resulting in increased risk of

damage.

In the Republic of South Africa

In the RSA, industrial and tertiary

electrical installations comply with

IEC standards.

All three earthing systems are used,

with a preference for the TN-S.

A country with a long mining tradition,

the RSA uses in its gold mines, for

a)

3

D1M25 + GFP

D2M10 + GFP

D3C161

short-circuit protection

insulation fault protection

R

3

R

b)

fig. 17: discrimination between downstream short-circuit protection (D3) and upstream earth

protection (GFP) (orange).

T1000 A - 0.1 s

T1200 A - 0.2 s

D3

ts D2

STR38

D1

STR58

IpA

0.10.2

5

875

1,000

1,200

2,500

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N

R = 27

525/900 V

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

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 in LV and

of 30 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 IECs 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.

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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 lectriquesen BT;c n 177 - Perturbations des systmeslectroniques et schmas des liaisons 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 1990s, 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.

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In TN and IT, do not connect the PE to

the metal frames of the building as thereturn currents may take a variety of

paths and turn into transmitting

antennae. The same also applies to thepower cable, incorporating the PE, in

which the sum of currents is no longer

zero.With respect to equipotentiality offrames, the TN and IT (on the 2nd fault)are equivalent since frame potential at

the fault point suddenly rises to Uo/2

whereas it remains at 0 V at the originof the installation.This leads to certain specialists

specifying in TN and IT the creation

of a low current frame circuit

separated from the earth circuit (PE),both being connected to the earth

connection at the origin of theLV installation.

The TT with distributed PE throughoutthe installation is the best system in this

respect (small Id and same potential

reference for all the communicating

devices), (see fig. 19).

Faced with lightning and operatingovervoltages

These overvoltages, of common or

differential mode and with a frequency

of 1 kHz to 1 MHz can damage certainelectronic devices if they are not fitted

with an isolating transformer with asmall primary/secondary capacitive

coupling.

As regards differential mode

overvoltages, all the earthing systemsare equivalent. The solution

consists in:

c implementing surge reducing at

disturbance source level (e.g. RC oncontactor coil);

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 areduction 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

2where as the devices close to the

source are at the earth potential.

PE

2 31

digital link

V

V

PE

digital link

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evolution ofearthing systems

Evolution of the TN

The original aim of this earthing system

was simplicity, efficiency and minimum

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

is i 10 mm2 Cu.

b) TN-C-S earthing system

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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

master the double fault.

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3

N

PhHV LV

RCD

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 /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

way (time discrimination)

fig. 22: evolution of the TT.

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The table in figure 23 reviews thestrong and weak points of eachearthing system:

For installation designersDesigning is simpler in TT, the same forextensions (no calculations). Designingcomplexity is equivalent in TN-S and IT.As regards costs:c the TN-S is the least costly to install,for example if the neutral is neitherprotected nor switched. But be warned:the cost of curative maintenance canbe high;c the IT is slightly more costly to install(insulation monitoring and insulationfault tracking devices). Search formaximum availability of electrical powerrequires the presence of an electrical

engineer, whose action will minimisecurative maintenance;c the TT, if enough discriminatingRCDs are installed, is slightly morecostly to install than the IT, but faulttracking is simple and curativemaintenance less costly than in TN.In terms of complete cost over 10 to20 years, all three earthing systems areequivalent.

The right choiceIn a certain number of countries, forsome buildings or parts of a building,the choice is laid down by legislations

or standards, e.g. for hospitals,schools, navy, worksites, mines, etc.In other cases, certain earthing systemsare strictly prohibited, for example theTN-C in premises with explosion risks.Apart from these compulsory choices,the DEPENDABILITY objectives(safety, availability, reliability,maintenability and proper operation oflow current communicating systems)are those which should determinewhich earthing system is chosen for aspecific building type.

The degree of development of the

country should also be taken intoconsideration, as should be nationalpractices, climate.... If we plot an axisfrom North to South, as regards publicdistribution, we find the IT earthingsystem in Norway, TN-C in Germany,TT in France and in most Africancountries.

In temperate, industrialised countries,all three earthing systems are used inprivate installations.

Finally, it should be noted that it ispossible and even advisable to mix theearthing systems (see fig. 24).

fig. 24: several earthing system included in the same LV installation.

TN-C TN-S TT IT(1) IT(2) Observations

safetyc of persons + + + ++ - Uc # 0 on 1st fault (IT)c fire - - - + ++ - TN-C not recommendedc explosions - - - + ++ - TN-C strictly prohibited

availability- + + + ++ + depends on discrimination(further to 1 fault) of the SCPDs or RCDs

(easier to implement)

maintenability - - + ++ - the IT authorises preventiveand even predictive maintenance

reliability - + ++ ++ + advantage for small Idsof the installation (damage- electrodynamic

forces)disturbances

c radiation - - + ++ - advantage for small Idtransmission EMc equipotentiality - - + ++ + + pay attention to harmonics

of PE in TN-C

(1) : 1st insulation fault.

(2) : 2nd fault.

b) "parallel" association of earthing systems

fig. 23: comparing the earthing system.

a) "series" association of earthing systems

- lighting

- heating

- computing centre

- machines

- communicating systems

. automation

. office

. TBM

- premises with fire hazards

- safety systems

- medical facilities

- industrial process

HV LV

TN-S

N

PE

3

TN-S

TT

PE

IT

HV LV

TN-C

PEN

TN-S

NPE

TT

3

IT

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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.

4. conclusion

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c364-1 - Electrical installations ofbuildings(NF C 15-100).

c364-1 - 1992Part 1: Scope, object andfundamental principles

c364-2-21 - 1993Part 2: Definitions - Chaper 21 - Guideto general terms

c364-3 - 1993Part 3: Assessment of general

characteristicsc364-4Part 4: Protection for safety

v364-4-41 - 1992Chapter 41 : Protection against ElectricShock

v364-4-42- 1980Chapter 42: Protection against thermaleffects

v364-4-43 - 1977Chapter 43: Protection againstovercurrent

v364-4-45 - 1984Chapter 45: Protection againstundervoltage

v364-4-46 - 1981Chapter 46: Isolation and switching

v364-4-47 - 1981Chapter 47: Application of protectivemeasures for safety - Section 470:General - Section 471: Measures ofprotection against Electric Shock

v364-4-442 - 1993Chapter 44: Protection againstovervoltages - Section 442: Protectionof low-voltage installations againstfaults between high-voltage systems

and earthv364-4-443 - 1993Chapter 44: Protection againstovervoltages - Section 443: Protectionagainst overvoltages of atmosphericorigin or due to switching

v364-4-473 - 1977Chapter 47: Application of protectivemeasures for safety - Section 473:Measures of protection againstovercurrent

v364-4-481 - 1993Chapter 48: Choice of protectivemeasures as a function of externalinfluences - Section 481: Selection ofmeasures for protection against ElectricShock in relation to external influences

v364-4-482 - 1982

Chapter 48: Choice of protectivemeasures as a function of externalinfluences - Section 482: Protectionagainst fire

c364-5Part 5: Selection and erection ofelectrical equipment.

v364-5-51 - 1979Chapter 51: Common rules

v364-5-51 - 1 - 1982Amendment No. 1364-5-51 - 1979.

v364-5-51 - 2 - 1993

Amendment No. 2364-5-51 - 1979.

v364-5-53 - 1986Chapter 53: Switchgear andcontrolgear

v364-5-53 - 2 - 1992Amendment No. 1364-5-53 - 1986.

v364-5-54 - 1980Chapter 54: Earthing arrangements andprotective conductors

v364-5-54 - 1 - 1982Amendment No. 1

364-5-54 - 1980.v364-5-56 - 1980 -Chapter 56: Safety services

v364-5-523 - 1983Chapter 52: Wiring systems - Section523: Current-carrying capacities

v364-5-537 - 1981Chapter 53: Switchgear andcontrolgear - Section 537: Devices forisolation and switching

v364-5-537 - 1 - 1989Amendment No. 1364-5-537 - 1981.

c364-6Part 6: Verification

v364-6-61 - 1986Chapter 61: Initial verification

v364-6-61 - 1993Amendment No. 1364-6-61 - 1986.

c364-7Part 7: Requirements for specialinstallations or locations

v364-7-701 - 1984Section 701: Locations containing abath tub or shower basin

v364-7-702 - 1983Section 702: Swimming pools

v364-7-703 - 1984Section 703: Locations containingsauna heaters

v364-7-704 - 1989Section 704: Construction anddemolition site installations

v364-7-705 - 1984Section 705: Electrical installationsof agricultural and horticulturalpremises

v364-7-706 - 1983Section 706: Restrictive conductinglocations

v364-7-707 - 1984

Section 707: Earthing requirements forthe installation of data processingequipment

v364-7-708 - 1988Section 708: Electrical installations incaravan parks and caravans

appendix 1: IEC 364 standard

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appendix 2: bibliography

Standardsc IEC 241: Fuses for domestic andsimilar purposes.

c IEC 269: Low voltage fuses.

c IEC 364: Electrical installations ofbuildings.

c IEC 479: Effects of currents flowingthrough the human body.

c IEC 755: General requirements forresidual current operated protectivedevices.

c IEC 947-2: Low voltage switchgearand controlgear.

c NF C 15-100 : Installationslectriques basse tension.

c NF C 63-080 : Dispositifs de contrlepermanent d'isolement et dispositifs delocalisation de dfauts associs.

c NF C 63-150 : Limiteurs desurtension : rgles.

French decree of the 14.11.88

Merlin Gerin's Cahiers Techniques

c Earthing of the neutral conductor inHigh-Voltage networks,Cahier Technique n 62F. SAUTRIAU

c Residual current devices,Cahier Technique n 114R. CALVAS

c Protections des personnes etalimentations statiques sans coupure,Cahier Technique n 129

J.-N. FIORINAc Les perturbations lectriques en BT,Cahier Technique n 141R. CALVAS

c Introduction to dependability design,Cahier Technique n 144P. BONNEFOI

c Lightning and HV electricalinstallations,Cahier Technique n 168B. DE METZ NOBLAT

c Earthing systems in LV,Cahier Technique n 172B. LACROIX and R. CALVAS

c Perturbations des systmeslectroniques et schmas des liaisons la terre,Cahier Technique n 177R. CALVAS

c Connaissance et emploi du SLTneutre isol,Cahier Technique n 178E. TISON et I. HERITIER

Other publications

c Guide de linstallation lectrique(partie G).Ed. FRANCE IMPRESSIONCONSEIL 1991.

c Guide de lingnierie lectrique.Ed. ELECTRA 1986.

c Electrical Review.Nov. 1991 - Oct. 1992.

c La protection diffrentielle.Cahier Technique J3E - 02/90.

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