172-Earthing Systems in LV

n° 172earthing systemsin LV

E/CT 172, first issued September 1995

Roland Calvas

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.

Cahier Technique Merlin Gerin n° 172 / p.2

glossary

Electric Shock Application of a voltage between two parts of the bodyElectrocution Electric Shock resulting in deathEMC Electro Magnetic CompatibilityI∆n Operating threshold of a RCDIMD Insulation Monitoring DeviceGFLD Insulation Fault Location DeviceMV/HV Medium Voltage: 1 to 35 kV as in CENELEC (circular of the 27.07.92)

High Voltage: 1 to 50 kV as in french standard (14.11.88)RCD Residual Current DeviceSCPD Short-Circuit Protection Device (circuit-breakers or fuses)STD Short Time Delay protection (protection against short-circuit overcurrents by circuit-breaker with rapid

trip release)TBM Technical Building ManagementTEM Technical Electrical Power Distribution ManagementUL Conventional limit voltage (maximum acceptable contact voltage) known as the «safety» voltage


earthing systems in LV

content

1. Introduction Evolution of needs p. 4Causes of insulation faults p. 4Hazards linked to insulation faults p. 4

2. Earthing systems and protection of persons p. 7TN system p. 8TT system p. 9IT system p. 9

3. Earthing systems confronted with Fire p. 12Electrical power unavailability p. 12

4. Influences of MV on BV, according to Lighning p. 14Operating overvoltages p. 14MV-frame disruptive breakdown p. 14of the transformerMV-LV disruptive breakdown p. 15inside the transformer

5. Switchgear linked to choice of TN system p. 17TT system p. 17IT system p. 18Neutral protection according to p. 19the earthing system

6. Choice of eathing system Methods for choosing the p. 21earthing systemConclusion p. 21

7. Bibliography p. 22

This «Cahier Technique» reviews thehazards that insulation faults representfor safety of persons and propery. Itemphasises the influence of earthingsystems and the availability of electricalpower.It presents the three earthing systemsdefined in standard IEC 364 and usedto varying degrees in all countries.Each earthing system is looked at interms of dependability (safety,maintenability and availability).There is no such thing as a badearthing system; they all ensure safetyof persons. Each system has its ownadvantages and disadvantages and theuser must therefore be guidedaccording to his needs, with theexception, however, of prescription orof standard or legislative bans.Readers interested in the practices ofvarious countries and in evolution ofearthing systems should read «CahierTechnique» n° 173.fire and electrical power unavailabiliy

hazards

the earthing systems

earthing system

and conclusion


It is normally a combination of theseprimary causes which results in theinsulation fault. The latter is:c either of differential mode (betweenlive conductors) and becomes ashort-circuit;c or of common mode (between liveconductors and frame or earth), a faultcurrent -said to be common mode orzero sequence (MV)- then flows in theprotective conductor (PE) and/or in theearth.

LV earthing systems are mainlyconcerned by common mode faultswhich mainly occur in loads and cables.

hazards linked toinsulation faultsAn insulation fault, irrespective of itscause, presents hazards for:c human life;c preservation of property;c availability of electrical power;the above all depending ondependability.

Electric Shock of personsA person (or animal) subjected toan electrical voltage is electrified.According to the gravity of the ElectricShock, this person may experience:c discomfort;c a muscular contraction;c a burn;c cardiac arrest (this is Electrocution)(see fig. 1).Since protection of persons against thedangerous effects of electric currenttakes priority, Electric Shock is thus thefirst hazard to be considered.The current strength I -in value andtime-, passing through the human body(in particular the heart) is thedangerous aspect. In LV, theimpedance value of the body (animportant aspect of which is skinresistance) virtually changes onlyaccording to environment (dry and wetpremises and damp premises).

1. introduction

c use of insulating materials;c distancing, which calls for clearancesin gases (e.g. in air) and creepagedistances (concerning switchgear, e.g.an insulator flash over path).

Insulation is characterised by specifiedvoltages which, in accordance withstandards, are applied to new productsand equipment:c insulating voltage (highest networkvoltage);c lightning impulse withstand voltage(1.2; 50 µs wave);c power frequency withstand voltage(2 U + 1,000 V/1mn).Example for a LV PRISMA typeswitchboard:c insulating voltage: 1,000 V;c impulse voltage: 12 kV.

When a new installation iscommissioned, produced as perproper practices with productsmanufactured as in standards,the risk of insulation faults is extremelysmall; as the installation ages,however, this risk increases.

In point of fact, the installation issubject to various aggressions whichgive rise to insulation faults, forexample:c during installation:v mechanical damage to a cableinsulator;c during operation:v conductive dust,v thermal ageing of insulators due toexcessive temperature caused by:- climate,- too many cables in a duct,- a poorly ventilated cubicle,- harmonics,- overcurrents, etc,v the electrodynamic forces developedduring a short-circuit which maydamage a cable or reduce a clearance,v the operating and lightningovervoltages,v the 50 Hz return overvoltages,resulting from an insulation fault in MV.

evolution of needsToday the 3 earthing systems such asdefined in IEC 364 and Frenchstandard NF C 15-100, are:c exposed-conductive parts connectedto neutral -TN-;c earthed neutral -TT-;c unearthed (or impedance-earthed)neutral -IT-.

The purpose of these three systems isidentical as regards protection ofpersons and property: mastery ofinsulation fault effects. They areconsidered to be equivalent withrespect to safety of persons againstindirect contacts.However, the same is not necessarilytrue for dependability of theLV electrical installation with respect to:c electrical power availability;c installation maintenance.

These quantities, which can becalculated, are subjected toincreasingly exacting requirements infactories and tertiary buildings.Moreover, the control and monitoringsystems of buildings -TBM- andelectrical power distributionmanagement systems -TEM- play anincreasingly important role inmanagement and dependability.

This evolution in dependabilityrequirements therefore affects thechoice of earthing system.

It should be borne in mind that theconcern with continuity of service(keeping a sound network in publicdistribution by disconnectingconsumers with insulation faults) playeda role when earthing systems firstemerged.

causes of insulation faultsIn order to ensure protection of personsand continuity of service, conductorsand live parts of electrical installationsare «insulated» from the framesconnected to the earth.Insulation is achieved by:


In each case, a safety voltage(maximum acceptable contact voltagefor at least 5 s) has been defined: it isknown as the conventional limit voltageUL in IEC 479.IEC 364 paragraph 413.1.1.1 andNF C 15-100 state that if there is a riskof contact voltage Uc exceedingvoltage UL, the application time of thefault voltage must be limited by the useof protection devices (see fig. 2).

FireThis hazard, when it occurs, can havedramatic consequences for bothpersons and property. A large numberof fires are caused by important andlocalised temperature rises or anelectric arc generated by an insulationfault. The hazard increases as the faultcurrent rises, and also depends on therisk of fire or explosion occurring in thepremises.

Unavailability of electrical powerIt is increasingly vital to master thishazard. In actual fact if the faulty part isautomatically disconnected to eliminatethe fault, the result is:c a risk for persons, for example:v sudden absence of lighting,v placing out of operation of equipmentrequired for safety purposes;c an economic risk due to productionloss. This risk must be mastered inparticular in process industries, whichare lengthy and costly to restart.

Moreover, if the fault current is high:c damage, in the installation or theloads, may be considerable andincrease repair costs and times;c circulation of high fault currents in thecommon mode (between network andearth) may also disturb sensitiveequipment, in particular if these are partof a «low current» systemgeographically distributed with galvaniclinks.Finally, on de-energising, theoccurrence of overvoltages and/orelectromagnetic radiation phenomenamay lead to malfunctioning or evendamage of sensitive equipment.

Direct and indirect contactsBefore beginning to study the earthingsystems, a review of Electric Shock bydirect and indirect contacts will certainlybe useful.

fig. 1: time/current zones of AC current effects (15 Hz to 100 Hz) on persons as in IEC 479-1.

fig. 2: maximum time for maintenance of contact voltage as in standard IEC 364.

c damp premises and places: UL i 25 Vpresumed contact voltage (V) maximum breaking time of the protection

device (s)AC (a) DC (b)

25 5 550 0.48 575 0.30 290 0.25 0.80110 0.18 0.50150 0.10 0.25220 0.05 0.06280 0.02 0.02

c dry or wet premises and places: UL i 50 Vpresumed contact voltage (V) maximum breaking time of the protection

device (s)AC DC

< 50 5 550 5 575 0.60 590 0.45 5120 0.34 5150 0.27 1220 0.17 0.40280 0.12 0.30350 0.08 0.20500 0.04 0.10

0.1 0.2 0.5 1 2 5 10 20 50 100 200 500 1,000 2,000 5,000 10,000

mA10

20

50

100

200

500

1,000

2,000

5,000

10,000

ms

time during which the human body is exposed

a b c2c1 c3

1 2 3 4

current passing through the human body

zone 1: perceptionzone 2: considerable discomfortzone 3: muscular contractionszone 4: risk of ventricular fibrillation (cardiac arrest)c1: likelihood 5 %c3: likelihood > 50 %


ccccc direct contact and protectionmeasuresThis is accidental contact of personswith a live conductor (phase or neutral)or a normally live conductive element(see fig. 3a).In cases where the risk is very great,the common solution consists indistributing electricity using a non-dangerous voltage, i.e. less than orequal to safety voltage. This issafety by extra-low voltage (SELVor PELV), in LV (230/400 V).

Protection measures consist in placingthese live parts out of reach or ininsulating them by means of insulators,enclosures or barriers. A complemen-tary measure against direct contactsconsists in using instantaneousi 30 mA High Sensitivity ResidualCurrent Devices known as HS-RCDs.

Note: this measure is necesssary in allcircuit supply cases whereimplementation of the earthing systemdownstream is not mastered.Consequently, some countries makethis measure a requirement:v for sockets of rating i 32 A,v in some types of installations(temporary, worksite, etc.).Treatment of protection against directcontacts is completely independentfrom the earthing system.

ccccc indirect contact, protection andprevention measuresContact of a person withaccidentally energised metal framesis known as indirect contact(see fig. 3b).This accidental energising is the resultof an insulation fault. A fault currentflows and creates a potential risebetween the frame and the earth, thuscausing a fault voltage to appear whichis dangerous if it exceeds voltage UL.As regards this hazard, the installationstandards (IEC 364 at internationallevel) have given official status to threeearthing systems and defined thecorresponding installation andprotection rules.

The protection measures againstindirect contacts are based on threebasic principles:c earthing of the frames of loadsand electrical equipment toprevent an insulation fault fromresulting in the equivalent of a directcontact;c equipotentiality of simultaneouslyaccessible framesInterconnection of these framesconsiderably helps to reduce contactvoltage. It is performed by theprotective conductor (PE) whichconnects the frames of electricalequipment for entire buildings,completed if required byadditional equipotential links(see fig. 4).

Reminder: equipotentiality cannotbe complete in all points (in particularin single level premises). Consequently,for the study of earthing systemsand their associated protectiondevices, the hypothesis chosen bystandard makers Uc = Ud is appliedsince Uc is at the most equal to Ud.v Ud = «fault» voltage, with respect tothe deep earth, of the frame of anelectrical device with an insulationfault,v Uc = contact voltage depending onthe potential Uc and the potentialreference of the person exposed tothe hazard, generally the ground.

c managing the electrical hazardv this management is optimised byprevention . For example, bymeasuring insulation of a device beforeenergising it, or by fault predictionbased on live monitoring of insulationevolution of an unearthed installation(IT system),v if an insulation fault occurs,generating a dangerous faultvoltage, it must be eliminated byautomatically disconnecting thepart of the installation where thisfault occurred. How the hazard isremoved then depends on the earthingsystem.

fig. 4: equipotentiality in a building.

fig. 3: direct and indirect contacts.

heating

mainprotectiveconductor

individualprotectiveconductors(PE)reinforcementmeshing

measuring strip

gas

earthing conductorditch bottom loop

water

3

Id Uc

a) direct contact

b) indirect contact

Uc

ph


2. earthing systems and protectionof persons

This section defines the Electric Shockand Electrocution hazards for thevarious earthing systems, such asspecified by the InternationalElectrotechnical Committee in standardIEC 364. The LV earthing systemcharacterises the earthing mode of thesecondary of the MV/LV transformerand the means of earthing theinstallation frames.Identification of the system types isthus defined by means of 2 letters:c the first one for transformer neutralconnection (2 possibilities):v T for «connected» to the earth,v I for «isolated» from the earth;c the second one for the typeof application frame connection(2 possibilities):v T for «directly connected» to theearth,v N for «connected to the neutral» atthe origin of the installation, which isconnected to the earth (see fig. 5).Combination of these two letters givesthree possible configurations:

transformer frameneutralc if T → T or N;c if I → T;i.e. TT, TN and IT.

Note 1:The TN system, as in IEC 364 includesseveral sub-systems:c TN-C; if the N and PE neutralconductors are one and thesame (PEN);c TN-S: if the N and PE neutralconductors are separate;c TN-C-S: use of a TN-S downstreamfrom a TN-C (the opposite is forbidden).Note that the TN-S is compulsory fornetworks with conductors of a cross-section i 10 mm2 Cu.

Note 2:Each earthing system can be applied toan entire LV electrical installation;

however several earthing systems maybe included in the same installation,see figure 6 as an example.

Note 3:In France, as in standard NF C 13-100concerning delivery substations, inorder to prevent hazards originating

in MV, the LV earthing system isexpressed by an additional letteraccording to interconnection ofthe various earth connections(see fig. 7).

Let us now see how to protect personsin each case.

N

T

N

3

T

3

I

N

3

N

N

3

fig. 5: connection mode of the neutral at the origin of the installation and of the frames of theelectrical loads.

PEN

TN-C TN-S TT

NN

PE PE

IT

PE

3

N

fig. 6: example of the various earthing systems included in the same installation.

fig. 7: linking of LV earth connections with that of the MV/LV substation.

additional earthing of the earthing of the earthing of theletter MV/LV substation LV neutral LV applicationR (connected) c c cN (of neutral) c c vS (separated) v v v(c = interconnected, v = separate)


.

TN systemWhen an insulating fault is present, thefault current Id is only limited by theimpedance of the fault loop cables(see fig. 8):

Id = Uo

Rph1 + Rd + RPE.

For a feeder and as soon as Rd ≈ 0:

Id = 0.8 Uo

Rph1 + RPE.

In point of fact, when a short-circuitoccurs, it is accepted that theimpedances upstream from the relevantfeeder cause a voltage drop ofaround 20 % on phase-to-neutralvoltage Uo, which is the nominalvoltage between phase and earth.

Id thus induces a fault voltage withrespect to earth:Ud = RPE Idi.e.:

Ud = 0.8 Uo RPE

Rph1 + RPE.

For 230/400 V networks, this voltage ofaround Uo/2 (if RPE = Rph) isdangerous since it exceeds the limitsafety voltage, even in dryatmospheres (UL = 50 V). Theinstallation or part of the installationmust then be automatically andpromptly de-energised (see fig. 9).

As the insulation fault resembles aphase-neutral short-circuit, breaking isachieved by the Short-Circuit ProtectionDevice (SCPD) with a maximumspecified breaking time dependingon UL.

ImplementationTo be sure that the protection devicereally is activated, the current Id mustbe greater than the operating thresholdof the protection device Ia (Id > Ia)irrespective of where the fault occurs.This condition must be verified at theinstallation design stage by calculatingthe fault currents for all the distributioncircuits.If the same path is taken by theprotective conductor - PE- and the liveconductors, this will simplify the

For the protection device to performits function properly, Ia must be lessthan Id, hence the expressionof Lmax, the maximum lengthauthorised by the protection device witha threshold Ia:

Lmax = 0.8 Uo Sph

ρ (1 + m) Iac Lmax: maximum length in m;c Uo: phase-to-neutral voltage 230 Vfor a three-phase 400 V network;c ρ: resistivity to normal operatingtemperature;c Ia: automatic breaking current:v for a circuit-breaker Ia = Im (Imoperating current of the magnetic orshort time delay trip release),v for a fuse, current such thattotal breaking time of the fuse(prearcing time + arcing time) complieswith the standard (see fig. 9),

c m = SphSPE

.

If the line is longer than Lmax, eitherconductor cross-section must beincreased or it must be protectedusing a Residual Current Device(RCD).

calculation. Certain country standardsrecommend this.To guarantee this condition, anotherapproach consists in imposing amaximum impedance value on thefault loops according to the type andrating of the SCPDs chosen (seeBritish standard BS 7671). Thisapproach may result in increasing thecross-section of the live and/orprotective conductors.Another means of checking that thedevice will ensure protection ofpersons is to calculate the maximumlength not to be exceeded by eachfeeder for a given protectionthreshold Ia.

To calculate Id and Lmax,three simple methods can be used(see «Cahier Technique» n° 158):c the impedance method;c the composition method;c the conventional method.The latter gives the followingequation:

Id = 0.8 Uo

Z =

0.8 UoRph + RPE

= 0.8 Uo Sphρ (1 + m) L

fig. 8: fault current and voltage in TN system.

Id = Uo

RAB + Rd + RCD ⇒

0.8 UoRph + RPE

Ud

Rd

N

A

BC

D

PE

Id

Ud ≈ 0.8 Uo

2 if R

PE= Rph and Rd = 0


TT systemWhen an insulation fault occurs, thefault current Id (see fig. 10) is mainlylimited by the earth resistances (if theearth connection of the frames and theearth connection of the neutral are notassociated).Still assuming that Rd = 0, the faultcurrent is:

Id ≈ Uo

Ra + RbThis fault current induces a faultvoltage in the earth resistance of theapplications:

Ud = Ra Id, or Ud = Uo Ra

Ra + RbAs earth resistances are normally lowand of the same magnitude (≈ 10 Ω),this voltage of the order of Uo/2 is

dangerous. The part of the installationaffected by the fault must therefore beautomatically disconnected(see fig. 11).

ImplementationAs the fault current beyond which a risk

is present ( Id0 = UL

Ra) is far lower

than the settings of the overcurrentprotection devices, at least one RCDmust be fitted at the supply end of theinstallation. In order to increaseavailability of electrical power, use ofseveral RCDs ensures time and currentdiscrimination on tripping.All these RCDs will have a nominalcurrent threshold I∆n less than Id0.The standard stipulates thatde-energising by the RCDs must occurin less than 1 s (this time may be

increased to 5 s if Ra and Rb areinterconnected).Note that protection by RCD:c does not depend on cable length;c authorises several separate Ra earthconnections (an unsuitable measuresince the PE is no longer a uniquepotential reference for the entireinstallation).«Cahier Technique» n° 114 gives adetailed description of RCD technologyand use.

IT systemThe neutral is unearthed, i.e. notconnected to the earth.The earth connections of the framesare normally interconnected (just likethe TN and TT earthing systems).

c in normal operation (withoutinsulation fault), the network is earthedby the network leakage impedance.We remind you that natural earthleakage impedance of a three-phase1 km long cable is characterised by thestandard values:v C = 1 µF / km,v R = 1 MΩ / km,which give (in 50 Hz):v Zcf = 1 / j C ω = 3,200 Ω,v Zrf = Rf = 1 MΩ,therefore Zf ≈ Zcf = 3,200 Ω.In order to properly set the potential ofa network in IT with respect to theearth, we advise that you place animpedance (Zn ≈ 1,500 Ω) betweentransformer neutral and the earth.... thisis the IT impedance-earthed system.

fig. 10: fault current and voltage in TT system.

Uo (volts) breaking time breaking timephase/neutral voltage (seconds) U L = 50 V (seconds) U L = 25 V

127 0.8 0.35230 0.4 0.2400 0.2 0.05≤> 400 0.1 0.02

fig. 9: breaking time in TN system (taken from IEC 364 tables 41 and 48A).

fig. 11: upper limit of the resistance of theframe earth connection not to be exceededaccording to RCD sensitivity andlimit voltage UL [I∆n = F (Ra)].

maximum resistance of earthconnection

UL 50 V 25 V

3 A 16 Ω 8 Ω1 A 50 Ω 25 Ω500 mA 100 Ω 50 Ω300 mA 166 Ω 83 Ω30 mA 1,660 Ω 833 Ω

Id ≈ Uo

Ra + RbUd = Uo

RaRa + Rb

Ud

N

PE

Rb Ra

Id

I∆n i

UL

Ra


N

If

If If

Rb

If

Ud

insulationmonitoringdevice

surgelimiter

321N

PE

Cf

IcN Ic1 Ic2

Cf Cf Cf

V1 V2

V2 3V1 3

V3

IcNIf

Ic2

Ic1

c behaviour on the first faultv unearthed neutral:The fault current is formed as follows(maximum value in the case of a fullfault and neutral not distributed)If = Ic1 + Ic2, where:Ic1 = j Cf ω V1 3 andIc2 = j Cf ω V2 3, where:Id = Uo 3 Cf ω.For 1 km of 230/400V network, the faultvoltage will be equal to:Uc = Rb Id, i.e. 0.7 V if Rb = 10 Ω.This voltage is not dangerous and theinstallation can thus be kept inoperation.If the neutral is distributed, the shift ofneutral potential with respect to theearth adds a current Icn = Uo Cf ω andId = Uo 4 Cf ω (see fig. 12).v impedance-earthed neutral:First fault current:

Id = U

Zeq where

1Zeq

= 1Zn

+ 3j Cf ω

The corresponding fault voltage is stilllow and not dangerous; the installationcan be kept in operation.Although risk-free continuity of serviceis a great advantage, it is necessary:- to know that there is a fault,- to track it and eliminate it promptly,before a second fault occurs.To meet this need:- the fault information is provided by anInsulation Monitoring Device (IMD)monitoring all live conductors, includingthe neutral,- locating is performed by means offault trackers.

c behaviour on the second faultWhen a second fault occurs and thefirst fault has not yet been eliminated,there are three possibilities:v the fault concerns the same liveconductor: nothing happens andoperation can continue,v the fault concerns two different liveconductors: if all the frames are inter-connected, the double fault is a short-circuit (via the PE). The Electric Shockhazard is similar to that encounteredwith the TN system. The mostunfavourable conditions for the SCPDs(smallest Id) are obtained when bothfaults occur on feeders with the samecharacteristics (cross-sections andlengths) (see fig. 13).

fig. 13: 2nd insulation fault current in IT system (distributed neutral) and relevant feeders withthe same cross-section and length.

fig. 12: first insulation fault current in IT system.

Ud ≈ Rb If

N

Id

Rb

RPE RphRPE Rph

Id

Ud Ud

321N

PE

0,8 Uo

Ud ≈ 0.8 Uo

4 Id ≈

0.8 Uo

2 (RPE

+ Rph)


Remember that:c ρ = 22 10-6 Ω/mm2/m for Cu (36 for Al);

Id Ud Lmax continuity of service

TN vertical discrimination

TT no constraint vertical discrimination

IT 1st fault < 1 A << UL no tripping

double fault vertical discriminationwith neutral and possibility ofdouble faultbetween phases

The SCPDs have to comply with thefollowing relationships:- if the neutral is distributed andone of the two faulty conductors isthe neutra:

Ia i

0.8 Uo2 Z

,

- or if the neutral is not distributed:

Ia i

0.8 Uo 32 Z

.

Note that if one of the two faults ison the neutral, the fault current andfault voltage are twice as low asin the TN system. This has resultedin standard makers authorisinglonger SCPD operating times(see fig. 14).Just as in the TN earthing system,protection by SCPD only applies tomaximum cable lengths:- distributed neutral:

Lmax = 12

0.8 Uo Sph

ρ (1 + m) Ia,

- non-distributed neutral:

Lmax = 3

2

0.8 Uo Sphρ (1 + m) Ia

.

This is provided that the neutral isprotected and its cross-section equal tophase cross-section... This is the mainreason why certain country standardsadvise against distributing the neutral.v case where all frames are notinterconnected.For frames earthed individually or ingroups, each circuit or group of circuitsmust be protected by a RCD.

In point of fact, should an insulation faultoccur in groups connected to two differentearth connections, the earthing system'sreaction to the insulation fault (Id, Ud) issimilar to that of a TT system (the faultcurrent flows through the earth).Protection of persons againstindirect contacts is thus ensured

in the same manner I∆n i

UL

Ra(see table in fig. 11).

Note that in view of the times specifiedby the standard, horizontal timediscrimination can be achieved to givepriority to continuity of service oncertain feeders.

Note: in order to protect LV unearthednetworks (IT) against voltagerises (arcing in the MV/LVtransformer, accidental contactwith a network of higher voltage,lightning on the MV network),French standard NF C 15-100stipulates that a surge limiter mustbe installed between the neutralpoint of the MV/LV transformer and theearth (Rb).

Readers wishing to study the ITearthing systems in greater detailshould read «Cahier Technique»n° 178.

So as to obtain a concise overview ofthe quantities characterising the variousearthing systems, as regards protectionof persons, the main formulas are listedin the table in figure 15.

fig. 14: maximum breaking times specified in IT system (as in IEC 364 tables 41B and 48A).

fig. 15: characteristic quantities of earthing systems.

horizontal discriminationto the advantage of hichcurrent feeders

c PE cross-section, normally equal to phase cross-section, can be equalto half of phase cross-section when the latter exceeds 35 mm2.... thusincreasing Ud in TN and IT.

Uo/U (volts) U L = 50 V UL = 25 VUo: phase/neutral voltage breaking time (seconds) breaking time (seconds)U: phase to phase voltage neutral not distributed neutral distributed neutral not distributed neutral distributed

127/220 0.8 5 0.4 1.00230/400 0.4 0.8 0.2 0.5400/690 0.2 0.4 0.06 0.2580/1 000 0.1 0.2 0.02 0.08

0.8 Uo Sphρ (1 + m) L

0.8 Uo1 + m

0.8 Uo Sphρ (1 + m) Ia

32

0.8 Uo Sph

ρ (1 + m) Ia

12

0.8 Uo Sph

ρ (1 + m) Ia i

12

0.8 Uo Sphρ (1 + m) L

i

32

0.8 Uo Sphρ (1 + m) L

i m 3

2 0.8 Uo1 + m

i

m2

0.8 Uo1 + m

Uo RaRa + Rb

UoRa + Rb

c m =

SphSPE

;


and/or explosion is present: as thePE and neutral conductors areone and the same, RCDs cannot beused.

electrical powerunavailabilityThis hazard is a major one foroperators, since it results innon-production and repair costs whichcan be high.It varies according to the earthingsystem chosen.We remind you that availability (D) is astatistical quantity (see fig. 16) equal tothe ratio between two periods of time:c time during which the mains ispresent;

c reference time equal to the time«mains present + mains absent».

Mean Up Time (MUT) depends on theoverall condition of network insulation.However, insulation deteriorates withtime as a result of thermal aggressionsand electrodynamic forces due inparticular to fault currents.Mean Down Time (MDT) also dependson the fault current and in particular onits strength which, according to itsvalue, may cause:c damage of varying degrees to loads,cables...;c fires;c malfunctionings on the low currentcontrol and monitoring equipment.Each earthing system must thereforebe examined as regards availability of

3. earthing systems confronted with fire andelectrical power unavailability hazards

fireIt has been proved, then acceptedby standard makers, that contactbetween a conductor and a metalpart can cause fire to break out, inparticularly vulnerable premises,when the fault current exceeds500 mA.

To give an example:c premises particularly at risk:petrochemical factories, farms;c premises averagely at risks, butwhere consequences may be veryserious: very high buildings receivingthe general public...In the unearthed neutral system, therisk of «fire»:c is very small on the first fault;c is as important as in TN on thesecond fault.

For the TT and TN earthing systems,the fault current is dangerous given thepower developed (P = Rd I2):c in TT = 5A < Id < 50 A;c in TN = 1 kA < Id < 100 kA.The power present where the fault hasoccurred is considerable, particularly inthe TN system, and prompt action isvital as from the lowest current levels inorder to limit the dissipated energy(∫ Rd i2 dt).

This protection, specified by the IECand a requirement of French standards(NF C 15-100, paragraph 482-2-10) isprovided by an instantaneous RCD withthreshold i 500 mA, regardless of theearthing system.

When risk of fire is especially high(manufacture/storage of inflammablematerials....) it is necessary andindeed compulsory to use an earthingsystem with earthed frames whichnaturally minimises this hazard(TT or IT).Note that the TN-C is banned incertain countries when a risk of fire

fig. 16: availability of electrical power.

D = availability of a systemMUT = Mean Up Time

MDT = Mean Down Time(detection + repair +resumption of operation)

D = MUT

MDT + MUTMean failure free time

MDT MUT MDT MUT MDT

de-energisingon fault

restorationof voltage





time

failure status operating status


of certain motors before startup.Bear in mind that 20 % of motorfailures are due to an insulation faultwhich occurs on energising. In pointof fact, an insulation loss, even small,on a hot motor cooling down in adamp atmosphere (condensation)degenerates into a full fault onrestarting, causing both considerabledamage to windings and productionloss and even major risks if the motorhas a safety function (drainage, fire,fan pump motor, etc.).This type of incident can be prevented,whatever the earthing system, by anInsulation Monitoring Devicemonitoring the load with power off. If afault occurs, startup is then prevented.To round off this section on «thehazard presented by electrical powerunavailability» it is clear that, regardingproper electrical power availability, the

earthing systems can be listed in thefollowing order of preference: IT,TT, TN.

Note:If, to ensure continuity of service,the installation is fitted with agenerator set or a UPS(Uninterruptible Power Supply)in «off line», there is a risk of failure tooperate or of delayed operation of theSCPDs (the short-circuit current islower) on changeover to thereplacement source (lowest Isc -see fig.17).

In TN and IT, for safety of personsand property, it is thus vital to checkthat the protection conditions arealways met (operating time andthreshold), especially for very longfeeders. If this is not so, then RCDsmust be used.

fig. 17: making a short-circuit in a network supplied by a diesel standby generator.

subtranscientstate

occurenceof fault

10 to20 ms

0.1 to0.3 s

transcientstate

generator with compoundexcitation or overexcitation

generator with serialexcitation 0.3 In

In

3 In

I rms

electrical power, with special emphasison the IT earthing system since it is theonly one that authorises non-tripping inthe presence of a fault.

c the IT earthing systemIn order to retain the advantage of thissystem, i.e. not interrupting electricaldistribution on the first fault, the secondfault must be prevented, since this thenpresents the same high risks as the TNsystem. The first fault must therefore beeliminated before a second faultoccurs. The use of efficient detectionand locating methods and the presenceof a reactive maintenance teamconsiderably reduces the likelihood ofthe «double fault».Moreover, monitoring devices arecurrently available which monitor intime the evolution in insulation of thevarious feeders, perform fault predictionand thus anticipate maintenance of thefirst fault.This ensures maximum availability withthe IT earthing system.

c the TN and TT earthing systemsThese systems use discrimination ontripping.In TN, this is acquired with short-circuitprotection devices if the installationprotection plan has been properlydesigned (current discrimination).In TT, it is easy to implement thanks tothe RCDs which ensure current andtime discrimination.

Remember that, in TN system, repairtime according to ∫ i2 dt, may be longerthan in TT system, wich also affectsavailability.c for all the earthing systemsIt is always useful to anticipateinsulation faults and in particular those

≈

≈


advisable to install surge limiters(lightning arresters) at the origin of theLV network, whatever earthing systemis used (see fig. 18).Likewise, to prevent coupling bycommon impedance, it is wise never toconnect the following to the earthconnection of the LV neutral:c MV lightning arresters;c lightning rods placed on the roof ofbuildings.In point of fact, the lightning currentwould cause a rise in potential of thePE and/or the LV neutral (risk ofdisruptive breakdown by return) andloss of earth connection effectivenessby vitrification.

operating overvoltagesSome MV switchgear (e.g. vacuumcircuit-breakers) cause considerableovervoltages when operated(see «Cahier Technique» n° 143).

Unlike lightning which is a commonmode disturbance (between networkand earth), these overvoltages are,in LV, differential mode disturbances(between live conductors) and are

transmitted to the LV network bycapacitive and magnetic coupling.Just like all differential modephenomena, operating overvoltages donot interfere, or only very slightly, withany of the earthing systems.

MV-frame disruptivebreakdown of thetransformerOn MV-frame disruptive breakdowninside the transformer and when thetransformer frame and LV installationneutral are connected to the sameearth connection, a MV «zerosequence» currrent (whose strengthdepends on the MV earthing system)can raise the frame of the transformerand neutral of the LV installation to adangerous potential.In point of fact, the value of thetransformer earth connection directlyconditions the contact voltage in thesubstation Ut i Rp IhMV and thedielectric withstand voltage ofthe LV equipment in the substationUtp = Rp IhMV (if the LV neutral earth is

LV networks, unless a replacementuninterruptible power supply (withgalvanic insulation) or a LV/LVtransformer is used, are influencedby MV.This influence takes the form of:c capacitive coupling: transmission ofovervoltage from MV windings to LVwindings;c galvanic coupling, should disruptivebreakdown occur between the MV andLV windings;c common impedance, if the variousearth connections are connected and aMV current flows off to earth.This results in LV disturbances, oftenovervoltages, whose generatingphenomena are MV incidents:c lightning;c operating overvoltages;c MV-frame disruptive breakdowninside the transformer;c MV-LV disruptive breakdown insidethe transformer.Their most common consequence isdestruction of LV insulators with theresulting risks of Electric Shock ofpersons and destruction of equipment.

lighningIf the MV network is an overhead one,the distributor installs ZnO lightningarresters to limit the effects of a director an indirect lightning stroke.Placed on the last pylon beforethe MV/LV substation, these lightningarresters limit overvoltage and causelightning current to flow off to earth(see «Cahiers Techniques» n° 151and 168).A lightning wave, however, istransmitted by capacitive effectbetween the transformer windings, tothe LV live conductors and can reach10 kV peak. Although it is progressivelyweakened by the stray capacities of thenetwork with respect to earth, it is

fig. 18: limitation and transmission of lighting overvoltages (whether or not the neutral isearthed, there are common mode overvoltages on phases).

3

33

N

i 125 kV i 10 kV

shortconnections

4. influences of MV on LV, according tothe earthing systems


diagrams (1) maximum resistance of the earthconnection of substation frames Rp (Ω)

no value stipulated but the following valuesprevent potential rise of the assembly

IhMV (A) RPAB (Ω)300 3 to 20

1,000 1 to 10

IhMV (A) RPB (Ω)300 31,000 1

Utp (kV) 2 4 10IhMV (A) RP (Ω)300 4 8 201,000 1 3 10

separate from the substation one). Theearth connections of the substation andof the LV neutral are not generallyconnected. If however they are, a limitis given to the common earthconnection value to prevent a rise inpotential of the LV network comparedwith the deep earth. Figure 19 gives thecommon earth connection values forthe IhMV values of French publicnetworks. Readers interested in thiscan consult standard IEC 364-4-442which explains the risks according toLV earthing systems.Still for public networks (except forAustralia and the USA where the faultcurrent can be very high), valuesencountered range from 10 A in Ireland(an impedance compensates thecapacitive current) to 1,000 A in France(underground networks) and inGreat Britain.MV industrial networks are normally runin impedance-earthed IT and have azero sequence current IhMV of a fewdozens of amps (see «CahierTechnique» n° 62).The maximum value authorised for theearth connection depends on theequipotentiality conditions of the framesof the LV network, i.e. on its earthingsystem.

MV-LV disruptivebreakdown insidethe transformerTo prevent potential with respect tothe earth of the LV network fromrising to the phase-to-neutral voltage ofthe MV network on MV-LV disruptivebreakdown inside the transformer, theLV network must be earthed.The consequences of this fault are:c in TNThe entire LV network, including thePE, is subjected to voltage IhMV RPABor RAB.If this overvoltage exceeds thedielectric withstand of the LV network(in practice of the order of 1,500 V),LV disruptive breakdowns are possibleif the equipotentiality of all the frames,electrical or not, of the building is notcomplete.

c in TTWhereas the load frames are at thepotential of the deep earth, the entireLV network is subjected to IhMV RPB

or RB: there is a risk of disruptivebreakdown «by return» of loadsif the voltage developed in RPB or RBexceeds their dielectric withstand.

fig. 19: maximum resistance of the earth connection of the substation frames according tonetwork earthing system.

TTN or ITN RPB RA

Z

TNR or ITR RPAB

Z

Z: direct earthing in TN and TTimpedance-earthed or unearthed in IT with presence of a discharger.

IhMV: maximum strength of the first earth single-phase fault current of the high voltage networksupplying the substation.Utp: power frequency withstand voltage of the low voltage equipment of the substation.

(1) the third letter of the earthing systems means:c all the frames are linked R;c the substation frame is connected to the Neutral frame: N;c the earth connections are Separated S.

TTS or ITSRBRP RA

Z


c in ITOperation of a discharger/short-circuiter(known as a surge limiter in France),which short-circuits itself as soon as itsarcing voltage is reached, then bringsthe problem to the level of the TNnetwork one (or TT if there are severalapplication earth connections).In all cases, MV/LV disruptivebreakdowns give rise to constraintswhich can be severe, both for theLV installation and loads, if the value ofthe LV neutral earth connection is notcontrolled. Interested readers can

consult IEC 364 which explains risksaccording to the earthing systems.The example of overhead publicdistribution in France provides asolution to a situation where risks oflightning, operating overvoltage andtransformer frame-MV andMV-LV disruptive breakdown arepresent (see fig. 20). It shows thatequipotentiality of the entiredistribution (all MV frames, neutralsand application frames connected) isnot vital: each risk is dealt withseparately.

This section has described theinfluence of the MV network. Itsconclusions are:c the value of using lightning arresters atthe origin of the LV installation, whateverthe earthing system type, if the MV andparticularly the LV supply is overhead;c connection of the earth connectionof the substation with the earthconnection of the LV neutral or withthose of the application frames,imposes variable constraints onthe LV network according tothe MV earthing system (value of Ih).

3Ih i 300 A

metering

earth trip

RA < 100 ΩRB < 4 ΩRp < 50 Ω

lightningarresterRCD

PE

u 30 m

u 8 m u 8 m

N

fig. 20: rural overhead public distribution in France.


shaping

threshold

time delay

output

Σ I ≠ 0

persons against indirect contacts.RCDs (see fig. 23) need to be used,associated with circuit-breakers orswitches (see IEC 364 - paragraph413.1.4.2).

These devices must meet the followingstandards in particular:c IEC 755: general rules;c IEC 1008: «household» residualcurrent switches;c IEC 1009: «household» residualcurrent SCPDs;c IEC 947-2: «industrial» residualcurrent circuit-breakers.

Their implementation must meet theobjectives for:c protection of persons, i.e.:v threshold I∆n i UL/RA,v breaking time i 1s;c continuity of service with thresholdsand time delays enabling current andtime discrimination;c fire protection with I∆n i 500 mA.

5. switchgear linked to choice ofearthing system

Choice of earthing system affects notonly dependability (in the largestsense) but also installation, in particularwith respect to the switchgear to beimplemented.

TN systemIn this system the SCPDs (circuit-breaker or fuses) generally provideprotection against insulation faults, withautomatic tripping according to aspecified maximum breaking time(depending on phase-to-neutral voltageUo: see fig. 9).

c with circuit-breakerAs soon as the fault current exceedsthe threshold of the short-circuitprotection trip release (generally«instantaneous»), opening occurs in atime far shorter than specifiedmaximum breaking time, for example5 s for distribution circuits and 0.4 s forterminal circuits (see fig. 21).

When impedance of the source andcables is high, either low threshold tripreleases must be used or RCDsassociated with the SCPDs. TheseRCDs may be separate residual currentdevices or be combined withcircuit-breakers (residual currentcircuit-breakers) of low sensitivity. Theirthreshold must be:

I∆n < 0.8 Uo

Rph + RPE

Use of a RCD has the advantage ofmaking loop impedance checkingunnecessary, a fact which is ofparticular value when the installation ismodified or extended.This solution is clearly not applicablewith a TN-C type earthing system (theprotective conductor being the same asthe neutral one).

c with fusesThe fuses used for short-circuitprotection are of the gG type and theirtime/current characteristics (see fig. 22)are defined by standards (householdfuses: IEC 241, industrial fuses:IEC 269). Checking suitability with themaximum specified breaking timetherefore calls for individual validationof the ratings provided for eachprotection device. If they are notsuitable, either fault loop impedancemust be reduced (increased cross-sections) or the fuse must be replacedby a low threshold or a residual currentcircuit-breaker.

TT systemWith this system, the small value of thefault currents (see previous section)does not allow the SCPDs to protect

fig. 24: fonctional diagram of a RCD.

fig. 23: Vigi module of a Compact NS.trip release type operating threshold

household B 3 In i Ia i 5 In(EN 60898) C 5 In i Ia i 10 In

D 10 In i Ia i 20 Inindustrial G (low threshold) 2 In i Ia i 5 In(IEC 947-2) D 5 In i Ia i 10 In

MA (for motor starter) 6.3 In i Ia i 12.5 In

fig. 21: tripping current (magnetic or short time delay) of LV circuit-breakers.

fig. 22: example of fuse operating threshold limits (as in IEC 269 paragraph 5-6-3).

In gG (A) Imin. 10 s Imax. 5 s Imin. 0.1 s Imax. 0.1 s63 160 320 450 82080 215 425 610 110100 290 580 820 1,450


In France, the IMDs and GFLDs haveto comply with manufacturing standardUTE 63080.The purpose of the IMDs is thus tomonitor the value of this resistance.They normally work by injecting an ACor DC current between the network andthe earth and by measuring the value ofthis current (see fig. 25).

Injection of a DC current ensurescontinuous knowledge of networkinsulation resistance. If this resistancedrops below a pre-set threshold, thenthe IMD reports the fault.Injection of low frequency AC current(F ≈ a few hertz) monitors faultresistance but with a distorsion due tothe presence of network leakagecapacitites. This minor drawbackcompared with injection frequency, ismade up for by an advantage in firstfault locating (one single injectiondevice).

LF current injection devices are nowavailable which can separately give thenetwork’s insulation resistance andreactance. Moreover, they enablelocating of the first fault without circuitopening and without the problems dueto highly capacitive feeders.

c operating principle of the GFLDsThe most common solution is toinject an identifiable current (with afrequency other than network one).The generator can be the IMD. Then,by means of magnetic Current Sensors(toroid transformers and/or clamp-onprobe) associated with an amplifiertuned to the injected current frequency,to trace its path up to the fault(see fig. 26).

Finally, another solution is also used,which consists in comparing, constantlyand for each feeder, the value of itsresistance with a pre-defined orprogrammable threshold value.This solution, computerised, enablesthe following actions, both locally andremotely:v reporting of the first fault (IMD),

IT systemRemember that in the event of a doublefault, safety of persons is provided bythe SCPDs. When the first insulationfault occurs, the calculation provedthere was no risk (contact voltagelower than limit safety voltage).Automatic de-energising is thereforenot compulsory: this is the mainadvantage of this system.To retain this advantage, standardsrecommend (IEC 364 - paragraph413.1.5.4) or stipulate (NF C 15-100)the use of an Insulation MonitoringDevice (IMD) and locating of the firstfault. In point of fact, if a second faultoccurs, automatic breaking is vital dueto the Electric Shock risk: this is thenthe role of the SCPDs backed up by theRCDs if required.

Locating the first fault for repairs(curative maintenance) is considerablysimplified by the use of a Ground FaultLocation Device (GFLD).

Predictive maintenance, based on themonitoring (recording) of variations ininsulation impedance of each circuit, isalso possible.LV networks, using the IT system,which take their origin at a MV/LVtransformer, must be protected againstrisks of insulation faults between MVand LV by a «surge limiter».Finally, to fix the potential of the LVnetwork with respect to the earth(short network supplied by a MV/LVtransformer) and avoid the risk offerromagnetic resonance, animpedance can be installed betweenthe transformer neutral and the earth.Its value in 50 Hz, of the order of1,500 Ω, is very high in DC and in verylow frequency so as not to obstructinsulation measurement and faultlocating.c operating principle of the IMDsA fault on a circuit results in a drop ininsulation, or more precisely inresistance of the network comparedwith earth.

v then locating of this fault (GFLD) toput it right (curative maintenance)(see fig. 27),v and knowledge of insulation evolutionin time, feeder by feeder, to takeaction on feeders with abnormal insula-tion drops (predictive maintenance);

c surge limiters: these are connectedbetween a live conductor (neutral orphase) of the installation and the earth.Their arcing voltage Ue must thereforebe adapted to the assembly planned:thus there are two models for a 50 Hz230/400 V network:v 250 V for connection to the neutral(400 V < Ue i 750 V),v 400 V, for connection to a phase(700 V < Ue i 1,100 V).Their purpose is twofold:v limit voltage on the LV network onMV/LV disruptive breakdown in thedistribution transformer. In this case,the limiter must flow off to earth the«residual» current of the MV network,v limit lightning overvoltages.This accounts for their characteristics,for example for the 250 V model:- rate voltage: 250 V,

fig. 25: functional diagram of an InsulationMonitoring Device (IMD).

N

Zimpedance( 100 kΩ at 50 Hz ; low in LF)

insulation measuringcurrent (Rd)

threshold → time delay→ alarm

PE

measuring

LF current generatora

≈


earth connection must be provided foreach zone/consumer.Figure 28, page 20, shows which typesof circuit-breaker should be used for

- disruptive breakdown voltage at50 Hz: min 400 V, max 750 V,- disruptive breakdown voltageaccording to the 1.2/50 µ wave:û < 1,570 V,- î lightning: 20 times 2,500 A (8/20 mswave): without short-circuiting,- î 50 Hz: 20,000 A / 0.2s,

5,000 A / 5 s,1,200 A / 2 mn.

This î 50 Hz: peak current withstand is fargreater than the value of the «residual»current of the MV network since alimiter which has been «arced» during avery high overvoltage may continue tobe short-circuited and must thereforebe still able to withstand a LVshort-circuit current resulting from a firstinsulation fault in the protectedLV network.The limiters marketed under theMerlin Gerin brand can withstand40 kA/0.2 s.

neutral protectionaccording to the earthingsystemThe neutral must be broken by a multi-pole device:c in TT and TN, if neutral cross-sectionis less than phase cross-section;c in terminal distribution in view of theNeutral/Phase reversal risk.

The neutral must be protected andbroken:c in IT for intervention of the protectiondevice on the double fault, withone of the faults possibly on the neutral;c in TT and TN-S if neutral cross-sectionis less than phase cross-section;c for all earthing systems if theinstallation generates harmoniccurrents of rank 3 and multiples(especially if neutral cross-section isreduced).In TN-C the neutral, which is alsothe PE, cannot be broken which isdangerous as a result of its potentialvariations, due to load currents andinsulation fault currents. To preventrisks, a local equipotentiality and an

which earthing system. Note that TTand TN can use the same devices (withan additional residual current modulein TT).

fig. 27: operating principle of an GFLD with LF impedance measurement.

PE

PE

a LF generator

The locating current flowing in the conductors is detected by Current Sensor s (CS). Each loadcomprising a discriminating amplifier (set to the frequency and phase of the locating current)calculates the resistance and capacity of the circuit (with the voltage and phase whosereference it obtains via a bus) and indicates the presence of the fault.

fig. 26: locating insulation faults by tracing the path of a low frequency current injected at theorigin of the installation.

PE

PE

a

"locating voltage" bus


circuits diagramsTN-C TN-S TT IT

single phase circuitssingle phase circuits with one protected pole no yes yes no

phase to neutral circuits with two protected poles no yes yes yes

three-phase circuits without neutralwith two-pole protection yes yes yes yes

three-phase circuits with neutralwithout overcurrent detection on neutral no yes yes no

yes yes yes no

with overcurrent detection on neutral no yes yes yes

fig. 28: examples of circuit-breakers according to earthing systems

four-polecircuit-breakerwith threeprotected poles

2

3

N

1 I>

I>

I>

four-polecircuit-breakerwith fourprotected poles

2

3

N

1 I>

I>

I>

I>

I>two-polecircuit-breaker(1 protected pole,2 de-energized poles)

N

two-polecircuit-breaker(with 2 protected poles)N

I>

I>

three-polecircuit-breaker

2

3

1 I>

I>

I>

three-polecircuit-breaker

2

3

N

1 I>

I>

I>


The three earthing systemsinternationally used and standardisedby IEC 364 have as their commonobjective the quest for optimumdependability .As regards protection of persons, the3 systems are equivalent if all installa-tion and operating rules are compliedwith. In view of the characteristicsspecific to each system, no one systemcan be preferred over another.Rather, choice of earthing system mustresult from a concertation between thenetwork user and designer (engineeringfirm, contractor, etc.) on:c installation characteristics;c operating conditions andrequirements.

methods for choosing theearthing systemc firstly do not forget that the threeearthing systems can all be includedin the same electrical installation: thisguarantees the best possible answer tosafety and availability needs;c then check that the choice is notspecified or stipulated by standardsor legislation (decrees, ministerialdecisions);c then dialogue with the user to get toknow his requirements and resources:v need for continuity of service,

6. choice of earthing system and conclusion

v loads with low natural insulation(furnaces) or with large HFfilter (large computers):prefer the TN-S,v supply of control and monitoringsystems: perfer the IT (continuity ofservice) or the TT (enhancedequipotentiality of communicatingdevices).

conclusionAs there is no ideal choice with a singleearthing system, it is thus advisable, inmany cases , to implement severalearthing systems in the sameinstallation.As a rule, a radial network installation,with a clear distinction between priorityand non-priority circuits and usingstandby sources or uninterruptiblepower supplies, is preferable to anarborescent monolithic installation.The purpose of this «CahierTechnique» was to perfect yourknowledge of earthing systems;we hope it will enable you tooptimise the dependability of yourinstallations.«Cahier Technique» n° 173 whichprovides an insight into use of earthingsystems worldwide and their evolutionwill usefully complete this firstdocument.

v whether or not there is amaintenance service,v fire hazard.

Generally:v continuity of service andmaintenance service: the IT will bechosen,v continuity of service and nomaintenance service: no fullysatisfactory solution: prefer the TTwhose discrimination on tripping iseasier to implement and whichminimises damage with respect tothe TN,The installation of additionnal output iseasily achieved without the necessity offurther calculations.v continuity of service not essential andcompent maintenance service: preferthe TN-S (rapid repairs and extensionsperformed according to rules),v continuity of service not essentialand no maintenance service: preferthe TT,v fire hazard: IT if maintenance serviceand use of 0.5 A RCD or TT.c allow for the special features ofnetwork and loads:v very long network or, even moreimportant, leakage current: prefer theTN-S,v use of replacement or standby powersupplies: prefer the TT,v loads sensitive to high fault currents(motors): prefer the TT or IT,


7. bibliography

Standardsc IEC 241: Fuses for domestic andsimilar purposes.

c IEC 269: Low voltage fuses.

c IEC 364: Electrical installation ofbuildings.

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

c IEC 755: General rules for residualcurrent devices

c IEC 947-2: Low voltage switchgear -2nd part: circuit-breakers.

c IEC 1008: Residual current operatedcircuit-breakers without integralovercurrent protection for householdand similar uses (RCCB's)

c IEC 1009: Residual current operatedcircuit-breakers with integralovercurrent protection for householdand similar uses (RCBO's)

c NF C 15-100: Installations électriquesà basse tension.

French decree of the 14.11.88.

Merlin Gerin's Cahiers Techniquesc Earthing of the neutral in a HVindustrial networkCahier Technique n° 62,F. SAUTRIAU

c Residual current devicesCahier Technique n° 114,R. CALVAS

c Protections des personnes etalimentations statiques sans coupure,Cahier Technique n° 129,J.-N. FIORINA

c Les perturbations électriques en BT,Cahier Technique n° 141R. CALVAS

c Introduction to dependability designCahier Technique n° 144,P. BONNEFOI

c EMC: Electromagnetic compatibilityCahier Technique n° 149,F. VAILLANT

c Overvoltages and insulationcoordination in MV and HVCahier Technique n° 151,D. FULCHIRON

c Lightning and HV electricalinstallationsCahier Technique n° 168,B. DE METZ NOBLAT

c Earthing systems worldwideand evolutionsCahier Technique n° 173,B. LACROIX and R. CALVAS

c Connaissances et emploi du SLTneutre isolé,Cahier Technique n° 178,E. TISON et I. HERITIER(available: end of 1995)

Other publications

c Guide de l’installation electriqueEd. France Impression Conseil 1991.

c Guide de l’ingénierie électriqueEd. ELECTRA 1986.

c Electrical ReviewNov. 1991 - Oct. 1992.

c La protection différentielleCahier Technique J3E - 02/90.



Real.: Illustration Technique - LyonEdition: DTE - Grenoble09-95 - 2500 - Printing: ClercPrinted in France

Documents

172-Earthing Systems in LV