Short Circuit

..........................................................................Collection Technique

Cahier technique no. 172

Earthing systems in LV

B. LacroixR. Calvas

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Furthermore, these "Cahiers Techniques" are often considered as helpful"tools" for training courses.They provide knowledge on new technical and technological developmentsin the electrotechnical field and electronics. They also provide betterunderstanding of various phenomena observed in electrical installations,systems and equipments.Each "Cahier Technique" provides an in-depth study of a precise subject inthe fields of electrical networks, protection devices, monitoring and controland industrial automation systems.

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no. 172Earthing systems in LV

ECT 172(e) updated March 2002

Bernard LACROIX

An ESPCI 74 graduate engineer (from the Ecole Supérieure dePhysique et Chimie Industrielle de Paris), he then worked 5 years forJeumont Schneider, where his activities included development of theTGV chopper.After joining Merlin Gerin in 1981, he was then in turn Sales Engineerfor UPS and sales manager for protection of persons.Since 1991 he is in charge of prescription for LV power distribution.

Roland CALVAS

An ENSERG 1964 graduate engineer (from the Ecole NationaleSupérieure d'Electronique et Radioélectricité de Grenoble) and anInstitut d'Administration des Entreprises graduate, he joinedMerlin Gerin in 1966.During his professional career, he was sales manager, then marketingmanager in the field of equipment for protection of persons, andfinally in charge of technical communication for Schneider Electric,until retirement early 1999.

Cahier Technique Schneider Electric no. 172 / p.2

Lexicon

Electric Shock: Application of a voltagebetween two parts of the body

Electrocution: Electric Shock resulting in death

EMC: Electro Magnetic Compatibility

I∆n: Operating threshold of a RCD

IMD: Insulation Monitoring Device

GFLD: Insulation Fault Location Device

MV/HV: Medium Voltage: 1 to 35 kV as inCENELEC (circular of the 27.07.92)High Voltage: 1 to 50 kV as in french standard(14.11.88)

RCD: Residual Current Device

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

STD: Short Time Delay protection (protectionagainst short-circuit overcurrents by circuit-breaker with rapid trip release)

TBM: Technical Building Management

TEM: Technical Electrical Power DistributionManagement

UL: Conventional limit voltage (maximumacceptable contact voltage) known as the"safety" voltage


Earthing systems in LV

Contents1 Introduction 1.1 Evolution of needs p. 4

1.2 Causes of insulation faults p. 4

1.3 Hazards linked to insulation faults p. 5

2 Earthing systems and protection of persons p. 8

2.1 TN system p. 9

2.2 TT system p. 10

2.3 IT system p. 11

3 Earthing systems confronted with fire 3.1 Fire p. 15

3.2 Electrical power unavailability p. 15

4 Influences of MV on BV, according to 4.1 Lightning p. 17

4.2 Operating overvoltages p. 17

4.3 MV-frame disruptive breakdown of the transformer p. 18

4.4 MV-LV disruptive breakdown inside the transformer p. 19

5 Switchgear linked to choice of 5.1 TN system p. 20

5.2 TT system p. 21

5.3 IT system p. 21

5.4 Neutral protection according to the earthing system p. 23

6 Choice of eathing system and conclusion 6.1 Methods for choosing the earthing system p. 25

6.2 Conclusion p. 25

Bibliography p. 26

This "Cahier Technique" reviews the hazards that insulation faultsrepresent for safety of persons and property. It emphasises the influence ofearthing systems and the availability of electrical power.

It presents the three earthing systems defined in standard IEC 60364 andused to varying degrees in all countries.

Each earthing system is looked at in terms of dependability (safety,maintainability and availability).

None of the earthing systems is basically bad. They all ensure safety ofpersons. Each system has its own advantages and disadvantages and theuser must therefore be guided according to his needs, with the exception,however, of prescription or of standard or legislative bans.

Readers interested in different practices of various countries and in theevolution of earthing systems should read "Cahier Technique" no. 173.

and electrical power unavailability hazards

the earthing systems

earthing system


1 Introduction

1.1 Evolution of needs

Today the 3 earthing systems such as defined inIEC 60364 and French standard NF C 15-100,are:

c exposed-conductive parts connected toneutral -TN-;

c earthed neutral -TT-;

c unearthed (or impedance-earthed) neutral -IT-.

The purpose of these three systems is identicalas regards protection of persons and property:mastery of insulation fault effects. They areconsidered to be equivalent with respect tosafety of persons against indirect contacts.However, the same is not necessarily true fordependability of the LV electrical installation withrespect to:

c electrical power availability;

c installation maintenance.

These quantities, which can be calculated, aresubjected to increasingly exacting requirementsin factories and tertiary buildings. Moreover, thecontrol and monitoring systems of buildings-TBM- and electrical power distributionmanagement systems -TEM- play anincreasingly important role in management anddependability.

This evolution in dependability requirementstherefore affects the choice of earthing system.

It should be borne in mind that the concern withcontinuity of service (keeping a sound network inpublic distribution by disconnecting consumerswith insulation faults) played a role whenearthing systems first emerged.

1.2 Causes of insulation faults

In order to ensure protection of persons andcontinuity of service, conductors and live parts ofelectrical installations are «insulated» from theframes connected to the earth.

Insulation is achieved by:

c use of insulating materials;

c distancing, which calls for clearances in gases(e.g. in air) and creepage distances (concerningswitchgear, e.g. an insulator flash over path).

Insulation is characterised by specified voltageswhich, in accordance with standards, are appliedto new products and equipment:

c insulating voltage (highest network voltage);

c lightning impulse withstand voltage (1.2; 50 mswave);

c power frequency withstand voltage(2 U + 1,000 V/1mn).

Example for a LV PRISMA type switchboard:

c insulating voltage: 1,000 V;

c impulse voltage: 12 kV.

When a new installation is commissioned,produced as per proper practices with productsmanufactured as in standards, the risk ofinsulation faults is extremely small; as theinstallation ages, however, this risk increases.

In point of fact, the installation is subject tovarious aggressions which give rise to insulationfaults, for example:

c during installation:v mechanical damage to a cable insulator;

c during operation:v conductive dust,v thermal ageing of insulators due to excessivetemperature caused by:- climate,- too many cables in a duct,- a poorly ventilated cubicle,- harmonics,- overcurrents, etc,v the electrodynamic forces developed during ashort-circuit which may damage a cable orreduce a clearance,v the operating and lightning overvoltages,v the 50 Hz return overvoltages, resulting froman insulation fault in MV.

It is normally a combination of these primarycauses which results in the insulation fault. Thelatter is:

c either of differential mode (between liveconductors) and becomes a short-circuit;

c or of common mode (between live conductorsand frame or earth), a fault current -said to becommon mode or zero sequence (MV)- thenflows in the protective conductor (PE) and/or inthe earth.

LV earthing systems are mainly concerned bycommon mode faults which mainly occur in loadsand cables.


1.3 Hazards linked to insulation faults

An insulation fault, irrespective of its cause,presents hazards for:

c human life;

c preservation of property;

c availability of electrical power;the above all depending on dependability.

Electric Shock of personsA person (or animal) subjected to an electricalvoltage is electrified. According to the gravity ofthe Electric Shock, 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 current takes

priority, Electric Shock is thus the first hazard tobe considered.

The current strength I -in value and time-,passing through the human body (in particularthe heart) is the dangerous aspect. In LV, theimpedance value of the body (an importantaspect of which is skin resistance) virtuallychanges only according to environment (dry andwet premises and damp premises).In each case, a safety voltage (maximumacceptable contact voltage for at least 5 s) hasbeen defined: it is known as the conventionallimit voltage UL in IEC 60479.

IEC 60364 paragraph 413.1.1.1 (andNF C 15-100) state that if there is a risk ofcontact voltage Uc exceeding voltage UL, theapplication time of the fault voltage must belimited by the use of protection devices(see fig. 2 ).

0.1 0.2 0.5 1 2 5 10 20Threshold = 30 mA

50 100 200 500 1000 2000 500010000

mA10

20

50

100

200

500

1 000

2 000

5 000

10 000

ms Time during whichthe human body is exposed

a b c2c1 c3

1 2 3

Current passing throughthe human body

4

Zone 1: perception Zone 2: considerable discomfortZone 3: muscular contractions Zone 4: risk of ventricular fibrillation (cardiac arrest)c2: likelyhood < 5 % c3: likelyhood u 50 %

Fig. 1 : time/current zones of ac effects (15 Hz to 100 Hz) on persons as in IEC 60479-1.

c Dry or humid premises and places: UL i 50 V

Presumed contact voltage (V) < 50 50 75 90 120 150 220 280 350 500

ac 5 5 0.60 0.45 0.34 0.27 0.17 0.12 0.08 0.04

dc 5 5 5 5 5 1 0.40 0.30 0.20 0.10

c Wet premises and places: UL i 25 V

Presumed contact voltage (V) 25 50 75 90 110 150 220 280

ac 5 0.48 0.30 0.25 0.18 0.10 0.05 0.02

dc 5 5 2 0.80 0.50 0.25 0.06 0.02

Maximum breaking time ofthe protection device (s)

Maximum breaking time ofthe protection device (s)

Fig. 2 : maximum time for maintenance of contact voltage as in standard IEC 60364.


FireThis hazard, when it occurs, can have dramaticconsequences for both persons and property. Alarge number of fires are caused by importantand localised temperature rises or an electric arcgenerated by an insulation fault. The hazardincreases as the fault current rises, and alsodepends on the risk of fire or explosion occurringin the premises.

Unavailability of electrical powerIt is increasingly vital to master this hazard. Inactual fact if the faulty part is automaticallydisconnected to eliminate the fault, the result is:

c a risk for persons, for example:v sudden absence of lighting,v placing out of operation of equipment requiredfor safety purposes;

c an economic risk due to production loss. Thisrisk must be mastered in particular in processindustries, which are lengthy and costly torestart.

Moreover, if the fault current is high:

c damage, in the installation or the loads, maybe considerable and increase repair costs andtimes;

c circulation of high fault currents in the commonmode (between network and earth) may alsodisturb sensitive equipment, in particular if theseare part of a "low current" system geographicallydistributed with galvanic links.

Finally, on de-energising, the occurrence ofovervoltages and/or electromagnetic radiationphenomena may lead to malfunctioning or evendamage of sensitive equipment.

Direct and indirect contactsBefore beginning to study the earthing systems,a review of Electric Shock by direct and indirectcontacts will certainly be useful.

c Direct contact and protection measuresThis is accidental contact of persons with a liveconductor (phase or neutral) or a normally liveconductive element (see fig. 3a ).

In cases where the risk is very great, thecommon solution consists in distributingelectricity using a non-dangerous voltage, i.e.less than or equal to safety voltage. This issafety by extra-low voltage (SELV or PELV).

In LV (230/400 V), protection measures consistin placing these live parts out of reach or ininsulating them by means of insulators,enclosures or barriers. A complemen-tarymeasure against direct contacts consists in usinginstantaneous i 30 mA High Sensitivity ResidualCurrent Devices known as HS-RCDs.

Treatment of protection against direct contacts iscompletely independent from the earthingsystem, but this measure is necesssary in all

circuit supply cases where implementation of theearthing system downstream is not mastered.Consequently, some countries make thismeasure a requirement:v for sockets of rating i 32 A,v in some types of installations (temporary,worksite, etc.).

c Indirect contact, protection and preventionmeasuresContact of a person with accidentally energisedmetal frames is known as indirect contact(see fig. 3b ).

This accidental energising is the result of aninsulation fault. A fault current flows and createsa potential rise between the frame and the earth,thus causing a fault voltage to appear which isdangerous if it exceeds voltage UL.

As regards this hazard, the installation standards(IEC 364 at international level) have given officialstatus to three earthing systems and defined thecorresponding installation and protection rules.

The protection measures against indirectcontacts are based on three basic principles:

v earthing of the frames of loads andelectrical equipment to prevent an insulationfault representing a risk equivalent of a directcontact;

v equipotentiality of simultaneouslyaccessible framesInterconnection of these frames considerablyhelps to reduce contact voltage. It is performedby the protective conductor (PE) which connects

Fig. 3 : direct and indirect contacts.

Uc

ph

3

Id Uc

a) direct contact

b) indirect contact


the frames of electrical equipment for entirebuildings, completed if required by additionalequipotential links (see fig. 4 ).Reminder: equipotentiality cannot be completein all points (in particular in single levelpremises). Consequently, for the study ofearthing systems and their associated protectiondevices, the hypothesis chosen by standardmakers Uc = Ud is applied since Uc is at themost equal to Ud.- Ud = "fault" voltage, with respect to the deepearth, of the frame of an electrical device with aninsulation fault,- Uc = contact voltage depending on thepotential Uc and the potential reference of theperson exposed to the hazard, generally theground;

v managing the electrical hazard- this management is optimised by prevention.For example, by measuring insulation of a devicebefore energising it, or by fault prediction basedon live monitoring of insulation evolution of anunearthed installation (IT system),- if an insulation fault occurs, generating adangerous fault voltage, it must be eliminated byautomatically disconnecting the part of theinstallation where this fault occurred. How thehazard is removed then depends on the earthingsystem.

Heating

Mainprotectiveconductor

Individualprotectiveconductors(PE)

Reinforcementmeshing

Measuring strip

Gas

Earthing conductorDitch bottom loop

Water

Fig. 4 : equipotentiality in a building.


2 Earthing systems and protection of persons

This section defines the Electric Shock andElectrocution hazards for the various earthingsystems, such as specified by the InternationalElectrotechnical Committee in standardIEC 60364.

The LV earthing system characterises theearthing mode of the secondary of the MV/LVtransformer and the means of earthing theinstallation frames.

Identification of the system types is thus definedby means of 2 letters:

c the first one for transformer neutral connection(2 possibilities):v T for "connected" to the earth,v I for "isolated" from the earth;

c the second one for the type of applicationframe connection (2 possibilities):v T for "directly connected" to the earth,v N for "connected to the neutral" at the origin ofthe installation, which is connected to the earth(see fig. 5 ).

Combination of these two letters gives threepossible configurations:

c TT: transformer neutral earthed, and frameearthed,

c TN: transformer neutral earthed, frameconnected to neutral,

c IT: unearthed transformer neutral, earthedframe.

Note 1:The TN system, as in IEC 60364 includesseveral sub-systems:

c TN-C; if the N and PE neutral conductors areone and the same (PEN);

c TN-S: if the N and PE neutral conductors areseparate;

c TN-C-S: use of a TN-S downstream from aTN-C (the opposite is forbidden).Note that the TN-S is compulsory for networkswith conductors of a cross-section i 10 mm2 Cu.

Note 2:Each earthing system can be applied to an entireLV electrical installation; however severalearthing systems may be included in the sameinstallation, see figure 6 as an example.

N

T

N

T

I

N N

N

3 3

33

Fig. 5 : connection mode of the neutral at the origin of the installation and of the frames of the electrical 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.


Note 3:In France, as in standard NF C 13-100concerning delivery substations, in order toprevent hazards originating in MV, theLV earthing system is expressed by an additional

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)

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

2.1 TN system

20 % on phase-to-neutral voltage Uo, which isthe nominal voltage between phase and earth.

Id thus induces a fault voltage with respect toearth:Ud = R dPE I

i.e.:

Ud = 0.8 Uo R

RphPE

1 +RPE

For 230/400 V networks, this voltage of aroundUo/2 (if RPE = Rph) is dangerous since itexceeds the limit safety voltage, even in dry

When an insulating fault is present, the faultcurrent Id is only limited by the impedance of thefault 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-circuit occurs, it isaccepted that the impedances upstream from therelevant feeder cause a voltage drop of around

Ud

Rd

N

A

BC

D

PE

Id

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

letter according to interconnection of the variousearth connections (see fig. 7 ).

Let us now see how to protect persons in eachcase.

Ud 0.8 Uo

2 if R = Rph and Rd = 0

d = Uo

R Rd R0.8 Uo

Rph+R

PE

AB CD PE

≈

+ +⇒I


atmospheres (UL = 50 V). The installationor part of the installation must then beautomatically and promptly de-energised(see fig. 9 ).

As the insulation fault resembles a phase-neutralshort-circuit, breaking is achieved by the Short-Circuit Protection Device (SCPD) with amaximum specified breaking time dependingon UL.

Implementation

To be sure that the protection device really isactivated, the current Id must be greater than theoperating threshold of the protection device Ia(Id > Ia) irrespective of where the fault occurs.This condition must be verified at the installationdesign stage by calculating the fault currents forall the distribution circuits.

If the same path is taken by the protectiveconductor - PE- and the live conductors, this willsimplify the calculation. Certain countrystandards recommend this.

To guarantee this condition, another approachconsists in imposing a maximum impedancevalue on the fault loops according to the typeand rating of the SCPDs chosen (see Britishstandard BS 7671). This approach may result inincreasing the cross-section of the live and/orprotective conductors.

Another means of checking that the device willensure protection of persons is to calculate themaximum length not to be exceeded by eachfeeder for a given protection threshold Ia.

To calculate Id and Lmax, three simple methodscan be used (see "Cahier Technique" n° 158):

c the impedance method;

c the composition method;

c the conventional method.The latter gives the following equation:

Id = 0.8 Uo

Z =

0.8 UoRph+R

= 0.8 Uo Sph

(1+m) PE ρ L

For the protection device to perform its functionproperly, Ia must be less than Id, hence theexpression of Lmax, the maximum lengthauthorised by the protection device with athreshold Ia:

Lmax = 0.8 Uo Sph

(1+m) aρ I

c Lmax: maximum length in m;

c Uo: phase-to-neutral voltage 230 V for a three-phase 400 V network;

c ρ: resistivity to normal operating temperature;

c Ia: automatic breaking current:v for a circuit-breaker Ia = Im (Im operatingcurrent of the magnetic or short time delay triprelease),v for a fuse, current such that total breaking timeof the fuse (prearcing time + arcing time)complies with the standard (see fig. 9 ),

c m = SphSPE

If the line is longer than Lmax, either conductorcross-section must be increased or it must beprotected using a Residual Current Device(RCD).

2.2 TT system

When an insulation fault occurs, the fault currentId (see fig. 10 ) is mainly limited by the earthresistances (if the earth connection of the framesand the earth connection of the neutral are notassociated).Still assuming that Rd = 0, the fault current is:

Id Uo

Ra + Rb≈

This fault current induces a fault voltage in theearth resistance of the applications:

Ud = Ra d, or Ud = Uo Ra

Ra + RbI

As earth resistances are normally low and of thesame magnitude (≈ 10 Ω), this voltage of theorder of Uo/2 is dangerous. The part of the

Uo (volts) Breaking time Breaking timephase/neutral voltage (seconds) UL = 50 V (seconds) UL = 25 V

127 0.8 0.35

230 0.4 0.2

400 0.2 0.05

> 400 0.1 0.02

Fig. 9 : breaking time in TN system (taken from IEC 60364 tables 41 and 48A).


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

Fig. 11 : upper limit of the resistance of the frame earthconnection not to be exceeded according to RCDsensitivity and limit voltage UL [I∆n = F (Ra)].

Maximum resistance of earthconnection

UL 50 V 25 V

3 A 16 Ω 8 Ω

1 A 50 Ω 25 Ω

500 mA 100 Ω 50 Ω

300 mA 166 Ω 83 Ω

30 mA 1,660 Ω 833 Ω

installation affected by the fault must thereforebe automatically disconnected (see fig. 11 ).

ImplementationAs the fault current beyond which a risk is

present ( Id = U

RaoL ) is far lower than the settings

of the overcurrent protection devices, at leastone RCD must be fitted at the supply end of theinstallation. In order to increase availability ofelectrical power, use of several RCDs ensurestime and current discrimination on tripping.All these RCDs will have a nominal currentthreshold I∆n less than Id0.The standard stipulates that de-energising by theRCDs must occur in less than 1 s.Note that protection by RCD:c does not depend on cable length;c authorises several separate Ra earthconnections (an unsuitable measure since the

PE is no longer a unique potential reference forthe entire installation)."Cahier Technique" no. 114 gives a detaileddescription of RCD technology and use.

2. 3 IT system

The neutral is unearthed, i.e. not connected tothe earth. The earth connections of the framesare normally interconnected (just like the TN andTT earthing systems).

c In normal operation (without insulation fault),the network is earthed by the network leakageimpedance.We remind you that natural earth leakageimpedance of a three-phase 1 km long cable ischaracterised by the standard 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 of a networkin IT with respect to the earth, we advise that youplace an impedance (Zn ≈ 1,500 Ω) betweentransformer neutral and the earth.... this is the ITimpedance-earthed system.

c Behaviour on the first fault

v Unearthed neutral:

The fault current is formed as follows (maximumvalue in the case of a full fault and neutral notdistributed).If = Ic1 + Ic2, where:Ic1 = j Cf ω V1 3,

Ud

N

PE

Rb Ra

Id

Id Uo

Ra + Rb≈

=+

Ud UoRa

Ra Rb

I∆n

ULRa

i


Ic2 = j Cf ω V2 3Id = Uo 3 Cf ω.

For 1 km of 230/400V network, the fault voltagewill be equal to:Uc = Rb Id, i.e. 0.7 V if Rb = 10 Ω.

This voltage is not dangerous and the installationcan thus be kept in operation.If the neutral is distributed, the shift of neutralpotential with respect to the earth adds a currentIcn = Uo Cf ω and Id = Uo 4 Cf ω (see fig. 12 ).v impedance-earthed neutral:First fault current:

Id U

Zeq where

1Zeq

= 1

Zn + 3j Cf

=

ω

The corresponding fault voltage is still low andnot dangerous; the installation can be kept inoperation.

Although risk-free continuity of service is a greatadvantage, it is necessary:- to know that there is a fault,- to track it and eliminate it promptly,before a second fault occurs.

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

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

To meet this need:- the fault information is provided by anInsulation Monitoring Device (IMD) monitoring alllive conductors, including the neutral,- locating is performed by means of faulttrackers.

c behaviour on the second faultWhen a second fault occurs and the first faulthas not yet been eliminated, there are threepossibilities:v the fault concerns the same live conductor:nothing happens and operation can continue,v the fault concerns two different live conductors:if all the frames are inter-connected, the doublefault is a short-circuit (via the PE). The ElectricShock hazard is similar to that encountered withthe TN system. The most unfavourableconditions for the SCPDs (smallest Id) areobtained when both faults occur on feeders withthe same characteristics (cross-sections andlengths) (see fig. 13 ).

The SCPDs have to comply with the followingrelationships:- if the neutral is distributed andone of the two faulty conductors is the neutral:

Ud Rb f . I≈


Ia

0.8 Uo2 Z

i

- or if the neutral is not distributed:

Ia

0.8 Uo 3i2Z

Note that if one of the two faults is on the neutral,the fault current and fault voltage are twice aslow as in the TN system. This has resulted instandard makers authorising longer SCPDoperating times (see fig. 14 ).Just as in the TN earthing system, protection bySCPD only applies to maximum cable lengths:- distributed neutral:

Lmax = 0.8 Uo Sph

(1+m) a12 ρ I

- non-distributed neutral:

Lmax = 0.8 Uo Sph

(1+m) a3

2 ρ I

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

N

Id

Rb

RPE RphRPE Rph

Id

Ud Ud

321N

PE

0,8 Uo

Fig. 14 : maximum breaking times specified in IT system (as in IEC 60364 tables 41B and 48A).

Uo/U (volts) UL = 50 V UL = 25 VUo: phase/neutral voltage breaking time (seconds) breaking time (seconds)U: phase to phase voltage Neutral Neutral Neutral Neutral

not distributed distributed not distributed distributed

127/220 0.8 5 0.4 1.00

230/400 0.4 0.8 0.2 0.5

400/690 0.2 0.4 0.06 0.2

580/1 000 0.1 0.2 0.02 0.08

Id 0.8 Uo

2 (R + Rph)PE≈ Ud

0.8 Uo2

≈

This is provided that the neutral is protectedand its cross-section equal to phase cross-section... This is the main reason why certaincountry standards advise against distributingthe neutral.v case where all frames are not interconnected.For frames earthed individually or in groups,each circuit or group of circuits must beprotected by a RCD.In point of fact, should an insulation fault occurin groups connected to two different earthconnections, the earthing system's reaction tothe insulation fault (Id, Ud) is similar to that of aTT system (the fault current flows through theearth).

Protection of persons against indirect contactsis thus ensured in the same manner

I∆n

U

RaLi (see table in figure 11 ).


32

0.8 Uo Sph

(1+m) Li

ρ i

m 32

0.8 Uo1+m

32

0.8 Uo Sph

(1+m) aρ I

i

12

0.8 Uo Sph

(1+m) Lρ i

m2

0.8 Uo1+m

12

0.8 Uo Sph

(1+m) aρ I

Id Ud Lmax Continuity of service

TN Vertical discrimination

TT No constraint Vertical discrimination

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

Double fault with Vertical discriminationdistributed neutral and possibility of

Double fault withnon distributed neutral

UoRa + Rb

Uo RaRa + Rb

0.8 Uo Sph (1+m) Lρ

0.8 Uo1+m

0.8 Uo Sph (1+m) aρ I

Note that in view of the times specified by thestandard, horizontal time discrimination can beachieved to give priority to continuity of serviceon certain feeders.

Note: in order to protect LV unearthed networks(IT) against voltage rises (arcing in the MV/LVtransformer, accidental contact with a network ofhigher voltage, lightning on the MV network),French standard NF C 15-100 stipulates that asurge limiter must be installed between the

neutral point of the MV/LV transformer and theearth (Rb).

Readers wishing to study the IT earthingsystems in greater detail should read "CahierTechnique" no. 178.

So as to obtain a concise overview of thequantities characterising the various earthingsystems, as regards protection of persons, themain formulas are listed in the table in figure 15.

horizontal discriminationto the advantage of hichcurrent feeders

Remember that:

c ρ = 22 10-3 Ω.mm2/m for Cu (36 for Al) at 75°C;

c m =SphSPE

;

c PE cross-section, normally equal to phase cross-section, can be equal to half ofphase cross-section when the latter exceeds 35 mm2.... thus increasing Ud in TNand IT.

Fig. 15 : characteristic quantities of earthing systems.


3 Earthing systems confronted with fireand electrical power unavailability hazards

3.1 Fire

It has been proved, then accepted by standardmakers, that contact between a conductor and ametal part can cause fire to break out, inparticularly vulnerable premises, when the faultcurrent exceeds 500 mA.

To give an example:c premises particularly at risk: petrochemicalfactories, farms;c premises averagely at risks, but whereconsequences may be very serious: very highbuildings receiving the general public...In the unearthed neutral system, the risk of "fire":c is very small on the first fault;c is as important as in TN on the second fault.

For the TT and TN earthing systems, the faultcurrent is dangerous given the power 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 has occurredis considerable, particularly in the TN system,and prompt action is vital as from the lowestcurrent levels in order to limit the dissipatedenergy (∫ Rd i2 dt).

This protection, specified by the IEC and arequirement of French standards (NF C 15-100,paragraph 482-2-10) is provided by aninstantaneous RCD with threshold i 500 mA,regardless of the earthing system.

When risk of fire is especially high (manufacture/storage of inflammable materials....) it isnecessary and indeed compulsory to use anearthing system with earthed frames whichnaturally minimises this hazard (TT or IT).

Note that the TN-C is banned in certain countrieswhen a risk of fire and/or explosion is present: asthe PE and neutral conductors are one and thesame, RCDs cannot be used.

3. 2 Electrical power unavailability

This hazard is a major one for operators, since itresults in non-production and repair costs whichcan be high.

It varies according to the earthing systemchosen.

We remind you that availability (D) is a statisticalquantity (see fig. 16 ) equal to the ratio betweentwo periods of time:

c time during which the mains is present;

c reference time equal to the time "mainspresent + mains absent".Mean Down Time (MDT) also depends on thefault current and in particular on its strengthwhich, according to its value, may cause:c damage of varying degrees to loads, cables...;c fires;

Fig. 16 : availability of electrical power.

D = Availability of a system MDT = Mean Down TimeMUT = Mean Up Time (detection + repair

Mean failure free time + resumption of operation)

D =MUT

MDT + MUT

MDT MUT MDT MUT MDT

De-energisingon fault

Restorationof voltage





Time

Failure status Operating status


c malfunctionings on the low current control andmonitoring equipment.

Each earthing system must therefore beexamined as regards availability of electricalpower, with special emphasis on the IT earthingsystem since it is the only one that authorisesnon-tripping in the presence of a fault.

c The IT earthing systemIn order to retain the advantage of this system,i.e. not interrupting electrical distribution on thefirst fault, the second fault must be prevented,since this then presents the same high risks asthe TN system. The first fault must therefore beeliminated before a second fault occurs. The useof efficient detection and locating methods andthe presence of a reactive maintenance teamconsiderably reduces the likelihood of the"double fault".

Moreover, monitoring devices are currentlyavailable which monitor in time the evolution ininsulation of the various feeders, perform faultprediction and thus anticipate maintenance ofthe first fault.This ensures maximum availability with the ITearthing system.

c The TN and TT earthing systemsThese systems use discrimination on tripping.In TN, this is acquired with short-circuitprotection devices if the installation protectionplan has been properly designed (disriminationby current and duration selectivity).In TT, it is easy to implement thanks to theRCDs which ensure current and timediscrimination.Remember that, in TN system, repair timeaccording to ∫ i2 dt, may be longer than in TTsystem, wich also affects availability.

c For all the earthing systemsIt is always useful to anticipate insulation faultsand in particular those of certain motors beforestartup.

Bear in mind that 20 % of motor failures are dueto an insulation fault which occurs on energising.In point of fact, an insulation loss, even small, ona hot motor cooling down in a damp atmosphere(condensation) degenerates into a full fault onrestarting, causing both considerable damage towindings and production loss and even majorrisks if the motor has a safety function (drainage,fire, fan pump motor, etc.).

This type of incident can be prevented, whateverthe earthing system, by an Insulation MonitoringDevice monitoring the load with power off. If afault occurs, startup is then prevented.To roundoff this section on "the hazard presented byelectrical power unavailability" it is clear that,regarding proper electrical power availability, theearthing systems can be listed in the followingorder of preference: IT, TT, TN.

Note:If, to ensure continuity of service, the installationis fitted with a generator set or a UPS(Uninterruptible Power Supply) in "off line", thereis a risk of failure to operate or of delayedoperation of the SCPDs (the short-circuit currentis lower) on changeover to the replacementsource (lowest Isc - see fig. 17 ).

In TN and IT, for safety of persons and property,it is thus vital to check that the protectionconditions are always met (operating time andthreshold), especially for very long feeders. Ifthis is not so, then RCDs must be used.

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

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


4 Influences of MV on LV, according tothe earthing systems

LV networks, unless a replacementuninterruptible power supply (with galvanicinsulation) or a LV/LV transformer is used, areinfluenced by MV.

This influence takes the form of:c capacitive coupling: transmission ofovervoltage from MV windings to LV windings;c galvanic coupling, should disruptivebreakdown occur between the MV and LVwindings;c common impedance, if the various earthconnections are connected and a MV currentflows off to earth.

This results in LV disturbances, oftenovervoltages, whose generating phenomenaare MV incidents:c lightning;c operating overvoltages;c MV-frame disruptive breakdown inside thetransformer;c MV-LV disruptive breakdown inside thetransformer.

Their most common consequence isdestruction of LV insulators with the resultingrisks of Electric Shock of persons anddestruction of equipment.

4.1 Lightning

If the MV network is an overhead one, thedistributor installs ZnO lightning arresters to limitthe effects of a direct or an indirect lightningstroke.

Placed on the last pylon before the MV/LVsubstation, these lightning arresters limitovervoltage and cause lightning current to flowoff to earth (see "Cahiers Techniques" no. 151and 168).

A lightning wave, however, is transmitted bycapacitive effect between the transformerwindings, to the LV live conductors and canreach 10 kV peak. Although it is progressivelyweakened by the stray capacities of the network

4.2 Operating overvoltages

with respect to earth, it is advisable to installsurge limiters (lightning arresters) at the origin ofthe LV network, whatever earthing system isused (see fig. 18 ).

Likewise, to prevent coupling by commonimpedance, it is wise never to connect thefollowing to the earth connection of theLV neutral:c MV lightning arresters;

c lightning rods placed on the roof of buildings.In point of fact, the lightning current would causea rise in potential of the PE and/or the LV neutral(risk of disruptive breakdown by return) and lossof earth connection effectiveness by vitrification.

3

33

N

i 125 kV i 10 kV

Shortconnections

Fig. 18 : limitation and transmission of lighting overvoltages (whether or not the neutral is earthed, there arecommon mode overvoltages on phases).

Some MV switchgear (e.g. vacuum circuit-breakers) cause considerable overvoltages whenoperated (see "Cahier Technique" no. 143).

Unlike lightning which is a common modedisturbance (between network and earth), theseovervoltages are, in LV, differential mode


disturbances (between live conductors) and aretransmitted to the LV network by capacitive andmagnetic coupling.

4.3 MV-frame disruptive breakdown of the transformer

On MV-frame disruptive breakdown inside thetransformer and when the transformer frame andLV installation neutral are connected to the sameearth connection, a MV "zero sequence" currrent(whose strength depends on the MV earthingsystem) can raise the frame of the transformerand neutral of the LV installation to a dangerouspotential.

In point of fact, the value of the transformer earthconnection directly conditions the contact voltage

Just like all differential mode phenomena,operating overvoltages do not interfere, or onlyvery slightly, with any of the earthing systems.

in the substation Ut i Rp IhMV and the dielectricwithstand voltage of the LV equipment in thesubstation Utp = Rp IhMV (if the LV neutral earthis separate from the substation one). The earthconnections of the substation and of theLV neutral are not generally connected. Ifhowever they are, a limit is given to the commonearth connection value to prevent a rise inpotential of the LV network compared with thedeep earth. Figure 19 gives the common earth

Z: direct earthing in TN and TT impedance-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 network supplying thesubstation.Utp: power frequency withstand voltage of the low voltage equipment of the substation.(1) the third letter of the earthing systems means:ccccc all the frames are linked R;ccccc the substation frame is connected to the Neutral frame: N;ccccc the earth connections are Separated S.

Fig. 19 : maximum resistance of the earth connection of the substation frames according to network earthingsystem.

Diagrams (1) Maximum resistance of the earthconnection of substation frames Rp (Ω)

No value stipulated but the following valuesprevent excessive potential rise of the assembly

IhMV (A) RPAB (Ω)

300 3 to 20

1,000 1 to 10

IhMV (A) RPB (Ω)

300 3

1,000 1

Utp (kV) 2 4 10

IhMV (A) RP (Ω)

300 4 8 20

1,000 1 3 10

TNR or ITR RPAB

Z

TTN or ITN RPB RA

Z

TTS or ITSRBRP RA

Z


connection values for the IhMV values of Frenchpublic networks. Readers interested in this canconsult standard IEC 364-4-442 which explainsthe risks according to LV earthing systems.

Still for public networks (except for Australia andthe USA where the fault current can be veryhigh), values encountered range from 10 A inIreland (an impedance compensates thecapacitive current) to 1,000 A in France(underground networks) and in Great Britain.

4.4 MV-LV disruptive breakdown inside the transformer

MV industrial networks are normally run inimpedance-earthed IT and have a zerosequence current IhMV of a few dozens of amps(see "Cahier Technique" no. 62).

The maximum value authorised for the earthconnection depends on the equipotentialityconditions of the frames of the LV network, i.e.on its earthing system.

To prevent potential with respect to the earth ofthe LV network from rising to thephase-to-neutral voltage of the MV network onMV-LV disruptive breakdown inside thetransformer, the LV network must be earthed.

The consequences of this fault are:

c in TNThe entire LV network, including the PE, issubjected to voltage IhMV RPAB or RAB.If this overvoltage exceeds the dielectricwithstand of the LV network (in practice of theorder of 1,500 V), LV disruptive breakdowns arepossible if the equipotentiality of all the frames,electrical or not, of the building is not complete;

c in TTWhereas the load frames are at the potential ofthe deep earth, the entire LV network issubjected to IhMV RPB or RB: there is a risk ofdisruptive breakdown "by return" of loadsif the voltage developed in RPB or RB exceedstheir dielectric withstand;

c in ITOperation of a discharger/short-circuiter (knownas a surge limiter in France), which short-circuitsitself as soon as its arcing voltage is reached,then brings the problem to the level of the

TN network one (or TT if there are severalapplication earth connections).

In all cases, MV/LV disruptive breakdowns giverise to constraints which can be severe, both forthe LV installation and loads, if the value of theLV neutral earth connection is not controlled.Interested readers can consult IEC 364 whichexplains risks according to the earthing systems.

The example of overhead public distribution inFrance provides a solution to a situation whererisks of lightning, operating overvoltage andtransformer frame-MV and MV-LV disruptivebreakdown are present (see fig. 20 ). It showsthat equipotentiality of the entire distribution (allMV frames, neutrals and application framesconnected) is not vital: each risk is dealt withseparately.

This section has described the influence of theMV network. Its conclusions are:c the value of using lightning arresters at theorigin of the LV installation, whatever theearthing system type, if the MV and particularlythe LV supply is overhead;c connection of the earth connection of thesubstation with the earth connection of theLV neutral or with those of the applicationframes, imposes variable constraints onthe LV network according to the MV earthingsystem (value of Ih).

3Ih ≤ 300 A

Metering

Earth trip

RA < 100 ΩRB < 4 ΩRp < 50 Ω

LightningarresterRCD

PE

≥ 30 m

≥ 8 m ≥ 8 m

N

Fig. 20 : rural overhead public distribution in France.


5 Switchgear linked to the choice of earthing system

Choice of earthing system affects not onlydependability (in the largest sense) but alsoinstallation, in particular with respect to theswitchgear to be implemented.

5.1 TN system

In this system the SCPDs (circuit-breaker orfuses) generally provide protection againstinsulation faults, with automatic trippingaccording to a specified maximum breaking time(depending on phase-to-neutral voltage Uo:see fig. 9).

c with circuit-breakerCircuit-breaker tripping occurs at a leveldetermined by the type of the tripping release(see fig. 21 ). As soon as the fault currentexceeds the threshold of the short-circuitprotection trip release (generally"instantaneous"), opening occurs in a time farshorter than specified maximum breaking time,for example 5 s for distribution circuits and 0.4 sfor terminal circuits.

When impedance of the source and cables ishigh, either low threshold trip releases must beused or RCDs associated with the SCPDs.These RCDs may be separate residual currentdevices or be combined with circuit-breakers(residual current circuit-breakers) of lowsensitivity. Their threshold must be:

I∆n < 0.8 Uo

Rph RPE+

Use of a RCD has the advantage of making loopimpedance checking unnecessary, a fact whichis of particular value when the installation ismodified or extended.

This solution is clearly not applicable with aTN-C type earthing system (the protectiveconductor being the same as the neutral one).

c with fusesThe fuses used for short-circuit protection are ofthe gG type and their time/current characteristics(see fig. 22 ) are defined by standards(household fuses: IEC 60241, industrial fuses:IEC 60269). Checking suitability with themaximum specified breaking time therefore callsfor individual validation of the ratings providedfor each protection device. If they are notsuitable, either fault loop impedance must bereduced (increased cross-sections) or the fusemust be replaced by a low threshold or aresidual current circuit-breaker.

Trip release type Operating threshold

Household (EN 60898) B 3 In i Ia i 5 InC 5 In i Ia i 10 InD 10 In i Ia i 20 In

Industrial (IEC 60947-2) G (low threshold) 2 In i Ia i 5 InD 5 In i Ia i 10 InMA (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.

In gG (A) Imin. 10 s Imax. 5 s Imin. 0.1 s Imax. 0.1 s

63 160 320 450 820

80 215 425 610 110

100 290 580 820 1,450

Fig. 22 : example of fuse operating threshold limits (as in IEC 60269 paragraph 5-6-3).


5.2 TT system

With this system, the small value of the faultcurrents (see previous section) does not allowthe SCPDs to protect persons against indirectcontacts. RCDs (see fig. 23 & 24 ) need to beused, associated with circuit-breakers orswitches (see IEC 60364 - paragraph 413.1.4.2).

These devices must meet the followingstandards in particular:

c IEC 60755: general rules;

c IEC 61008: "household" residual currentswitches;

c IEC 61009: «household» residual currentSCPDs;

c IEC 60947-2: «industrial» residual currentcircuit-breakers.

Their implementation must meet the objectives for:

c protection of persons, i.e.:v threshold I∆n i UL/RA,v breaking time i 1s;

c continuity of service with thresholds and timedelays enabling current and time discrimination;

c fire protection with I∆n i 500 mA.

Fig. 24 : functional diagram of an RCD.Fig. 23 : Vigi module of a Compact NS.

5.3 IT system

Remember that in the event of a double fault,safety of persons is provided by the SCPDs.When the first insulation fault occurs, thecalculation proved there was no risk (contactvoltage lower than limit safety voltage).Automatic de-energising is therefore notcompulsory: this is the main advantage of thissystem.

To retain this advantage, standards recommend(IEC 60364 - paragraph 413.1.5.4) or stipulate(NF C 15-100) the use of an InsulationMonitoring Device (IMD) and locating of the firstfault. In point of fact, if a second fault occurs,automatic breaking is vital due to the ElectricShock risk: this is then the role of the SCPDsbacked up by the RCDs if required.

Locating the first fault for repairs (curativemaintenance) is considerably simplified by theuse of a Ground Fault Location Device (GFLD).

Predictive maintenance, based on the monitoring(recording) of variations in insulation impedanceof each circuit, is also possible.

LV networks, using the IT system, which taketheir origin at a MV/LV transformer, must beprotected against risks of insulation faultsbetween MV and LV by a «surge limiter».Finally, to fix the potential of the LV network withrespect to the earth (short network supplied by aMV/LV transformer), an impedance can beinstalled between the transformer neutral andthe earth. Its value in 50 Hz, of the order of1,500 W, is very high in DC and in very lowfrequency so as not to obstruct insulationmeasurement and fault locating.

c Operating principle of the IMDs

A fault on a circuit results in a drop in insulation,or more precisely in resistance of the networkcompared with earth.

In France, the IMDs and GFLDs have to complywith manufacturing standard UTE 63080.

The purpose of the IMDs is thus to monitor thevalue of this resistance. They normally work byinjecting an AC or DC current between the

Shaping

Threshold

Time delay

Output

Σ I ≠ 0


network and the earth and by measuring thevalue of this current (see fig. 25 ).

Injection of a DC current ensures continuousknowledge of network insulation resistance. Ifthis resistance drops below a pre-set threshold,then the IMD reports the fault.

Injection of low frequency AC current (F ≈ a fewhertz) monitors fault resistance but with adistorsion due to the presence of networkleakage capacitites. This minor drawbackcompared with injection frequency, is made upfor by an advantage in first fault locating (onesingle injection device).

LF current injection devices are now availablewhich can separately give the network’sinsulation resistance and reactance. Moreover,they enable locating of the first fault withoutcircuit opening and without the problems due tohighly capacitive feeders.

c Operating principle of the GFLDsThe most common solution is to inject anidentifiable current (with a frequency other thannetwork one). The generator can be the IMD.Then, by means of magnetic Current Sensors(toroid transformers and/or clamp-on probe)associated with an amplifier tuned to the injectedcurrent frequency, to trace its path up to the fault(see fig. 26 ).

Finally, another solution is also used, whichconsists in comparing, constantly and for each

feeder, the value of its resistance with a pre-defined or programmable threshold value.This solution, computerised, enables thefollowing actions, both locally and remotely:v reporting of the first fault (IMD),

N

ZImpedance(≈ 100 kΩ at 50 Hz ; low in LF)

Insulation measuringcurrent (Rd)

Threshold → time delay→ alarm

PE

Measuring

LF current generatora

Fig. 25 : functional diagram of an Insulation MonitoringDevice (IMD).

Fig. 26 : locating insulation faults by tracing the path of a low frequency current injected at the origin of theinstallation.

PE

PE

a LF generator


v then locating of this fault (GFLD) to put it right(curative maintenance) (see fig. 27 ),v and knowledge of insulation evolution in time,feeder by feeder, to take action on feeders withabnormal insulation drops (predictivemaintenance).

c Surge limiters: these are connected between alive conductor (neutral or phase) of theinstallation and the earth. Their arcing voltageUe must therefore be adapted to the assemblyplanned: 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 on MV/LVdisruptive breakdown in the distributiontransformer. In this case, the limiter must flow offto earth the "residual" current of the MV network,v limit lightning overvoltages.

This accounts for their characteristics, forexample for the 250 V model:- rate voltage: 250 V,- disruptive breakdown voltage at50 Hz: min 400 V, max 750 V,- disruptive breakdown voltage according to the1.2/50 µ wave: û < 1,570 V,- î lightning: 20 times 2,500 A (8/20 ms wave):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 far greaterthan the value of the "residual" current of the MVnetwork since a limiter which has been "arced"during a very high overvoltage may continue tobe short-circuited and must therefore be still ableto withstand a LV short-circuit current resultingfrom a first insulation fault in the protectedLV network.The limiters marketed under the Merlin Gerinbrand can withstand 40 kA/0.2 s.

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

The locating current flowing in the conductors is detected by Current Sensors (CS). Each load comprising adiscriminating amplifier (set to the frequency and phase of the locating current) calculates the resistance andcapacity of the circuit (with the voltage and phase whose reference it obtains via a bus) and indicates the presenceof the fault.

PE

PE

a

"locating voltage" bus

5.4 Neutral protection according to the earthing system

The neutral must be broken by a multi-poledevice:c in TT and TN, if neutral cross-section is lessthan phase cross-section;c in terminal distribution in view of the Neutral/Phase reversal risk.

The neutral must be protected and broken:

c in IT for intervention of the protection device onthe double fault, with one of the faults possiblyon the neutral;c in TT and TN-S if neutral cross-section is lessthan phase cross-section;


c for all earthing systems if the installationgenerates harmonic currents of rank 3 andmultiples (especially if neutral cross-section isreduced).

In TN-C the neutral, which is also the PE, cannotbe broken which is dangerous as a result of itspotential variations, due to load currents andinsulation fault currents.

To prevent risks, a local equipotentiality and anearth connection must be provided for eachzone/consumer.

Figure 28 shows which types of circuit-breakershould be used for which earthing system.Note that TT and TN can use the same devices(with an additional residual current modulein TT).

Circuits DiagramsTN-C TN-S TT IT

Single phase circuits

Single 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 neutral

With three-pole protection yes yes yes yes

Three-phase circuits with neutral

Without overcurrent detection on neutral no yes yes no

yes yes yes no

With overcurrent detection on neutral no yes yes yes

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>

Four-polecircuit-breakerwith threeprotected poles

2

3

N

1 I>

I>

I>

Three-polecircuit-breaker

2

3

N

1 I>

I>

I>

Four-polecircuit-breakerwith fourprotected poles

2

3

N

1 I>

I>

I>

I>

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


6 Choice of earthing system and conclusion

The three earthing systems internationally usedand standardised by IEC 60364 have as theircommon objective the quest for optimum safety.

As regards protection of persons, the 3 systemsare equivalent if all installation and operatingrules are complied with. In view of thecharacteristics specific to each system, no onesystem can be preferred over another.

6.1 Methods for choosing the earthing system

Rather, choice of earthing system must resultfrom a concertation between the network userand designer (engineering firm, contractor, etc.)on:

c installation characteristics;

c operating conditions and requirements.

c Firstly do not forget that the three earthingsystems can all be included in the sameelectrical installation: this guarantees the bestpossible answer to safety and availability needs;

c Then check that the choice is not specifiedor stipulated by standards or legislation(decrees, ministerial decisions);

c Then dialogue with the user to get to knowhis requirements and resources:v need for continuity of service,v whether or not there is a maintenance service,v fire hazard.

Generally:v continuity of service andmaintenance service: the IT will be chosen,v continuity of service and no maintenanceservice: no fully satisfactory solution: prefer theTT whose discrimination on tripping is easier toimplement and which minimises damage withrespect to the TN.

The installation of additionnal output is easilyachieved without the necessity of furthercalculations.

v continuity of service not essential andcompent maintenance service: prefer the TN-S(rapid repairs and extensions performedaccording to rules),v continuity of service not essentialand no maintenance service: prefer the TT,v fire hazard: IT if maintenance service and useof 0.5 A RCD or TT.

c Allow for the special features of network andloads:v very long network or, even more important,leakage current: prefer the TN-S,v use of replacement or standby powersupplies: prefer the TT,v loads sensitive to high fault currents (motors):prefer the TT or IT,v loads with low natural insulation (furnaces) orwith large HFfilter (large computers):prefer the TN-S,v supply of control and monitoring systems:perfer the IT (continuity of service) or the TT(enhanced equipotentiality of communicatingdevices).

6.2 Conclusion

As there is no ideal choice with a single earthingsystem, it is thus advisable, in many cases, toimplement several earthing systems in the sameinstallation.As a rule, a radial network installation, with aclear distinction between priority and non-prioritycircuits and using standby sources oruninterruptible power supplies, is preferable toan arborescent monolithic installation.

The purpose of this "Cahier Technique" was toperfect your knowledge of earthing systems;we hope it will enable you to optimise thedependability of your installations.

"Cahier Technique" no. 173 which provides aninsight into use of earthing systems worldwideand their evolution will usefully complete this firstdocument.


Bibliography

Standardsc IEC 60241: Fuses for domestic and similarpurposes.

c IEC 60269: Low voltage fuses.

c IEC 60364: Electrical installation of buildings.

c IEC 60479: Effects of currents flowing throughthe human body.

c IEC 60755: General rules for residual currentdevices

c IEC 60947-2: Low voltage switchgear2nd part: circuit-breakers.

c IEC 61008: Residual current operated circuit-breakers without integral overcurrent protectionfor household and similar uses (RCCB's)

c IEC 61009: Residual current operated circuit-breakers with integral overcurrent protection forhousehold and similar uses (RCBO's)

c NF C 15-100: Installations électriques à bassetension.

c French decree of the 14.11.88.

Schneider Electric's Cahiers Techniquesc Earthing of the neutral in a HV industrialnetwork,Cahier Technique no. 62,F. SAUTRIAU.

c Residual current devices,Cahier Technique no.114,R. CALVAS.

c Protections des personnes et alimentationsstatiques sans coupure,Cahier Technique no. 129,J.-N. FIORINA.

c Les perturbations électriques en BT,Cahier Technique no. 141,R. CALVAS.

c Introduction to dependability design,Cahier Technique no. 144,P. BONNEFOI.

c EMC: Electromagnetic compatibility,Cahier Technique no. 149,F. VAILLANT

c Overvoltages and insulation coordination inMV and HV,Cahier Technique no. 151,D. FULCHIRON

c Lightning and HV electrical installations,Cahier Technique no. 168,B. DE METZ NOBLAT

c Earthing systems worldwide and evolutions,Cahier Technique no. 173,B. LACROIX and R. CALVAS

Other publicationsc 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.

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03-0263714

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