178-The IT Earthing System

8/11/2019 178-The IT Earthing System

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Collection Technique

Cahier technique no. 178

The IT earthing system(unearthed neutral)in LV

F. JullienI. Hritier


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Cahiers Techniques is a collection of documents intended for engineersand technicians, people in the industry who are looking for more in-depthinformation in order to complement that given in product catalogues.

Furthermore, these Cahiers Techniques are often considered as helpfultools 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.

The latest publications can be downloaded from the Schneider Electricinternet web site.Code: http://www.schneider-electric.comSection: Experts place

Please contact your Schneider Electric representative if you want either aCahier Technique or the list of available titles.

The Cahiers Techniques collection is part of the Schneider ElectricsCollection technique.

ForewordThe author disclaims all responsibility subsequent to incorrect use ofinformation or diagrams reproduced in this document, and cannot be heldresponsible for any errors or oversights, or for the consequences of usinginformation and diagrams contained in this document.

Reproduction of all or part of a Cahier Technique is authorised with theprior consent of the Scientific and Technical Division. The statementExtracted from Schneider Electric Cahier Technique no. ..... (pleasespecify) is compulsory.


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Cahier Technique Schneider Electric no. 178 / p.2

Lexicon

BB: busbar.

C1for phase 1, C2for phase 2, and C3for

phase 3: earthing impedance capacitive

components for each phase.

CR: network overall capacity (leakage capacities

of cables and filters if any).

IC: capacitive current.

Id: fault current flowing in the earth connection

resistance RAof the application frame.

Ifu: fuse blowing current within a maximum time

stipulated by standards.

Im: short time delay (magnetic or electronic)

tripping current (threshold) of a circuit-breaker.

IN: capacitive current flowing through the

earthed neutral connection, in particular through

the impedance ZN, when present.

L: length of faulty circuits.

m: ratio of live conductor/protective conductor

cross-section (Sa/ Spe).

: resistivity of copper.

Ra: resistance of the live conductor (phase orneutral) of the circuit where the fault occurred.

RA: resistance of the earth connection of theapplication frames.

RB: resistance of the neutral earth connection.RCD: Residual Current Device.

Rd: fault resistance.

Rpe: resistance of the PE protective conductor.

R1for phase 1, R2for phase 2, and R3forphase 3: earthing impedance resistive

components of each phase.

Sa: live conductor cross-section.

SCPD: Short-Circuit Protection Device.

Spe: protective conductor cross-section.

UC: contact voltage between the frame of afaulty device and another frame or the earth.

U0: phase to neutral voltage.

UL: limit safety voltage (24 V) not to beexceeded between the frame of a device and

another frame or the earth.

Un: nominal voltage or phase-to-phase voltage

(U1, U2, U3), equal to eU0for a three-phase

electrical circuit.

Ur: network voltage.

ZN: additional impedance connected betweenthe neutral point of a network in the IT earthing

system and the earth.

ZR: overall impedance of a network with respectto the earth, made up of the capacitive

components C1, C2, C3and the resistive

components R1, R2, R3.


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The IT earthing system(unearthed neutral)in LV

Although all Earthing Systems offer users the same degree of safety, they

do not all have the same operating characteristics.

This is why, in certain countries, a specific earthing system is stipulated by

legislation or standards according to buildings. For example, in France the

IT system is compulsory in hospital operating theatres, and the TN-C is

forbidden in premises where there is a risk of explosion.

These stipulations apart, dependability objectives (safety, availability,

reliability, maintenability and proper operation of low current

communication systems) determine which earthing system should be

chosen for which installation.

The aim of this Cahier Technique is to describe the advantages and

areas of application of the IT earthing system.

After a brief introduction of the electrical hazard and the various earthingsystems, the first fault situations, followed by the double fault specific to theIT system, are studied, and the advantages and disadvantages of this

particular earthing system are developed. This Cahier Technique also

offers solutions for the surge limiter with the various types of possibleovervoltages.

Finally, a choice table is provided for all earthing systems, based on criteria

for safety, availability, electromagnetic compatibility and operators

professional requirements.

Contents

1 Introduction 1.1 Protection of persons against electrical shocks p. 4

1.2 The various standardised earthing systems p. 41.3 Choosing an earthing system p. 7

1.4 Type of insulation p. 7

1.5 Equivalent system for an unearthed or impedance-earthedneutral network p. 8

2 The 1stinsulation fault with the 2.1 Calculating fault currents and contact voltage p. 9

IT earthing system on the first fault

2.2 Permanent insulation monitors, history and principles p. 11

2.3 Tracking the 1stinsulation fault p. 13

3 The 2ndinsulation fault with the 3.1 Analysis of the double insulation fault p. 15

IT earthing system 3.2 Elimination of the double insulation fault p. 16

4 Special features of the IT earthing system 4.1 Overvoltages in the IT system p. 184.2 Surge limiters p. 20

4.3 Why use an impedance? p. 21

5 Advantages and disadvantages 5.1 Increased availability p. 22

of the IT earthing system in LV 5.2 Increased safety against risk of fire p. 22

5.3 Less downtime on control and monitoring circuits p. 23

5.4 Usage limits and precautions of the IT earthing system p. 23

6 Conclusion 6.1 Availability: an increasing need to be satisfied p. 26

6.2 The IT earthing system finds its true place p. 26

6.3 The added advantage of safety p. 26

6.4 In short p. 27

Bibliography p. 28


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PE

N

PEN

RB

Id

UdRd

SCPD

123N

PE

N

SCPD

PEN

RB

UdRd

Id

N

PE

RB

UdRd

Id

SCPD

Fig. 1 : insulation fault on a network operated in TN-C [a], TN-S [b] and TN-C-S [c].

Fig. 2: insulation fault on a network operated in TT.

N

RCD

RB RA

UdRd

Id

a) b)

c)

Protection is provided by the Residual CurrentDevices (RCD): the faulty part is disconnected

as soon as the threshold In, of the RCD placed

upstream, is overshot by the fault current, so that

In RBi UL.

The IT system

cIts principle:

vthe transformer neutral is not earthed, but istheoretically unearthed. In actual fact, it isnaturally earthed by the stray capacities of the

network cables and/or voluntarily by a high

impedance of around 1,500 (impedance-earthed neutral);

vthe electrical load frames are earthed.


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Fig. 3: single [a] and double [bandc] insulation fault on a network operated in IT.

Ud

N

RB

Permanentinsulationmonitor(PIM)

ZN: optional

impedance

Surgelimiter

321

PE

IC1 IC2 IC3

C1 C2 C3

IC

IC

IN

Id

Id

Id

ZN

a)

N

Id

Id

RB

Permanent

insulation

monitor

(PIM)

Surge

limiter

321N

PE

SCPD

Ud1Rd1

SCPD

Rd2Ud2

Id

Id

b)

N

Id

Id

Id

RB

Permanent

insulation

monitor

(PIM)

Surge

limiter

3

21N

PEPE

RCD

SCPD

RA

Ud1Rd1

SCPD

Rd2Ud2

Id

Id

c)

cIts operation:

vshould an insulation fault occur, a low currentdevelops as a result of the networks straycapacities (see fig. 3a ).The contact voltage developed in the frame

earth connection (no more than a few volts) isnot dangerous;

vif a second fault occurs on another phasebefore the first fault has been eliminated(see fig. 3band 3c ), the frames of the loads inquestion are brought to the potential developed

by the fault current in the protective conductor(PE) connecting them. The SCPDs (for theframes interconnected by the PE) or the RCDs(for the frames with separate earth connections)provide the necessary protection.

This deliberately brief presentation of the variousearthing systems clearly cannot cover all thespecific installation possibilities. Readersrequiring more details can consult CahiersTechniques no. 114, 172 and 173.


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1.3 Choosing an earthing system

Although all three earthing systems offer users thesame degree of safety against indirect contact,only the IT system guarantees risk-free continuity

of supply in the presence of an insulation fault.This undeniable advantage also has certaindrawbacks: for example the need to locate thisfirst fault and the possibility of overvoltagesoccurring that may affect operation of sensitiveloads.

However, choice of earthing system for aninstallation also depends on parameters otherthan safety of persons and continuity of supply,namely:

cthe environment (e.g. premises with a risk offire or sites frequently struck by lightning);

celectromagnetic compatibility (EMC) (presence

in the installation of harmonics and radiating

fields, and sensitivity of equipment to suchphenomena);

ctechnicity of installation designers and

operators;cmaintenance quality and cost;

cnetwork size;

cetc.

Although consideration of the above parametersguarantees choice of the earthing system mostsuited to the installation, it should be emphasisedthat the advantage offered by the IT system interms of availability (2ndfault most unlikely)generates installation and operating costs thatshould be compared with the downtime costsgenerated by other earthing systems (operatinglosses and repair costs caused by the first

insulation fault).

1.4 Type of insulation

Common mode impedance

All electrical networks have an impedance withrespect to earth known as the common modeimpedance, the origin of which is insulation ofnetwork cables and loads. This impedanceconsists of the leakage capacity and resistancebetween each live conductor and the earth.

In LV, the leakage resistance of a new cable is

around 10 Mper kilometer and per phase,whereas its capacity evenly distributed withrespect to earth is approximately 0.25 F, i.e.12.7 kat 50 Hz.It should also be noted that in MV and HV thisleakage capacity is even greater and MUST betaken into account when drawing up a protectionplan (see Cahier Technique no. 62).

Loads also have a natural leakage capacity,usually negligible.

Effect of distributed capacity in the IT system

In electrical installations, other capacities areadded to the network cable ones. This is the

case of certain electronic loads that aregenerators of HF harmonic currents, in particularwhen they use the chopper principle (e.g. pulse

with modulation converters). However,ElectroMagnetic Compatibility (EMC) standardsstate that these HF currents must be shunted toearth, resulting in the presence of filters and thuscapacitors between phases and frame.According to the number of loads, theircontribution to the networks leakage capacitycan be significant or even important.Measurements taken on a variety of electricalpower networks show that capacity variesconsiderably from network to network andcovers a range of a few F to a few dozen F.

Excessively high capacities may question theadvantage of the IT earthing system: if, on thefirst fault, the value of network impedance withrespect to earth means that contact voltageexceeds 50 V, safety of persons is notguaranteed. This is rare, however, as with a10earth connection, the networks earthleakage capacity must exceed 70F (23F perphase if the neutral is not distributed).

An IT network must therefore have a limited

capacity with respect to earth, and the presenceof loads equipped with HF filters must be takeninto account in the network design stage.


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Fig. 4 : equivalent system of a network with unearthed or impedance-earthed neutral.

N

ZN

RB

321N

PE

CN C1 C2 C3RN R1 R2 R3

RA

1.5 Equivalent system for a network with unearthed or impedance-earthed neutral

A few definitions and assumptions are givenbelow in order to define the equivalent systemfor this network (see fig. 4 ):

c the neutral point is unearthed or earthed by animpedance (ZN) of high value (normally 1 kto2 k) whose earth connection is equivalent to aresistance (RB);

c the load frames are interconnected either fullyor by group. For EMC reasons (seeCahier Technique no. 187), it is advisable to

interconnect all the application frames of thesame installation and to connect them to thesame earth connection (resistance RA);

c the earth connections (RAand RB) areinterconnected (in most cases), or separate.

NB:Two earth connections are considered to beseparate if they are more than 8 m apart;

c each live conductor has, with respect to earth,an impedance made up of a resistance and acapacity.


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2 The 1stinsulation fault with the IT earthing system

In normal operating conditions, safety of personsis guaranteed when contact voltage is less than50 V as per standard IEC 60364 (NF C 15-100).When this contact voltage is exceeded, these

standards require automatic opening of thecircuit. The following section shows how use ofthe IT earthing system for network operationprevents tripping on the first insulation fault.

2.1 Calculating fault currents and contact voltage on the first fault

General case (resistive fault)

Should a fault with a resistive value Rd occurbetween phase 3 and the earth, a fault current Idflows in the neutral impedance and in the

capacities C1, C2 and C3 (see fig. 3a).Assuming that the phase-to-earth capacities arebalanced (C1 = C2 = C3 = C), the fault currenthas the following value:

IdN dZ R

= U 1+ 3j C Z

R 3j C Z0

N

d N

+ +

The capacitive current is written as:

IcN dZ R

= U 3j C Z

R 3j C Z0

N

d N

+ +

and the current in the impedance ZN:

INN dZ R

= U

R 3j C Z0

d N+ +

The contact voltage UC

(contact voltage betweenthe frame of a faulty device and another frame orthe earth) is calculated from the fault current Idflowing in the earth connection resistance RAofthe application frames if they are notinterconnected, else RB(only network earthconnection):UC= RA Id.

Case of the full fault

This paragraph calculates the configurationgenerating the highest contact voltage (UC):thus for a fault occuring on a frame with earthconnection separate from that of ZN.By application of the above formulae, whereRd= 0, we obtain:

IdN

U

Z=

3j C0

+

U R U

Zc A

N

=3j C0

+

The capacitive current is equal to:IC= +3j CU0and the current in impedance ZN:

INN

U

Z= 0

In the various examples below, studied forZN = (unearthed neutral) and ZN = 1 k

Rd (k) 0 0.5 1 10

Case 1 ZN= UC(V) 0.72 0.71 0.69 0.22CR = 1F Id(A) 0.07 0.07 0.07 0.02

ZN = 1 k UC(V) 2.41 1.6 1.19 0.21

Id(A) 0.24 0.16 0.12 0.02Case 2 ZN= UC(V) 3.61 2.84 1.94 0.23CR = 5F Id(A) 0.36 0.28 0.19 0.02

ZN = 1 k UC(V) 4.28 2.53 1.68 0.22

Id(A) 0.43 0.25 0.17 0.02

Case 3 ZN= UC(V) 21.7 4.5 2.29 0.23CR = 30F Id(A) 2.17 0.45 0.23 0.02

ZN = 1 k UC(V) 21.8 4.41 2.26 0.23

Id(A) 2.18 0.44 0.23 0.02

Fig. 5: comparison of fault currents and contact

voltages on a first fault.

(impedance-earthed neutral), the calculationsare made for a network in the IT system,400 VAC (U0= 230 V), where:RA, earth connection resistance = 10 Rd , insulation fault value = 0 to 10 k.

cCase 1:

Low capacity network (e.g. limited to anoperating theatre)C1 = C2 = C3 = C = 0.3F per phase.

cCase 2:

Power network, whereC1 = C2 = C3 = C = 1.6F per phase.

cCase 3:

Very long power network, whereC1 = C2 = C3 = C = 10F per phase, i.e.roughly 40 km of cables!The results of all these calculations, grouped inthe table in figure 5 , confirm the low faultvoltage ( 20 V in the most unfavourable cases),ensuring continuity of operation, without risk forpersons, of a network designed using the ITsystem. They prove that addition of animpedance between the neutral and the earthhas very little effect on contact voltage.


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Fig. 6: contact voltage on a first insulation fault is always less than safety voltage.

Fig. 7: the networks distributed capacities create a connection between neutral and earth.

1

23

1

3 2

C C C 3C

Artificial neutral

100

50

10

500

(recommended threshold)

1

1 10 100 1,000 1040.1 Rd()

Uc(V) where

Zn= 1,000

CR = 5 F

CR = 30 F

CR = 70 F

CR = 1 F

The curves in figure 6 representing these resultsshow the considerable effect of network capacityon the value of UC.

In point of fact, regardless of the distributedcapacity of the sound network or network on

which a first fault has occurred, users can becertain that this voltage will always be less thanthe conventional safety voltage and thus withoutrisk for persons. Also, the currents of a first fullfault are low and thus have minimum destructiveor disturbing (EMC) effect.

Effect of distributed capacities, vector chartand neutral potential

cEffect of distributed capacities on a soundnetwork

The capacities of all 3 phases create an artificial

neutral point. In the absence of an insulation fault,if network capacities are balanced, this neutralpoint is then at earth potential (see fig. 7 ).In the absence of a fault, the phase-to-earthpotential is thus equal to phase to neutralvoltage for each phase.


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Fig. 11: principle of PIM with current injection.

N

RB

321N

PE

IPIM

IPIM

V

mA

V

BF~

IBF

IBF

= UBF

UBF

RNetwork CNetwork

ZNetwork

IR-BF

IR-BF

IC-BFIC-BF

Fig. 12: the low frequency AC current injection technique has been improved by means of synchronous

demodulation, which enables the insulation drop (resistive leakage) to be distinguished from capacitive leakage.

The principle consists of measuring andcomparing voltages between the (+) polarity andthe earth, on the one hand, and the (-) polarityand the earth on the other. This principle makesauxiliary power sources unnecessary, since thenetwork supplies the PIM directly viameasurement sensors (resistances).

This technique applies to two-phase AC and DCnetworks and does not allow live fault tracking.

For AC networks

The most commonly used PIM are those withinsulation measurement by DC current injection.

Permanent measurement of insulationresistance required use of active systems inplace of the previously used passive systems.

This resistance can be measured accurately inDC (see fig. 11), which is why the first PIMs,placed between the network and the earth,

injected a low DC current which flowed throughthe fault. This simple, reliable technique is stillextensively used today, but does not allow livefault tracking.

Note that when these PIMs are used on mixednetworks (containing rectifiers without galvanicinsulation), they may be disturbed or even

blinded if a fault is present on the DC part ofthe network.These were followed by PIMs with AC currentinjection at low frequency (< 10 Hz), operatingon the same principle. Although these PIMsallow live fault tracking, they can be misled bycable capacities that are seen as insulationfaults and disturbed by frequency converters(variable speed controllers).

For all AC and DC networks

Finally, nowadays, given that networks arefrequently of the mixed AC/DC kind as well asvariable frequency, the new PIMs are able tomonitor insulation on all types of networks.

cSome use squared wave pulses at very lowfrequency ( 1Hz). They allow PIM not to bedisturbed by earth leakage capacities, as theyare then immediately loaded then unloaded by

the next strobe pulse of opposite sign. They areuniversal in use and easily adapted to modernnetworks, in particular to those supplying powerelectronic devices which often deform the ACpulse. However, their response time, dependingon the networks earth leakage capacity, may beas much as a few minutes and does not allowthe detection of intermittent faults.

cIn order to compensate the usage restrictionsof these PIMs for very long networks andnetworks with a large number of capacitivefilters, the low frequency AC current injectiontechnique has been improved by means ofsynchronous demodulation (see fig. 12): this

type of PIM applies a low frequency AC voltagebetween the network and the earth, measuresthe current flowing back via network insulationimpedance and calculates the voltage-currentshift.

It is then possible to determine the resistive andcapacitive components of this current and thusrelate the threshold to the resistive componentonly. This upgrade, the result of digitaltechnology, combines the advantages of DCcurrent and low frequency AC current injectionwithout their disadvantages.


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PIM standards

cThe manufacturing standards

Standard IEC 61557-8 is in existence sinceFebruary 1997. It defines the specialspecifications governing insulation monitors

designed for permanent monitoring, irrespectiveof the measurement principle, of insulationresistance with respect to earth of unearthed ACand DC IT system networks, and of AC ITsystem networks containing rectifiers suppliedwithout galvanic separation (transformer withseparate windings).It places particular emphasis on three points.

v Properly inform specifiers and contractors. Themanufacturer must provide the characteristics ofthe devices he produces and in particular thosethat are dependent on network capacity(response time and threshold values).

v Ensure that these devices are properly

integrated in their electrical environment. Thisrequires compliance with the specifications ofstandards IEC 61326-1 and 61326-10concerning ElectroMagnetic Compatibility(EMC).

v Guarantee operating safety for users.The main stipulations are: device operatingtesting must be possible without inserting anadditional impedance between the monitorednetwork and the earth, settings must beprotected to prevent modification by error or byunauthorised users, and impossibility of devicedisconnection (the need to use a tool fordisassembly).

cThe operating standards

As concerns PIM setting, standard IEC 60364provides an initial answer: A PIM designedaccording to is set at a value less than theminimum value of the insulation resistance

defined for the installation in question, i.e.greater than or equal to 0.5 Mfor a circuit witha nominal voltage greater than or equal to 500 V.Guide NF C 15-100 states: set at a valueroughly less than 20% of the resistance of theinstallation as a whole

However, a clear distinction must be madebetween the insulation resistance of theinstallation, which only takes electricaldistribution into account, and the insulation levelwhich is set for overall network monitoring,including the various machines and switchgearconnected to it.

In the previous chapter we saw that for faults

greater than 500, contact voltage does notexceed 5 V with an earth connection of 10 (see fig. 5). In practice, for a normal industrialinstallation, it is thus reasonable, without takingrisks, to set the lower alarm threshold at a valueof between 500and 1,000, ensuringeffective fault tracking (and thus location of thereported insulation fault). To organise preventivetracking, it is useful to have a first level thresholdaround 10 kfor example. This threshold mustbe adapted according to installationcharacteristics and operating requirements. Notethat short networks allow a higher preventionthreshold.

2.3 Tracking the 1stinsulation fault

When tracking a fault, although certain operatorsmerely identify the faulty feeder, accuratedetermination of the location of this fault isrecommended (e.g. damaged cable or insulationfault in a device) in order to put it right as quicklyas possible.

Tracking by successive de-energisation offeeders

This means of fault tracking is quoted formemory only. It consists of opening the feedersone by one, beginning with the main feeders.When the faulty feeder is opened, the currentinjected by the PIM decreases markedly anddrops below the detection threshold. The audiblealarm normally controlled by the PIM then stops,enabling remote identification of the faultyfeeder.This procedure, which requires interruption ofoperation on each feeder, is contrary to theoperating philosophy of the IT earthing system,which stipulates continuity of supply. Althoughfrequently used in the past, it is graduallydisappearing with the development of the new

fault tracking systems which allow live tracking(without power breaking).

Live tracking

cDetecting the fault current

As seen above (see fig. 3a), a current Idflowsthrough the first insulation fault at the samefrequency as that of the network (50 Hz or60 Hz), returning to the source via the capacities

of the other sound phases and via the neutralimpedance if any.An initial live tracking method (withoutinterrupting distribution) consisted of using aclamp-on probe to measure the earth leakagecurrent on each feeder. The faulty feeder wasthe one on which the highest value wasmeasured.

This method has two drawbacks, namely:

vIt is not reliable for networks with a largenumber of feeders some of which are highlycapacitive (how can the earth current of a shortfaulty feeder be distinguished from that of a longcapacitive feeder?).


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Fig. 13: tracking can take place with fixed or portable devices sensitive to the injected pulse.

N

RB

LF generator

(G)

PIM ( )

PE

PE

G

Fixed load with manual

or automatic scanning

Manual load

////12 3N

vIt is not applicable on networks with fewcapacitive leakages (the fault current is virtuallyundetectable).

In order to improve detection of the fault currentpath (at power frequency) using a clamp-on

probe, two tricks were used.The first consisted of increasing this fault currentby temporarily placing a low impedance inparallel on the PIM.The second consisted of distinguishing thecapacitive leakage currents from the fault currentby the periodic use of the above-quotedimpedance by means of a beating relay(approx. 2 Hz).

cDetecting an injected current

This method uses a low frequency sinusoidalpulse (i10 Hz) injected by a generator or a PIM.Choice of low frequency for fault trackingensures no disturbance by network leakage

capacities, but this frequency cannot be lessthan 2.5 Hz, as detection with a magnetic sensorbecomes difficult. This method uses devicessensitive to the injected pulse only, that caneither be fixed with detection toroids placed onall feeders, or portable with a clamp-on probetuned to signal frequency in order to locate theexact position of the fault (see fig. 13 ).

When the devices (generator, sensors and load)are fixed, live fault tracking can be automatic ondetection of a fault, with transmission of an orderby the PIM.

cMeasuring insulation of each feeder

Operators, with their ever-increasing need forcontinuity of supply, are no longer prepared evento wait for the first fault, but want to be able toprogramme maintenance work and thusanticipate the next feeder likely to be affected byan insulation fault.It is thus necessary to monitor the changes ininsulation of each feeder and to carefully identifythe resistive and capacitive insulation components.The synchronous demodulation principle canalso be used by measuring, first, the injectioncurrent flowing in the feeders (by the toroidsensors) and, second, the injection voltage.Development of this tracking method is

encouraged by application of digital techniquesto the management of electrical power distribution(see Cahier Technique no. 186): the user cannow remotely and continually monitor insulationchanges of the various feeders. Use of digitalbuses enables data to be centralised on asupervisor, displayed and logged, thus allowingintelligent, predictive maintenance.


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3 The 2ndinsulation fault with the IT earthing system

As we have already seen in the previouschapter, the advantage of using the IT system innetwork operation lies in the possibility ofcontinuity of supply even though an insulationfault has occurred on a circuit.This message has been received loud and clearby standard drafters who, in order to maintain ahigh level of availability, stipulate in installationstandards indication and tracking of this first faultso as not to fear a second fault. Protectiondevices are also provided for this second fault inorder to guarantee the same level of safety ofpersons as for the TN and TT earthing systems.

The two sections below study the fault currentsand contact voltage which depend on how theframes are earthed. There are two possibilities,namely:

cThe load frames are all interconnected by aPE protective conductor: this is the generalcase.

cThe frames are not interconnected and areconnected to separate earth connections(configuration to be avoided due to EMC:see Cahier Technique no. 187).

3.1 Analysis of the double insulation fault

In this section, fault currents and contact voltageare calculated by considering two full insulationfaults on two different live conductors (on onephase and the neutral if the neutral is distributed,or on two different phase conductors if theneutral is not distributed) of two circuits ofidentical cross-section and length.

This assumption, which results in a minimumfault current, is the one normally chosen to

calculate the maximum lengths protected by theshort-circuit protection devices.

Contact voltage and double fault currentwhen the frames are interconnected

When a fault current occurs between two faultyframes, a current flows in the phase conductorsand the PE protective conductor ensuringinterconnection of frames (see fig. 3b).This current is only limited by the impedance ofthe fault loop equal to the sum of theimpedances of the live conductors concernedand the circuit of the equipotential links (PE).There are a number of methods for calculating

fault currents for an electrical installation (seeCahier Technique no. 158).

In this case, the conventional method has beenchosen, as it enables calculation of fault currentand contact voltage values without making toomany assumptions on installation characteristics.

It will thus be used from now on in this CahierTechnique to give an idea of the value of thecurrents and voltages involved on a double faultin the IT system.It is based on the simplified assumption thatconsiders that, during the duration of the fault,the voltage at the origin of the feeder consideredis equal to 80% of installation nominal voltage.

This assumes that the impedance of the feederin question accounts for 80% of total impedanceof the faulty loop, and that upstream impedanceaccounts for 20%.

For the following calculations:

U = phase to neutral voltage, (= U0if one of thetwo faults is on the distributed neutral),

or

U = phase-to-phase voltage, (= eU0if the

neutral is not distributed).

Ra= L

Sa= resistance of the live conductor

(phase or neutral) of the circuit on which the faultoccurred.

Rpe= L

Spe= resistance of the circuit protective

conductor.Sa= cross-section of the live conductor.

Spe= cross-section of the protective conductor.

L = length of the faulty circuits.

m=S

S

a

pe

= ratio of live conductor cross-section

over protective conductor cross-section(normally i 1).

c If we consider that the live and PE conductorsof the two faulty feeders have the same cross-section and length and if we ignore their reactance:

vif one of the faults is on the neutral

Id0

a

= 0,8U

2 R +( )Rpe, i.e. Id 0

a= 0 , 8 U S

2 1 +( )m L,

vif the double fault concerns two phase

conductors Id 0a= 0,8 U

S

2 1e

+( )m L.


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cThe corresponding contact voltage isUC = Rpe Id, i.e.:

vif one of the faults is on the neutral

U = 0 , 8 U m

2 1c 0

+( )m,or

vif the double fault concerns two phase

conductors U = 0,8 U m

2 1c 0e

+( )m.

NB: this method is not applicable for installationssupplied by generator set, as, due to highgenerator impedance compared with theimpedance of the supplied network, voltage atthe origin of the network in question is low whena fault occurs (> 400 0.02 0.08

Fig. 16: maximum breaking time specified for the IT

earthing system by installation standards (* for single-phase networks).

3.2 Elimination of the double insulation fault

Case of interconnected application frames

In view of the importance of the fault current,comparable with a short-circuit current, automaticshort-circuit protection devices (SCPD) can beused for tripping if cable lengths are compatiblewith their operating thresholds. Otherwiseresidual current devices (RCD) are used.

Elimination of the double fault must also satisfyother requirements which apply regardless of thetype of SCPD installed (fuse or circuit-breaker):

cThe contact voltages calculated in the previouschapter, for all SCPD types, leave little time forfault elimination. In order to simplify the networkdesigners task, standard IEC 60364 specifiesmaximum breaking times as a function ofoperating voltage (see fig. 16).

cMulti-pole breaking, including the neutralconductor when distributed.The reasons for this are:

vbreaking only of the faulty phase conductor ofa feeder means that three-phase machines aresupplied by the two other phases,

vbreaking of the neutral exposes to phase-to-phase voltage, single-phase loads normally

supplied by phase to neutral voltage.

cProtection of the neutral conductor when it isdistributed.

Figure 3b shows that when a double fault occurs,the two SCPDs detect the fault current but eachone on a single phase or on the neutral.

This situation calls for particular monitoring ofSCPD characteristics: this is because if thecables of the two feeders have similar cross-sections, the two SCPDs play an equal role inbreaking, but if the cross-sections are different,

there is a risk of only one SCPD, the one with

cDigital example

The results presented in the table in figure 14confirm that a double insulation fault is a risk forsafety of persons since contact voltage is greaterthan limit safety voltage UL. The automatic

protection devices must then de-energise theinstallation.

Contact voltage and double fault currentwhen the frames are not interconnected

If the two faults occur on two loads connected totwo separate earth connections (see fig. 3c), thefault current Idis then closed by the earth and islimited by the earth connection resistances RAand RB.

A simple calculation shows that this secondinsulation fault is just as dangerous (see fig. 15 ),and must therefore be automatically eliminated,and that the threshold of the short-circuit

protection devices cannot be reached.


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the lowest rating, performing breaking. It is thusnecessary to verify that its breaking capacity onone phase, thus under e U0, is greater than Id.For this reason, circuit-breaker manufacturersspecify the single-phase breaking capacities oftheir devices according to each nominal voltage,and standard IEC 60947-2 specifies a testsequence for circuit-breakers designed forprotection of IT networks. Devices failing tosatisfy the requirements of these tests must bemarked: IT

Protection must also be confirmed for the neutralconductor when its cross-section is less thanthat of the phase conductors. Note that four-polecircuit-breakers (the fourth pole has a half rating)can be used to protect cables with neutral cross-section half of phase cross-section.

It should be stressed that four-pole SCPDs arebecoming increasingly necessary, irrespective of

the installation earthing system used (TN, TT orIT), due to the proliferation of harmonics innetworks, and thus that the neutral can beoverloaded by harmonic currents of rank 3 andmultiples.

cFuse protection

The fuse blowing zone is located between twoenvelope curves.Using the expression of current Id, defined in theprevious chapter, and the condition Ifu< Id, it ispossible to determine the maximum length of theprotected circuit.

vIf the neutral conductor is distributed:

L = 0,8U1+ m

max0S

fu

12 ( )I

.

vIf the neutral conductor is not distributed:

L =0,8 U

1+ mmax

0e Sph

fu2 ( )I.

Ifucorresponds to the fuse blowing current withina maximum time stipulated by the standards.It should be checked that this time is compatiblewith protection of persons in event of a doublefault.Note that use of fuses in the IT earthing systemoften clashes with the need for multi-pole

breaking, including that of the neutral conductorwhen distributed.

cCircuit-breaker protection

Protection of persons is guaranteed when thefault current is greater than the circuit-breakersshort time delay protection setting.Just as with fuses, the maximum length of theprotected circuit can be determined according tothe expression of current Id, defined in theprevious chapter and the condition Im< Id.The maximum length of the circuit protected by acircuit-breaker is:

vwith the distributed neutral conductor:

L = 0,8 U S

2 1+ mmax 0

a

m ( )I.

vwith the non-distributed neutral conductor:

L = 0,8 U S

2 1+ mmax 0

a

m

e ( )I .

Note that whether protection is provided by fuse orcircuit-breaker, the fact of distributing the neutralin IT divides byethe maximum length protected.

cImprovement of tripping conditions.

When tripping conditions are not satisfied(lengths greater than maximum lengthsprotected), the following measures can be taken:

vreduce the value of Imof the circuit-breakers:however current discrimination between circuit-breakers may be reduced as a result;

vincrease PE conductor cross-section. Theimpedance of the return circuit of the double fault

current is thus reduced and enables an increasein maximum length for protection of persons.However, although contact voltage will bereduced, the electrodynamic stresses on thecables will increase.

vIncrease live conductor cross-section. This isthe most expensive solution and also results inan increase in three-phase short-circuit currents.

vFinally, there is a simple solution that requiresno calculation: use of low sensitivity RCDs onvery long feeders. This solution is also possiblein IT, as the PE conductor is separate from theneutral conductor which is not the case in TN-C.

Case of application frames with separateearth connections

When an installation supplies a number ofseparate buildings at a distance from oneanother, their application frames are oftenconnected to separate earth connections. Theimpedance of the path of fault current Idis thenincreased by the resistance of the two earthconnections in question, and the conditionnecessary for protection of persons (respect ofmaximum breaking times) can no longer beguaranteed by the short-circuit protection devices.The simplest study and installation solution is touse RCDs. Their settings follow the same rulesas in TT.

To derive maximum benefit from the continuity ofsupply offered by the IT system, the RCDs mustbe prevented from tripping on the first fault bynot setting their threshold IDn at too low a level,particularly for circuits with a high leakagecapacity, while at the same time respecting the

inequation: In 5

U0+ 1,200 V (i.e. 1600 V in IT)* i5

(*) For an IT network, the voltage U0must be replaced

by the voltage e U0.

Fig. 17: acceptable AC voltage constraints on LV

installation equipment in the IT system for a 230/400 V

network.


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N

MVHV MV

RT(Rp)

LV

IhMT

These overvoltages are rare and their suddenappearance means that the surge limiter, whosecertain arcing voltage is set at least at 2.5 timestype voltage (NF C 63-150), i.e. for example750 V for a limiter placed on the neutral of a230/400 V network, immediately earths the LVnetwork, preventing it from rising to MV potential.

cMV/frame internal disruptive breakdown alsoknown as return disruptive breakdownWhen the transformer frame and the LV networkare connected to the same earth connection(see fig. 18 ) there is a risk of LV equipmentdisruptive breakdown if the voltage RpIhMTexceeds equipment dielectric withstand, with Rp(earth connection resistance) and IhMT(zerosequence current due to MV disruptivebreakdown).One solution is to connect the LV installationframes to an earth connection that is electricallyseparate from that of the substation frames.

However, in practice, this separation is difficultdue to frame meshing in MV/LV substations.

Consequently standard IEC 60364-4-442 statesthat the LV installation frames can be connectedto the earth connection of the transformersubstation frames if the voltage RpIhMTiseliminated within the stipulated times.

Overvoltages due to lightning striking theupstream MV network

When lightning strikes the MV network, a waveis transmitted to the live conductors on the LVside as a result of capacitive coupling between

the transformer windings.If the installation is in IT, the surge limiterabsorbs the overvoltage occurring on the liveconductor to which it is connected (neutral orphase) and is short-circuited if this overvoltage isvery high: the network can then be compared toa network in TN-S. Experience andmeasurements have resulted in the followingobservations:

cOvervoltages of around 2 kV occur at the endof short cables (10 m) irrespective of load andearthing system.

cHigher overvoltages occur at the end of cableswith open end or which supply loads likely togenerate resonance. Even with a resistive load,overvoltages exist (see fig. 19 ), caused by wavepropagation and reflection phenomena and bycapacitive coupling between conductors.

In view of the waveform of these overvoltages,the surge limiter is effective on the conductor towhich it is connected. Consequently, regardlessof the earthing system, we strongly recommendthat surge arresters be installed at the origin ofthe LV network, between all live conductors and

Fig. 18: when the substation frames (MV) and the PE

earth connection (LV) are connected to the same earth

connection, the LV load frames are brought to the

potential IhMTRp.

(kV) Ph/Ph Ph/PE Ph/N N/PE PE/deep

earthSystem :

cIT 0.38 4.35 0.20 4.30 1.62

cTN-S 0.36 4.82 0.20 4.72 1.62

Fig. 19: overvoltages, caused by a lightning shock

wave, measured at the end of a 50m cable supplying a

resistive load.

the earth, if there is a risk of the upstream MVnetwork being directly struck by lightning (caseof overhead lines) and especially if the LVnetwork is also at risk. The surge limiter

continues to perform its function for MV/LVdisruptive breakdown.

Overvoltages due to lightning striking thebuilding housing the installation

These overvoltages are caused by lightningcurrent flowing through the buildings earthconnection, particularly when lightning strikes abuilding equipped with a lightning rod.The entire earth network then markedly rises inpotential with respect to the deep earth. The LVnetwork, immediately earthed by the surgelimiter, changes from the IT to the TN-S system ifall the application frames are interconnected.

The lightning energy thus flown off can beconsiderable and require replacement of thelimiter.In order to minimise these overvoltages onelectrical installations, the buildings horizontaland vertical equipotentiality must be the bestpossible in low and high frequency. A singleearth circuit (PE network) is naturallyrecommended, and use of metal cable trays withproper electrical connections (braids) is highlyadvisable for distribution.


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4.3 Why use an impedance?

A reading of the table in figure 5 shows thatwhen the network is very slightly capacitive(case 1), the neutral impedance ZNcauses thefault current to increase, which neverthelessremains very low ( 250 mA in figure 5). Thiseffect is even slighter when the network is highlycapacitive (cases 2 and 3). In practice, thisimpedance effects only very slightly the contactvoltage UCwhich remains less than ULin soundnetworks.Finally, presence of a resistance in the impedanceenables a reduction of the ferromagneticresonance hazard.

An impedance can be connected between thenetwork and the earth, normally between thetransformer neutral and the earth. Its value isapproximately 1,700at 50 Hz.

Its purpose is to reduce variations in potentialbetween network and earth, caused by MVdisturbances or fluctuations in potential of thelocal earth. It is therefore particularlyrecommended for short networks supplyingmeasurement instruments sensitive to thispotential and for networks placed next tocommunication networks (Bus).


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Fig. 22: a Markof graph shows that average electrical

power availability is 91 times better in IT than in TN or

TT.

No

fault

2

faults

1

fault

= 1j

=1

90j

= 1j

=1

90j

5 Advantages and disadvantages of

the IT earthing system in LV

The main advantage of using the IT earthingsystem for network operation is without doubtthe continuity of supply it offers, as there is noneed to trip on the first fault (as described in thesection below). Another of this systems strongpoints is guaranteed safety against the fire

hazard and for control and monitoring circuits ofmachine tools.However, to benefit fully from such advantages,the restrictions of this system must also beconsidered.

5.1 Increased availability

This advantage can be confirmed by a simple

probability calculation.Let us assume that the occurrence of aninsulation fault in an electrical installation is onefault every three months (90 days),

i.e. = 1

90j

and the time needed to track and put right thefaulty part is one day,i.e. = 1 j .The Markof graph technique gives therepresentation shown in figure 22and enablesus to calculate that the average time betweentwo double faults is 8,190 days!This corresponds to an average electrical poweravailability that is 91 times better in IT than in TNor TT.Consequently, preference is frequently given tothe IT earthing system for use in:

chospitals,

cairport take-off runways,

cvessels,cplants with continuous manufacturingprocesses,

claboratories,

ccold storage units,

celectrical power plants.

5.2 Increased safety against the fire hazard

Electricity is often the cause of fire. Standardsset the threshold for this risk at 500mA on aninsulation fault (NF C 15-100, part 482.2.10). Thisvalue can be considerably exceeded, particularlywith stray currents that flow through buildingstructures when faults occur in the TN system.

Also worthy of note is that the IT is the onlyearthing system that monitors insulation of theneutral conductor, compared with the TNS whichcan insidiously turn into a TNC on a neutral-PEfault with an increase in the fire hazard.

It is because the current of the first fault isparticularly low that the IT earthing system hasbeen chosen for use in certain establishments atrisk from fire and explosion (see chapter 1).Furthermore the first PIMs were used in

firedamp mines.


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5.3 Less downtime on control and monitoring circuits

The relay diagram illustrated in figure 23withthe TN earthing system shows three possibleinsulation faults which, when full, result in

immediate downtime, whose material andeconomic consequences are rarely negligible.These faults have the same consequences withthe TT system.In particular, faults cand dcause tripping of themaster protection device, and prevent allsubsequent operations, such as for example theorder to change direction on a transporter bridge!

These same first faults which can causeoperating malfunctions or even accidents withthe TN and TT systems, have no effect with theIT system, except if they occur as the secondfault (extremely unlikely, see section 5.1).

These examples show that even if safety of

persons with respect to the electrical hazard isguaranteed by the various earthing systems, or

5.4 Restrictions and precautions for using the IT earthing systemThe restrictions for using the IT system arelinked to loads and networks.

Limits linked to loads

cWith a high earth capacitive coupling(presence of filters).A number of devices fitted with capacitive filters(see fig. 24 ) offer the same disadvantage, dueto their number, as very long networks when theIT system is used.These capacitive leakages have a particularity,with respect to distributed capacity mainly due to

network cables, i.e. they can be unbalanced.

by use of Safety by Extra Low Voltage (SELV),safety of persons with respect to mechanicalhazards may not be guaranteed in certain cases.

More care must therefore be taken when wiringsuch circuits in the TT and TN than in the ITsystem, as the latter warns the operator of theincident (first insulation fault), thereby guardingagainst electrical and mechanical hazards. PIMsare increasingly used for just this purpose, tomonitor automation networks.

An additional solution is often advisable,particularly with relays using electronic devicessensitive to electromagnetic disturbances. Theaim is to supply all the control and monitoringcircuits separately by means of a LV/LVtransformer with separate windings.

Despite this, as stated in chapter 2, use of the IT

earthing system has its limits which aredescribed in the section below.

Fig. 23: monitoring circuit may be concerned by several types of insulation faults always resulting in downtime

with TT and TN system.

Device Network/earth capacity

Micro-computer 20 nF to 40 nF

UPS 40 nF

Variable speed controlers 70 nF

Fluorescent tubes 20 nF

(in ramps of 10)

Fig. 24: guideline capacitive values for HF filters built

into various devices.

N

RB

321N

PE

MA

Id

a b c d

M A

a b c d

Fault acannot be detected.

Faults b, c anddcause a

short-circuit.

Fault acannot be detected.

Fault bprevents the off function.

Faults c anddcause a

short-circuit.


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Db

Da

RCD

30 mA

(A)

(B)

1

2

3

NCFCF

Fig. 25: in the IT system, capacitive current flow can cause nuisance tripping of the RCDs by sympathy.

In this case, in presence of a fault on feeder B, circuit-breaker Da placed on a highly capacitive feeder (presence of

several filters) may open instead of Db.

Fig. 26: insulation monitoring of the various parts of a

network with a replacement source.

PIM

A

PIM

B

PIM

C

R

Office computer equipment: micro-computers,monitors and printers, concentrated on the samesingle-phase feeder, is an example of this. Itshould be borne in mind that interferencesuppression filters (compulsory according to theEuropean Directive for EMC) placed on thesedevices, generate in single-phase permanentleakage currents at 50 Hz that can reach 3.5 mAper device (see IEC 95); these leakage currentsadd up if the devices are connected on the samephase.

To prevent nuisance tripping (see fig. 25 ),especially when the RCDs installed have lowthresholds, the permanent leakage current mustnot exceed 0.17 In in IT. In practice, the supplyby a 30 mA RCD of three micro-computerstations is the maximum recommended.This problem also exists with the TT and TNsystems.

For memory:

vto guarantee safety of persons (UCi UL), thelimit not to be exceeded is 3C i 70F.

vfor insulation monitoring, PIMs with DC currentinjection are not affected by these capacities.

Note that if the devices are connected on allthree phases, these capacitive currents canceleach other out when they are balanced (vectorsum).

cWith a low insulation resistance

This particularly applies to induction furnacesand arc welding machines, as well as very oldcables.A low insulation resistance is equivalent to apermanent insulation fault: the IT system istransformed into a TN or TT system, with a PIMon permanent alert.

Limits due to the physical characteristics ofnetworks

High capacitive leakages disturb insulationmonitoring using PIMs with AC current injectionand tracking of the first fault using a very LFgenerator (see chapter 2).When an insulation fault occurs, they can alsocause flow of residual currents likely to generatenuisance tripping by sympathy of the RCDs

placed on very long or highly capacitive feeders(see Cahier Technique no. 114).

Use of the IT system is thus advised against forvery long networks, containing long feeders, forexample for electrical power distribution in anumber of buildings at a distance from one another.

Case of networks with replacement powersupply

The fact that a network can be supplied byseveral sources makes it necessary to detect thefirst fault and to trip on the second fault,irrespective of the voltage source in operation.


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6 Conclusion

Evolution of the various earthing systems shouldmirror the changing needs of electrical powerusers.

6.1 Availability: an increasing need to be satisfied

The increasing number of computer, automationand control/monitoring equipment has resulted inall major economic entities (industrial,commercial, etc.) calling for greater availability ofelectrical power.Today, electrical power is considered to be a

simple product with which a number of qualitycriteria, particularly availability, are associated.To ensure that users benefit from this increasedavailability, this demand, already acknowledgedby electricity utilities, must also be incorporated

in the design of the new internal and privatedistribution installations precisely there wherethe IT earthing system assumes its fullimportance by indicating the very first fault (notdangerous) and preventing tripping.However, for the network to benefit from all the

advantages of the IT system, designers mustcarefully consider the future operation of thenetwork and have excellent knowledge of thedevices to be supplied.

6.2 The IT earthing system finds its true place

Usable in a very large number of electricalinstallations

The IT earthing system can be used in a verylarge number of electrical installations inindustrialised countries, with the exception ofapplications (e.g. arc furnace, old lighting circuit)and situations (e.g. damp environment, very longnetwork) normally or frequently exhibiting a lowinsulation level. These countries possess skilledelectricians, sufficiently reactive to offer rapidinstallation servicing (the same day). Moreover,their infrastructures allow use of remotesupervision.

For adapted distribution circuits

Changes in continuity of service requirementsand implementation of new machines withspecific characteristics, particularly in the field ofelectromagnetic compatibility (EMC) mean thatthe electrical power supply sometimes requiresspecially adapted distribution circuits. Thisaccounts for the emergence of privatedistribution networks comprising a variety of sub-networks with an appropriate earthing system.In these conditions, the IT system easilyguarantees the necessary continuity of supply.

6.3 The added advantage of safety

Installation designers must also identify fire and

explosion hazards and satisfy EMC requirements(disturbance of measurements andcommunications).

The IT earthing system offers the mostadvantages and best meets operatorsrequirements with such specific features as:

c better EMC (interconnection of frames and intheory a single earth connection),

c minimum fire and explosion hazards (low firstfault currents).

Moreover, its use is encouraged by the

upgrading of equipment (PIM, tracking device,supervisor, etc.) allowing:

canticipation of maintenance (prediction),

cquicker tracking of the first insulation faults(automation), or even remote tracking (remotesupervision via digital connections),

cpreparation of troubleshooting (remotediagnosis).


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6.4 In short

Our readers now understand the importance ofproperly listing the requirements relating toequipment used, the environment and the study

conditions of the installation and subsequentmodifications, before choosing the earthingsystem for an electrical distribution network.

A brief reminder of the advantages anddisadvantages inherent in each earthing system

is essential at this point: this is the purpose offigure 28.

NB:The installation cost is not included in this

table as the possible additional cost of an ITsystem (PIM, fault tracking system) must becompared with the financial loss generated byunexpected downtime on the first fault thismust be evaluated for each activity.

TT TN-C TN-S IT

Safety of persons (perfect installation) c c c c c c c c c c c c

Safety of equipment

cagainst the fire hazard c c c v v v c c c

cfor machine protection on an insulation fault c c c v v c c c

Availability of electrical power v v v v v v c c c c

Electromagnetic compatibility v v v v v v v

For installation and maintenance

cskill c c c c c c c c c c c c c

cavailability v v v v v c c c

c c c c excellent

c c c good

v v average

v poor

Fig. 28: summary of the advantages and disadvantages of the various earthing systems.


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Bibliography

Standards and decrees

cIEC 60364: Electrical installations of buildings.

cIEC 60479-1: Effects of current on humanbeings and livestock.

cIEC 60947-2: Low voltage switchgear andcontrolgear - Part 2: Circuit-breakers.

cIEC 60950: Safety of information technologyequipment.

cIEC 61000: Electromagnetic compatibility.

cIEC 61557, NF EN 61557: Electrical safety inlow voltage distribution systems up to 1,000 V AC

and 1,500 V DC - Equipment for testing, measu-ring or monitoring of protective measures -.Part 8: Insulation monitoring devices forIT systems.

cNF C 15-100: Installations lectriques bassetension.

Schneider Electrics Cahiers Techniques

cNeutral earthing in an industrial HV network.F. SAUTRIAU, Cahier Technique no. 62

cResidual current devices.R. CALVAS, Cahier Technique no. 114

cEMC: Electromagnetic compatibility.

F. VAILLANT, Cahier Technique no. 149

cHarmonics in industrial networks.N. QUILLON, P. ROCCIA,Cahier Technique no. 152

cCalculation of short-circuit currents.B. De METZ-NOBLAT, G. THOMASSET,R. CALVAS and A. DUCLUZAUX,Cahier Technique no. 158

cEarthing systems in LV.R. CALVAS, B. LACROIX,Cahier Technique no. 172

cEarthing systems worldwide and evolutions.

R. CALVAS, B. LACROIX,Cahier Technique no. 173

cPerturbations des systmes lectroniques etschmas des liaisons la terre.R. CALVAS, Cahier Technique no. 177

cLV surges and surge arresters - LV insulationco-ordination -.Ch. SERAUDIE, Cahier Technique no. 179

cIntelligent LV switchboards.A. JAMMES , Cahier Technique no. 186

cCohabitation of high and low currents.R. CALVAS, J. DELABALLE,Cahier Technique no. 187


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178-The IT Earthing System