ECT114-Residual Current Devices in LV

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

Cahier technique no. 114

Residual current devices in LV

R. Calvas

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ECT 114 updated, February 1999

no. 114Residual current devices in LV

Roland CALVAS

With an engineering degree from “Ecole Nationale Supérieured’Electronique et de Radioélectricité de Grenoble” (1964) and aBusiness Administration Institute diploma, he joined Merlin Gerinin 1966.In the course of his career, he has held the position of sales manager,followed by marketing manager for the activity dealing with theprotection of people against electrical hazards. He is currentlycharged with technical communication within Schneider Electric.

Cahier Technique Schneider Electric no. 114 / p.2

Lexicon

Cardiac fibrillation:A malfunctioning of the heart corresponding toloss of synchronism of the activity of its walls(diastole and systole). The flow of AC currentthrough the body may be responsible for this dueto the periodic excitation that it generates. Theultimate consequence is stoppage of blood flow.

Direct contact:Contact of a person with the live parts ofelectrical devices (normally energised parts andconductors).

Earthing system:Standard IEC 60364 stipulates three main officialearthing systems which define the possibleconnections of the neutral of the source andframes to the earth or neutral. The electricalprotection devices are then defined for each one.

Electrification:Application of voltage between two parts of thebody of a living being.Electrocution:Electrification resulting in death.

Fault current I d:Current resulting from an insulation fault.

Frame:Conductive part likely to be touched and which,although normally insulated from live parts, maybe brought to a dangerous voltage further to aninsulation fault.Indirect contact:Contact of a person with accidentally energisedframes (usually further to an insulation fault).

Insulation:Arrangement preventing transmission of voltage(and current flow) between a normally energisedelement and a frame or the earth.

Insulation fault:Insulation rupture causing an earth fault currentor a short-circuit via the protection conductor.

Leakage current:Current which, in the absence of an insulationfault, returns to the source via the earth or theprotection conductor.

Limit safety voltage (U L):Voltage UL below which there is no risk ofelectrocution.

Live conductors:Set of conductors assigned to electrical powertransmission, including the neutral in AC and thecompensator in DC, with the exception of thePEN conductor whose “protection conductor”(PE) function takes priority over the “neutral”function.

Operating residual current I f:Value of the residual current causing a residualcurrent device to trip.According to construction standards, at 20°Cand for a threshold set at IDn, low voltageresidual current devices must comply with:

Ι∆ Ι Ι∆n2

< < nf

In high voltage, the “zero phase-sequence”relays have, allowing for operating accuracy, anoperating current equal to the thresholddisplayed in amperes.

Protection conductors (PE or PEN):Conductors which, according to specifications,connect the frames of electrical devices andsome conductive elements to the earthingconnection.Residual current:Rms value of the vector sum of the currentsflowing through all live conductors in a circuit at apoint of the electrical installation.Residual current device (RCD):Device whose decisive quantity is the residualcurrent. It is normally associated with orincorporated in a breaking device.


Residual current devices in LV

Today, the residual current device is recognised the world over as aneffective means of guaranteeing protection of people against electricalhazards in low voltage, as a result of indirect or direct contact.Its choice and optimum use require sound knowledge of the electricalinstallations and in particular of the earthing systems, existing technologiesand their possibilities.All these aspects are dealt with in this “Cahier Technique”, completed bynumerous answers provided by Schneider Electric’s technical andmaintenance departments to the questions which they are frequentlyasked.

Contents

1 Introduction 1.1 The RCD: its scope p. 4

1.2 “Residual current protection” and “Earth leakage protection”: p. 4two separate notions

1.3 The RCD, a useful protection device p. 5

2 The patho-physiological effects 2.1 Effects according to current strength p. 6of electrical current on people 2.2 Effects according to exposure time p. 6

2.3 Effects according to frequency p. 8

3 Insulation fault protection 3.1 The installation standards p. 10

3.2 The direct contact risk p. 11

3.3 Fire protection p. 11

3.4 The “TT” earthing system p. 11

3.5 The “TN” earthing system p. 12

3.6 The “IT” earthing system p. 12

4 RCD operating principle and description 4.1 Operating principle p. 14

4.2 Sensors p. 14

4.3 Measuring relays and actuators p. 17

4.4 Product manufacturing standards p. 19

4.5 The various devices p. 21

5 Optimised use of the RCD 5.1 EMC: manufacturers’ obligations and what this implies p. 22for contractors

5.2 A need: discrimination p. 23

5.3 Avoiding known problems p. 26

5.4 RCDs for mixed and DC networks p. 27

6 Conclusion p. 31

Bibliography p. 32


1 Introduction

In electrical installations, direct and indirectcontacts are always associated with a faultcurrent which does not return to the source viathe live conductors. These contacts aredangerous for people and equipment(see “Cahiers Techniques” no. 172 and 173).For this reason the use of Residual CurrentDevices (RCD), whose basic function isdetection of residual currents, is widespread.

1.1 The RCD: its scope

Fig. 1 : a current leakage results in a residual fault current id.

Fig. 2 : earth leakage protection.

Fig. 3 : current leakage protection.

1.2 “Residual current protection” and “Earth leakage protection”:two separate notions

It is important not to confuse these two notions.

A “residual current device” (RCD) is aprotection device associated with a toroidalsensor surrounding the live conductors. Itsfunction is detection of current difference or, tobe more precise, residual current (see fig. 1 ).Existence of a residual current indicatespresence of an insulation fault between a liveconductor and a frame or the earth. This currenttakes an abnormal path, normally the earth, toreturn to the source.The RCD is normally combined with a breakingdevice (switch, circuit-breaker, contactor) whichautomatically de-energises the faulty circuit.

“Earth leakage protection” consists of one ormore measuring devices whose function is todetect a difference between the input current andthe output current on part of the installation: line,cable, transformer or machine (generator, motor,etc.).This protection is mainly used in medium andhigh voltage. Earth leakage protection (zerophase-sequence current) for insulation faultprotection (see fig. 2 ) and current leakageprotection for phase-to-phase fault protection(see fig. 3 ) are both found.

Moreover, RCDs monitor insulation of cablesand electrical loads, and are thereforefrequently used to indicate insulation drops or toreduce the destructive effects of a strong faultcurrent.

G

Outgoing current

Source

Load

InId I1

I2

I3

id = ia - ir

i3

in

Load

SourceReturn current

Fault current

i2

i1


Fig. 4 : graph showing the evolution of deaths by electrocution due to the use of hand-held tools in Japanesecompanies. This figure begins to drop in 1970, the year after that in which a law was decreed making the use ofhigh sensitivity RCDs compulsory.

The first decisive factor in choosing and usingRCDs in an installation is the earthing systemprovided.

c In the TT earthing system (directly earthedneutral), protection of people against indirectcontact relies on the use of RCDs.

c In the IT and TN earthing systems, the mediumand low sensitivity (MS and LS) RCDs are used:

v to limit the risk of fire,v to prevent the destructive effects of a strongfault current,v to protect people against indirect contact (verylong outgoers).

c For all earthing systems, the high sensitivity(HS) RCDs provide additional protection againstdirect contact. They are compulsory in finaldistribution in a large number of countries.

1.3 The RCD, a useful protection device

Their efficiency was confirmed at the end of thiscentury by the remarked reduction in the numberof people electrocuted. The result of anIEC survey conducted in August 1982 in Japanalready proved the efficiency of these devices(see fig. 4 ).

“The residual current device is generallyrecognised (throughout the industrialised world)as being the best and most reliable of theprotection devices developed to provideprotection against indirect contact in the lowvoltage field”.

Such were the words of professor C.F. DALZIEL(Berkeley-USA), one of the pioneers of the studyof the effects of electrical current on people inthe fifth international conference of the AISS(Lucerne 1978).

Decree of the law making HS-RCDs compulsory40

30

20

10

66 67 69 7068 71 72 73 74 75 76 78 79 8077

Annual number of deaths by electrocution

Years


2 The patho-physiological effects of electrical current on people

The patho-physiological effects of electricalcurrent on people (tetanisation, external andinternal burns, ventricular fibrillation and cardiacarrest) depend on a variety of factors: thephysiological characteristics of the person inquestion, the environment (e.g. dry or wet) andthe characteristics of the current passing throughthe body.

As protection of people is the main function ofthe RCD, it is clear that optimum implementationof these devices requires knowledge of the

Effects (for t < 10s) Current strength (mA)DC 50/60 Hz 10 kHz

Slight tingling, perception threshold 3.5 0.5 8

Painful shock, but no loss of muscular control 41 6 37

Non-release threshold 51 10 50

Considerable breathing difficulty 60 15 61

Respiratory paralysis threshold 30

Fig. 5 : effects of weak electrical currents on human beings.

sensitivity thresholds of people and of the risksincurred.

The International Electrotechnical Committee(IEC) has looked into the problem in order topool, at international level, a variety of viewpointsreflecting and even often defending nationalpractices, habits and standards. Many scientistshave participated in this undertaking and havehelped clarify the subject (Dalziell, Kisslev,Osypka, Bielgelmeier, Lee, Koeppen, Tolazzi,etc.).

2.1 Effects according to current strength

The effects of the electrical current passingthrough the human body depend on the frequencyand strength of this current (see fig. 5 ).

2.2 Effects according to exposure time

The risks of non-release, respiratory arrest orirreversible cardiac fibrillation (see lexicon)increase in proportion to the time during whichthe human body is exposed to the electricalcurrent (see fig. 6 ).

The chart in figure 6 identifies in particular zones3 and 4 in which danger is real.c Zone 3 (situated between curves b and c1).For people in this situation, there is normally noorganic damage. However there is a likelihood ofmuscular contractions, breathing difficulties andreversible perturbation of the formation ofimpulses in the heart and of their propagation. Allthese phenomena increase with current strengthand exposure time.

c Zone 4 (situated to the right of curve c1)In addition to the effects of zone 3, the likelihoodof ventricular fibrillation is:v approximately 5 % between curves c1 and c2,v less than 50 % between curves c2 and c3,v more than 50 % beyond curve c3.

Patho-physiological effects such as cardiacarrest, respiratory arrest and serious burnsincrease with current strength and exposure time.For this reason it is accepted that use of an RCDwith instantaneous operation and with a thresholdof less than 30 mA, ensures this situation isnever reached and such risks never incurred.With a more general approach, IEC 60364(NF C 15-100 in France) stipulates the operating


times for the Residual Current Devices accordingto contact voltage. These times are recalled inthe two tables in figure 7 .

Limit safety voltage (U L)According to environmental conditions andparticularly presence or absence of water, limit

Fig. 6 : duration of current flow in the body as a function of current strength. In this chart, the effects of AC current(15 to 100 Hz) have been divided into four zones (as per IEC 60479-1).

Prospective contact voltage (V) Maximum breaking time of theprotection device (s)AC DC

ccccc Dry or wet premises or locations: UL i 50 V< 50 5 550 5 575 0.60 590 0.45 5120 0.34 5150 0.27 1220 0.17 0.40280 0.12 0.30350 0.08 0.20500 0.04 0.10

c Wet premises or locations: UL i 25 V

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

safety voltage UL (voltage below which there isno risk for people, according to standardNF C 15-100) is, in AC:v 50 V for dry and wet premises,v 25 V for damp premises, for example foroutdoor worksites.

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

50 100200 5001000 2000500010000

mA10

20

50

100

200

500

1000

2000

5000

10000ms

Duration of current flow

a b c2c1 c3

1 2 3

Current flowing through the body

4

Fig. 7 : maximum duration of contact voltage holding as per standard IEC 60364.


Direct contactDirect contact with normally energised parts isdangerous for voltages in excess of UL. Themain protection precautions to be taken aredistance and insulation.

The RCD can detect a fault current flowingthrough a person and, as such, is specified,regardless of the earthing system, in finaldistribution as an additional protection. Itsoperating threshold, as shown in the table infigure 5, must be less than or equal to 30 mA,and its operation must be instantaneous sincethe value of the fault current, dependent on theexposure conditions, may exceed 1A.

Indirect contactOn contact with an accidentally energised frame,the danger threshold is also fixed by the limitsafety voltage UL.

To ensure there is no danger when networkvoltage is greater than UL, contact voltage mustbe less than UL.

Fig. 8 : fault voltage generation principle RCD.

In the diagram in figure 8 , when the installationneutral is earthed (TT earthing system) where:RA = earthing resistance of the installationframes,RB = earthing resistance of the neutral,this implies choosing an operating threshold(I∆n) of the RCD such that:U R Ud A d L= ≤Ι

and thus: I∆n i UR

L

A

The protection operating time must be chosenaccording to fault voltage

UR

R R Ud

A

A B

=

+(see fig. 7).

Note that if the equipotentiality of the site is notensured or is badly ensured, contact voltage isequal to fault voltage.

2.3 Effects according to frequency

IEC 60479-2 deals with the effects ofAC current of a frequency in excess of 100 Hz.Skin impedance decreases in reverseproportion to frequency. The standard statesthat the frequency factor, which is the ratio ofcurrent at the frequency (f) over current at thefrequency of 50/60 Hz for the same physiologicaleffect considered, increases with frequency.Moreover, it has been observed that between10 and 100 kHz the perception threshold

increases approximately by 10 mA to 100 mAin rms value.Although standards do not yet stipulate specificoperating rules, the major manufacturers,aware of the potential risks of such currents,ensure that the thresholds of the protectiondevices they propose are below the ventricularfibrillation curve defined in standardIEC 60479-2 (see fig. 9 ).

Ud

N

PE

RB RA

Id

RCD RCD


Fig. 9 : variations in ventricular fibrillation threshold (as per IEC 60479-2) and thresholds of various RCDs set on30 mA, for frequencies of between 50/60 Hz and 2 kHz (source: Merlin Gerin).

25.00

15.00

10.00

5.00

0.0010 100 50 1 000 10 000

20.00

Frequency (Hz)

Id(ƒ) / Id(50 Hz)

LimitA type ID AC type IDVigirex RH328A


3 Insulation fault protection

3.1 The installation standards

Fig. 10 : the three main earthing systems are the TT, TN and IT systems, defined by IEC 60364-3. The TN may beeither TN-C (neutral and PE combined) or TN-S (separate neutral and PE).

c insulation: class II devices and safetytransformers,c earthing of frames,c equipotentiality.

General rulesWhatever earthing system is chosen for aninstallation, the standards require that:c Each application frame be connected to anearthing connection by a protection conductor.c Simultaneously accessible application framesbe connected to the same earthing connection.c A breaking device automatically disconnectsall parts of the installation where a dangerouscontact voltage develops.c The breaking time of this device be less thanthe maximum time defined (see fig. 7).

RCDs are used in electrical, domestic andindustrial installations. Their use depends onstandards and mainly on the IEC 60364(in France NF C 15-100).

This standard officially stipulates three mainsystems for earthing the electrical network: theearthing systems (see fig. 10 ), used to a greateror lesser extent depending on the country.

Furthermore, for each of these systems itdefines more precisely the use of the RCDs, asthe electrical hazard is greatly influenced bychoice of earthing system (see “CahierTechnique” no. 172).It also describes the basic precautions which, innormal operating conditions, considerablyreduce electrical hazards, for example:c distance and obstacles,

Directly earthed neutral (TT)123N

Multiple earthed neutral (TN-C)123PEN

: Permanent insulation monitor.

Multiple earthed neutral (TN-S)123NPEPE

PE

Unearthed neutral (IT)123N

RB RA

RB

RB

RB


3.2 The direct contact risk

This risk is the same for people whateverearthing system is used. The protectionmeasures stipulated by standards are thereforeidentical and use the possibilities offered by thehigh sensitivity RCDs.

This is because:c As the fault current flows through a person incontact with a live conductor, he or she isexposed to the patho-physiological risksdescribed above.

c An RCD placed upstream of the contact pointcan measure the current flowing through theperson and break the dangerous current.

Regulations recognise the use of an RCD withhigh or very high sensitivity (i 30 mA) as anadditional protection measure when the risk ofdirect contact exists due to the environment, theinstallation or people (article 412.5.1 ofIEC 60364). This risk also exists when theprotection conductor is likely to be broken ordoes not exist (hand-held devices).In this case use of a high sensitivity RCD iscompulsory. Standard NF C 15-100, paragraph

532-2.6.1, states that RCDs with a threshold atmost equal to 30 mA must protect the circuitssupplying power outlets when they are:

c Placed in damp premises or in temporaryinstallations.

c Of rating i 32 A in all the other installationcases.

NoteStandard IEC 60479 states that the resistance ofthe human body is greater than or equal to1000 Ω for 95 % of people exposed to a 230 Vcontact voltage and thus through whom a 0.23 Acurrent flows.An RCD with a 30 mA threshold does not limitcurrent, but its instantaneous operation ensuressafety up to 0.5 A (see fig. 6).Use of an RCD with a sensitivity of 5 or 10 mAtherefore does not increase safety. However itmakes the risk of nuisance tripping not negligibleas a result of capacitive leakage (distributedcapacitances of cables and filters).

3.4 The “TT” earthing system

Protection of people against indirect contact

In this system protection relies on use of RCDs.

The fault current depends on the resistance ofthe insulation fault (Rd) and the resistances ofthe earthing connection. A person in contact withthe metal enclosure of a load with an insulationfault (see fig. 8) may be subjected to the voltagedeveloped in the load earthing connection (RA).

For example

Where U = 230V, RA = RB = 10 Ω and Rd = 0, ifthe person is not on an equipotential site, he orshe is subjected to Uc = Ud = 115 V.

Protection must be provided by use of an RCD ofmedium or low sensitivity which must

de-energise the faulty device as soon as thevoltage Ud exceeds the limit safety voltage UL.We remind you that their operating thresholdmust be set at:

I∆n i UR

L

A

.

Protection of machines and equipmentThe level of the RCD tripping thresholdsnecessary for protection of people in the TTearthing system is well below that of the faultcurrents able to damage the magnetic circuits ofmachines (motor) or cause fires.The RCDs therefore prevent such electricaldestruction.

3.3 Fire protection

Whatever earthing system is used, the electricalinstallations of premises where risk of fire ispresent must be equipped with RCDs of asensitivity I∆n i 500 mA, as it is acknowledged

that a 500 mA current can result inincandescence of two metal parts coming intooccasional contact.


3.6 The “IT” earthing system

3.5 The “TN” earthing system

Remindersc With this earthing system, the current of a fullinsulation fault is a short-circuit current.

c In TN-C, in view of the fact that the neutral andthe protection conductor are combined, RCDscannot be used. The following text thereforemainly concerns the TN-S.

Protection of people against indirect contact

As the fault current depends on the impedanceof the fault loop, protection is normally providedby overcurrent protection devices (calculation/measurement of loop impedances).

If the impedance is too great and does not allowthe fault current to trip the overcurrent protectiondevices (very long cables), one solution is to usea low sensitivity RCD (I∆n u 1 A).Moreover this system cannot be used when, forexample, the network is supplied by atransformer whose zero phase-sequenceimpedance is too great (star-star connection).

Protection of electrical devices and circuitsIn the multiple earthed neutral system, insulationfaults are responsible for strong fault currentsequivalent to short-circuit currents. The flow ofsuch currents results in serious damage, forexample: perforation of the magnetic circuitplates of a motor, requiring replacement insteadof rewinding of the motor. Such damage can begreatly limited by use of a low sensitivity RCD(e.g. 3 A) with instantaneous operation, which isthus able to react before the current reaches ahigh value.

Note that the need of protection increases asoperating voltage rises, as the energy lost at thefault point is proportional to the voltage square.The economic consequence of such destructionmust be estimated as it is a vital criterion inchoice of earthing system.

Detection of insulation faults between theNeutral and the protection conductor (PE)or building frames

This type of fault insidiously transforms the TN-Ssystem into a TN-C system. Part of the neutralcurrent (increased by the sum of 3rd order andmultiple of 3 harmonic currents) permanentlyflows in the PE and in the metal structures of thebuilding with two consequences:

c Equipotentiality of the PE is no longer ensured(a few volts may disturb the operation of thedigital systems connected by bus and whichmust have the same potential reference).

c Current flow in the structures increases the riskof fire.

The RCDS are used to highlight this type offault.

Detection of insulation faults without trippingand protection of equipment

In the TN-S earthing system, unlike the ITsystem, there are no safety rules stipulatinginsulation monitoring. However, all trippingfurther to an insulation fault is the cause ofoperating losses and often of costly repairs priorto re-energisation. For this reason, more andmore often operators request preventiondevices in order to take action before theinsulation loss becomes a short-circuit.

The answer to this need is the use in indication,in TN-S, on critical outgoers, of an RCD with athreshold of around 0.5 to a few amperes, whichcan detect insulation drops (on the phases orneutral) and alert operators.

On the other hand, the risk of electrical fire isreduced and destruction of equipment avoidedby using an RCD with tripping for I∆n i 500 mA.

Protection of people against indirect contactWhen the first insulation fault occurs, the faultcurrent is very weak and the fault voltage notdangerous: the standards require that this faultbe indicated (by the permanent insulationmonitors) and tracked (by the power on faulttracking devices).

When the second fault occurs, the installationfinds itself in a situation similar to a fault in theTN earthing system. However there are twopossibilities: that of a single earthing connectionfor all the frames and that of multiple earthingconnections.


c Case of a single earthing connectionIn this case protection is usually ensured by theovercurrent protection devices (calculation/measurement of the loop impedances).

c Case of multiple earthing connectionsWhen both faults affect devices not connected tothe same earthing connection, the fault currentmay not reach the operating threshold of theovercurrent releases. The standards stipulateuse of RCDs on each group of framesinterconnected with the same earthingconnection.

c In all cases, simple or multiple earthingconnectionsIf the impedance of a fault loop is too great (verylong cables), a simple, practical solution is to usea low sensitivity RCD (1 to 30 A).

Protection of equipment, electrical devicesand circuits

Although there is no particular danger forequipment when the first fault occurs, a secondfault is normally responsible for strong faultcurrents equivalent to short-circuit currents, as inthe TN earthing system.

RCDs with medium or low sensitivity can then beprovided for the more critical cases (premiseswith risk of fire, sensitive and expensivemachines), bearing in mind that the risk of thesecond fault is particularly low, especially whentracking of the first faults is systematic. In actualfact, assuming a fault once every three monthsand that this fault is eliminated the same day, theaverage time between two “double faults” isapproximately 22 years!


4 RCD operating principle and description

4.1 Operating principle

All residual current devices are made up of atleast two components:

c The sensorThe sensor must be able to supply an electricalsignal which is useful when the sum of thecurrents flowing in the live conductors isdifferent from zero.

c The measurement relayThe relay compares the electrical signalsupplied by the sensor with a setpoint value and

Fig. 11 : functional diagram showing an electronic RCD with auxiliary supply source.

4.2 Sensors

sends, with a possible deliberate delay, theopening order to the associated breakingdevice.

The unit controlling the opening of the device(switch or circuit-breaker) placed upstream ofthe electrical circuit monitored by the RCD isknown as the trip unit or actuator.

The entire RCD is shown in the diagram infigure 11 .

Two types of sensors are normally used onAC circuits:

c The toroidal transformer, which is the mostcommon for measuring leakage currents.

c The current transformers, used in HV and MVand sometimes in LV.

The toroidal transformerThis covers all the live conductors and is thusexcited by the residual magnetic fieldcorresponding to the vector sum of the currentsflowing through the phases and the neutral.

Induction in the toroid and the electrical signalavailable at the terminals of the secondarywinding are thus the image of the residual current.

This type of sensor is used to detect residualcurrents from a few milliamperes to severaldozen amperes.

The current transformers (CT)

To measure the residual current of a three-phase electrical circuit without neutral, threecurrent transformers must be installed as shownin figure 12 .

I1 I2 I3

AIh

B

RCD

Toroid ThresholdShapingStatic or relay output

Auxiliary supply source

Time delay relay

Fig. 12 : the vector sum of the phase currents yieldsthe residual current.


The three CTs are parallel-connected currentgenerators, causing circulation between A and Bof a current which is the vector sum of the threecurrents and thus the residual current.

This circuit, known as the Nicholson circuit, iscommonly used in MV and HV when the earthfault current can reach several dozen or evenseveral hundred amperes.

During use, care must be taken with theCT accuracy class: with CTs of 5 % class, it isprudent not to set earth protection below 10 % oftheir nominal current. The HV electricalinstallation standard NF C 13-200 of December1989 specifies 10 %.

Special casesc High power supply

The Nicholson CT circuit, which would be usefulin LV when the conductors are large cross-section bars or cables for the transmission ofstrong currents, does not allow, even withcoupled CTs, settings that are compatible withprotection of people (threshold I∆n i UL / Ru).

There are a number of solutions:v If the problem occurs in a main switchboarddownstream of the transformer, the followingmay be considered:- either installation of a toroid at the supply endof the installation on the earthing connection ofthe transformer LV neutral (see fig. 13 ). This isbecause, according to the Kirchhoff node law,the residual current detected by (N) is strictly thesame as that detected by (G) for a fault occurringin LV distribution,- or installation of a toroid on each outgoer, allparallel-connected to a single relay (see fig. 14 ).When the measurement relay (normallyelectronic) only needs a very weak electricalsignal to operate, the toroids can be made tooperate as “current generators”. When parallel-connected, they give the image of the vectorsums of the primary currents.

Although this circuit is laid down in theinstallation standards, the approval of the RCDmanufacturer is preferable. However, fordiscrimination reasons, it is preferable to use oneRCD per outgoer.v If the problem arises with parallel-connectedcables which cannot all cross a toroid, a toroidcan be placed on each cable (including all thelive conductors), and all the toroids can beparallel-connected (see fig. 15 ).

However the following must be noted:v That each toroid detects n turns in short-circuit(3 in the figure) which may reduce sensitivity.

Fig. 13 : toroid N delivers the same information astoroid G.

Fig. 14 : toroids placed on the outgoers and parallel-connected to a single relay compensate theimpossibility of placing a toroid on the incomer.

Fig. 15 : layout of toroids on parallel-connected largediameter single-line cables.

RCD

31 2

31 2

HV / LV

RCD

1

N

G

23

RCD


v If the connections represent impedancedifferences, each toroid will indicate a false zerophase-sequence current. However proper wiringconsiderably limits these currents.v That this circuit implies for each toroid that theoutput terminals S1-S2 be marked according tothe energy flow direction. This solution calls forthe approval of the RCD manufacturer.

c High power outgoerTo ensure a reliable, linear toroid “response”, thelive conductors must be placed as close aspossible to the centre of the toroid so that theirmagnetic effects are completely compensated inthe absence of residual current. In actual fact,the magnetic field developed by a conductordecreases in proportion to distance: thus infigure 16 , phase 3 causes at point A a localmagnetic saturation and thus no longer has aproportional effect. The same applies if the toroidis placed near or in a bend of the cables that itsurrounds (see fig. 17 ). The appearance of astray residual induction, for strong currents, willgenerate on the toroid secondary a signal thatmay cause nuisance tripping. The risk increasesas the RCD threshold drops with respect tophase current, particularly on a short-circuit.

In problem cases (Max. Iph. / high I∆n), twosolutions can be used to counter the risk ofnuisance tripping:v Use a toroid that is far larger than necessary,for example with a diameter that is twice the onejust right for conductor insertion.v Place a sleeve in the toroid.This sleeve must be made of magnetic materialin order to homogenise the magnetic field (softiron - magnetic plate), (see fig. 18 ).When all these precautions have been taken:- centring of conductors,- large toroid,- and magnetic sleeve,

the ratio max. phase

nΙ

Ι∆ may reach 50,000.

Using an RCD with built-in toroid

It must be pointed out that RCDs with built-intoroids provide contractors and operators with aready-made solution since it is the manufacturerwho studies and works out the technicalsolutions. This is because he:c Masters the problem of centring the liveconductors and, for weak currents, cananticipate and properly distribute several primaryturns around the toroid.

Fig. 16 : incorrect centring of conductors in the toroid isresponsible for its local magnetic saturation at point A,which may be the cause of nuisance tripping.

Fig. 17 : the toroid must be far enough from the cablebend so as not to be the cause of nuisance tripping.

Fig. 18 : a magnetic sleeve placed around theconductors, in the toroid, reduces the risk of trippingdue to the magnetic effects of the current peaks.

c Can “operate” the toroid at higher induction inorder to maximise the energy sensed andminimise sensitivity to stray inductions (strongcurrents).

L u 2 Ø

Ø

L u 2 Ø

Ø

A3

1

2


4.3 Measurement relays and actuators

The RCDs can be classed in three categoriesaccording to their supply mode or theirtechnology.

According to their supply mode“With own current”: in this device the trippingenergy is supplied by the fault current. Thissupply mode is considered by most specialistsas the most reliable. In many countries andparticularly in Europe, this category of RCD isrecommended for domestic and similarinstallations (standards EN 61008 and 61009).

“With auxiliary supply source”: in this devicethe tripping energy requires a source of energythat is independent from the fault current. Thesedevices (normally electronic) can therefore onlycause tripping if this auxiliary energy source isavailable when the fault current appears.

“With own voltage” : this is a device with an“auxiliary supply source” but whose source is themonitored circuit. Thus, when this circuit isenergised, the RCD is supplied, and when thiscircuit is not energised, the RCD is not activatedbut there is no danger. An additional guaranteeis provided by these devices when they aredesigned to operate correctly with voltage dropsof up to 50 V (safety voltage). This is the case ofthe Vigi modules which are RCDs associatedwith the Merlin Gerin “Compact” circuit-breakers.However, as far as power supply is concerned,the RCDs are also classed according to whetheror not their operation is of the “fail-safe” kind.Two types of devices are considered to be fail-safe:

c Those whose tripping only depends on thefault current: all own current devices are fail-safedevices.

c And those, more seldom used, whose trippingdoes not only depend on the fault current butwhich are automatically placed in the trippingposition (safety position) when the conditions nolonger guarantee tripping in the presence of thefault current (e.g. a voltage drop up to 25 V).

According to their technology

“Electromagnetic devices” (see fig. 19 ).These modern devices are of the “own current”type and use the principle of magnetic latching.A very low electrical power (100 µVA for some)is sufficient to overcome the latching force andcause the contacts to open by means of amechanical amplifier.

They are very widespread (with the “fail-safe”function) and particularly suitable for the creationof an RCD with a single sensitivity.

“Electronic devices”

These devices are particularly used in industryas electronics ensures:

c A very low acquisition power,c Accurate, adjustable thresholds and timedelays (thus ensuring optimum trippingdiscrimination).

Due to these two characteristics, these devicesare ideal for the creation of:

c RCDs with separate toroids, which areassociated with high rating circuit-breakers andcontactors.c RCDs associated with industrial circuit-breakers up to 630 A.Electronic devices require a certain energy, oftenvery weak, to operate. RCDs with electronicdevices are therefore available with the varioussupply modes described above, either “with ownvoltage” or “with auxiliary supply source”.

Fig. 19 : the fault current, via the toroid, suppliesenergy to an electromagnet whose moving part is“stuck down” by a permanent magnet. When theoperating threshold is reached, the electromagnetdestroys the attraction of the permanent magnet andthe moving part, drawn by a spring, opens themagnetic circuit and mechanically controls circuit-breaker opening.

IrIa

A


“Mixed devices” (own current)

This solution consists of inserting between thetoroid and the magnetic latching relay a signalprocessing device, allowing:

c An accurate, precise operating threshold.c Excellent immunity to interference and steepfront current transients, while respecting anoperating time compatible with safety curves. Asan example, Merlin Gerin “si” type RCDs aremixed devices.

c Creation of time-delayed RCDs.

A similar principle is used in MV. In point of fact,a few years ago tripping in electrical powersupply consumer substations (MV/LV substation)required an accumulator bank which was thesource of many problems. The combination of anown current electronic device and anelectromechanical trip unit with magnetic latchingoffers a satisfactory solution with respect to costand reliability with removal of the battery.

Operational requirementsIEC 60364, paragraph 531-2-2-2 states thefollowing for non fail-safe devices with auxiliarysupply source:“Their use is permitted if they are installed ininstallations operated by experienced andqualified people”.Standard NF C 15-100, paragraph 532.2.2 alsostates that they must not be used in householdinstallations or for similar applications.

Proper operating test

An RCD is a safety device. Whether it iselectromagnetic, electronic or mixed, it is thusessential for it to be equipped with a test device.Although own current devices appear the mostreliable, implementation of fail-safe safety withthe other “own voltage” or “auxiliary supplysource” energy sources grant the RCDs anincreased degree of safety which does not,however, replace the periodical test.

c Recommend periodical RCD testingIn practice perfect fail-safe safety, particularlyconcerning internal faults, does not exist. For thisreason, in France, RCDs using an auxiliarysupply source are reserved for industrial andlarge tertiary installations and own current RCDsfor domestic and similar installations: anarrangement which is consistent with theirinherent possibilities described above.In all cases, periodical testing should berecommended for highlighting internal faults.

c The manner in which the test is conducted isimportant.

This test must allow for the fact that capacitiveearth leakage currents are always present in anelectrical installation, as are often resistiveleakage currents resulting from damagedinsulation.

The vector addition of all these leakage currents(Id) is detected by the toroidal sensor and mayaffect test operation, in particular when the testcircuit is the one shown in figure 20 . Despitethis, this test principle is widespread as it checksthe toroid/relay/breaking device assembly.

Construction standards limit the test current,which may account for a certain number of RCDoperating failures during the test, as shown bythe vector addition (see figure 20) of the leakagecurrent (Id) and the test current (test I). Forexample standards IEC 61008 and 61009 statethat the test current must not exceed 2.5 I∆n foran RCD usable in 230 or 400 V, i.e. 1.15 I∆n if itis supplied in 230 V - 20%.

The test principle described above is used onearth leakage protection sockets, residualcurrent circuit-breakers and residual currentdevices. With respect to residual current relayswith separate toroid, the same principle issometimes chosen when the contractor is theperson producing the test circuit. However somerelays, for example the Merlin Gerin Vigirex, areequipped with the “test” function, and alsopermanently monitor continuity of the detectioncircuit (toroid/relay link and toroid winding).

location of If

1

2

3

I testR

Id

→

→

→

→

Test

Id

Ir

I test

→ → → → →Ir = Id + I test ⇒ Ir u If

Fig. 20 : some test circuits created on installation maynot operate in the presence of weak fault currents.


Fig. 21 : fault currents stipulated in the RCD construction standards.

c Verification of the operating thresholdEven more so than for the test, it is important tobear in mind when carrying out this verificationthat leakage currents of the downstream circuit,

whether or not they are natural, may flowthrough the sensor.For reliable measurement, the downstreamcircuit will always be disconnected.

Sensitivities (I∆n)

These are standardised by the IEC:

c high sensitivity -HS-: 6-10-30 mA,

c medium sensitivity -MS-: 100-300 and500 mA,

c low sensitivity -LS-: 1-3-5-10 and 20 A.

It is clear that HS is most often used for directcontact protection, whereas the othersensitivities (MS and LS) are used for all otherprotection needs, such as indirect contact

For RCDs of the type:

AC

B

A

t

t

t

Id

Id

Id

(TT earthing system), fire hazards and machinedestruction protection.

Tripping curves

These curves take into account the internationalstudies performed on electrical hazards(IEC 60479) and in particular:c the effects of current in the case of directcontact protection,

c limit safety voltage in the case of indirectcontact protection.

4.4 Product manufacturing standards

The main RCD manufacturing standards arelisted in the appendix.

The IEC has standardised for the RCDs, types,threshold values, sensitivities and operatingcurves.

AC, A and B type RCDs to be chosenaccording to the current to be detected

The current conveyed in electrical networks isincreasingly less sinusoidal. Consequentlystandard IEC 60755 has defined three types ofRCD: the AC, A and B types, according to theresidual current to be detected (see fig. 21 ).

c The AC type, for sinusoidal AC currents.c The A type, for sinusoidal AC currents, pulsedDC currents or pulsed DC currents with aDC component of 0.006 A, with or without phaseangle monitoring, whether they are suddenlyapplied or slowly increase.

c The B type, for the same currents as theA type, but in addition for rectifier currents:v with simple halfwave with a capacitive loadproducing a smoothed DC current,v three-phase with simple or double halfwave.


With respect to the domestic and similar sector,standards IEC 61008 (residual current circuit-breakers) and 61009 (residual current devices)define standardised operating time values (seetable in figure 22 for the operating curves G andS in figure 23 ):

c The G curve for the instantaneous RCDs.c The S curve for the selective RCDs with thelowest time delay level, used in France forincomer circuit-breakers for example.

For power residual current devices, they aregiven in appendix B of standard IEC 60947-2.The above standards define the maximumoperating time as a function of the Id/I f ratio forinverse response time RCDs (oftenelectromagnetic).Electronic RCDs, mainly used in industry andlarge tertiary, normally have an adjustablethreshold and time delay, and their responsetime is not dependent on the fault current.IEC 60364 (NF C 15-100) defines maximumbreaking times on final circuits for the TN and ITearthing systems (see fig. 24 ). For the TTearthing system, RCD operating time must be

Type In I∆n Standardised value of operating(A) (A) and non-operating times (in seconds) at:

I∆n 2 I∆n 5 I∆n 500 A

General All All 0.3 0.15 0.04 0.04 Maximum operating(instantaneous) values values time

Selective > 25 > 0.030 0.5 0.2 0.15 0.15 Maximum operatingtime

0.13 0.06 0.05 0.04 Minimum non -operating time

Fig. 22 : standardised values of the maximum operating times and non-operating times as per IEC 61008.

chosen according to fault voltage. In practice “G”and “S” type RCDs are suitable on final circuitsfor i 230/440 V network voltages. The standardalso stipulates that a time of 1 second isacceptable in the TT system, for distributioncircuits, in order to create the discriminationstages required for continuity of supply.

In addition to the above-mentionedcharacteristics of the residual current function,product standards also stipulate:

c impact strength and jarring withstand,c ambient temperature and humidity,

c mechanical and electrical durability,

c insulation voltage, impulse voltage withstand,c EMC limits.

The standards also make provision for type testsand for periodical checking of quality andperformance carried out either by themanufacturer or by approved organisations.They thus guarantee users product quality andsafety of people.RCDs are also marked for quality, for exampleNF-USE marking in France.

Fig. 23 : maximum operating time curves for “S” (selective) and “G” (general purpose) residual current circuit-breaker or device.

1 2 5 1010

20

50

100

200

500

t (ms)

Id / I∆n.

500

G

S max.

A


Nominal Maximum breaking time (s)phase-to-earth TN IT ITnetwork voltage Non-distributed Distributed(VCA). neutral neutral

120-127 0.8 0.8 5

220-230 0.4 0.4 0.8

400 0.2 0.2 0.4

580 0.1 0.1 0.2

Fig. 24 : maximum breaking times.

4.5 The various devices

Areas - Types Network earthing system Sensitivity Time delay

Domestic and similar

Extension with earth TT - TN - IT i 30 mA 0leakage protection(breaking by built-in contact)

Earth leakage TT - TN - IT 30 mA 0protection socket(breaking by built-in contact)

Residual current TT - TN - IT 30 - 300 mA 0circuit-breaker

Residual current device

c Incomer TT In France “S” type as optionI∆n = 500 mA (disturbed network withis the most common or without surge arrester)

c Final distribution TT 30 - 300 mA 0

Industry and large tertiary

Residual current TT - (TN and IT in socket 30 - 300 mA 0circuit-breaker circuit protection)

Residual current device

c Power TT - (TN and IT in fire, 30 mA to 30 A 0 to 1 smachine and long outgoerprotection)

c Final distribution TT - (TN and IT in fire and 30 - 300 mA 0machine protection)

Residual current relay TT - (TN and IT in fire, 30 mA to 30 A 0 to 1 swith separate toroid machine and long

outgoer protection)

Fig. 25 : general presentation of the various RCDs.

c Analyse operating requirements (discriminationneeds, fail-safe safety needs, etc.), in order todetermine:v the required threshold level (sensitivity),v the time delay ranges (delay).

The table in figure 25 gives a concisepresentation of the various devices.

The standards state that technologically differentRCDs exist that are suited to the two mainsectors: domestic and industry.

The RCD must be chosen according to thenetwork earthing system and the protectiontarget (direct contact, indirect contact, loadprotection, etc.). However it is also necessary to:

c Define its type (A, AC or B) from the networkcharacteristics (AC, mixed, etc.),


5 Optimised use of the RCD

5.1 EMC: manufacturers’ obligations and what this implies for contractors

EMC (Electro Magnetic Compatibility) is thecontrol of electrical interference and its effects:a device must neither be disturbed nor disturb itsenvironment.

All electrical equipment manufacturers mustnaturally comply with certain EMC standards.RCDs are tested for electromagneticcompatibility (emission and susceptibility)according to the European Directive whichspecifies compliance with a certain number ofstandards (for example: EN 61543 for domesticRCDs).However, electrical installations generate ortransmit disturbances (see “Cahier Technique”no. 187), which can be permanent or temporary,alternating or impulse, low or high frequency, aswell as conducted or radiated, common ordifferential mode, internal or external tobuildings. Overvoltage is one of the mosttroublesome disturbances.

Overvoltage withstand

RCDs can be sensitive to lightning strokes,particularly on overhead networks which aremore likely to be affected by atmosphericdisturbances. In point of fact, according to thedistance of the cause of the disturbance, an LVnetwork can be subjected to:

c An overvoltage occurring between the liveconductors and the earth: the disturbance flowsoff to the earth well upstream of the RCDs(see fig. 26a ).c An overcurrent, a part of which flows off in thenetwork downstream of the RCD, particularly viathe stray capacitances (see fig. 26b ).

c An overcurrent detected by the RCD andwhich is due to breakdown downstream of thisRCD (see fig. 26c ).

Technically speaking, solutions are known andnormally implemented by RCD manufacturers.Such solutions include:c For electromagnetic relays, installation of aparallel diode on the relay exciting circuit. Thissolution is used for incomer circuit-breakers.c For electronic relays, use of a low-pass filter atsignal shaping level (see fig. 11).

Manufacturing standards make provision forRCDs immunised against these stray currents:the “S” type RCDs (I∆n u 100 mA). However

Fig. 26 : standardised voltage and current wavesrepresentative of lightning.

manufacturers also propose devices with highsensitivity and reinforced immunity such as theMerlin Gerin RCDs of the “si” type (I∆n i 30 mA).Thus, confronted with this problem, installationservice quality is only dependent on the devicechosen.

t

u

50 µs1,2 µs

a

t

I

20µs8 µs

c

t

I b

10 µs


Influence of choices when designing aninstallation

Installation designers and installers, whilerespecting proper procedures, are also active inthis area, particularly when choosing the earthingsystem for the installation. For example theymust know that in the TN system, severalcurrents are responsible for the disturbance dueto radiation of sensitive devices:

c On an insulation fault, strong currents flow inthe PE, the device frames and the structures.

Note

Problems may be encountered whenimplementing discrimination if it is necessary tocombine residual current devices and residualcurrent relays, since:

c The residual current device is defined in delaytime -tr-.

Fig. 27 : vertical discrimination.

5.2 A need: discrimination

Ensure that only the faulty outgoer is de-energised by the tripping of the protectiondevice: this is the purpose of discrimination andthe aim of protection co-ordination.

“Vertical” discrimination

This type of discrimination presides over theoperation of two protection devices connected inseries on a circuit (see fig. 27 ).

In view of RCD operating requirements as wellas of their manufacturing standards,discrimination must be both current and time.

c Current, as, according to the standards, anRCD must trip at I∆n and not trip at I∆n / 2. Inpractice, a ratio of 3 is required:I∆n (upstream) u 3 I∆n (downstream).

c Time, as, in order to react, all mechanismsneed a period of time, even the smallest, towhich a deliberate time delay or delay mustsometimes be added.

The double condition of non tripping of Da for afault downstream of Db is therefore:I∆n (Da) > 2 I∆n (Db).

and

tr (Da) > tr (Db) + tc (Db) or tr (Da) > tf (Db)

where:

tr = tripping delay = time of non operationtc = time separating the moment of breaking(including arcing time) from the moment whenthe breaking order was given by themeasurement relay,tf = operating time, from detection of the faultthrough to complete breaking of the fault current.

Time-delayable electronic relays may exhibit afault memorisation phenomenon by theirthreshold circuit. It is then necessary to take intoaccount a “memory time” -tm- (see fig. 28 ) toensure that they do not trip after opening of thedownstream device:tr (Da) > tf (Db) + tm.

Fig. 28 : the time delay of an upstream RCD must takeaccount of the breaking time associated with thedownstream RCD and of the memory time of theupstream relay.

c In the TN-C earthing system, load unbalancecurrents flow continuously in the metal structuresof the buildings.

c In the TN-S earthing system, these unbalancecurrents also appear on an insulation faultbetween the neutral and the protectionconductor. Moreover, this fault, which cannot bedetected by the overcurrent protection devices,insidiously changes the TN-S system into aTN-C system.

tr tc

tm

tc

tm

tc

tr

(1)

(2)

(3)

RCD

RCD

Da

Db


c The residual current relay is defined in specificoperating time or time delayed to a value t, whichcorresponds to the time elapsing between theoccurrence of the fault and the transmission of theopening order to the breaking device (see fig. 29 ).

Fig. 29 : two examples of time discrimination, associating a residual current device of the Vigicompact type and aVigirex relay (Merlin Gerin). Note that these times are far shorter than the authorised tripping times in figure 24.

Fig. 30 : example of horizontal discrimination.

The successive times tf and tr (or t) must then becalculated (at 2 I∆n) for each RCD, starting atfinal distribution and moving back towards theorigin of the installation.

“Horizontal” discrimination

Sometimes referred to as circuit selection,stipulated in standard NF C 15-100 paragraph536.3.2, it means that a residual current deviceplaced in a cubicle at the supply end of theinstallation is not necessary when all theoutgoers in this cubicle are protected by residualcurrent devices. Only the faulty outgoer is de-energised: the residual current devices placedon the other outgoers (parallel to the faultyoutgoer) do not detect the fault current(see fig. 30 ). The residual current devices maythen have the same tr (or t).

In practice, horizontal discrimination may gowrong. Indeed nuisance tripping known as“sympathy tripping” has been observed,particularly on networks containing very longoutgoers (stray capacitances of unbalancedcables) or capacitive filters (computer).Two examples are given below:

c Case 1 (see fig. 31 )The opening of Db placed on the supply circuitof a load R, a powerful overvoltage generator

(e.g. welding machine) causes an overvoltageon the network.

This overvoltage causes on outgoer A,protected by Da, the occurrence of a capacitiveearthing current which may be due to the straycapacitances of the cables or to a capacitiveearthing filter.

RCDRCD

tr = 15 ms

tc = 30 ms

tf = 45 ms

Vigicompact

tr = 60 ms

RCD

Vigicompact

RCD

Vigirex

tr = 60 ms

tf < 140 ms

t = 200 ms

RCD

Vigirex RH

RCD


Fig. 31 : the presence of a capacitance on outgoer Amay cause:c on opening of Db, the tripping of Da,and/orc on energisation of outgoer A, the tripping of Da.The use of time-delayed RCDs is often necessary toprotect against the nuisance tripping caused bylightning overvoltages or equipment switchings.

Fig. 32 : transient current wave occurring on closing ofa highly capacitive circuit.

A solution: the RCD of Db may be instantaneousand the RCD of Da must be time-delayed.

Note that for this configuration, the time delay ofthe RCD (Da) is often vital as, when circuit A isenergised, the capacitances (stray or otherwise)cause the appearance of a damped oscillatingresidual current (see fig. 32 ).

As a guideline, a measurement taken on a largecomputer containing an interference filterrevealed a current with the followingcharacteristics:v 40 A (first peak),v f = 11.5 kHz,v damping time (66 %): 5 periods.

c Case 2 (see fig. 33 )A full insulation fault on phase 1 of outgoer Bplaces this phase at the potential of the earth.

Db

Da

RCD

(A)

(B)

RCD

1

2

3

Extended network

Cp

Fig. 33 : in the presence of a fault, Da may open instead of Db. Use of time-delayed RCDs is often necessary toprotect against nuisance tripping on healthy outgoers.

The capacitive current supplied by outgoer A willcause “by sympathy” the tripping of thecorresponding RCD. This phenomenon exists forall earthing systems, but mainly affects networksusing the IT system.

Both examples show the need to time delay theRCDs of long outgoers and those containingfilters.

Use of directional RCDs is another solution toprevent tripping due to the “return” of capacitivecurrent via the healthy outgoer.This type of RCD detects the fault current,compares its amplitude with the scheduledthreshold level and only trips if this currentpasses through the toroid from upstream todownstream.

t

I

Da Db

(A) (B)

R

RCDRCD


5.3 Avoiding known problems

Fig. 34 : in an installation containing a surge arrester,according to local obligations, the RCD may be placeddifferently: in A a time-delayed or “S” type RCD and inB a standard RCD.

Taking leakage currents into account

The last sub-chapter emphasises the attentionthat must be paid to these currents, oftencapacitive, which by “deceiving” the RCDs areable to seriously disturb operation.c 50 Hz - 60 Hz leakage currents

As from the design stage of the installation, thelengths of the various outgoers should beevaluated, together with the future equipmentcontaining capacitive earthed devices. It is thennecessary to design a distribution system able toreduce the importance of this phenomenon.Consequently, interference filters (compulsoryaccording to the European directive on EMC)placed on the microcomputers and otherelectronic devices, generate in single-phasepermanent leakage currents at 50 Hz of theorder of 0.5 to 1.5 mA per device.These leakage currents add up if the devices areconnected to the same phase. And if thesedevices are connected to all three phases, thesecurrents cancel each other out when they arebalanced (vector sum). This reflection is all themore true when the RCDs installed have lowthresholds. In order to guard against nuisancetripping, the permanent leakage current must notexceed 0.3 I∆n in the TT and TN systems, and0.17 I∆n in the IT system.

c Transient leakage currentsThese currents appear on energisation of acircuit with a capacitive unbalance (see fig. 33)or on a common mode overvoltage. “S” typeRCDs (I∆n u 300 mA) and “si” type RCDs(I∆n = 30 mA and 300 mA) prevent nuisancetripping as do also slightly time-delayed RCDs.

c High Frequency leakage currentsExamples of large EMC polluters are thyristorrectifiers whose filters contain capacitors whichgenerate an HF leakage current able to attain5% of nominal current. Unlike the 50 Hz - 60 Hzleakage currents whose vector sum is zero,these HF currents are not synchronous over allthree phases and their sum constitutes aleakage current. In order to prevent nuisancetripping, RCDs must be protected against theseHF currents (equipped with low-pass filters): thisis the case for industrial RCDs and for theMerlin Gerin “S” and “si” type RCDs.c Lightning currents

If the installation is equipped with a surgearrester, the RCD sensor should not be placedon the flow path of the current generated by thelightning (see fig. 34 ). Otherwise,

implementation of RCDs immunised againstthese currents (time-delayed or “S” type) is thesolution.

Maintaining the earthing system

When replacement sources are provided,protection of people and equipment should bestudied in the various configurations of theinstallation, as the position of the neutral withrespect to the earth may be different. Thesupply, even temporary, of an installation with agenerator set requires interconnecting the set’sframe with the existing earthing networkwhatever the earthing system and, in the TTsystem, earthing of the generator neutral, sinceotherwise the fault currents would not reach theRCD threshold.When the installation in the TT earthing systemcontains an Uninterruptible Power Supply (UPS),earthing of the neutral downstream of the UPS isessential for proper operation of the RCDs(K contactor on figure 35 ), but not for protectionof people as:

c The installation is then in the IT system andthe first fault is not dangerous (see standardC 15-402, paragraph 6.2.2.2.).

c The likelihood of a second insulation faultoccurring during the period of operation limitedby back-up time of the UPS batteries is veryslight.

ARCD

BRCD

Flow off of currentgenerated by lightning

Surge arrester


Fig. 35 : on detection of mains power loss on the UPS supply, the contactor K reproduces the TT system downstream of the UPS.

Fig. 36 : curves established from the maximum breaking times of an RCD laid down by NF C 15-100,paragraph 413.1.1.1.

5.4 RCDs for mixed and DC networks

An insulation fault with DC current is far lessdangerous than with AC current

Experiments (see fig. 5) have shown that forweak currents people are approximately 5 timesless sensitive to DC current than to 50/60 HzAC current.The risk of ventricular fibrillation only appearsover 300 mA.

Installation standards NF C 15-100 andIEC 60479 have chosen a ratio close to 2, taking

100 230 400 50050100 200 250 300 400 500

1

2.5

5

0.2

0.3

0.5

0.75

0.04

0.080.1

200 (V AC)(V DC)120

Contact voltage

t (s)

3L

3L

3LN

Transfer switches

3L

3L

N

3L

N

3L

N

Bypass circuit

N

Backed up equipment

Non backed up outgoers

NPower loss detection relay

Transfer switches

(Maintenance) K

Fault supplied by the self-generating UPS

Fault supplied by the mains

account of the fact that in practice fault currentsare directional but not always smoothed.

This is illustrated by figure 36 drawn up usingthe table in figure 7.

Note that a three-phase rectifier supplied by a400 V AC phase-to-phase voltage generates adirect contact voltage of 270 V DC, whichcorresponds to a maximum breaking time of0.3 s.


RCD manufacturing standards take into accountthe existence of non AC currents, andparticularly define the standard cases shown infigure 21 and describe the relevant tests. To givean example, residual current circuit-breakersmust operate for Id i 1.4 I∆n in all casescorresponding to figure 37 , with or withoutsuperimposition of a smoothed DC current of6 mA: the fault current is applied either suddenlyor by slowly increasing from 0 to 1.4 I∆n in 30 s.

The RCDs satisfying these tests can beidentified by the following symbol on their frontface:

kkkkk

Real fault currentsThese currents reflect the voltages existingbetween the fault point and the neutral of theinstallation. The waveform of the fault current isseldom the same as that of voltage or appliedcurrent, delivered to the load. Fault voltages andcurrents of the pure DC type (zero ripple factor)are very rare.

c In the domestic sector, distribution and rectifiercircuits are single-phase, and correspond to thediagrams marked A to F in figure 38 . A typeRCDs provide protection of people. However, fordiagram B, they only detect fault currents if theiroccurrence is sudden. Note that circuit E isincreasingly common as it is placed at the inputof switch mode power supplies that arewidespread in electrical household appliances(TV, microwave, etc.) as well as in professionalequipment (microcomputers, photocopiers, etc.).

c In industry most rectifier circuits are three-phase (diagrams G to K in figure 39 , seepage 30).

Some circuits may generate a DC fault currentwith a small ripple factor:

Fig. 40 : evolution of the sensitivity of an electronicRCD placed upstream of a thyristor rectifier.Fig. 37 : waveform of the A type RCD test currents.

v Circuits G and HCircuit G supplies a rectified voltage with apermanent small ripple factor, and consequentlyfault currents that are hard to detect by the RCD.On the other hand, circuit H generates highlychopped fault currents which are thus visible bythe RCD. However this circuit is equivalent tocircuit G for full wave conduction.v Circuit JThis common circuit type is particularly used forvariable speed controllers used in DC motors.The back-electromotive force and reactor of themotors generate smoother fault currents than theG and H circuits described above. However,regardless of the thyristor conduction angle, theRCDs placed upstream of the variable speedcontrollers must be able to provide protection.Some standard RCDs may be suitable providedtheir lDn threshold receives a suitable setting.

To give an example, figure 40 shows thesensitivity of an RCD, with analogue electronictechnology, according to the variable speedcontroller output voltage applied at the motor.v Circuit KWith this circuit type, a fault on the DC circuitdoes not produce dϕ / dt in the magnetic sensorsof the RCDs which are then “blinded”. This circuitis dangerous unless a transformer is usedinstead of an autotransformer, as AC and A typeRCDs are inoperative.

Special case: DC current return

Let us now see what happens when a secondfault occurs on the AC part of a network(see fig. 41 ) containing a rectifier according tocircuit G described above. If the power supply(A) of the rectifier is not monitored by an RCD, orif this RCD has been incorrectly chosen or isinoperative for any reason, the insulation faultexisting on the DC part remains.

and

and

and

90°

135°

Sensitivity

50 %

0.15

20 %

100 %

100 %

Ud/Udo

Sinusoidal AC fault

On-load motor

Off-load motor


ph

N

R

A/Soldering iron or two "setting"light dimmer switch

Id

ωt

Id

ωt

B/Television, battery charger, etc.

Id

ωt

Id

ωt

C/Light dimmer, arc welding machine

D/Household appliances with motor (universal)

ph

N

R

ph

N

R

Id

ωt

E/

F/

ph

N

M_

Id

ωt

ph

NR

ph

NR

Fig. 38 : form of the fault currents detected on the single-phase supply of rectifiers when an insulation fault occurs on their positive output.


Fig. 39 : form of fault currents detected on the three-phase supply of rectifiers when an insulation fault occurs on their output.

G/Welding machinec electromagnetc electrolysisc etc.

H/Rectifier set for:c industrial DC networkc electrophoresis

J/Variable speed controller for DC motor.

K/Stationary battery charger for:c DC auxiliary networkc UPS

NB : The fault current is “pulsed” at low speeds and is very close to pure DC current at high speeds.

NB: In this diagram, the smoothing reactor (L) causes conduction (cyclic and in pairs) of the thyristors such that the fault point (+) or (-) is always electrically connected to the neutral, resulting in a virtually pure DC fault current.

1

2 R

3

(+)

(-)

(-)

1

2 R

3

(+)

1

2

3

(+)

M_

(-)

(+)

1

2

3

(-)

L

Id

ωtN

Id

ωtN

Id

ωtN

+

-

Fault on (-)

Fault on (-)

Fault on (+)

Fault on (-)

Fault on (+)

Fault on (+)

NB: The fault current in (+) follows the upper limit of the conduction zones. Likewise, the fault current in (-) follows the lower limit.


Fig. 41 : the current of a latched fault at the rectifier output (non-opening of Da) may “blind ” the RCD placedon B.

However, should a fault occur on an AC outgoerB, the current of this fault is equal to i1 + i2, andthere is no certainty that the RCD placed on thisoutgoer, if it is of the AC type, will trip at thedisplayed threshold. For this reason standardC 15-100, paragraph 532-2-1-4 stipulates:“When electrical devices likely to produce DCcurrents are installed downstream of an RCD,precautions must be taken so that, should an

earth fault occur, the DC currents do not affectoperation of the RCDs and do not jeopardisesafety”.

It is thus advisable to:

ccccc choose the right RCD placed just upstream ofa rectifier system,

ccccc if necessary, use A type RCDs in theremainder of the installation.

32

1

(A)

N

(B)

+

-

N

311 V

i1

i2Ru

Ru

Da

Db

At a time when electricity, as an energy source,is playing an increasingly dominant role inhousing, tertiary and industry, it is useful to pointout and quantify the electrical hazard and tofurther knowledge of Residual Current Devices.These devices, like any others, have their strongand weak points. Not yet fully perfected, theyplay an increasingly important role in theprotection of people and equipment. Allindustrialised countries make extensive use ofRCDs, with a variety of earthing systems, both inindustry and housing.Generally speaking, the following information isimportant for installation standards andpractices:

c For protection of people against the indirectcontact risk, an RCD is:v compulsory in the TT system,v necessary in the IT system if there are severalearthing connections,

6 Conclusion

v to be provided in the case of very longoutgoers in the TN and IT systems.

c For protection of people against direct contactrisk, an RCD is very useful and often stipulatedby standards as an additional precautionirrespective of the earthing system.

c RCDs also provide protection against:v fires of electrical origin,v destruction of machines in the TN system,v electromagnetic disturbances in the TN-Ssystem (neutral insulation monitoring).

Present day RCDs comply with constructionstandards (see chapter 4) and continue toprogress in terms of reliability and immunity tointerference phenomena which are notascribable to insulation faults.

The purpose of this study is to further knowledgeof residual current devices and therebycontribute to the safety of us all.


Bibliography

Standards

From 1997 onwards the new publications,issues, versions and IEC amendments toexisting publications have a designation in the60000 series.We would like to draw the users’ attention to thefact that the former publications printed before1997 continue to bear the old numbers on theprinted copies, while waiting to be revised.

“Product” standards

c IEC 60479: Guide to the effects of currentpassing through the human body.c IEC 60755: General rules concerning residualcurrent protection devices.

c IEC 60947-2: Low voltage switchgear - Part 2:Circuit-breakers.

c IEC 61008, NF C 61-150 and 151: Automaticresidual current circuit-breakers for domestic andsimilar purposes.

c IEC 61009, NF C 61-440 and 441: Circuit-breakers for domestic and similar purposes.c IEC 61557-6, NF EN 61557-6: Electricalsafety in low voltage distribution systems up to1000 V AC and 1500 V DC - Part 6: Residualcurrent devices (RCD) in TT and TN systems.

c UTE C 60-130: Residual current protectiondevices.

c NF C 61-420: Small residual current devices.c NF C 62-411: Connection and similarequipment, residual current devices for firstcategory installation monitoring switchboards.

c Draft standard: earth leakage protectionsocket.

“Installation” standards

c IEC 60364, NF C 15-100: LV electricalinstallationsc UTE C 15-401: practical guide, installation ofthermal motor/generator sets

c UTE C 15-402: practical guide, staticuninterruptible power supplies (UPS).

Schneider Electric “Cahiers Techniques”

c Protection of people and uninterruptible powersuppliesJ.-N. FIORINA, “Cahier Technique” no. 129

c Evolution of LV circuit-breakers with standardIEC 60947-2E. BLANC, “Cahier Technique” no. 150

c Earthing layouts in LVB. LACROIX and R. CALVAS,“Cahier Technique” no. 172c Earthing systems worldwide and evolutionsB. LACROIX and R. CALVAS,“Cahier Technique” no. 173

c Disturbances of electronic systems andearthing systemsR. CALVAS, “Cahier Technique” no. 177

c Cohabitation of strong and weak currentsR. CALVAS and J. DELABALLE,“Cahier Technique” no. 187

Other publications

The Schneider Electric guide to the LV electricalinstallationEditor: CITEF S.A.

Schneider Electric Direction Scientifique et Technique,Service Communication TechniqueF-38050 Grenoble cedex 9Fax: (33) 04 76 57 98 60

DTP: AXESS - Saint-Péray (07)Edition: Schneider ElectricPrinting: Imprimerie du Pont de Claix - Claix - France - 1000- 100 FF - ©

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Documents

ECT114-Residual Current Devices in LV