Residual current devices in LV (no 114) by hamada1331


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

Cahier technique no. 114

Residual current devices in LV

                                                         R. Calvas
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no. 114
Residual current devices in LV


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

ECT 114 updated, February 1999

                             Cardiac fibrillation:                               Leakage current:
                             A malfunctioning of the heart corresponding to      Current which, in the absence of an insulation
                             loss of synchronism of the activity of its walls    fault, returns to the source via the earth or the
                             (diastole and systole). The flow of AC current      protection conductor.
                             through the body may be responsible for this due    Limit safety voltage (UL):
                             to the periodic excitation that it generates. The   Voltage UL below which there is no risk of
                             ultimate consequence is stoppage of blood flow.     electrocution.
                             Direct contact:                                     Live conductors:
                             Contact of a person with the live parts of          Set of conductors assigned to electrical power
                             electrical devices (normally energised parts and    transmission, including the neutral in AC and the
                             conductors).                                        compensator in DC, with the exception of the
                             Earthing system:                                    PEN conductor whose “protection conductor”
                             Standard IEC 60364 stipulates three main official   (PE) function takes priority over the “neutral”
                             earthing systems which define the possible          function.
                             connections of the neutral of the source and        Operating residual current If:
                             frames to the earth or neutral. The electrical      Value of the residual current causing a residual
                             protection devices are then defined for each one.   current device to trip.
                             Electrification:                                    According to construction standards, at 20°C
                             Application of voltage between two parts of the     and for a threshold set at IDn, low voltage
                             body of a living being.                             residual current devices must comply with:
                             Electrocution:                                      Ι∆n
                             Electrification resulting in death.                     < Ι f < Ι∆n
                             Fault current Id:
                                                                                 In high voltage, the “zero phase-sequence”
                             Current resulting from an insulation fault.
                                                                                 relays have, allowing for operating accuracy, an
                             Frame:                                              operating current equal to the threshold
                             Conductive part likely to be touched and which,     displayed in amperes.
                             although normally insulated from live parts, may
                                                                                 Protection conductors (PE or PEN):
                             be brought to a dangerous voltage further to an
                                                                                 Conductors which, according to specifications,
                             insulation fault.
                                                                                 connect the frames of electrical devices and
                             Indirect contact:                                   some conductive elements to the earthing
                             Contact of a person with accidentally energised     connection.
                             frames (usually further to an insulation fault).
                                                                                 Residual current:
                             Insulation:                                         Rms value of the vector sum of the currents
                             Arrangement preventing transmission of voltage      flowing through all live conductors in a circuit at a
                             (and current flow) between a normally energised     point of the electrical installation.
                             element and a frame or the earth.                   Residual current device (RCD):
                             Insulation fault:                                   Device whose decisive quantity is the residual
                             Insulation rupture causing an earth fault current   current. It is normally associated with or
                             or a short-circuit via the protection conductor.    incorporated in a breaking device.

Cahier Technique Schneider Electric no. 114 / p.2
                                            Residual current devices in LV

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

1 Introduction                              1.1 The RCD: its scope                                                       p. 4
                                            1.2 “Residual current protection” and “Earth leakage protection”:            p. 4
                                            two 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. 6
of 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. 22
                                            for 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

                                                                               Cahier Technique Schneider Electric no. 114 / p.3
1 Introduction

1.1 The RCD: its scope
                             In electrical installations, direct and indirect                   Moreover, RCDs monitor insulation of cables
                             contacts are always associated with a fault                        and electrical loads, and are therefore
                             current which does not return to the source via                    frequently used to indicate insulation drops or to
                             the live conductors. These contacts are                            reduce the destructive effects of a strong fault
                             dangerous for people and equipment                                 current.
                             (see “Cahiers Techniques” no. 172 and 173).
                             For this reason the use of Residual Current
                             Devices (RCD), whose basic function is
                             detection of residual currents, is widespread.

                                                            current                        i1                          In
                                                                       Load                                                      Id    I1
                                        Source                         Fault               i3                               I3
                                                       Return                                          Load
                                                                       current                                                          I2
                                                       current                   Source    in

                                        id = ia - ir

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

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 a
                             protection device associated with a toroidal
                             sensor surrounding the live conductors. Its
                             function is detection of current difference or, to
                             be more precise, residual current (see fig. 1 ).
                             Existence of a residual current indicates
                             presence of an insulation fault between a live
                             conductor and a frame or the earth. This current
                             takes an abnormal path, normally the earth, to
                             return to the source.
                             The RCD is normally combined with a breaking                       Fig. 2 : earth leakage protection.
                             device (switch, circuit-breaker, contactor) which
                             automatically de-energises the faulty circuit.
                             “Earth leakage protection” consists of one or
                             more measuring devices whose function is to
                             detect a difference between the input current and
                             the output current on part of the installation: line,
                             cable, transformer or machine (generator, motor,
                             This protection is mainly used in medium and
                             high voltage. Earth leakage protection (zero
                             phase-sequence current) for insulation fault
                             protection (see fig. 2 ) and current leakage
                             protection for phase-to-phase fault protection                     Fig. 3 : current leakage protection.
                             (see fig. 3 ) are both found.

Cahier Technique Schneider Electric no. 114 / p.4
1.3 The RCD, a useful protection device
                 The first decisive factor in choosing and using            Their efficiency was confirmed at the end of this
                 RCDs in an installation is the earthing system             century by the remarked reduction in the number
                 provided.                                                  of people electrocuted. The result of an
                 c In the TT earthing system (directly earthed              IEC survey conducted in August 1982 in Japan
                 neutral), protection of people against indirect            already proved the efficiency of these devices
                 contact relies on the use of RCDs.                         (see fig. 4 ).
                 c In the IT and TN earthing systems, the medium            “The residual current device is generally
                 and low sensitivity (MS and LS) RCDs are used:             recognised (throughout the industrialised world)
                                                                            as being the best and most reliable of the
                 v to limit the risk of fire,                               protection devices developed to provide
                 v to prevent the destructive effects of a strong           protection against indirect contact in the low
                 fault current,                                             voltage field”.
                 v to protect people against indirect contact (very         Such were the words of professor C.F. DALZIEL
                 long outgoers).                                            (Berkeley-USA), one of the pioneers of the study
                 c For all earthing systems, the high sensitivity           of the effects of electrical current on people in
                 (HS) RCDs provide additional protection against            the fifth international conference of the AISS
                 direct contact. They are compulsory in final               (Lucerne 1978).
                 distribution in a large number of countries.

                       Annual number of deaths
                       by electrocution

                                                          Decree of the law making HS-RCDs compulsory




                                66    67    68    69    70    71    72    73    74    75    76    77    78    79     80
                 Fig. 4 : graph showing the evolution of deaths by electrocution due to the use of hand-held tools in Japanese
                 companies. This figure begins to drop in 1970, the year after that in which a law was decreed making the use of
                 high sensitivity RCDs compulsory.

                                                                                 Cahier Technique Schneider Electric no. 114 / p.5
2 The patho-physiological effects of electrical current on people

                             The patho-physiological effects of electrical               sensitivity thresholds of people and of the risks
                             current on people (tetanisation, external and               incurred.
                             internal burns, ventricular fibrillation and cardiac        The International Electrotechnical Committee
                             arrest) depend on a variety of factors: the                 (IEC) has looked into the problem in order to
                             physiological characteristics of the person in              pool, at international level, a variety of viewpoints
                             question, the environment (e.g. dry or wet) and             reflecting and even often defending national
                             the characteristics of the current passing through          practices, habits and standards. Many scientists
                             the body.                                                   have participated in this undertaking and have
                             As protection of people is the main function of             helped clarify the subject (Dalziell, Kisslev,
                             the RCD, it is clear that optimum implementation            Osypka, Bielgelmeier, Lee, Koeppen, Tolazzi,
                             of these devices requires knowledge of the                  etc.).

2.1 Effects according to current strength
                             The effects of the electrical current passing
                             through the human body depend on the frequency
                             and strength of this current (see fig. 5 ).

                                    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.

2.2 Effects according to exposure time
                             The risks of non-release, respiratory arrest or             c Zone 4 (situated to the right of curve c1)
                             irreversible cardiac fibrillation (see lexicon)             In addition to the effects of zone 3, the likelihood
                             increase in proportion to the time during which             of ventricular fibrillation is:
                             the human body is exposed to the electrical                 v approximately 5 % between curves c1 and c2,
                             current (see fig. 6 ).                                      v less than 50 % between curves c2 and c3,
                             The chart in figure 6 identifies in particular zones        v more than 50 % beyond curve c3.
                             3 and 4 in which danger is real.                            Patho-physiological effects such as cardiac
                             c Zone 3 (situated between curves b and c1).                arrest, respiratory arrest and serious burns
                             For people in this situation, there is normally no          increase with current strength and exposure time.
                             organic damage. However there is a likelihood of            For this reason it is accepted that use of an RCD
                             muscular contractions, breathing difficulties and           with instantaneous operation and with a threshold
                             reversible perturbation of the formation of                 of less than 30 mA, ensures this situation is
                             impulses in the heart and of their propagation. All         never reached and such risks never incurred.
                             these phenomena increase with current strength              With a more general approach, IEC 60364
                             and exposure time.                                          (NF C 15-100 in France) stipulates the operating

Cahier Technique Schneider Electric no. 114 / p.6
times for the Residual Current Devices according                safety voltage UL (voltage below which there is
to contact voltage. These times are recalled in                 no risk for people, according to standard
the two tables in figure 7 .                                    NF C 15-100) is, in AC:
                                                                v 50 V for dry and wet premises,
Limit safety voltage (UL)                                       v 25 V for damp premises, for example for
According to environmental conditions and                       outdoor worksites.
particularly presence or absence of water, limit

                                                  Duration of current flow
                                     a                  b    c1 c2 c3

                                 1            2                    3                    4

                           0.1 0.2 0.5 1   2   5 10 20 50 100 200 5001000 2000 500010000
                                           Threshold = 30 mA Current flowing through the body
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 the
                                                            protection device (s)
                                                            AC                                         DC

             c Dry or wet premises or locations: UL i 50 V
             < 50                                      5                                               5
             50                                        5                                               5
             75                                        0.60                                            5
             90                                        0.45                                            5
             120                                       0.34                                            5
             150                                       0.27                                            1
             220                                       0.17                                            0.40
             280                                       0.12                                            0.30
             350                                       0.08                                            0.20
             500                                       0.04                                            0.10

             c Wet premises or locations: UL i 25 V
             25                                             5                                          5
             50                                             0.48                                       5
             75                                             0.30                                       2
             90                                             0.25                                       0.80
             110                                            0.18                                       0.50
             150                                            0.10                                       0.25
             220                                            0.05                                       0.06
             280                                            0.02                                       0.02
Fig. 7 : maximum duration of contact voltage holding as per standard IEC 60364.

                                                                       Cahier Technique Schneider Electric no. 114 / p.7
                             Direct contact                                               In the diagram in figure 8 , when the installation
                             Direct contact with normally energised parts is              neutral is earthed (TT earthing system) where:
                             dangerous for voltages in excess of UL. The                  RA = earthing resistance of the installation
                             main protection precautions to be taken are                  frames,
                             distance and insulation.                                     RB = earthing resistance of the neutral,
                             The RCD can detect a fault current flowing                   this implies choosing an operating threshold
                             through a person and, as such, is specified,                 (I∆ n) of the RCD such that:
                             regardless of the earthing system, in final                  Ud = RA Ι d ≤ UL
                             distribution as an additional protection. Its
                             operating threshold, as shown in the table in                and thus: I∆ n i
                             figure 5, must be less than or equal to 30 mA,                                  RA
                             and its operation must be instantaneous since                The protection operating time must be chosen
                             the value of the fault current, dependent on the             according to fault voltage
                             exposure conditions, may exceed 1A.
                                                                                          Ud =               U
                             Indirect contact                                                     RA + RB
                             On contact with an accidentally energised frame,             (see fig. 7).
                             the danger threshold is also fixed by the limit              Note that if the equipotentiality of the site is not
                             safety voltage UL.                                           ensured or is badly ensured, contact voltage is
                             To ensure there is no danger when network                    equal to fault voltage.
                             voltage is greater than UL, contact voltage must
                             be less than UL.



                                                              RCD                    PE


                                                    RB                               RA

                             Fig. 8 : fault voltage generation principle RCD.

2.3 Effects according to frequency
                             IEC 60479-2 deals with the effects of                        increases approximately by 10 mA to 100 mA
                             AC current of a frequency in excess of 100 Hz.               in rms value.
                             Skin impedance decreases in reverse                          Although standards do not yet stipulate specific
                             proportion to frequency. The standard states                 operating rules, the major manufacturers,
                             that the frequency factor, which is the ratio of             aware of the potential risks of such currents,
                             current at the frequency (f) over current at the             ensure that the thresholds of the protection
                             frequency of 50/60 Hz for the same physiological             devices they propose are below the ventricular
                             effect considered, increases with frequency.                 fibrillation curve defined in standard
                             Moreover, it has been observed that between                  IEC 60479-2 (see fig. 9 ).
                             10 and 100 kHz the perception threshold

Cahier Technique Schneider Electric no. 114 / p.8
          Id(ƒ) / Id(50 Hz)






                   10                        100                              1 000             10 000
                                                                                         Frequency (Hz)
                        A type ID
                        AC type ID
                        Vigirex RH328A
Fig. 9 : variations in ventricular fibrillation threshold (as per IEC 60479-2) and thresholds of various RCDs set on
30 mA, for frequencies of between 50/60 Hz and 2 kHz (source: Merlin Gerin).

                                                                   Cahier Technique Schneider Electric no. 114 / p.9
3 Insulation fault protection

3.1 The installation standards
                             RCDs are used in electrical, domestic and                c insulation: class II devices and safety
                             industrial installations. Their use depends on           transformers,
                             standards and mainly on the IEC 60364                    c earthing of frames,
                             (in France NF C 15-100).                                 c equipotentiality.
                             This standard officially stipulates three main
                             systems for earthing the electrical network: the         General rules
                             earthing systems (see fig. 10 ), used to a greater       Whatever earthing system is chosen for an
                             or lesser extent depending on the country.               installation, the standards require that:
                             Furthermore, for each of these systems it                c Each application frame be connected to an
                             defines more precisely the use of the RCDs, as           earthing connection by a protection conductor.
                             the electrical hazard is greatly influenced by           c Simultaneously accessible application frames
                             choice of earthing system (see “Cahier                   be connected to the same earthing connection.
                             Technique” no. 172).                                     c A breaking device automatically disconnects
                             It also describes the basic precautions which, in        all parts of the installation where a dangerous
                             normal operating conditions, considerably                contact voltage develops.
                             reduce electrical hazards, for example:                  c The breaking time of this device be less than
                             c distance and obstacles,                                the maximum time defined (see fig. 7).

                                       Directly earthed neutral (TT)                   Multiple earthed neutral (TN-C)
                                                                             1                                              1
                                                                             2                                              2
                                                                             3                                              3
                                                                             N                                              PEN

                                              RB              RA                               RB

                                       Unearthed neutral (IT)                          Multiple earthed neutral (TN-S)
                                                                             1                                              1
                                                                             2                                              2
                                                                             3                                              3
                                                                             N                                              N
                                                                             PE                                             PE

                                              RB                                               RB

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

Cahier Technique Schneider Electric no. 114 / p.10
3.2 The direct contact risk
                  This risk is the same for people whatever            532-2.6.1, states that RCDs with a threshold at
                  earthing system is used. The protection              most equal to 30 mA must protect the circuits
                  measures stipulated by standards are therefore       supplying power outlets when they are:
                  identical and use the possibilities offered by the   c Placed in damp premises or in temporary
                  high sensitivity RCDs.                               installations.
                  This is because:                                     c Of rating i 32 A in all the other installation
                  c As the fault current flows through a person in     cases.
                  contact with a live conductor, he or she is
                  exposed to the patho-physiological risks             Note
                  described above.                                     Standard IEC 60479 states that the resistance of
                  c An RCD placed upstream of the contact point        the human body is greater than or equal to
                  can measure the current flowing through the          1000 Ω for 95 % of people exposed to a 230 V
                  person and break the dangerous current.              contact voltage and thus through whom a 0.23 A
                  Regulations recognise the use of an RCD with         current flows.
                  high or very high sensitivity (i 30 mA) as an        An RCD with a 30 mA threshold does not limit
                  additional protection measure when the risk of       current, but its instantaneous operation ensures
                  direct contact exists due to the environment, the    safety up to 0.5 A (see fig. 6).
                  installation or people (article 412.5.1 of           Use of an RCD with a sensitivity of 5 or 10 mA
                  IEC 60364). This risk also exists when the           therefore does not increase safety. However it
                  protection conductor is likely to be broken or       makes the risk of nuisance tripping not negligible
                  does not exist (hand-held devices).                  as a result of capacitive leakage (distributed
                  In this case use of a high sensitivity RCD is        capacitances of cables and filters).
                  compulsory. Standard NF C 15-100, paragraph

3.3 Fire protection
                  Whatever earthing system is used, the electrical     that a 500 mA current can result in
                  installations of premises where risk of fire is      incandescence of two metal parts coming into
                  present must be equipped with RCDs of a              occasional contact.
                  sensitivity I∆n i 500 mA, as it is acknowledged

3.4 The “TT” earthing system
                  Protection of people against indirect contact        de-energise the faulty device as soon as the
                  In this system protection relies on use of RCDs.     voltage Ud exceeds the limit safety voltage UL.
                                                                       We remind you that their operating threshold
                  The fault current depends on the resistance of
                                                                       must be set at:
                  the insulation fault (Rd) and the resistances of
                  the earthing connection. A person in contact with    I∆n i UL .
                  the metal enclosure of a load with an insulation            RA
                  fault (see fig. 8) may be subjected to the voltage
                                                                       Protection of machines and equipment
                  developed in the load earthing connection (RA).
                                                                       The level of the RCD tripping thresholds
                  For example                                          necessary for protection of people in the TT
                                                                       earthing system is well below that of the fault
                  Where U = 230V, RA = RB = 10 Ω and Rd = 0, if        currents able to damage the magnetic circuits of
                  the person is not on an equipotential site, he or    machines (motor) or cause fires.
                  she is subjected to Uc = Ud = 115 V.                 The RCDs therefore prevent such electrical
                  Protection must be provided by use of an RCD of      destruction.
                  medium or low sensitivity which must

                                                                          Cahier Technique Schneider Electric no. 114 / p.11
3.5 The “TN” earthing system
                             Reminders                                              Detection of insulation faults between the
                             c With this earthing system, the current of a full     Neutral and the protection conductor (PE)
                             insulation fault is a short-circuit current.           or building frames
                             c In TN-C, in view of the fact that the neutral and    This type of fault insidiously transforms the TN-S
                             the protection conductor are combined, RCDs            system into a TN-C system. Part of the neutral
                             cannot be used. The following text therefore           current (increased by the sum of 3rd order and
                             mainly concerns the TN-S.                              multiple of 3 harmonic currents) permanently
                                                                                    flows in the PE and in the metal structures of the
                             Protection of people against indirect contact          building with two consequences:
                             As the fault current depends on the impedance          c Equipotentiality of the PE is no longer ensured
                             of the fault loop, protection is normally provided     (a few volts may disturb the operation of the
                             by overcurrent protection devices (calculation/        digital systems connected by bus and which
                             measurement of loop impedances).                       must have the same potential reference).
                             If the impedance is too great and does not allow       c Current flow in the structures increases the risk
                             the fault current to trip the overcurrent protection   of fire.
                             devices (very long cables), one solution is to use     The RCDS are used to highlight this type of
                             a low sensitivity RCD (I∆n u 1 A).                     fault.
                             Moreover this system cannot be used when, for
                             example, the network is supplied by a                  Detection of insulation faults without tripping
                             transformer whose zero phase-sequence                  and protection of equipment
                             impedance is too great (star-star connection).
                                                                                    In the TN-S earthing system, unlike the IT
                             Protection of electrical devices and circuits          system, there are no safety rules stipulating
                                                                                    insulation monitoring. However, all tripping
                             In the multiple earthed neutral system, insulation
                                                                                    further to an insulation fault is the cause of
                             faults are responsible for strong fault currents
                                                                                    operating losses and often of costly repairs prior
                             equivalent to short-circuit currents. The flow of
                                                                                    to re-energisation. For this reason, more and
                             such currents results in serious damage, for
                                                                                    more often operators request prevention
                             example: perforation of the magnetic circuit
                                                                                    devices in order to take action before the
                             plates of a motor, requiring replacement instead
                             of rewinding of the motor. Such damage can be          insulation loss becomes a short-circuit.
                             greatly limited by use of a low sensitivity RCD        The answer to this need is the use in indication,
                             (e.g. 3 A) with instantaneous operation, which is      in TN-S, on critical outgoers, of an RCD with a
                             thus able to react before the current reaches a        threshold of around 0.5 to a few amperes, which
                             high value.                                            can detect insulation drops (on the phases or
                             Note that the need of protection increases as          neutral) and alert operators.
                             operating voltage rises, as the energy lost at the     On the other hand, the risk of electrical fire is
                             fault point is proportional to the voltage square.     reduced and destruction of equipment avoided
                             The economic consequence of such destruction           by using an RCD with tripping for I∆n i 500 mA.
                             must be estimated as it is a vital criterion in
                             choice of earthing system.

3.6 The “IT” earthing system
                             Protection of people against indirect contact          When the second fault occurs, the installation
                             When the first insulation fault occurs, the fault      finds itself in a situation similar to a fault in the
                             current is very weak and the fault voltage not         TN earthing system. However there are two
                             dangerous: the standards require that this fault       possibilities: that of a single earthing connection
                             be indicated (by the permanent insulation              for all the frames and that of multiple earthing
                             monitors) and tracked (by the power on fault           connections.
                             tracking devices).

Cahier Technique Schneider Electric no. 114 / p.12
c Case of a single earthing connection                 Protection of equipment, electrical devices
In this case protection is usually ensured by the      and circuits
overcurrent protection devices (calculation/           Although there is no particular danger for
measurement of the loop impedances).                   equipment when the first fault occurs, a second
c Case of multiple earthing connections                fault is normally responsible for strong fault
When both faults affect devices not connected to       currents equivalent to short-circuit currents, as in
the same earthing connection, the fault current        the TN earthing system.
may not reach the operating threshold of the           RCDs with medium or low sensitivity can then be
overcurrent releases. The standards stipulate          provided for the more critical cases (premises
use of RCDs on each group of frames                    with risk of fire, sensitive and expensive
interconnected with the same earthing                  machines), bearing in mind that the risk of the
connection.                                            second fault is particularly low, especially when
c In all cases, simple or multiple earthing            tracking of the first faults is systematic. In actual
connections                                            fact, assuming a fault once every three months
If the impedance of a fault loop is too great (very    and that this fault is eliminated the same day, the
long cables), a simple, practical solution is to use   average time between two “double faults” is
a low sensitivity RCD (1 to 30 A).                     approximately 22 years!

                                                           Cahier Technique Schneider Electric no. 114 / p.13
4 RCD operating principle and description

4.1 Operating principle
                             All residual current devices are made up of at              sends, with a possible deliberate delay, the
                             least two components:                                       opening order to the associated breaking
                             c The sensor                                                device.
                             The sensor must be able to supply an electrical             The unit controlling the opening of the device
                             signal which is useful when the sum of the                  (switch or circuit-breaker) placed upstream of
                             currents flowing in the live conductors is                  the electrical circuit monitored by the RCD is
                             different from zero.                                        known as the trip unit or actuator.
                             c The measurement relay                                     The entire RCD is shown in the diagram in
                             The relay compares the electrical signal                    figure 11 .
                             supplied by the sensor with a setpoint value and

                                                                                                   Time delay       Static or relay
                                              Toroid        Shaping          Threshold             relay            output


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

4.2 Sensors
                             Two types of sensors are normally used on                   The current transformers (CT)
                             AC circuits:                                                To measure the residual current of a three-
                             c The toroidal transformer, which is the most               phase electrical circuit without neutral, three
                             common for measuring leakage currents.                      current transformers must be installed as shown
                             c The current transformers, used in HV and MV               in figure 12 .
                             and sometimes in LV.

                             The toroidal transformer                                         I1            I2           I3
                             This covers all the live conductors and is thus
                             excited by the residual magnetic field                                                               A
                             corresponding to the vector sum of the currents                                                          Ih
                             flowing through the phases and the neutral.                                                                   RCD
                             Induction in the toroid and the electrical signal
                             available at the terminals of the secondary                                                          B
                             winding are thus the image of the residual current.
                             This type of sensor is used to detect residual              Fig. 12 : the vector sum of the phase currents yields
                             currents from a few milliamperes to several                 the residual current.
                             dozen amperes.

Cahier Technique Schneider Electric no. 114 / p.14
The three CTs are parallel-connected current
generators, causing circulation between A and B
                                                              HV / LV                          G
of a current which is the vector sum of the three
currents and thus the residual current.                                                                      1
This circuit, known as the Nicholson circuit, is                                                             2
commonly used in MV and HV when the earth                                                                    3
fault current can reach several dozen or even
several hundred amperes.
                                                               N               RCD             RCD
During use, care must be taken with the
CT accuracy class: with CTs of 5 % class, it is
prudent not to set earth protection below 10 % of
their nominal current. The HV electrical
installation standard NF C 13-200 of December
1989 specifies 10 %.
                                                       Fig. 13 : toroid N delivers the same information as
Special cases                                          toroid G.
c High power supply
The Nicholson CT circuit, which would be useful
in LV when the conductors are large cross-
section bars or cables for the transmission of
strong currents, does not allow, even with
coupled CTs, settings that are compatible with
protection of people (threshold I∆n i UL / Ru).
There are a number of solutions:
v If the problem occurs in a main switchboard
downstream of the transformer, the following
may be considered:                                                               RCD
- either installation of a toroid at the supply end
of the installation on the earthing connection of
the transformer LV neutral (see fig. 13 ). This is
because, according to the Kirchhoff node law,
the residual current detected by (N) is strictly the
same as that detected by (G) for a fault occurring
in LV distribution,                                    Fig. 14 : toroids placed on the outgoers and parallel-
- or installation of a toroid on each outgoer, all     connected to a single relay compensate the
parallel-connected to a single relay (see fig. 14 ).   impossibility of placing a toroid on the incomer.
When the measurement relay (normally
electronic) only needs a very weak electrical
                                                              1                   2                   3
signal to operate, the toroids can be made to
operate as “current generators”. When parallel-
connected, they give the image of the vector
sums of the primary currents.
Although this circuit is laid down in the
installation standards, the approval of the RCD
manufacturer is preferable. However, for
discrimination reasons, it is preferable to use one
RCD per outgoer.
v If the problem arises with parallel-connected
cables which cannot all cross a toroid, a toroid
can be placed on each cable (including all the
live conductors), and all the toroids can be
parallel-connected (see fig. 15 ).                            1                   2                    3
However the following must be noted:                   Fig. 15 : layout of toroids on parallel-connected large
v That each toroid detects n turns in short-circuit    diameter single-line cables.
(3 in the figure) which may reduce sensitivity.

                                                           Cahier Technique Schneider Electric no. 114 / p.15
                             v If the connections represent impedance               c Can “operate” the toroid at higher induction in
                             differences, each toroid will indicate a false zero    order to maximise the energy sensed and
                             phase-sequence current. However proper wiring          minimise sensitivity to stray inductions (strong
                             considerably limits these currents.                    currents).
                             v That this circuit implies for each toroid that the
                             output terminals S1-S2 be marked according to
                             the energy flow direction. This solution calls for
                             the approval of the RCD manufacturer.
                             c High power outgoer
                             To ensure a reliable, linear toroid “response”, the                         1
                             live conductors must be placed as close as                                                       A
                             possible to the centre of the toroid so that their                         2
                             magnetic effects are completely compensated in
                             the absence of residual current. In actual fact,
                             the magnetic field developed by a conductor
                             decreases in proportion to distance: thus in
                             figure 16 , phase 3 causes at point A a local          Fig. 16 : incorrect centring of conductors in the toroid is
                             magnetic saturation and thus no longer has a           responsible for its local magnetic saturation at point A,
                             proportional effect. The same applies if the toroid    which may be the cause of nuisance tripping.
                             is placed near or in a bend of the cables that it
                             surrounds (see fig. 17 ). The appearance of a
                             stray residual induction, for strong currents, will
                             generate on the toroid secondary a signal that                         Ø
                             may cause nuisance tripping. The risk increases
                             as the RCD threshold drops with respect to
                             phase current, particularly on a short-circuit.
                             In problem cases (Max. Iph. / high I∆n), two
                             solutions can be used to counter the risk of
                             nuisance tripping:                                                                              L u2Ø
                             v Use a toroid that is far larger than necessary,
                             for example with a diameter that is twice the one
                             just right for conductor insertion.                    Fig. 17 : the toroid must be far enough from the cable
                             v Place a sleeve in the toroid.                        bend so as not to be the cause of nuisance tripping.
                             This sleeve must be made of magnetic material
                             in order to homogenise the magnetic field (soft
                             iron - magnetic plate), (see fig. 18 ).
                             When all these precautions have been taken:
                             - centring of conductors,
                             - large toroid,
                             - and magnetic sleeve,
                                         max. Ι phase
                             the ratio                may reach 50,000.
                             Using an RCD with built-in toroid
                             It must be pointed out that RCDs with built-in
                             toroids provide contractors and operators with a                                          Lu2Ø
                             ready-made solution since it is the manufacturer
                             who studies and works out the technical
                             solutions. This is because he:
                             c Masters the problem of centring the live             Fig. 18 : a magnetic sleeve placed around the
                             conductors and, for weak currents, can                 conductors, in the toroid, reduces the risk of tripping
                             anticipate and properly distribute several primary     due to the magnetic effects of the current peaks.
                             turns around the toroid.

Cahier Technique Schneider Electric no. 114 / p.16
4.3 Measurement relays and actuators
                 The RCDs can be classed in three categories            They are very widespread (with the “fail-safe”
                 according to their supply mode or their                function) and particularly suitable for the creation
                 technology.                                            of an RCD with a single sensitivity.

                 According to their supply mode                         “Electronic devices”
                 “With own current”: in this device the tripping        These devices are particularly used in industry
                 energy is supplied by the fault current. This          as electronics ensures:
                 supply mode is considered by most specialists
                 as the most reliable. In many countries and            c A very low acquisition power,
                 particularly in Europe, this category of RCD is        c Accurate, adjustable thresholds and time
                 recommended for domestic and similar                   delays (thus ensuring optimum tripping
                 installations (standards EN 61008 and 61009).          discrimination).
                 “With auxiliary supply source”: in this device         Due to these two characteristics, these devices
                 the tripping energy requires a source of energy        are ideal for the creation of:
                 that is independent from the fault current. These      c RCDs with separate toroids, which are
                 devices (normally electronic) can therefore only       associated with high rating circuit-breakers and
                 cause tripping if this auxiliary energy source is      contactors.
                 available when the fault current appears.              c RCDs associated with industrial circuit-
                 “With own voltage”: this is a device with an           breakers up to 630 A.
                 “auxiliary supply source” but whose source is the      Electronic devices require a certain energy, often
                 monitored circuit. Thus, when this circuit is          very weak, to operate. RCDs with electronic
                 energised, the RCD is supplied, and when this          devices are therefore available with the various
                 circuit is not energised, the RCD is not activated     supply modes described above, either “with own
                 but there is no danger. An additional guarantee        voltage” or “with auxiliary supply source”.
                 is provided by these devices when they are
                 designed to operate correctly with voltage drops
                 of up to 50 V (safety voltage). This is the case of
                 the Vigi modules which are RCDs associated
                 with the Merlin Gerin “Compact” circuit-breakers.
                 However, as far as power supply is concerned,
                                                                          Ia                     Ir
                 the RCDs are also classed according to whether
                 or not their operation is of the “fail-safe” kind.
                 Two types of devices are considered to be fail-
                 c Those whose tripping only depends on the
                 fault current: all own current devices are fail-safe
                 c And those, more seldom used, whose tripping
                 does not only depend on the fault current but
                 which are automatically placed in the tripping
                 position (safety position) when the conditions no
                 longer guarantee tripping in the presence of the
                 fault current (e.g. a voltage drop up to 25 V).

                 According to their technology                          Fig. 19 : the fault current, via the toroid, supplies
                                                                        energy to an electromagnet whose moving part is
                 “Electromagnetic devices” (see fig. 19 ).              “stuck down” by a permanent magnet. When the
                 These modern devices are of the “own current”          operating threshold is reached, the electromagnet
                 type and use the principle of magnetic latching.       destroys the attraction of the permanent magnet and
                 A very low electrical power (100 µVA for some)         the moving part, drawn by a spring, opens the
                 is sufficient to overcome the latching force and       magnetic circuit and mechanically controls circuit-
                 cause the contacts to open by means of a               breaker opening.
                 mechanical amplifier.

                                                                            Cahier Technique Schneider Electric no. 114 / p.17
                             “Mixed devices” (own current)                          This test must allow for the fact that capacitive
                             This solution consists of inserting between the        earth leakage currents are always present in an
                             toroid and the magnetic latching relay a signal        electrical installation, as are often resistive
                             processing device, allowing:                           leakage currents resulting from damaged
                             c An accurate, precise operating threshold.
                                                                                    The vector addition of all these leakage currents
                             c Excellent immunity to interference and steep
                                                                                    (Id) is detected by the toroidal sensor and may
                             front current transients, while respecting an
                                                                                    affect test operation, in particular when the test
                             operating time compatible with safety curves. As
                                                                                    circuit is the one shown in figure 20 . Despite
                             an example, Merlin Gerin “si” type RCDs are
                                                                                    this, this test principle is widespread as it checks
                             mixed devices.
                                                                                    the toroid/relay/breaking device assembly.
                             c Creation of time-delayed RCDs.
                                                                                    Construction standards limit the test current,
                             A similar principle is used in MV. In point of fact,   which may account for a certain number of RCD
                             a few years ago tripping in electrical power           operating failures during the test, as shown by
                             supply consumer substations (MV/LV substation)         the vector addition (see figure 20) of the leakage
                             required an accumulator bank which was the             current (Id) and the test current (test I). For
                             source of many problems. The combination of an         example standards IEC 61008 and 61009 state
                             own current electronic device and an                   that the test current must not exceed 2.5 I∆n for
                             electromechanical trip unit with magnetic latching     an RCD usable in 230 or 400 V, i.e. 1.15 I∆n if it
                             offers a satisfactory solution with respect to cost    is supplied in 230 V - 20%.
                             and reliability with removal of the battery.
                                                                                    The test principle described above is used on
                             Operational requirements                               earth leakage protection sockets, residual
                             IEC 60364, paragraph 531-2-2-2 states the              current circuit-breakers and residual current
                             following for non fail-safe devices with auxiliary     devices. With respect to residual current relays
                             supply source:                                         with separate toroid, the same principle is
                             “Their use is permitted if they are installed in       sometimes chosen when the contractor is the
                             installations operated by experienced and              person producing the test circuit. However some
                             qualified people”.                                     relays, for example the Merlin Gerin Vigirex, are
                             Standard NF C 15-100, paragraph 532.2.2 also           equipped with the “test” function, and also
                             states that they must not be used in household         permanently monitor continuity of the detection
                             installations or for similar applications.             circuit (toroid/relay link and toroid winding).

                             Proper operating test
                             An RCD is a safety device. Whether it is                              Test     R          I test
                             electromagnetic, electronic or mixed, it is thus
                             essential for it to be equipped with a test device.                                                Id
                             Although own current devices appear the most                 1
                             reliable, implementation of fail-safe safety with
                             the other “own voltage” or “auxiliary supply
                             source” energy sources grant the RCDs an                     3
                             increased degree of safety which does not,
                             however, replace the periodical test.                                                                 →
                                                                                                                       location of If
                             c Recommend periodical RCD testing
                             In practice perfect fail-safe safety, particularly
                             concerning internal faults, does not exist. For this
                             reason, in France, RCDs using an auxiliary                                           Id
                             supply source are reserved for industrial and                         →
                                                                                                   I test    →
                             large tertiary installations and own current RCDs                               Ir
                             for domestic and similar installations: an
                             arrangement which is consistent with their
                             inherent possibilities described above.                      →    →     →            → →
                             In all cases, periodical testing should be                   Ir = Id + I test ⇒ Ir u If
                             recommended for highlighting internal faults.
                                                                                    Fig. 20 : some test circuits created on installation may
                             c The manner in which the test is conducted is         not operate in the presence of weak fault currents.

Cahier Technique Schneider Electric no. 114 / p.18
                 c Verification of the operating threshold                   whether or not they are natural, may flow
                 Even more so than for the test, it is important to          through the sensor.
                 bear in mind when carrying out this verification            For reliable measurement, the downstream
                 that leakage currents of the downstream circuit,            circuit will always be disconnected.

4.4 Product manufacturing standards
                 The main RCD manufacturing standards are                    c The AC type, for sinusoidal AC currents.
                 listed in the appendix.                                     c The A type, for sinusoidal AC currents, pulsed
                 The IEC has standardised for the RCDs, types,               DC currents or pulsed DC currents with a
                 threshold values, sensitivities and operating               DC component of 0.006 A, with or without phase
                 curves.                                                     angle monitoring, whether they are suddenly
                                                                             applied or slowly increase.
                 AC, A and B type RCDs to be chosen                          c The B type, for the same currents as the
                 according to the current to be detected                     A type, but in addition for rectifier currents:
                 The current conveyed in electrical networks is              v with simple halfwave with a capacitive load
                 increasingly less sinusoidal. Consequently                  producing a smoothed DC current,
                 standard IEC 60755 has defined three types of               v three-phase with simple or double halfwave.
                 RCD: the AC, A and B types, according to the
                 residual current to be detected (see fig. 21 ).

                                                                              For RCDs of the type:





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

                 Sensitivities (I∆n)                                          (TT earthing system), fire hazards and machine
                 These are standardised by the IEC:                           destruction protection.
                 c high sensitivity -HS-: 6-10-30 mA,                         Tripping curves
                 c medium sensitivity -MS-: 100-300 and                       These curves take into account the international
                 500 mA,                                                      studies performed on electrical hazards
                 c low sensitivity -LS-: 1-3-5-10 and 20 A.                   (IEC 60479) and in particular:
                 It is clear that HS is most often used for direct            c the effects of current in the case of direct
                 contact protection, whereas the other                        contact protection,
                 sensitivities (MS and LS) are used for all other             c limit safety voltage in the case of indirect
                 protection needs, such as indirect contact                   contact protection.

                                                                                  Cahier Technique Schneider Electric no. 114 / p.19
                             With respect to the domestic and similar sector,                 chosen according to fault voltage. In practice “G”
                             standards IEC 61008 (residual current circuit-                   and “S” type RCDs are suitable on final circuits
                             breakers) and 61009 (residual current devices)                   for i 230/440 V network voltages. The standard
                             define standardised operating time values (see                   also stipulates that a time of 1 second is
                             table in figure 22 for the operating curves G and                acceptable in the TT system, for distribution
                             S in figure 23 ):                                                circuits, in order to create the discrimination
                             c The G curve for the instantaneous RCDs.                        stages required for continuity of supply.
                             c The S curve for the selective RCDs with the                    In addition to the above-mentioned
                             lowest time delay level, used in France for                      characteristics of the residual current function,
                             incomer circuit-breakers for example.                            product standards also stipulate:
                             For power residual current devices, they are                     c impact strength and jarring withstand,
                             given in appendix B of standard IEC 60947-2.                     c ambient temperature and humidity,
                             The above standards define the maximum
                                                                                              c mechanical and electrical durability,
                             operating time as a function of the Id/If ratio for
                             inverse response time RCDs (often                                c insulation voltage, impulse voltage withstand,
                             electromagnetic).                                                c EMC limits.
                             Electronic RCDs, mainly used in industry and                     The standards also make provision for type tests
                             large tertiary, normally have an adjustable                      and for periodical checking of quality and
                             threshold and time delay, and their response                     performance carried out either by the
                             time is not dependent on the fault current.                      manufacturer or by approved organisations.
                             IEC 60364 (NF C 15-100) defines maximum                          They thus guarantee users product quality and
                             breaking times on final circuits for the TN and IT               safety of people.
                             earthing systems (see fig. 24 ). For the TT                      RCDs are also marked for quality, for example
                             earthing system, RCD operating time must be                      NF-USE marking in France.

                                        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 operating
                                                                                       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.


                                                    200                                                      S max.
                                                     50                                                        G

                                                                                                                      500      A
                                                                 1   2             5                   10               Id / I∆n.
                             Fig. 23 : maximum operating time curves for “S” (selective) and “G” (general purpose) residual current circuit-
                             breaker or device.

Cahier Technique Schneider Electric no. 114 / p.20
                                                  Nominal             Maximum breaking time (s)
                                                  phase-to-earth      TN   IT              IT
                                                  network 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
                 The standards state that technologically different              c Analyse operating requirements (discrimination
                 RCDs exist that are suited to the two main                      needs, fail-safe safety needs, etc.), in order to
                 sectors: domestic and industry.                                 determine:
                 The RCD must be chosen according to the                         v the required threshold level (sensitivity),
                 network earthing system and the protection                      v the time delay ranges (delay).
                 target (direct contact, indirect contact, load                  The table in figure 25 gives a concise
                 protection, etc.). However it is also necessary to:             presentation of the various devices.
                 c Define its type (A, AC or B) from the network
                 characteristics (AC, mixed, etc.),

                 Areas - Types                     Network earthing system            Sensitivity            Time delay
                 Domestic and similar
                 Extension with earth              TT - TN - IT                       i 30 mA                0
                 leakage protection
                 (breaking by built-in contact)
                 Earth leakage                     TT - TN - IT                       30 mA                  0
                 protection socket
                 (breaking by built-in contact)
                 Residual current                  TT - TN - IT                       30 - 300 mA            0
                 Residual current device
                 c Incomer                         TT                                 In France              “S” type as option
                                                                                      I∆n = 500 mA           (disturbed network with
                                                                                      is 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            0
                 circuit-breaker                   circuit protection)
                 Residual current device
                 c Power                           TT - (TN and IT in fire,           30 mA to 30 A          0 to 1 s
                                                   machine and long outgoer
                 c Final distribution              TT - (TN and IT in fire and        30 - 300 mA            0
                                                   machine protection)
                 Residual current relay            TT - (TN and IT in fire,           30 mA to 30 A          0 to 1 s
                 with separate toroid              machine and long
                                                   outgoer protection)

                 Fig. 25 : general presentation of the various RCDs.

                                                                                    Cahier Technique Schneider Electric no. 114 / p.21
5 Optimised use of the RCD

5.1 EMC: manufacturers’ obligations and what this implies for contractors
                             EMC (Electro Magnetic Compatibility) is the            manufacturers also propose devices with high
                             control of electrical interference and its effects:    sensitivity and reinforced immunity such as the
                             a device must neither be disturbed nor disturb its     Merlin Gerin RCDs of the “si” type (I∆n i 30 mA).
                             environment.                                           Thus, confronted with this problem, installation
                             All electrical equipment manufacturers must            service quality is only dependent on the device
                             naturally comply with certain EMC standards.           chosen.
                             RCDs are tested for electromagnetic
                             compatibility (emission and susceptibility)
                             according to the European Directive which
                                                                                             u                                 a
                             specifies compliance with a certain number of
                             standards (for example: EN 61543 for domestic
                             However, electrical installations generate or
                             transmit disturbances (see “Cahier Technique”
                             no. 187), which can be permanent or temporary,
                             alternating or impulse, low or high frequency, as
                             well as conducted or radiated, common or
                             differential mode, internal or external to
                             buildings. Overvoltage is one of the most                                                             t
                             troublesome disturbances.                                           1,2 µs             50 µs

                             Overvoltage withstand                                           I                                 b
                             RCDs can be sensitive to lightning strokes,
                             particularly on overhead networks which are
                             more likely to be affected by atmospheric
                             disturbances. In point of fact, according to the
                             distance of the cause of the disturbance, an LV
                             network can be subjected to:
                             c An overvoltage occurring between the live
                             conductors and the earth: the disturbance flows
                             off to the earth well upstream of the RCDs                                                            t
                             (see fig. 26a ).                                                               10 µs
                             c An overcurrent, a part of which flows off in the
                             network downstream of the RCD, particularly via
                             the stray capacitances (see fig. 26b ).
                             c An overcurrent detected by the RCD and                                                          c
                             which is due to breakdown downstream of this
                             RCD (see fig. 26c ).
                             Technically speaking, solutions are known and
                             normally implemented by RCD manufacturers.
                             Such solutions include:
                             c For electromagnetic relays, installation of a
                             parallel diode on the relay exciting circuit. This
                             solution is used for incomer circuit-breakers.
                             c For electronic relays, use of a low-pass filter at                                                  t
                             signal shaping level (see fig. 11).                                          8 µs              20µs
                             Manufacturing standards make provision for             Fig. 26 : standardised voltage and current waves
                             RCDs immunised against these stray currents:           representative of lightning.
                             the “S” type RCDs (I∆n u 100 mA). However

Cahier Technique Schneider Electric no. 114 / p.22
                 Influence of choices when designing an                 c In the TN-C earthing system, load unbalance
                 installation                                           currents flow continuously in the metal structures
                 Installation designers and installers, while           of the buildings.
                 respecting proper procedures, are also active in       c In the TN-S earthing system, these unbalance
                 this area, particularly when choosing the earthing     currents also appear on an insulation fault
                 system for the installation. For example they          between the neutral and the protection
                 must know that in the TN system, several               conductor. Moreover, this fault, which cannot be
                 currents are responsible for the disturbance due       detected by the overcurrent protection devices,
                 to radiation of sensitive devices:                     insidiously changes the TN-S system into a
                 c On an insulation fault, strong currents flow in      TN-C system.
                 the PE, the device frames and the structures.

5.2 A need: discrimination
                 Ensure that only the faulty outgoer is de-             Note
                 energised by the tripping of the protection            Problems may be encountered when
                 device: this is the purpose of discrimination and      implementing discrimination if it is necessary to
                 the aim of protection co-ordination.                   combine residual current devices and residual
                 “Vertical” discrimination                              current relays, since:
                 This type of discrimination presides over the          c The residual current device is defined in delay
                 operation of two protection devices connected in       time -tr-.
                 series on a circuit (see fig. 27 ).
                 In view of RCD operating requirements as well
                 as of their manufacturing standards,
                 discrimination must be both current and time.
                 c Current, as, according to the standards, an
                 RCD must trip at I∆ n and not trip at I∆ n / 2. In
                 practice, a ratio of 3 is required:                                                            RCD
                 I∆n (upstream) u 3 I∆n (downstream).
                 c Time, as, in order to react, all mechanisms
                 need a period of time, even the smallest, to
                 which a deliberate time delay or delay must                                    Db
                 sometimes be added.
                 The double condition of non tripping of Da for a                                               RCD
                 fault downstream of Db is therefore:
                 I∆n (Da) > 2 I∆n (Db).
                 and                                                    Fig. 27 : vertical discrimination.
                 tr (Da) > tr (Db) + tc (Db) or tr (Da) > tf (Db)
                 where:                                                                              tr               tc
                 tr = tripping delay = time of non operation
                 tc = time separating the moment of breaking                (1)                                tm
                 (including arcing time) from the moment when
                 the breaking order was given by the                                      tr              tc
                 measurement relay,
                 tf = operating time, from detection of the fault           (2)                tm
                 through to complete breaking of the fault current.
                 Time-delayable electronic relays may exhibit a                      tc
                 fault memorisation phenomenon by their                     (3)
                 threshold circuit. It is then necessary to take into   Fig. 28 : the time delay of an upstream RCD must take
                 account a “memory time” -tm- (see fig. 28 ) to         account of the breaking time associated with the
                 ensure that they do not trip after opening of the      downstream RCD and of the memory time of the
                 downstream device:                                     upstream relay.
                 tr (Da) > tf (Db) + tm.

                                                                            Cahier Technique Schneider Electric no. 114 / p.23
                             c The residual current relay is defined in specific         The successive times tf and tr (or t) must then be
                             operating time or time delayed to a value t, which          calculated (at 2 I∆ n) for each RCD, starting at
                             corresponds to the time elapsing between the                final distribution and moving back towards the
                             occurrence of the fault and the transmission of the         origin of the installation.
                             opening order to the breaking device (see fig. 29 ).

                                                                        RCD                                                RCD
                                                     Vigicompact                                        Vigirex
                                                     tr = 60 ms                                         t = 200 ms

                                                                        RCD                                                 RCD
                                                     Vigirex RH                                         Vigicompact

                                                     tr = 15 ms                                         tr = 60 ms
                                                     tc = 30 ms                                         tf < 140 ms
                                                     tf = 45 ms

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

                             “Horizontal” discrimination                                 (e.g. welding machine) causes an overvoltage
                             Sometimes referred to as circuit selection,                 on the network.
                             stipulated in standard NF C 15-100 paragraph                This overvoltage causes on outgoer A,
                             536.3.2, it means that a residual current device            protected by Da, the occurrence of a capacitive
                             placed in a cubicle at the supply end of the                earthing current which may be due to the stray
                             installation is not necessary when all the                  capacitances of the cables or to a capacitive
                             outgoers in this cubicle are protected by residual          earthing filter.
                             current devices. Only the faulty outgoer is de-
                             energised: the residual current devices placed
                             on the other outgoers (parallel to the faulty
                             outgoer) do not detect the fault current
                             (see fig. 30 ). The residual current devices may
                             then have the same tr (or t).
                             In practice, horizontal discrimination may go
                             wrong. Indeed nuisance tripping known as
                             “sympathy tripping” has been observed,
                             particularly on networks containing very long
                             outgoers (stray capacitances of unbalanced
                             cables) or capacitive filters (computer).                                       RCD                      RCD
                             Two examples are given below:
                             c Case 1 (see fig. 31 )
                             The opening of Db placed on the supply circuit              Fig. 30 : example of horizontal discrimination.
                             of a load R, a powerful overvoltage generator

Cahier Technique Schneider Electric no. 114 / p.24
A solution: the RCD of Db may be instantaneous             The capacitive current supplied by outgoer A will
and the RCD of Da must be time-delayed.                    cause “by sympathy” the tripping of the
Note that for this configuration, the time delay of        corresponding RCD. This phenomenon exists for
the RCD (Da) is often vital as, when circuit A is          all earthing systems, but mainly affects networks
energised, the capacitances (stray or otherwise)           using the IT system.
cause the appearance of a damped oscillating               Both examples show the need to time delay the
residual current (see fig. 32 ).                           RCDs of long outgoers and those containing
As a guideline, a measurement taken on a large             filters.
computer containing an interference filter                 Use of directional RCDs is another solution to
revealed a current with the following                      prevent tripping due to the “return” of capacitive
characteristics:                                           current via the healthy outgoer.
v 40 A (first peak),                                       This type of RCD detects the fault current,
v f = 11.5 kHz,                                            compares its amplitude with the scheduled
v damping time (66 %): 5 periods.                          threshold level and only trips if this current
c Case 2 (see fig. 33 )                                    passes through the toroid from upstream to
A full insulation fault on phase 1 of outgoer B
places this phase at the potential of the earth.


             (A)                     (B)
       Da                      Db

                   RCD                       RCD


Fig. 31 : the presence of a capacitance on outgoer A
may cause:
c on opening of Db, the tripping of Da ,
c on energisation of outgoer A, the tripping of Da .
The use of time-delayed RCDs is often necessary to
protect against the nuisance tripping caused by            Fig. 32 : transient current wave occurring on closing of
lightning overvoltages or equipment switchings.            a highly capacitive circuit.

                                    Da              (A)                                  Extended network

       1                                                                            Cp
                                    Db              (B)


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

                                                                Cahier Technique Schneider Electric no. 114 / p.25
5.3 Avoiding known problems
                             Taking leakage currents into account                 implementation of RCDs immunised against
                             The last sub-chapter emphasises the attention        these currents (time-delayed or “S” type) is the
                             that must be paid to these currents, often           solution.
                             capacitive, which by “deceiving” the RCDs are
                             able to seriously disturb operation.                 Maintaining the earthing system
                             c 50 Hz - 60 Hz leakage currents                     When replacement sources are provided,
                                                                                  protection of people and equipment should be
                             As from the design stage of the installation, the
                                                                                  studied in the various configurations of the
                             lengths of the various outgoers should be
                                                                                  installation, as the position of the neutral with
                             evaluated, together with the future equipment
                                                                                  respect to the earth may be different. The
                             containing capacitive earthed devices. It is then
                                                                                  supply, even temporary, of an installation with a
                             necessary to design a distribution system able to
                                                                                  generator set requires interconnecting the set’s
                             reduce the importance of this phenomenon.
                                                                                  frame with the existing earthing network
                             Consequently, interference filters (compulsory
                                                                                  whatever the earthing system and, in the TT
                             according to the European directive on EMC)
                                                                                  system, earthing of the generator neutral, since
                             placed on the microcomputers and other
                                                                                  otherwise the fault currents would not reach the
                             electronic devices, generate in single-phase
                                                                                  RCD threshold.
                             permanent leakage currents at 50 Hz of the
                             order of 0.5 to 1.5 mA per device.                   When the installation in the TT earthing system
                             These leakage currents add up if the devices are     contains an Uninterruptible Power Supply (UPS),
                             connected to the same phase. And if these            earthing of the neutral downstream of the UPS is
                             devices are connected to all three phases, these     essential for proper operation of the RCDs
                             currents cancel each other out when they are         (K contactor on figure 35 ), but not for protection
                             balanced (vector sum). This reflection is all the    of people as:
                             more true when the RCDs installed have low           c The installation is then in the IT system and
                             thresholds. In order to guard against nuisance       the first fault is not dangerous (see standard
                             tripping, the permanent leakage current must not     C 15-402, paragraph
                             exceed 0.3 I∆n in the TT and TN systems, and         c The likelihood of a second insulation fault
                             0.17 I∆n in the IT system.                           occurring during the period of operation limited
                             c Transient leakage currents                         by back-up time of the UPS batteries is very
                             These currents appear on energisation of a           slight.
                             circuit with a capacitive unbalance (see fig. 33)
                             or on a common mode overvoltage. “S” type
                             RCDs (I∆n u 300 mA) and “si” type RCDs
                             (I∆n = 30 mA and 300 mA) prevent nuisance
                             tripping as do also slightly time-delayed RCDs.
                             c High Frequency leakage currents                                                   A             B
                             Examples of large EMC polluters are thyristor                                      RCD           RCD
                             rectifiers whose filters contain capacitors which
                             generate an HF leakage current able to attain
                             5% of nominal current. Unlike the 50 Hz - 60 Hz
                             leakage currents whose vector sum is zero,
                             these HF currents are not synchronous over all
                             three phases and their sum constitutes a                           Flow off of current
                             leakage current. In order to prevent nuisance                      generated by               Surge arrester
                             tripping, RCDs must be protected against these
                             HF currents (equipped with low-pass filters): this
                             is the case for industrial RCDs and for the
                             Merlin Gerin “S” and “si” type RCDs.
                             c Lightning currents
                                                                                  Fig. 34 : in an installation containing a surge arrester,
                             If the installation is equipped with a surge         according to local obligations, the RCD may be placed
                             arrester, the RCD sensor should not be placed        differently: in A a time-delayed or “S” type RCD and in
                             on the flow path of the current generated by the     B a standard RCD.
                             lightning (see fig. 34 ). Otherwise,

Cahier Technique Schneider Electric no. 114 / p.26
                                                Non backed up

                                         3L                                      N




                               Bypass circuit
                                                           Transfer switches
                             Power loss                        (Maintenance)                                        Backed up
                             detection relay                                               K                        equipment
                                                                                            Fault supplied by the self-generating UPS
               Fault supplied by the mains

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

5.4 RCDs for mixed and DC networks
                            An insulation fault with DC current is far less               account of the fact that in practice fault currents
                            dangerous than with AC current                                are directional but not always smoothed.
                            Experiments (see fig. 5) have shown that for                  This is illustrated by figure 36 drawn up using
                            weak currents people are approximately 5 times                the table in figure 7.
                            less sensitive to DC current than to 50/60 Hz                 Note that a three-phase rectifier supplied by a
                            AC current.                                                   400 V AC phase-to-phase voltage generates a
                            The risk of ventricular fibrillation only appears             direct contact voltage of 270 V DC, which
                            over 300 mA.                                                  corresponds to a maximum breaking time of
                            Installation standards NF C 15-100 and                        0.3 s.
                            IEC 60479 have chosen a ratio close to 2, taking

                                                 t (s)




                                                 0.04                                                            Contact voltage
                                                                   50         100     200 230 400 500 (V AC)
                                                         100       120      200 250 300 400 500   (V DC)
                            Fig. 36 : curves established from the maximum breaking times of an RCD laid down by NF C 15-100,
                            paragraph 413.1.1.1.

                                                                                               Cahier Technique Schneider Electric no. 114 / p.27
                             RCD manufacturing standards take into account          v Circuits G and H
                             the existence of non AC currents, and                  Circuit G supplies a rectified voltage with a
                             particularly define the standard cases shown in        permanent small ripple factor, and consequently
                             figure 21 and describe the relevant tests. To give     fault currents that are hard to detect by the RCD.
                             an example, residual current circuit-breakers          On the other hand, circuit H generates highly
                             must operate for Id i 1.4 I∆n in all cases             chopped fault currents which are thus visible by
                             corresponding to figure 37 , with or without           the RCD. However this circuit is equivalent to
                             superimposition of a smoothed DC current of            circuit G for full wave conduction.
                             6 mA: the fault current is applied either suddenly     v Circuit J
                             or by slowly increasing from 0 to 1.4 I∆n in 30 s.     This common circuit type is particularly used for
                             The RCDs satisfying these tests can be                 variable speed controllers used in DC motors.
                             identified by the following symbol on their front      The back-electromotive force and reactor of the
                             face:                                                  motors generate smoother fault currents than the
                                                                                    G and H circuits described above. However,
                              k                                                     regardless of the thyristor conduction angle, the
                                                                                    RCDs placed upstream of the variable speed
                             Real fault currents                                    controllers must be able to provide protection.
                             These currents reflect the voltages existing           Some standard RCDs may be suitable provided
                             between the fault point and the neutral of the         their lDn threshold receives a suitable setting.
                             installation. The waveform of the fault current is     To give an example, figure 40 shows the
                             seldom the same as that of voltage or applied          sensitivity of an RCD, with analogue electronic
                             current, delivered to the load. Fault voltages and     technology, according to the variable speed
                             currents of the pure DC type (zero ripple factor)      controller output voltage applied at the motor.
                             are very rare.                                         v Circuit K
                             c In the domestic sector, distribution and rectifier   With this circuit type, a fault on the DC circuit
                             circuits are single-phase, and correspond to the       does not produce dϕ / dt in the magnetic sensors
                             diagrams marked A to F in figure 38 . A type           of the RCDs which are then “blinded”. This circuit
                             RCDs provide protection of people. However, for        is dangerous unless a transformer is used
                             diagram B, they only detect fault currents if their    instead of an autotransformer, as AC and A type
                             occurrence is sudden. Note that circuit E is           RCDs are inoperative.
                             increasingly common as it is placed at the input
                             of switch mode power supplies that are                 Special case: DC current return
                             widespread in electrical household appliances
                                                                                    Let us now see what happens when a second
                             (TV, microwave, etc.) as well as in professional
                                                                                    fault occurs on the AC part of a network
                             equipment (microcomputers, photocopiers, etc.).
                                                                                    (see fig. 41 ) containing a rectifier according to
                             c In industry most rectifier circuits are three-       circuit G described above. If the power supply
                             phase (diagrams G to K in figure 39 , see              (A) of the rectifier is not monitored by an RCD, or
                             page 30).                                              if this RCD has been incorrectly chosen or is
                             Some circuits may generate a DC fault current          inoperative for any reason, the insulation fault
                             with a small ripple factor:                            existing on the DC part remains.

                                                                                       100 %          Sinusoidal AC fault

                                                                                                               On-load motor

                                                                                         50 %

                                                                                                            Off-load motor
                                                                                         20 %
                                                      and                                                                          Ud/Udo
                                  135°                                                               0.15                         100 %

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

Cahier Technique Schneider Electric no. 114 / p.28
                                                    Soldering iron or two "setting"                Id
ph                                                  light dimmer switch


ph                                                  B/
                                                    Television, battery charger, etc.              Id



ph                                                  C/
                                                    Light dimmer, arc welding machine              Id



ph                                                  D/
                                                    Household appliances with                      Id
                                                    motor (universal)








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

                                                                                                 Cahier Technique Schneider Electric no. 114 / p.29
                                   (+)             G/
                                                   Welding machine                                     Id
                                                                                                                       Fault on (+)
                                                   c electromagnet
                                                   c electrolysis
 1                                                 c etc.
 2                                                                                                                                              ωt

                                   (-)                                                                                       Fault on (-)

                                   (+)             H/
                                                   Rectifier set for:                                  Id
                                                   c industrial DC network                                                    Fault on (+)
                                                   c electrophoresis
 2                                       R
 3                                                                                                     N

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

                                   (+)              J/
                                                    Variable speed controller for DC motor.

 2                                  M

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

                                   L     (+)       K/
                                                   Stationary battery charger for:                     Id
                                                   c DC auxiliary network                                                       Fault on (+)
                                                   c UPS                                                +
 2                                                                                                                                              ωt

                                                    NB: In this diagram, the smoothing                                           Fault on (-)
                                         (-)        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.

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

Cahier Technique Schneider Electric no. 114 / p.30
               However, should a fault occur on an AC outgoer               earth fault occur, the DC currents do not affect
               B, the current of this fault is equal to i1 + i2, and        operation of the RCDs and do not jeopardise
               there is no certainty that the RCD placed on this            safety”.
               outgoer, if it is of the AC type, will trip at the           It is thus advisable to:
               displayed threshold. For this reason standard
               C 15-100, paragraph 532-2-1-4 stipulates:                    c choose the right RCD placed just upstream of
               “When electrical devices likely to produce DC                a rectifier system,
               currents are installed downstream of an RCD,                 c if necessary, use A type RCDs in the
               precautions must be taken so that, should an                 remainder of the installation.


                                             3                      Da           (A)
                                             2                                                                  N
                                                                                                           311 V


                                                        i2                  Ru

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

6 Conclusion

               At a time when electricity, as an energy source,             v to be provided in the case of very long
               is playing an increasingly dominant role in                  outgoers in the TN and IT systems.
               housing, tertiary and industry, it is useful to point        c For protection of people against direct contact
               out and quantify the electrical hazard and to                risk, an RCD is very useful and often stipulated
               further knowledge of Residual Current Devices.               by standards as an additional precaution
               These devices, like any others, have their strong            irrespective of the earthing system.
               and weak points. Not yet fully perfected, they               c RCDs also provide protection against:
               play an increasingly important role in the
                                                                            v fires of electrical origin,
               protection of people and equipment. All
               industrialised countries make extensive use of               v destruction of machines in the TN system,
               RCDs, with a variety of earthing systems, both in            v electromagnetic disturbances in the TN-S
               industry and housing.                                        system (neutral insulation monitoring).
               Generally speaking, the following information is             Present day RCDs comply with construction
               important for installation standards and                     standards (see chapter 4) and continue to
               practices:                                                   progress in terms of reliability and immunity to
               c For protection of people against the indirect              interference phenomena which are not
               contact risk, an RCD is:                                     ascribable to insulation faults.
               v compulsory in the TT system,                               The purpose of this study is to further knowledge
               v necessary in the IT system if there are several            of residual current devices and thereby
               earthing connections,                                        contribute to the safety of us all.

                                                                                  Cahier Technique Schneider Electric no. 114 / p.31

                             Standards                                            “Installation” standards
                             From 1997 onwards the new publications,              c IEC 60364, NF C 15-100: LV electrical
                             issues, versions and IEC amendments to               installations
                             existing publications have a designation in the      c UTE C 15-401: practical guide, installation of
                             60000 series.                                        thermal motor/generator sets
                             We would like to draw the users’ attention to the
                                                                                  c UTE C 15-402: practical guide, static
                             fact that the former publications printed before
                                                                                  uninterruptible power supplies (UPS).
                             1997 continue to bear the old numbers on the
                             printed copies, while waiting to be revised.
                                                                                  Schneider Electric “Cahiers Techniques”
                             “Product” standards                                  c Protection of people and uninterruptible power
                             c IEC 60479: Guide to the effects of current
                                                                                  J.-N. FIORINA, “Cahier Technique” no. 129
                             passing through the human body.
                                                                                  c Evolution of LV circuit-breakers with standard
                             c IEC 60755: General rules concerning residual
                                                                                  IEC 60947-2
                             current protection devices.
                                                                                  E. BLANC, “Cahier Technique” no. 150
                             c IEC 60947-2: Low voltage switchgear - Part 2:
                                                                                  c Earthing layouts in LV
                                                                                  B. LACROIX and R. CALVAS,
                             c IEC 61008, NF C 61-150 and 151: Automatic          “Cahier Technique” no. 172
                             residual current circuit-breakers for domestic and
                                                                                  c Earthing systems worldwide and evolutions
                             similar purposes.
                                                                                  B. LACROIX and R. CALVAS,
                             c IEC 61009, NF C 61-440 and 441: Circuit-           “Cahier Technique” no. 173
                             breakers for domestic and similar purposes.
                                                                                  c Disturbances of electronic systems and
                             c IEC 61557-6, NF EN 61557-6: Electrical             earthing systems
                             safety in low voltage distribution systems up to     R. CALVAS, “Cahier Technique” no. 177
                             1000 V AC and 1500 V DC - Part 6: Residual
                             current devices (RCD) in TT and TN systems.          c Cohabitation of strong and weak currents
                                                                                  R. CALVAS and J. DELABALLE,
                             c UTE C 60-130: Residual current protection          “Cahier Technique” no. 187
                             c NF C 61-420: Small residual current devices.       Other publications
                             c NF C 62-411: Connection and similar                The Schneider Electric guide to the LV electrical
                             equipment, residual current devices for first        installation
                             category installation monitoring switchboards.       Editor: CITEF S.A.
                             c Draft standard: earth leakage protection

Cahier Technique Schneider Electric no. 114 / p.32
                                                                                                                              © 1999 Schneider Electric

Schneider Electric   Direction Scientifique et Technique,   DTP: AXESS - Saint-Péray (07)
                     Service Communication Technique        Edition: Schneider Electric
                     F-38050 Grenoble cedex 9               Printing: Imprimerie du Pont de Claix - Claix - France - 1000
                     Fax: (33) 04 76 57 98 60               - 100 FF -

063139L                                                                                                               06-99

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