Collection Technique .......................................................................... Cahier technique no. 114 Residual current devices in LV R. Calvas “Cahiers Techniques” is a collection of documents intended for engineers and technicians, people in the industry who are looking for more in-depth information in order to complement that given in product catalogues. Furthermore, these “Cahiers Techniques” are often considered as helpful “tools” for training courses. They provide knowledge on new technical and technological developments in the electrotechnical field and electronics. They also provide better understanding of various phenomena observed in electrical installations, systems and equipments. Each “Cahier Technique” provides an in-depth study of a precise subject in the fields of electrical networks, protection devices, monitoring and control and industrial automation systems. The latest publications can be downloaded from the Schneider Electric internet web site. 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The statement “Extracted from Schneider Electric “Cahier Technique” no. .....” (please specify) is compulsory. no. 114 Residual current devices in LV Roland CALVAS 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 Lexicon 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 2 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 asked. Contents 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. Outgoing current i1 In Load Id I1 i2 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. G “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, etc.). 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 40 30 20 10 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 Years 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 ms 10000 a b c1 c2 c3 5000 2000 1000 500 1 2 3 4 200 100 50 20 mA 10 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 UL 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. RA 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. Id N RCD PE RCD Ud 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) 25.00 20.00 15.00 10.00 5.00 0.00 10 100 1 000 10 000 50 Frequency (Hz) Limit 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 PE 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 Auxiliary supply source 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 3 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 ). Ø When all these precautions have been taken: - centring of conductors, - large toroid, - and magnetic sleeve, max. Ι phase the ratio may reach 50,000. Ι∆n 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- safe: A c Those whose tripping only depends on the fault current: all own current devices are fail-safe devices. 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 insulation. 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 2 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. important. 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: Id AC t Id A t Id B t 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 time 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. t (ms) 500 200 S max. 100 50 G 20 500 A 10 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 circuit-breaker 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 protection) 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 RCDs). 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 I 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. Da 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 downstream. A full insulation fault on phase 1 of outgoer B places this phase at the potential of the earth. I (A) (B) Da Db RCD RCD R Fig. 31 : the presence of a capacitance on outgoer A may cause: c on opening of Db, the tripping of Da , and/or t 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 RCD 2 3 Db (B) RCD 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 188.8.131.52.). 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 lightning 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 outgoers 3L 3L 3L N 3L 3L 3L N N Transfer N switches Bypass circuit 3L N Transfer switches N 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) 5 2.5 1 0.75 0.5 0.3 0.2 0.1 0.08 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 4184.108.40.206. 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. Sensitivity and 100 % Sinusoidal AC fault On-load motor and 50 % 90° 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 A/ Soldering iron or two "setting" Id ph light dimmer switch R ωt N ph B/ Television, battery charger, etc. Id R ωt N ph C/ Light dimmer, arc welding machine Id R ωt N ph D/ Household appliances with Id motor (universal) M _ ωt N E/ Id ph R N ωt F/ Id ph R N ωt 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. R 2 ωt N 3 (-) Fault on (-) (+) H/ Rectifier set for: Id c industrial DC network Fault on (+) c electrophoresis 1 2 R ωt 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. 1 2 M _ 3 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 + 1 2 ωt N 3 - 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 N 1 311 V Db - (B) i1 Ru 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 Bibliography 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 supplies 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 Circuit-breakers. 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 devices. 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 socket. 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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