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Technical collection 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. Code: http://www.schneider-electric.com Section: Experts' place Please contact your Schneider Electric representative if you want either a “Cahier Technique” or the list of available titles. The “Cahiers Techniques” collection is part of the Schneider Electric’s “Collection technique”. Foreword The author disclaims all responsibility subsequent to incorrect use of information or diagrams reproduced in this document, and cannot be held responsible for any errors or oversights, or for the consequences of using information and diagrams contained in this document. Reproduction of all or part of a “Cahier Technique” is authorised with the prior consent of the Scientific and Technical Division. 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: A malfunctioning of the heart corresponding to loss of synchronism of the activity of its walls (diastole and systole). The flow of AC current through the body may be responsible for this due to the periodic excitation that it generates. The ultimate consequence is stoppage of blood flow. Direct contact: Contact of a person with the live parts of electrical devices (normally energised parts and conductors). Earthing system: Standard IEC 60364 stipulates three main official earthing systems which define the possible connections of the neutral of the source and frames to the earth or neutral. The electrical protection devices are then defined for each one. Electrification: Application of voltage between two parts of the body of a living being. Electrocution: Electrification resulting in death. Fault current Id: Current resulting from an insulation fault. Frame: Conductive part likely to be touched and which, although normally insulated from live parts, may be brought to a dangerous voltage further to an insulation fault. Indirect contact: Contact of a person with accidentally energised frames (usually further to an insulation fault). Insulation: Arrangement preventing transmission of voltage (and current flow) between a normally energised element and a frame or the earth. Insulation fault: Insulation rupture causing an earth fault current or a short-circuit via the protection conductor. Cahier Technique Schneider Electric no. 114 / p.2 Leakage current: Current which, in the absence of an insulation fault, returns to the source via the earth or the protection conductor. Limit safety voltage (UL): Voltage UL below which there is no risk of electrocution. Live conductors: Set of conductors assigned to electrical power transmission, including the neutral in AC and the compensator in DC, with the exception of the PEN conductor whose “protection conductor” (PE) function takes priority over the “neutral” function. Operating residual current If: Value of the residual current causing a residual current device to trip. According to construction standards, at 20°C and for a threshold set at IDn, low voltage residual current devices must comply with: Ι∆n < Ι f < Ι∆n 2 In high voltage, the “zero phase-sequence” relays have, allowing for operating accuracy, an operating current equal to the threshold displayed in amperes. Protection conductors (PE or PEN): Conductors which, according to specifications, connect the frames of electrical devices and some conductive elements to the earthing connection. Residual current: Rms value of the vector sum of the currents flowing through all live conductors in a circuit at a point of the electrical installation. Residual current device (RCD): Device whose decisive quantity is the residual current. It is normally associated with or incorporated in a breaking device. 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 1.2 “Residual current protection” and “Earth leakage protection”: two separate notions p. 4 p. 4 1.3 The RCD, a useful protection device p. 5 2.1 Effects according to current strength 2.2 Effects according to exposure time p. 6 p. 6 2.3 Effects according to frequency p. 8 3 Insulation fault protection 3.1 3.2 3.3 3.4 3.5 3.6 The installation standards The direct contact risk Fire protection The “TT” earthing system The “TN” earthing system The “IT” earthing system p. 10 p. 11 p. 11 p. 11 p. 12 p. 12 4 RCD operating principle and description 4.1 4.2 4.3 4.4 4.5 Operating principle Sensors Measuring relays and actuators Product manufacturing standards The various devices p. 14 p. 14 p. 17 p. 19 p. 21 5 Optimised use of the RCD 5.1 EMC: manufacturers’ obligations and what this implies for contractors p. 22 5.2 A need: discrimination 5.3 Avoiding known problems 5.4 RCDs for mixed and DC networks p. 23 p. 26 p. 27 2 The patho-physiological effects of electrical current on people 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 contacts are always associated with a fault current which does not return to the source via the live conductors. These contacts are dangerous for people and equipment (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 Return current In i1 Load Source Moreover, RCDs monitor insulation of cables and electrical loads, and are therefore frequently used to indicate insulation drops or to reduce the destructive effects of a strong fault current. Id i2 Fault current i3 Source Load I3 in I1 I2 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 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, 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 (see fig. 3 ) are both found. Cahier Technique Schneider Electric no. 114 / p.4 Fig. 2 : earth leakage protection. G Fig. 3 : current leakage protection. 1.3 The RCD, a useful protection device The first decisive factor in choosing and using RCDs in an installation is the earthing system provided. c In the TT earthing system (directly earthed neutral), protection of people against indirect contact relies on the use of RCDs. c In the IT and TN earthing systems, the medium and low sensitivity (MS and LS) RCDs are used: v to limit the risk of fire, v to prevent the destructive effects of a strong fault current, v to protect people against indirect contact (very long outgoers). c For all earthing systems, the high sensitivity (HS) RCDs provide additional protection against direct contact. They are compulsory in final distribution in a large number of countries. Their efficiency was confirmed at the end of this century by the remarked reduction in the number of people electrocuted. The result of an IEC survey conducted in August 1982 in Japan already proved the efficiency of these devices (see fig. 4 ). “The residual current device is generally recognised (throughout the industrialised world) as being the best and most reliable of the protection devices developed to provide protection against indirect contact in the low voltage field”. Such were the words of professor C.F. DALZIEL (Berkeley-USA), one of the pioneers of the study of the effects of electrical current on people in the fifth international conference of the AISS (Lucerne 1978). 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 current on people (tetanisation, external and internal burns, ventricular fibrillation and cardiac arrest) depend on a variety of factors: the physiological characteristics of the person in question, the environment (e.g. dry or wet) and the characteristics of the current passing through the body. As protection of people is the main function of the RCD, it is clear that optimum implementation of these devices requires knowledge of the sensitivity thresholds of people and of the risks incurred. The International Electrotechnical Committee (IEC) has looked into the problem in order to pool, at international level, a variety of viewpoints reflecting and even often defending national practices, habits and standards. Many scientists have participated in this undertaking and have helped clarify the subject (Dalziell, Kisslev, Osypka, Bielgelmeier, Lee, Koeppen, Tolazzi, 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 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 10 kHz 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 irreversible cardiac fibrillation (see lexicon) increase in proportion to the time during which the human body is exposed to the electrical current (see fig. 6 ). The chart in figure 6 identifies in particular zones 3 and 4 in which danger is real. c Zone 3 (situated between curves b and c1). For people in this situation, there is normally no organic damage. However there is a likelihood of muscular contractions, breathing difficulties and reversible perturbation of the formation of impulses in the heart and of their propagation. All these phenomena increase with current strength and exposure time. Cahier Technique Schneider Electric no. 114 / p.6 c Zone 4 (situated to the right of curve c1) In addition to the effects of zone 3, the likelihood of ventricular fibrillation is: v approximately 5 % between curves c1 and c2, v less than 50 % between curves c2 and c3, v more than 50 % beyond curve c3. Patho-physiological effects such as cardiac arrest, respiratory arrest and serious burns increase with current strength and exposure time. For this reason it is accepted that use of an RCD with instantaneous operation and with a threshold of less than 30 mA, ensures this situation is never reached and such risks never incurred. With a more general approach, IEC 60364 (NF C 15-100 in France) stipulates the operating times for the Residual Current Devices according to contact voltage. These times are recalled in the two tables in figure 7 . Limit safety voltage (UL) According to environmental conditions and particularly presence or absence of water, limit safety voltage UL (voltage below which there is no risk for people, according to standard NF C 15-100) is, in AC: v 50 V for dry and wet premises, v 25 V for damp premises, for example for outdoor worksites. Duration of current flow ms 10000 b a 5000 c1 c2 c3 2000 1000 500 200 1 2 3 4 100 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 50 5 75 0.60 90 0.45 120 0.34 150 0.27 220 0.17 280 0.12 350 0.08 500 0.04 5 5 5 5 5 1 0.40 0.30 0.20 0.10 c Wet premises or locations: UL i 25 V 25 50 75 90 110 150 220 280 5 5 2 0.80 0.50 0.25 0.06 0.02 5 0.48 0.30 0.25 0.18 0.10 0.05 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 Direct contact with normally energised parts is dangerous for voltages in excess of UL. The main protection precautions to be taken are distance and insulation. The RCD can detect a fault current flowing through a person and, as such, is specified, regardless of the earthing system, in final distribution as an additional protection. Its operating threshold, as shown in the table in figure 5, must be less than or equal to 30 mA, and its operation must be instantaneous since the value of the fault current, dependent on the exposure conditions, may exceed 1A. Indirect contact On contact with an accidentally energised frame, the danger threshold is also fixed by the limit safety voltage UL. To ensure there is no danger when network voltage is greater than UL, contact voltage must be less than UL. In the diagram in figure 8 , when the installation neutral is earthed (TT earthing system) where: RA = earthing resistance of the installation frames, RB = earthing resistance of the neutral, this implies choosing an operating threshold (I∆ n) of the RCD such that: Ud = RA Ι d ≤ UL and thus: I∆ n i UL RA The protection operating time must be chosen according to fault voltage RA Ud = U RA + RB (see fig. 7). Note that if the equipotentiality of the site is not ensured or is badly ensured, contact voltage is equal to fault voltage. 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 AC current of a frequency in excess of 100 Hz. Skin impedance decreases in reverse proportion to frequency. The standard states that the frequency factor, which is the ratio of current at the frequency (f) over current at the frequency of 50/60 Hz for the same physiological effect considered, increases with frequency. Moreover, it has been observed that between 10 and 100 kHz the perception threshold Cahier Technique Schneider Electric no. 114 / p.8 increases approximately by 10 mA to 100 mA in rms value. Although standards do not yet stipulate specific operating rules, the major manufacturers, aware of the potential risks of such currents, ensure that the thresholds of the protection devices they propose are below the ventricular fibrillation curve defined in standard IEC 60479-2 (see fig. 9 ). Id(ƒ) / Id(50 Hz) 25.00 20.00 15.00 10.00 5.00 0.00 10 100 50 1 000 10 000 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 industrial installations. Their use depends on standards and mainly on the IEC 60364 (in France NF C 15-100). This standard officially stipulates three main systems for earthing the electrical network: the earthing systems (see fig. 10 ), used to a greater or lesser extent depending on the country. Furthermore, for each of these systems it defines more precisely the use of the RCDs, as the electrical hazard is greatly influenced by choice of earthing system (see “Cahier Technique” no. 172). It also describes the basic precautions which, in normal operating conditions, considerably reduce electrical hazards, for example: c distance and obstacles, Directly earthed neutral (TT) c insulation: class II devices and safety transformers, c earthing of frames, c equipotentiality. General rules Whatever earthing system is chosen for an installation, the standards require that: c Each application frame be connected to an earthing connection by a protection conductor. c Simultaneously accessible application frames be connected to the same earthing connection. c A breaking device automatically disconnects all parts of the installation where a dangerous contact voltage develops. c The breaking time of this device be less than the maximum time defined (see fig. 7). Multiple earthed neutral (TN-C) 1 2 3 N PE RB RA 1 2 3 PEN RB Unearthed neutral (IT) Multiple earthed neutral (TN-S) 1 2 3 N PE RB 1 2 3 N PE 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 earthing system is used. The protection measures stipulated by standards are therefore identical and use the possibilities offered by the high sensitivity RCDs. This is because: c As the fault current flows through a person in contact with a live conductor, he or she is exposed to the patho-physiological risks described above. c An RCD placed upstream of the contact point can measure the current flowing through the person and break the dangerous current. Regulations recognise the use of an RCD with high or very high sensitivity (i 30 mA) as an additional protection measure when the risk of direct contact exists due to the environment, the installation or people (article 412.5.1 of IEC 60364). This risk also exists when the protection conductor is likely to be broken or does not exist (hand-held devices). In this case use of a high sensitivity RCD is compulsory. Standard NF C 15-100, paragraph 532-2.6.1, states that RCDs with a threshold at most equal to 30 mA must protect the circuits supplying power outlets when they are: c Placed in damp premises or in temporary installations. c Of rating i 32 A in all the other installation cases. Note Standard IEC 60479 states that the resistance of the human body is greater than or equal to 1000 Ω for 95 % of people exposed to a 230 V contact voltage and thus through whom a 0.23 A current flows. An RCD with a 30 mA threshold does not limit current, but its instantaneous operation ensures safety up to 0.5 A (see fig. 6). Use of an RCD with a sensitivity of 5 or 10 mA therefore does not increase safety. However it makes the risk of nuisance tripping not negligible as a result of capacitive leakage (distributed capacitances of cables and filters). 3.3 Fire protection Whatever earthing system is used, the electrical installations of premises where risk of fire is present must be equipped with RCDs of a sensitivity I∆n i 500 mA, as it is acknowledged that a 500 mA current can result in incandescence of two metal parts coming into occasional contact. 3.4 The “TT” earthing system Protection of people against indirect contact In this system protection relies on use of RCDs. The fault current depends on the resistance of the insulation fault (Rd) and the resistances of the earthing connection. A person in contact with the metal enclosure of a load with an insulation fault (see fig. 8) may be subjected to the voltage developed in the load earthing connection (RA). For example Where U = 230V, RA = RB = 10 Ω and Rd = 0, if the person is not on an equipotential site, he or she is subjected to Uc = Ud = 115 V. Protection must be provided by use of an RCD of medium or low sensitivity which must de-energise the faulty device as soon as the voltage Ud exceeds the limit safety voltage UL. We remind you that their operating threshold must be set at: I∆n i UL . RA Protection of machines and equipment The level of the RCD tripping thresholds necessary for protection of people in the TT earthing system is well below that of the fault currents able to damage the magnetic circuits of machines (motor) or cause fires. The RCDs therefore prevent such electrical destruction. Cahier Technique Schneider Electric no. 114 / p.11 3.5 The “TN” earthing system Reminders c With this earthing system, the current of a full insulation fault is a short-circuit current. c In TN-C, in view of the fact that the neutral and the protection conductor are combined, RCDs cannot be used. The following text therefore mainly concerns the TN-S. Protection of people against indirect contact As the fault current depends on the impedance of the fault loop, protection is normally provided by overcurrent protection devices (calculation/ measurement of loop impedances). If the impedance is too great and does not allow the fault current to trip the overcurrent protection devices (very long cables), one solution is to use a low sensitivity RCD (I∆n u 1 A). Moreover this system cannot be used when, for example, the network is supplied by a transformer whose zero phase-sequence impedance is too great (star-star connection). Protection of electrical devices and circuits In the multiple earthed neutral system, insulation faults are responsible for strong fault currents equivalent to short-circuit currents. The flow of such currents results in serious damage, for example: perforation of the magnetic circuit plates of a motor, requiring replacement instead of rewinding of the motor. Such damage can be greatly limited by use of a low sensitivity RCD (e.g. 3 A) with instantaneous operation, which is thus able to react before the current reaches a high value. Note that the need of protection increases as operating voltage rises, as the energy lost at the fault point is proportional to the voltage square. The economic consequence of such destruction must be estimated as it is a vital criterion in choice of earthing system. Detection of insulation faults between the Neutral and the protection conductor (PE) or building frames This type of fault insidiously transforms the TN-S system into a TN-C system. Part of the neutral current (increased by the sum of 3rd order and multiple of 3 harmonic currents) permanently flows in the PE and in the metal structures of the building with two consequences: c Equipotentiality of the PE is no longer ensured (a few volts may disturb the operation of the digital systems connected by bus and which must have the same potential reference). c Current flow in the structures increases the risk of fire. The RCDS are used to highlight this type of fault. Detection of insulation faults without tripping and protection of equipment In the TN-S earthing system, unlike the IT system, there are no safety rules stipulating insulation monitoring. However, all tripping further to an insulation fault is the cause of operating losses and often of costly repairs prior to re-energisation. For this reason, more and more often operators request prevention devices in order to take action before the insulation loss becomes a short-circuit. The answer to this need is the use in indication, in TN-S, on critical outgoers, of an RCD with a threshold of around 0.5 to a few amperes, which can detect insulation drops (on the phases or neutral) and alert operators. On the other hand, the risk of electrical fire is reduced and destruction of equipment avoided by using an RCD with tripping for I∆n i 500 mA. 3.6 The “IT” earthing system Protection of people against indirect contact When the first insulation fault occurs, the fault current is very weak and the fault voltage not dangerous: the standards require that this fault be indicated (by the permanent insulation monitors) and tracked (by the power on fault tracking devices). Cahier Technique Schneider Electric no. 114 / p.12 When the second fault occurs, the installation finds itself in a situation similar to a fault in the TN earthing system. However there are two possibilities: that of a single earthing connection for all the frames and that of multiple earthing connections. c Case of a single earthing connection In this case protection is usually ensured by the overcurrent protection devices (calculation/ measurement of the loop impedances). c Case of multiple earthing connections When both faults affect devices not connected to the same earthing connection, the fault current may not reach the operating threshold of the overcurrent releases. The standards stipulate use of RCDs on each group of frames interconnected with the same earthing connection. c In all cases, simple or multiple earthing connections If the impedance of a fault loop is too great (very long cables), a simple, practical solution is to use a low sensitivity RCD (1 to 30 A). Protection of equipment, electrical devices and circuits Although there is no particular danger for equipment when the first fault occurs, a second fault is normally responsible for strong fault currents equivalent to short-circuit currents, as in the TN earthing system. RCDs with medium or low sensitivity can then be provided for the more critical cases (premises with risk of fire, sensitive and expensive machines), bearing in mind that the risk of the second fault is particularly low, especially when tracking of the first faults is systematic. In actual fact, assuming a fault once every three months and that this fault is eliminated the same day, the average time between two “double faults” is 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 least two components: c The sensor The sensor must be able to supply an electrical signal which is useful when the sum of the currents flowing in the live conductors is different from zero. c The measurement relay The relay compares the electrical signal supplied by the sensor with a setpoint value and Toroid Shaping sends, with a possible deliberate delay, the opening order to the associated breaking device. The unit controlling the opening of the device (switch or circuit-breaker) placed upstream of the electrical circuit monitored by the RCD is known as the trip unit or actuator. The entire RCD is shown in the diagram in figure 11 . Time delay relay Threshold Static or 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 AC circuits: c The toroidal transformer, which is the most common for measuring leakage currents. c The current transformers, used in HV and MV and sometimes in LV. The toroidal transformer This covers all the live conductors and is thus excited by the residual magnetic field corresponding to the vector sum of the currents flowing through the phases and the neutral. Induction in the toroid and the electrical signal available at the terminals of the secondary winding are thus the image of the residual current. This type of sensor is used to detect residual currents from a few milliamperes to several dozen amperes. Cahier Technique Schneider Electric no. 114 / p.14 The current transformers (CT) To measure the residual current of a threephase electrical circuit without neutral, three current transformers must be installed as shown in figure 12 . I1 I2 I3 A Ih RCD B Fig. 12 : the vector sum of the phase currents yields the residual current. The three CTs are parallel-connected current generators, causing circulation between A and B of a current which is the vector sum of the three currents and thus the residual current. This circuit, known as the Nicholson circuit, is commonly used in MV and HV when the earth fault current can reach several dozen or even several hundred amperes. 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 %. Special cases c High power supply The Nicholson CT circuit, which would be useful in LV when the conductors are large crosssection 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: - 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, - or installation of a toroid on each outgoer, all parallel-connected to a single relay (see fig. 14 ). When the measurement relay (normally electronic) only needs a very weak electrical signal to operate, the toroids can be made to operate as “current generators”. When parallelconnected, 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 ). However the following must be noted: v That each toroid detects n turns in short-circuit (3 in the figure) which may reduce sensitivity. HV / LV G 1 2 3 N RCD RCD Fig. 13 : toroid N delivers the same information as toroid G. RCD Fig. 14 : toroids placed on the outgoers and parallelconnected to a single relay compensate the impossibility of placing a toroid on the incomer. 1 2 3 1 2 3 Fig. 15 : layout of toroids on parallel-connected large diameter single-line cables. Cahier Technique Schneider Electric no. 114 / p.15 v If the connections represent impedance differences, each toroid will indicate a false zero phase-sequence current. However proper wiring considerably limits these 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 live conductors must be placed as close as possible to the centre of the toroid so that their 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 magnetic saturation and thus no longer has a proportional effect. The same applies if the toroid 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 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: 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. v Place a sleeve in the toroid. 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, the ratio c Can “operate” the toroid at higher induction in order to maximise the energy sensed and minimise sensitivity to stray inductions (strong currents). 1 3 A 2 Fig. 16 : incorrect centring of conductors in the toroid is responsible for its local magnetic saturation at point A, which may be the cause of nuisance tripping. Ø L u2Ø Fig. 17 : the toroid must be far enough from the cable bend so as not to be the cause of nuisance tripping. Ø max. Ι phase 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 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 conductors and, for weak currents, can anticipate and properly distribute several primary turns around the toroid. Cahier Technique Schneider Electric no. 114 / p.16 Lu2Ø Fig. 18 : a magnetic sleeve placed around the conductors, in the toroid, reduces the risk of tripping due to the magnetic effects of the current peaks. 4.3 Measurement relays and actuators The RCDs can be classed in three categories according to their supply mode or their technology. They are very widespread (with the “fail-safe” function) and particularly suitable for the creation of an RCD with a single sensitivity. According to their supply mode “With own current”: in this device the tripping energy is supplied by the fault current. This supply mode is considered by most specialists as the most reliable. In many countries and particularly in Europe, this category of RCD is recommended for domestic and similar installations (standards EN 61008 and 61009). “With auxiliary supply source”: in this device the tripping energy requires a source of energy that is independent from the fault current. These devices (normally electronic) can therefore only cause tripping if this auxiliary energy source is available when the fault current appears. “With own voltage”: this is a device with an “auxiliary supply source” but whose source is the monitored circuit. Thus, when this circuit is energised, the RCD is supplied, and when this circuit is not energised, the RCD is not activated but there is no danger. An additional guarantee 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, 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 failsafe: 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). “Electronic devices” These devices are particularly used in industry as electronics ensures: c A very low acquisition power, c Accurate, adjustable thresholds and time delays (thus ensuring optimum tripping discrimination). Due to these two characteristics, these devices are ideal for the creation of: c RCDs with separate toroids, which are associated with high rating circuit-breakers and contactors. c RCDs associated with industrial circuitbreakers up to 630 A. Electronic devices require a certain energy, often very weak, to operate. RCDs with electronic devices are therefore available with the various supply modes described above, either “with own voltage” or “with auxiliary supply source”. According to their technology “Electromagnetic devices” (see fig. 19 ). These modern devices are of the “own current” type and use the principle of magnetic latching. A very low electrical power (100 µVA for some) is sufficient to overcome the latching force and cause the contacts to open by means of a mechanical amplifier. Fig. 19 : the fault current, via the toroid, supplies energy to an electromagnet whose moving part is “stuck down” by a permanent magnet. When the operating threshold is reached, the electromagnet destroys the attraction of the permanent magnet and the moving part, drawn by a spring, opens the magnetic circuit and mechanically controls circuitbreaker opening. Ia Ir A Cahier Technique Schneider Electric no. 114 / p.17 “Mixed devices” (own current) This solution consists of inserting between the toroid and the magnetic latching relay a signal processing device, allowing: c An accurate, precise operating threshold. c Excellent immunity to interference and steep front current transients, while respecting an operating time compatible with safety curves. As an example, Merlin Gerin “si” type RCDs are mixed devices. c Creation of time-delayed RCDs. A similar principle is used in MV. In point of fact, a few years ago tripping in electrical power supply consumer substations (MV/LV substation) required an accumulator bank which was the source of many problems. The combination of an own current electronic device and an electromechanical trip unit with magnetic latching offers a satisfactory solution with respect to cost and reliability with removal of the battery. Operational requirements IEC 60364, paragraph 531-2-2-2 states the following for non fail-safe devices with auxiliary supply source: “Their use is permitted if they are installed in installations operated by experienced and qualified people”. Standard NF C 15-100, paragraph 532.2.2 also states that they must not be used in household installations or for similar applications. Proper operating test An RCD is a safety device. Whether it is electromagnetic, electronic or mixed, it is thus essential for it to be equipped with a test device. Although own current devices appear the most reliable, implementation of fail-safe safety with the other “own voltage” or “auxiliary supply source” energy sources grant the RCDs an increased degree of safety which does not, however, replace the periodical test. 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 supply source are reserved for industrial and large tertiary installations and own current RCDs for domestic and similar installations: an arrangement which is consistent with their inherent possibilities described above. In all cases, periodical testing should be recommended for highlighting internal faults. c The manner in which the test is conducted is important. Cahier Technique Schneider Electric no. 114 / p.18 This test must allow for the fact that capacitive earth leakage currents are always present in an electrical installation, as are often resistive leakage currents resulting from damaged insulation. The vector addition of all these leakage currents (Id) is detected by the toroidal sensor and may affect test operation, in particular when the test circuit is the one shown in figure 20 . Despite this, this test principle is widespread as it checks the toroid/relay/breaking device assembly. Construction standards limit the test current, which may account for a certain number of RCD operating failures during the test, as shown by the vector addition (see figure 20) of the leakage current (Id) and the test current (test I). For example standards IEC 61008 and 61009 state that the test current must not exceed 2.5 I∆n for an RCD usable in 230 or 400 V, i.e. 1.15 I∆n if it is supplied in 230 V - 20%. The test principle described above is used on earth leakage protection sockets, residual current circuit-breakers and residual current devices. With respect to residual current relays with separate toroid, the same principle is sometimes chosen when the contractor is the person producing the test circuit. However some relays, for example the Merlin Gerin Vigirex, are equipped with the “test” function, and also permanently monitor continuity of the detection circuit (toroid/relay link and toroid winding). Test R I test Id 1 2 3 → location of If → → I test Id → Ir → → → → → Ir = Id + I test ⇒ Ir u If Fig. 20 : some test circuits created on installation may not operate in the presence of weak fault currents. c Verification of the operating threshold Even more so than for the test, it is important to bear in mind when carrying out this verification that leakage currents of the downstream circuit, whether or not they are natural, may flow through the sensor. For reliable measurement, the downstream circuit will always be disconnected. 4.4 Product manufacturing standards The main RCD manufacturing standards are listed in the appendix. The IEC has standardised for the RCDs, types, threshold values, sensitivities and operating curves. AC, A and B type RCDs to be chosen according to the current to be detected The current conveyed in electrical networks is increasingly less sinusoidal. Consequently standard IEC 60755 has defined three types of RCD: the AC, A and B types, according to the residual current to be detected (see fig. 21 ). c The AC type, for sinusoidal AC currents. c The A type, for sinusoidal AC currents, pulsed DC currents or pulsed DC currents with a DC component of 0.006 A, with or without phase angle monitoring, whether they are suddenly applied or slowly increase. c The B type, for the same currents as the A type, but in addition for rectifier currents: v with simple halfwave with a capacitive load producing a smoothed DC current, v three-phase with simple or double halfwave. 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) These are standardised by the IEC: c high sensitivity -HS-: 6-10-30 mA, c medium sensitivity -MS-: 100-300 and 500 mA, c low sensitivity -LS-: 1-3-5-10 and 20 A. It is clear that HS is most often used for direct contact protection, whereas the other sensitivities (MS and LS) are used for all other protection needs, such as indirect contact (TT earthing system), fire hazards and machine destruction protection. Tripping curves These curves take into account the international studies performed on electrical hazards (IEC 60479) and in particular: c the effects of current in the case of direct contact protection, c limit safety voltage in the case of indirect contact protection. Cahier Technique Schneider Electric no. 114 / p.19 With respect to the domestic and similar sector, standards IEC 61008 (residual current circuitbreakers) and 61009 (residual current devices) define standardised operating time values (see table in figure 22 for the operating curves G and S in figure 23 ): c The G curve for the instantaneous RCDs. c The S curve for the selective RCDs with the lowest time delay level, used in France for incomer circuit-breakers for example. For power residual current devices, they are given in appendix B of standard IEC 60947-2. The above standards define the maximum operating time as a function of the Id/If ratio for inverse response time RCDs (often electromagnetic). Electronic RCDs, mainly used in industry and large tertiary, normally have an adjustable threshold and time delay, and their response time is not dependent on the fault current. IEC 60364 (NF C 15-100) defines maximum breaking times on final circuits for the TN and IT earthing systems (see fig. 24 ). For the TT earthing system, RCD operating time must be Type In I∆n (A) (A) chosen according to fault voltage. In practice “G” and “S” type RCDs are suitable on final circuits for i 230/440 V network voltages. The standard also stipulates that a time of 1 second is acceptable in the TT system, for distribution circuits, in order to create the discrimination stages required for continuity of supply. In addition to the above-mentioned characteristics of the residual current function, product standards also stipulate: c impact strength and jarring withstand, c ambient temperature and humidity, c mechanical and electrical durability, c insulation voltage, impulse voltage withstand, c EMC limits. The standards also make provision for type tests and for periodical checking of quality and performance carried out either by the manufacturer or by approved organisations. They thus guarantee users product quality and safety of people. RCDs are also marked for quality, for example NF-USE marking in France. Standardised value of operating and non-operating times (in seconds) at: I∆n 2 I∆n 5 I∆n 500 A General All (instantaneous) values All values 0.3 0.15 0.04 0.04 Maximum operating time Selective > 0.030 0.5 0.2 0.15 0.15 Maximum operating time 0.13 0.06 0.05 0.04 Minimum nonoperating time > 25 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 10 1 2 5 10 A Id / I∆n. Fig. 23 : maximum operating time curves for “S” (selective) and “G” (general purpose) residual current circuitbreaker or device. Cahier Technique Schneider Electric no. 114 / p.20 Nominal phase-to-earth network voltage (VCA). Maximum breaking time (s) TN IT IT Non-distributed Distributed 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 RCDs exist that are suited to the two main sectors: domestic and industry. The RCD must be chosen according to the network earthing system and the protection target (direct contact, indirect contact, load protection, etc.). However it is also necessary to: c Analyse operating requirements (discrimination needs, fail-safe safety needs, etc.), in order to determine: v the required threshold level (sensitivity), v the time delay ranges (delay). The table in figure 25 gives a concise 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 Extension with earth leakage protection (breaking by built-in contact) TT - TN - IT i 30 mA 0 Earth leakage protection socket (breaking by built-in contact) TT - TN - IT 30 mA 0 Residual current circuit-breaker TT - TN - IT 30 - 300 mA 0 In France Domestic and similar Residual current device c Incomer TT is the most common “S” type as option (disturbed network with or without surge arrester) c Final distribution TT 30 - 300 mA 0 TT - (TN and IT in socket circuit protection) 30 - 300 mA 0 c Power TT - (TN and IT in fire, machine and long outgoer protection) 30 mA to 30 A 0 to 1 s c Final distribution TT - (TN and IT in fire and machine protection) 30 - 300 mA 0 Residual current relay with separate toroid TT - (TN and IT in fire, machine and long outgoer protection) 30 mA to 30 A 0 to 1 s I∆n = 500 mA Industry and large tertiary Residual current circuit-breaker Residual current device 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 control of electrical interference and its effects: a device must neither be disturbed nor disturb its environment. All electrical equipment manufacturers must naturally comply with certain EMC standards. RCDs are tested for electromagnetic compatibility (emission and susceptibility) according to the European Directive which 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 troublesome disturbances. Overvoltage withstand 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 (see fig. 26a ). 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 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 signal shaping level (see fig. 11). Manufacturing standards make provision for RCDs immunised against these stray currents: the “S” type RCDs (I∆n u 100 mA). However Cahier Technique Schneider Electric no. 114 / p.22 manufacturers also propose devices with high sensitivity and reinforced immunity such as the Merlin Gerin RCDs of the “si” type (I∆n i 30 mA). Thus, confronted with this problem, installation service quality is only dependent on the device chosen. a u t 1,2 µs 50 µs b I t 10 µs c I t 8 µs 20µs Fig. 26 : standardised voltage and current waves representative of lightning. Influence of choices when designing an installation Installation designers and installers, while respecting proper procedures, are also active in this area, particularly when choosing the earthing system for the installation. For example they must know that in the TN system, several currents are responsible for the disturbance due to radiation of sensitive devices: c On an insulation fault, strong currents flow in the PE, the device frames and the structures. c In the TN-C earthing system, load unbalance currents flow continuously in the metal structures of the buildings. c In the TN-S earthing system, these unbalance currents also appear on an insulation fault between the neutral and the protection conductor. Moreover, this fault, which cannot be detected by the overcurrent protection devices, insidiously changes the TN-S system into a TN-C system. 5.2 A need: discrimination Ensure that only the faulty outgoer is deenergised by the tripping of the protection device: this is the purpose of discrimination and the aim of protection co-ordination. “Vertical” discrimination This type of discrimination presides over the operation of two protection devices connected in 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: 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 sometimes be added. The double condition of non tripping of Da for a fault downstream of Db is therefore: I∆n (Da) > 2 I∆n (Db). and tr (Da) > tr (Db) + tc (Db) or tr (Da) > tf (Db) where: tr = tripping delay = time of non operation tc = time separating the moment of breaking (including arcing time) from the moment when the breaking order was given by the measurement relay, tf = operating time, from detection of the fault through to complete breaking of the fault current. Time-delayable electronic relays may exhibit a fault memorisation phenomenon by their threshold circuit. It is then necessary to take into account a “memory time” -tm- (see fig. 28 ) to ensure that they do not trip after opening of the downstream device: tr (Da) > tf (Db) + tm. Note Problems may be encountered when implementing discrimination if it is necessary to combine residual current devices and residual current relays, since: c The residual current device is defined in delay time -tr-. Da RCD Db RCD Fig. 27 : vertical discrimination. tr tc (1) tm tr (2) (3) tc tm tc Fig. 28 : the time delay of an upstream RCD must take account of the breaking time associated with the downstream RCD and of the memory time of the upstream relay. Cahier Technique Schneider Electric no. 114 / p.23 c The residual current relay is defined in specific operating time or time delayed to a value t, which corresponds to the time elapsing between the occurrence of the fault and the transmission of the opening order to the breaking device (see fig. 29 ). The successive times tf and tr (or t) must then be calculated (at 2 I∆ n) for each RCD, starting at final distribution and moving back towards the origin of the installation. 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 Sometimes referred to as circuit selection, stipulated in standard NF C 15-100 paragraph 536.3.2, it means that a residual current device placed in a cubicle at the supply end of the installation is not necessary when all the outgoers in this cubicle are protected by residual current devices. Only the faulty outgoer is deenergised: 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). Two examples are given below: c Case 1 (see fig. 31 ) The opening of Db placed on the supply circuit of a load R, a powerful overvoltage generator Cahier Technique Schneider Electric no. 114 / p.24 (e.g. welding machine) causes an overvoltage on the network. This overvoltage causes on outgoer A, protected by Da, the occurrence of a capacitive earthing current which may be due to the stray capacitances of the cables or to a capacitive earthing filter. RCD RCD Fig. 30 : example of horizontal discrimination. A solution: the RCD of Db may be instantaneous and the RCD of Da must be time-delayed. Note that for this configuration, the time delay of the RCD (Da) is often vital as, when circuit A is energised, the capacitances (stray or otherwise) cause the appearance of a damped oscillating residual current (see fig. 32 ). As a guideline, a measurement taken on a large computer containing an interference filter revealed a current with the following characteristics: v 40 A (first peak), v f = 11.5 kHz, v damping time (66 %): 5 periods. c Case 2 (see fig. 33 ) A full insulation fault on phase 1 of outgoer B places this phase at the potential of the earth. The capacitive current supplied by outgoer A will cause “by sympathy” the tripping of the corresponding RCD. This phenomenon exists for all earthing systems, but mainly affects networks using the IT system. Both examples show the need to time delay the RCDs of long outgoers and those containing filters. Use of directional RCDs is another solution to prevent tripping due to the “return” of capacitive current via the healthy outgoer. This type of RCD detects the fault current, compares its amplitude with the scheduled threshold level and only trips if this current passes through the toroid from upstream to downstream. 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 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 lightning overvoltages or equipment switchings. Fig. 32 : transient current wave occurring on closing of a highly capacitive circuit. Extended network (A) Da 1 t Cp RCD 2 3 (B) Db 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 The last sub-chapter emphasises the attention that must be paid to these currents, often capacitive, which by “deceiving” the RCDs are able to seriously disturb operation. c 50 Hz - 60 Hz leakage currents As from the design stage of the installation, the lengths of the various outgoers should be evaluated, together with the future equipment containing capacitive earthed devices. It is then necessary to design a distribution system able to reduce the importance of this phenomenon. Consequently, interference filters (compulsory according to the European directive on EMC) placed on the microcomputers and other electronic devices, generate in single-phase permanent leakage currents at 50 Hz of the order of 0.5 to 1.5 mA per device. These leakage currents add up if the devices are connected to the same phase. And if these devices are connected to all three phases, these currents cancel each other out when they are balanced (vector sum). This reflection is all the more true when the RCDs installed have low thresholds. In order to guard against nuisance tripping, the permanent leakage current must not exceed 0.3 I∆n in the TT and TN systems, and 0.17 I∆n in the IT system. c Transient leakage currents These currents appear on energisation of a 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 Examples of large EMC polluters are thyristor 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 leakage current. In order to prevent nuisance 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 If the installation is equipped with a surge arrester, the RCD sensor should not be placed on the flow path of the current generated by the lightning (see fig. 34 ). Otherwise, Cahier Technique Schneider Electric no. 114 / p.26 implementation of RCDs immunised against these currents (time-delayed or “S” type) is the solution. Maintaining the earthing system When replacement sources are provided, protection of people and equipment should be studied in the various configurations of the installation, as the position of the neutral with respect to the earth may be different. The supply, even temporary, of an installation with a generator set requires interconnecting the set’s frame with the existing earthing network whatever the earthing system and, in the TT system, earthing of the generator neutral, since otherwise the fault currents would not reach the RCD threshold. When the installation in the TT earthing system contains an Uninterruptible Power Supply (UPS), earthing of the neutral downstream of the UPS is essential for proper operation of the RCDs (K contactor on figure 35 ), but not for protection of people as: c The installation is then in the IT system and the first fault is not dangerous (see standard C 15-402, paragraph 6.2.2.2.). c The likelihood of a second insulation fault occurring during the period of operation limited by back-up time of the UPS batteries is very slight. A RCD Flow off of current generated by lightning B RCD Surge arrester Fig. 34 : in an installation containing a surge arrester, according to local obligations, the RCD may be placed differently: in A a time-delayed or “S” type RCD and in B a standard RCD. 3L Non backed up outgoers Bypass circuit 3L N N 3L N Transfer switches 3L 3L 3L N 3L N Transfer switches N Power loss detection relay (Maintenance) K Backed up 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 dangerous than with AC current Experiments (see fig. 5) have shown that for weak currents people are approximately 5 times less sensitive to DC current than to 50/60 Hz AC current. The risk of ventricular fibrillation only appears over 300 mA. Installation standards NF C 15-100 and IEC 60479 have chosen a ratio close to 2, taking account of the fact that in practice fault currents are directional but not always smoothed. This is illustrated by figure 36 drawn up using the table in figure 7. Note that a three-phase rectifier supplied by a 400 V AC phase-to-phase voltage generates a direct contact voltage of 270 V DC, which corresponds to a maximum breaking time of 0.3 s. t (s) 5 2.5 1 0.75 0.5 0.3 0.2 0.1 0.08 0.04 Contact voltage 100 50 120 100 200 230 400 500 (V AC) 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 the existence of non AC currents, and particularly define the standard cases shown in figure 21 and describe the relevant tests. To give an example, residual current circuit-breakers must operate for Id i 1.4 I∆n in all cases corresponding to figure 37 , with or without superimposition of a smoothed DC current of 6 mA: the fault current is applied either suddenly or by slowly increasing from 0 to 1.4 I∆n in 30 s. The RCDs satisfying these tests can be identified by the following symbol on their front face: k Real fault currents These currents reflect the voltages existing between the fault point and the neutral of the installation. The waveform of the fault current is seldom the same as that of voltage or applied current, delivered to the load. Fault voltages and currents of the pure DC type (zero ripple factor) are very rare. c In the domestic sector, distribution and rectifier circuits are single-phase, and correspond to the diagrams marked A to F in figure 38 . A type RCDs provide protection of people. However, for diagram B, they only detect fault currents if their occurrence is sudden. Note that circuit E is increasingly common as it is placed at the input of switch mode power supplies that are widespread in electrical household appliances (TV, microwave, etc.) as well as in professional equipment (microcomputers, photocopiers, etc.). c In industry most rectifier circuits are threephase (diagrams G to K in figure 39 , see page 30). Some circuits may generate a DC fault current with a small ripple factor: v Circuits G and H Circuit G supplies a rectified voltage with a permanent small ripple factor, and consequently fault currents that are hard to detect by the RCD. On the other hand, circuit H generates highly chopped fault currents which are thus visible by the RCD. However this circuit is equivalent to circuit G for full wave conduction. v Circuit J This common circuit type is particularly used for variable speed controllers used in DC motors. The back-electromotive force and reactor of the motors generate smoother fault currents than the G and H circuits described above. However, regardless of the thyristor conduction angle, the RCDs placed upstream of the variable speed controllers must be able to provide protection. Some standard RCDs may be suitable provided their lDn threshold receives a suitable setting. To give an example, figure 40 shows the sensitivity of an RCD, with analogue electronic technology, according to the variable speed controller output voltage applied at the motor. v Circuit K With this circuit type, a fault on the DC circuit does not produce dϕ / dt in the magnetic sensors of the RCDs which are then “blinded”. This circuit is dangerous unless a transformer is used instead of an autotransformer, as AC and A type RCDs are inoperative. Special case: DC current return Let us now see what happens when a second fault occurs on the AC part of a network (see fig. 41 ) containing a rectifier according to circuit G described above. If the power supply (A) of the rectifier is not monitored by an RCD, or if this RCD has been incorrectly chosen or is inoperative for any reason, the insulation fault existing on the DC part remains. Sensitivity and 100 % Sinusoidal AC fault On-load motor and 50 % 90° Off-load motor 20 % and 135° Fig. 37 : waveform of the A type RCD test currents. Cahier Technique Schneider Electric no. 114 / p.28 Ud/Udo 0.15 100 % Fig. 40 : evolution of the sensitivity of an electronic RCD placed upstream of a thyristor rectifier. A/ Soldering iron or two "setting" light dimmer switch ph Id R ωt N B/ Television, battery charger, etc. ph Id R ωt N C/ Light dimmer, arc welding machine ph Id R ωt N D/ Household appliances with motor (universal) ph Id 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 c electromagnet c electrolysis c etc. (+) 1 Id Fault on (+) R 2 ωt N 3 (-) Fault on (-) H/ Rectifier set for: c industrial DC network c electrophoresis (+) Id Fault on (+) 1 R 2 ωt N 3 NB: The fault current in (+) follows the upper limit of the conduction zones. Likewise, the fault current in (-) follows the lower limit. (-) (+) Fault on (-) 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: c DC auxiliary network c UPS Id + Fault on (+) 1 ωt 2 N 3 (-) NB: In this diagram, the smoothing reactor (L) causes conduction (cyclic and in pairs) of the thyristors such that the fault point (+) or (-) is always electrically connected to the neutral, resulting in a virtually pure DC fault current. - Fault on (-) 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 B, the current of this fault is equal to i1 + i2, and there is no certainty that the RCD placed on this outgoer, if it is of the AC type, will trip at the displayed threshold. For this reason standard C 15-100, paragraph 532-2-1-4 stipulates: “When electrical devices likely to produce DC currents are installed downstream of an RCD, precautions must be taken so that, should an earth fault occur, the DC currents do not affect operation of the RCDs and do not jeopardise safety”. It is thus advisable to: c choose the right RCD placed just upstream of a rectifier system, c if necessary, use A type RCDs in the remainder of the installation. + N Da 3 2 1 (A) N 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, is playing an increasingly dominant role in housing, tertiary and industry, it is useful to point out and quantify the electrical hazard and to further knowledge of Residual Current Devices. These devices, like any others, have their strong and weak points. Not yet fully perfected, they play an increasingly important role in the protection of people and equipment. All industrialised countries make extensive use of RCDs, with a variety of earthing systems, both in industry and housing. Generally speaking, the following information is important for installation standards and practices: c For protection of people against the indirect contact risk, an RCD is: v compulsory in the TT system, v necessary in the IT system if there are several earthing connections, v to be provided in the case of very long outgoers in the TN and IT systems. c For protection of people against direct contact risk, an RCD is very useful and often stipulated by standards as an additional precaution irrespective of the earthing system. c RCDs also provide protection against: v fires of electrical origin, v destruction of machines in the TN system, v electromagnetic disturbances in the TN-S system (neutral insulation monitoring). Present day RCDs comply with construction standards (see chapter 4) and continue to progress in terms of reliability and immunity to interference phenomena which are not ascribable to insulation faults. The purpose of this study is to further knowledge of residual current devices and thereby contribute to the safety of us all. Cahier Technique Schneider Electric no. 114 / p.31 Bibliography Standards From 1997 onwards the new publications, issues, versions and IEC amendments to existing publications have a designation in the 60000 series. We would like to draw the users’ attention to the fact that the former publications printed before 1997 continue to bear the old numbers on the printed copies, while waiting to be revised. “Product” standards c IEC 60479: Guide to the effects of current passing through the human body. c IEC 60755: General rules concerning residual current protection devices. c IEC 60947-2: Low voltage switchgear - Part 2: Circuit-breakers. c IEC 61008, NF C 61-150 and 151: Automatic residual current circuit-breakers for domestic and similar purposes. c IEC 61009, NF C 61-440 and 441: Circuitbreakers for domestic and similar purposes. c IEC 61557-6, NF EN 61557-6: Electrical safety in low voltage distribution systems up to 1000 V AC and 1500 V DC - Part 6: Residual current devices (RCD) in TT and TN systems. c UTE C 60-130: Residual current protection devices. c NF C 61-420: Small residual current devices. c NF C 62-411: Connection and similar equipment, residual current devices for first category installation monitoring switchboards. c Draft standard: earth leakage protection socket. Cahier Technique Schneider Electric no. 114 / p.32 “Installation” standards c IEC 60364, NF C 15-100: LV electrical installations c UTE C 15-401: practical guide, installation of thermal motor/generator sets c UTE C 15-402: practical guide, static uninterruptible power supplies (UPS). Schneider Electric “Cahiers Techniques” c Protection of people and uninterruptible power supplies J.-N. FIORINA, “Cahier Technique” no. 129 c Evolution of LV circuit-breakers with standard IEC 60947-2 E. BLANC, “Cahier Technique” no. 150 c Earthing layouts in LV B. LACROIX and R. CALVAS, “Cahier Technique” no. 172 c Earthing systems worldwide and evolutions B. LACROIX and R. CALVAS, “Cahier Technique” no. 173 c Disturbances of electronic systems and earthing systems R. CALVAS, “Cahier Technique” no. 177 c Cohabitation of strong and weak currents R. CALVAS and J. DELABALLE, “Cahier Technique” no. 187 Other publications The Schneider Electric guide to the LV electrical installation Editor: CITEF S.A. 063139L Direction Scientifique et Technique, Service Communication Technique F-38050 Grenoble cedex 9 Fax: (33) 04 76 57 98 60 DTP: AXESS - Saint-Péray (07) Edition: Schneider Electric Printing: Imprimerie du Pont de Claix - Claix - France - 1000 - 100 FF 06-99 © 1999 Schneider Electric Schneider Electric