Technical collection. Cahier technique. no Residual current devices in LV. R. Calvas

Transcription

1 Technical collection Cahier technique no. 114 Residual current devices in LV R. Calvas

2 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: 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.

3 no. 114 Residual current devices in LV Roland CALVAS With an engineering degree from Ecole ationale Supérieure d Electronique et de Radioélectricité de Grenoble (1964) and a Business Administration Institute diploma, he joined Merlin Gerin in 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

4 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 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 I d : 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. 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 (U L ): 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 PE conductor whose protection conductor (PE) function takes priority over the neutral function. Operating residual current I f : 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 < Ιf < 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 PE): 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 (): Device whose decisive quantity is the residual current. It is normally associated with or incorporated in a breaking device. Cahier Technique Schneider Electric no. 114 / p.2

5 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 : its scope p Residual current protection and Earth leakage protection : p. 4 two separate notions 1.3 The, 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 Effects according to frequency p. 8 3 Insulation fault protection 3.1 The installation standards p The direct contact risk p Fire protection p The TT earthing system p The T earthing system p The IT earthing system p operating principle and description 4.1 Operating principle p Sensors p Measuring relays and actuators p Product manufacturing standards p The various devices p Optimised use of the 5.1 EMC: manufacturers obligations and what this implies p. 22 for contractors 5.2 A need: discrimination p Avoiding known problems p s for mixed and DC networks p Conclusion p. 31 Bibliography p. 32 Cahier Technique Schneider Electric no. 114 / p.3

6 1 Introduction 1.1 The : 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 (), whose basic function is detection of residual currents, is widespread. Moreover, s 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. Source Return current Outgoing current Load Fault current Source i 1 i 2 i 3 i n Load I n I d I 1 I 3 I 2 i d = i a - i r Fig. 1 : a current leakage results in a residual fault current i d. 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 () 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 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. Fig. 2 : earth leakage protection. Fig. 3 : current leakage protection. G Cahier Technique Schneider Electric no. 114 / p.4

7 1.3 The, a useful protection device The first decisive factor in choosing and using s 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 s. c In the IT and T earthing systems, the medium and low sensitivity (MS and LS) s 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) s 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 40 Decree of the law making HS-s compulsory 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 s compulsory. Cahier Technique Schneider Electric no. 114 / p.5

8 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, 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 10 khz Slight tingling, perception threshold Painful shock, but no loss of muscular control on-release threshold Considerable breathing difficulty 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 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. 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 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 (F C in France) stipulates the operating Cahier Technique Schneider Electric no. 114 / p.6

9 times for the Residual Current Devices according to contact voltage. These times are recalled in the two tables in figure 7. Limit safety voltage (U L ) According to environmental conditions and particularly presence or absence of water, limit safety voltage U L (voltage below which there is no risk for people, according to standard F C ) is, in AC: v 50 V for dry and wet premises, v 25 V for damp premises, for example for outdoor worksites. ms Duration of current flow a b c1 c2c ma 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 ). Prospective contact voltage (V) Maximum breaking time of the protection device (s) AC DC c Dry or wet premises or locations: U L i 50 V < c Wet premises or locations: U L i 25 V Fig. 7 : maximum duration of contact voltage holding as per standard IEC Cahier Technique Schneider Electric no. 114 / p.7

10 Direct contact Direct contact with normally energised parts is dangerous for voltages in excess of U L. The main protection precautions to be taken are distance and insulation. The 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 U L. To ensure there is no danger when network voltage is greater than U L, contact voltage must be less than U L. In the diagram in figure 8, when the installation neutral is earthed (TT earthing system) where: R A = earthing resistance of the installation frames, R B = earthing resistance of the neutral, this implies choosing an operating threshold (I n) of the such that: U d = R A Ιd UL and thus: I n i U RL A The protection operating time must be chosen according to fault voltage R Ud = A R R U A + B (see fig. 7). ote that if the equipotentiality of the site is not ensured or is badly ensured, contact voltage is equal to fault voltage. I d PE U d R B R A Fig. 8 : fault voltage generation principle. 2.3 Effects according to frequency IEC 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 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 (see fig. 9 ). Cahier Technique Schneider Electric no. 114 / p.8

11 I d (ƒ) / I d (50 Hz) Limit A type ID AC type ID Vigirex RH328A Frequency (Hz) Fig. 9 : variations in ventricular fibrillation threshold (as per IEC ) and thresholds of various s 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

12 3 Insulation fault protection 3.1 The installation standards s are used in electrical, domestic and industrial installations. Their use depends on standards and mainly on the IEC (in France F C ). 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 s, 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, 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). Directly earthed neutral (TT) PE Multiple earthed neutral (T-C) PE R B R A R B Unearthed neutral (IT) PE Multiple earthed neutral (T-S) PE R B R B : Permanent insulation monitor. Fig. 10 : the three main earthing systems are the TT, T and IT systems, defined by IEC The T may be either T-C (neutral and PE combined) or T-S (separate neutral and PE). Cahier Technique Schneider Electric no. 114 / p.10

13 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 s. 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 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 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 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 is compulsory. Standard F C , paragraph , states that s 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. ote Standard IEC 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 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 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 s 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 s. 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 (R A ). For example Where U = 230V, R A = R B = 10 Ω and R d = 0, if the person is not on an equipotential site, he or she is subjected to U c = U d = 115 V. Protection must be provided by use of an of medium or low sensitivity which must de-energise the faulty device as soon as the voltage U d exceeds the limit safety voltage U L. We remind you that their operating threshold must be set at: I n i RL U. A Protection of machines and equipment The level of the 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 s therefore prevent such electrical destruction. Cahier Technique Schneider Electric no. 114 / p.11

14 3.5 The T earthing system Reminders c With this earthing system, the current of a full insulation fault is a short-circuit current. c In T-C, in view of the fact that the neutral and the protection conductor are combined, s cannot be used. The following text therefore mainly concerns the T-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 (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 (e.g. 3 A) with instantaneous operation, which is thus able to react before the current reaches a high value. ote 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 eutral and the protection conductor (PE) or building frames This type of fault insidiously transforms the T-S system into a T-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 S are used to highlight this type of fault. Detection of insulation faults without tripping and protection of equipment In the T-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 T-S, on critical outgoers, of an 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 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). When the second fault occurs, the installation finds itself in a situation similar to a fault in the T earthing system. However there are two possibilities: that of a single earthing connection for all the frames and that of multiple earthing connections. Cahier Technique Schneider Electric no. 114 / p.12

15 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 s 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 (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 T earthing system. s 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

16 4 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 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 is known as the trip unit or actuator. The entire is shown in the diagram in figure 11. Toroid Shaping Threshold Time delay relay Static or relay output Auxiliary supply source Fig. 11 : functional diagram showing an electronic 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. 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 B Ih Fig. 12 : the vector sum of the phase currents yields the residual current. Cahier Technique Schneider Electric no. 114 / p.14

17 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 icholson 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 F C of December 1989 specifies 10 %. Special cases c High power supply The icholson 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 U L / 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 () 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 manufacturer is preferable. However, for discrimination reasons, it is preferable to use one 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 Fig. 13 : toroid delivers the same information as toroid G. Fig. 14 : toroids placed on the outgoers and parallelconnected to a single relay compensate the impossibility of placing a toroid on the incomer Fig. 15 : layout of toroids on parallel-connected large diameter single-line cables. G Cahier Technique Schneider Electric no. 114 / p.15

18 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 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 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, max. Ι phase the ratio may reach 50,000. Ι n Using an with built-in toroid It must be pointed out that s 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. c Can operate the toroid at higher induction in order to maximise the energy sensed and minimise sensitivity to stray inductions (strong currents). 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 1 2 Fig. 17 : the toroid must be far enough from the cable bend so as not to be the cause of nuisance tripping. L u 2 Ø 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. 3 Ø A L u 2 Ø Cahier Technique Schneider Electric no. 114 / p.16

19 4.3 Measurement relays and actuators The s can be classed in three categories according to their supply mode or their technology. 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 is recommended for domestic and similar installations (standards E 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 is supplied, and when this circuit is not energised, the 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 s associated with the Merlin Gerin Compact circuit-breakers. However, as far as power supply is concerned, the s 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). 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. They are very widespread (with the fail-safe function) and particularly suitable for the creation of an with a single sensitivity. 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 s with separate toroids, which are associated with high rating circuit-breakers and contactors. c s associated with industrial circuitbreakers up to 630 A. Electronic devices require a certain energy, often very weak, to operate. s with electronic devices are therefore available with the various supply modes described above, either with own voltage or with auxiliary supply source. Ia Ir 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. A Cahier Technique Schneider Electric no. 114 / p.17

20 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 s are mixed devices. c Creation of time-delayed s. 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 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 F C , paragraph also states that they must not be used in household installations or for similar applications. Proper operating test An 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 s an increased degree of safety which does not, however, replace the periodical test. c Recommend periodical testing In practice perfect fail-safe safety, particularly concerning internal faults, does not exist. For this reason, in France, s using an auxiliary supply source are reserved for industrial and large tertiary installations and own current s 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. 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 (I d ) 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 operating failures during the test, as shown by the vector addition (see figure 20) of the leakage current (I d ) and the test current (test I). For example standards IEC and state that the test current must not exceed 2.5 I n for an usable in 230 or 400 V, i.e 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 I test R I d I r I test I r = I d + I test I r u I f I d location of I f Fig. 20 : some test circuits created on installation may not operate in the presence of weak fault currents. Cahier Technique Schneider Electric no. 114 / p.18

21 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 manufacturing standards are listed in the appendix. The IEC has standardised for the s, types, threshold values, sensitivities and operating curves. AC, A and B type s to be chosen according to the current to be detected The current conveyed in electrical networks is increasingly less sinusoidal. Consequently standard IEC has defined three types of : 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 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 s of the type: I d AC t I d A t I d B t Fig. 21 : fault currents stipulated in the construction standards. Sensitivities (I n) These are standardised by the IEC: c high sensitivity -HS-: ma, c medium sensitivity -MS-: and 500 ma, c low sensitivity -LS-: 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

22 With respect to the domestic and similar sector, standards IEC (residual current circuitbreakers) and (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 s. c The S curve for the selective s 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 The above standards define the maximum operating time as a function of the I d /I f ratio for inverse response time s (often electromagnetic). Electronic s, 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 (F C ) defines maximum breaking times on final circuits for the T and IT earthing systems (see fig. 24 ). For the TT earthing system, operating time must be chosen according to fault voltage. In practice G and S type s 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. s are also marked for quality, for example F-USE marking in France. Type I n I n Standardised value of operating (A) (A) and non-operating times (in seconds) at: I n 2 I n 5I n 500 A General All All Maximum operating (instantaneous) values values time Selective > 25 > Maximum operating time Minimum nonoperating time Fig. 22 : standardised values of the maximum operating times and non-operating times as per IEC t (ms) S max. G A I d / 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

23 Fig. 24 : maximum breaking times. ominal Maximum breaking time (s) phase-to-earth T IT IT network voltage on-distributed Distributed (VCA). neutral neutral The various devices The standards state that technologically different s exist that are suited to the two main sectors: domestic and industry. The 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 Define its type (A, AC or B) from the network characteristics (AC, mixed, etc.), 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. Areas - Types etwork earthing system Sensitivity Time delay Domestic and similar Extension with earth TT - T - IT i 30 ma 0 leakage protection (breaking by built-in contact) Earth leakage TT - T - IT 30 ma 0 protection socket (breaking by built-in contact) Residual current TT - T - IT 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 ma 0 Industry and large tertiary Residual current TT - (T and IT in socket ma 0 circuit-breaker circuit protection) Residual current device c Power TT - (T and IT in fire, 30 ma to 30 A 0 to 1 s machine and long outgoer protection) c Final distribution TT - (T and IT in fire and ma 0 machine protection) Residual current relay TT - (T 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 s. Cahier Technique Schneider Electric no. 114 / p.21

24 5 Optimised use of the 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. s are tested for electromagnetic compatibility (emission and susceptibility) according to the European Directive which specifies compliance with a certain number of standards (for example: E for domestic s). 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 s 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 s (see fig. 26a ). c An overcurrent, a part of which flows off in the network downstream of the, particularly via the stray capacitances (see fig. 26b ). c An overcurrent detected by the and which is due to breakdown downstream of this (see fig. 26c ). Technically speaking, solutions are known and normally implemented by 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 s immunised against these stray currents: the S type s (I n u 100 ma). However manufacturers also propose devices with high sensitivity and reinforced immunity such as the Merlin Gerin s of the si type (I n i 30 ma). Thus, confronted with this problem, installation service quality is only dependent on the device chosen. u I I 1,2 µs 10 µs 8 µs 50 µs 20µs Fig. 26 : standardised voltage and current waves representative of lightning. a b c t t t Cahier Technique Schneider Electric no. 114 / p.22