MXPA00008028A - Zone arc fault detection - Google Patents

Abstract

A number of methods can be used for zone arc protection to detect series and/or shunt arcing faults in various electrical components and/or circuits (22, 20). A differential current detector, or a di/dt based detector, may be used to detect shunt arcs (26), while a differential voltage detector or a zero-sequence voltage detector, is used to detectseries arcs. A differential phase current detection scheme may be used for both shunt arcs (26) and series arcs. Series arcs may also be detected using a voltage drop system or a line power loss system. Detection of arcing in joints may be monitored directly. A ground fault detector may be combined with one or more of these systems for detecting a ground fault in the circuit to be protected. Two or more of the above systems or detectors may be combined for arc monitoring and detection.

Description

DETECTION OF ARCO FAULT ZONE FIELD OF THE INVENTION The present invention relates to the protection of electrical circuits and, more particularly, to the detection of electrical faults in an electrical circuit, and more particularly to the detection of arc faults in individual areas of electrical circuits. . BACKGROUND OF THE INVENTION A Electrical systems in residential, commercial and industrial applications usually include a connection board to receive electric power from a supply service. The energy is routed to through protection devices to designated branch circuits that supply one or more loads. These protection devices are typically circuit breakers such as circuit breakers and fuses that are I designed to interrupt the electric current if the limits of the conductors supplying the loads are exceeded. The interruption of the circuit reduces the risk of damage or the potential for property damage due to a resulting fire. Circuit breakers are the preferred type of circuit breaker because a reset mechanism allows its reuse. Typically, Circuit breakers interrupt an electrical circuit due to a trip or trip condition such as a current overload or ground fault. The condition of ^ current overload results when a current exceeds the 5 normal conditions of continuous operation of the switch during a time interval determined by the current disconnect. A disconnection condition due to ground fault is created by the imbalance of currents flowing between a line conductor and a neutral conductor that could be caused by leakage current or arcing fault to ground. Arcing faults are commonly defined as ionized gas through the current between two ends of a broken conductor or in a contact or connector with a fault, between two conductors supplying a load, or between a conductor and ground. However, arcing faults may not cause a conventional circuit breaker to disconnect. The fault current levels by arcing can be reduced by shunt impedance or ™ load at a level below the curve settings of disconnection of the circuit breaker. In addition, an arcing fault that does not make contact with an earth conductor or a person will not disconnect an earth fault protector. There are many conditions that can cause a failure by arc formation. For example, wires, connectors.

Corroded, worn or old contacts or insulation, loose connections, cables damaged by nails or staples through the insulation, and electrical voltage caused by overload ^ repeated, lightning strikes, et cetera. These failures can damage the driver's insulation and reach an unacceptable temperature. Arcing faults can cause fires if combustible materials are in close proximity. There are many conditions that can cause a failure of "false" arc formation. For example, the presence of an arcing fault event in a bypass circuit of an electrical distribution system can cause a false arcing fault signal in another bypass circuit as a serial path is created between the branched circuits through a load center. As a result, circuit breakers in more than one branch circuit are disconnected. Another example is a noisy load such as an arc weld, an electric drill, ^ etcetera which produce a disturbance at high frequency in the electrical circuit, which appears to be a failure of arc formation. In general, a "zone" refers to any length of wiring with power that is bounded by some definable end of zone device such as a sensor. current, a line-to-ground voltage sensor, or a Terminal lug connector between conductors of the same phase. In general, arc faults are of two types, arcs in series and parallel arcs. The series arcs are ^ Interruptions not triggered in normal current path 5 such as broken wires, loose terminations or contacts with low contact force. Parallel arcs generally mix conduction through an insulating path between conductors of different voltages. These parallel arcs can be line-to-ground arcs (ground faults) or line-to-line arcs (phase faults). In general, the current through a series arc is usually limited by the load impedance while the current through a parallel arc is controlled by the impedance of the line and the arc voltage. arch of the prior art allow room for improvement in several areas. Some serial arc detection devices of the prior art have been based on the detection of noise in the »Load current to detect an arc. In general, the 0 noise signatures of the arcs are broadband current fluctuations that vary in amplitude and frequency content depending on the type of load. Many loads also transmit electrical noise due to electronic power connection components, brushless motors or other loads generally "noisy". Some devices of the technique previous have difficulty distinguishing between the noise of the load and the noise of the arc. Second, many arcs start as a series arc in a faulty connection, but they are not detected by the prior art detectors until more serious arcs have occurred from line to ground or from line to line. That is, some methods and detection devices of the prior art do not detect a substantial arc in series and immediately as soon as it is formed. Third, in line-to-line arc faults, the current is sometimes limited by the arc voltage or line impedance as noted above. In this way, line-to-line faults typically can not be detected by the instantaneous current set to (magnetic disconnection) of a circuit breaker or the time characteristic I2t of a fuse. Moreover, some arcs are reduced arcs of firing with I2t which also slows the detection of overcurrent by the breakers and fuses. Thus, it is difficult with the methods and devices of the prior art to detect failures of this type in a timely manner. Fourth, some methods and devices of the prior art use algorithms that require some time in which to perform signal processing, such as detecting the difference between the present waveform and a reference waveform or image delayed in time. These methods may include calculating the average load impedances at different frequencies for use in the ^ P algorithm The algorithm must then determine whether any observed change is due to the arcs or is due to fluctuating load conditions. Since charges can be turned on and off and have unpredictable characteristics, arc detection with these methods typically takes a long time and is not substantially instantaneous or immediate after the presence of an arc fault. This type of detection can therefore result in annoying disconnections, for example disconnecting a circuit breaker due to a fluctuating load condition which is inappropriately identified as an arc fault, while also potentially ignoring some arc faults. OBJECTS AND SUMMARY OF THE INVENTION It is a general object of the present invention to provide a system and method of arc fault detection that reliably detects arc faults. A related object is to provide a system and method of arc fault detection that detects both arcs in parallel and in parallel in each protection zone of a cable system without considering the nature of the arc fault waveform or the direction of the load current in the circuit Still another object is to provide an arc fault detection system that is not affected by the noise content in the load or by external sources. ^ P Another object of the invention is to provide a arc fault detection system and method which substantially instantaneously detects a arc in seine. Still another object of the invention is to provide a system and an arc fault detection method that detects line-to-line faults substantially at the time of the start of the fault current regardless of the fault voltage signature. Still another object of the invention is to provide a system and method of arc fault detection that does not require historical information about the charges or forms of wave and that is insensitive to changes in load conditions or waveforms of the load current. A related object of the invention is to provide an arc fault detection system which substantially exceeds the aforementioned problems in some methods and devices of the prior art. According to one aspect of the invention, there is provided an arc fault zone detector comprising a summing element of voltages to sum the voltages of all the phases of the circuit both at the ends of origin as cargo of each zone; and comparative elements for compare the sum of the voltages at one of the load and origin ends of each phase and the sum of the voltages at the other load and origin ends of each phase and to produce a difference signal corresponding to any difference between them. With another aspect of the invention, an arc fault zone detection system for detecting arcing faults in a defined area of an electrical circuit comprises a differential phase current detector including a pair of substantially identical conductors. parallel isolates for each zone of each phase to be protected by the differential phase current detector whereby a protected zone is defined which comprises the length of the parallel conductors between two points where the two conductors are coupled to each other, a transformer current sensor inductively coupled to each pair of parallel conductors, the transformer current sensor and the The conductors are respectively configured and arranged so that the current induced in said current transformer sensor of one of the conductors is subtracted from the current induced in the current transformer sensor from the other conductor to produce a different current output. According to another aspect of the invention, an arc fault zone detector system comprises a detector of differential current arc to detect line-to-line and ground fault arc formation conditions; and a differential voltage arc detection to detect faults ™ series arc training. In accordance with another aspect of the invention, an arc zone detector system comprises a differential current arc detector for detecting line-to-line arc and ground fault conditions; and a differential phase current to detect both arc faults in series as parallel. According to another aspect of the invention, an arc fault zone detection system for detecting arc faults in a defined area of an electrical circuit comprises voltage sensing elements for sensing the voltage in a line conductor of the electrical circuit both at the origin and load end of said area; and arc detector elements coupled with the sensor element to produce a difference signal corresponding to any P difference between voltages at the ends of origin and load from the area. According to another aspect of the invention, an arc fault zone detection system for detecting arc faults in a defined area of an electrical circuit comprises an operationally coupled current transformer. with a line conductor of said electrical circuit to producing a current signal corresponding to an electric current within the line conductor; an arc detector coupled with the current transformer to detect a ™ change in current due to a failure of arc formation in 5 senes in each line conductor; and a line impedance stabilization network coupled with the origin and line ends within the defined area of the line driver. According to another aspect of the invention, an arc fault zone detection system for detecting arc faults in a defined area of an electrical circuit, comprises voltage sensing elements coupled with each of the source and load ends within a region of a line conductor of the electrical circuit; a current sensor coupled to the line conductor of the circuit electric adjacent to the origin end of the zone; and an arc energy sensor operatively coupled with the voltage sensors and with the current sensors to measure the energy loss through the line conductor in the area corresponding to a solid arc in said line inside. from the area. According to another aspect of the invention, an arc fault zone detection system for detecting arc faults in a defined area of an electrical circuit comprises a voltage summing element to sum the voltages of all phases of the circuit at both ends of origin and loading of the area; comparison elements to compare the sum of the voltages in one of the load and origin ends of each phase and the sum of the voltage in the other ^ load end and origin of each phase and to produce a difference signal corresponding to any difference between them; a differential current arc detector; and a ground fault sensing element comprising a current transformer sensor operatively coupled with the line and neutral conductors of the electrical circuit for TQ detect a difference in the current between them to produce a corresponding difference signal. According to another aspect of the invention, a system for monitoring arc-forming faults in a junction of an electric circuit comprises inclusion elements 15 for monitoring a voltage differential across the junction and elements to produce a signal of failure of corresponding arc formation. BRIEF DESCRIPTION OF THE DRAWINGS * In the drawings: Figure 1 is a simplified schematic circuit showing a ground fault detector which can be used to detect arcs to ground according to an aspect of the invention. Figure 2 is a schematic circuit of a differential current derivative arc detection system.

Figure 3 is a schematic circuit of a differential voltage arc detection system with zero sequence. Figure 4 is a simplified schematic circuit ^ which illustrates the arc detection of phase current differential. Figure 5 shows an alternative form of differential phase current detection functionally equivalent to Figure 4. Figures 6 and 7 show yet another form of differential phase current detection functionally equivalent to Figures 4 and 5. The Figures 8, 9 and 10 show still other variations in apparatus for differential phase current detection that are functionally equivalent to those illustrated in the previous Figures 4-7. Figures 11 and 12 show apparatuses for accommodating leads in apparatus for differential phase current detection according to any of Figures 4-10.

Figure 13 is a schematic circuit of a simplified apparatus 20 for the detection of arc in low voltage. Figure 14 is a simplified schematic circuit of an inline energy loss arc detection system. 25 Figure 15 is a simplified schematic circuit of a detection system of arc di sene di / dt. Figure 16 is a simplified diagram of a busbar joining monitor using a toroidal sensor, • and 5 Figure 17 is a simplified diagram of a busbar junction monitor using an optical sensor. DESCRIPTION OF ILLUSTRATIVE MODALITIES Although the information may be susceptible to various modifications and alternative forms, the modalities .10 specific thereto have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that it is not intended to limit the invention to particular forms described, but on the contrary, the intention is to cover all modifications, equivalents and alternatives that fall within the spirit and e of the invention as defined by the appended claims. Several different aspects of the invention are described, each with its own characteristics and ™ alternative modalities. Permutations and combinations of these characteristics can lead to still other modalities. Referring now to the drawings and initially to Figure 1, a differential current sensing arrangement of zero sequence or ground fault in an electric circuit of alternating current. For simplicity, a single phase circuit including a line conductor 20 and a neutral conductor 22 is shown in Figure 1. One type of sensor coil current transformer 24 generally ^ P comprises a toroidal coil through which both the line and neutral conductors pass. The toroidal coil 24 can be wound helically in a core made of magnetic material, these sensors or ground fault transformers are generally known in the art. Since the flow or direction of the current in the line and neutral conductors should be opposite and equal in the absence of any ground fault in the circuit, an output coil 26 wound with respect to the toroidal coil 24 should essentially produce zero unless a ground fault occurs. The impedance of the conductors to ground is usually Relatively high and some transient capacitance in the conductors or in the load (not shown) can count as a small error current. Although the ground fault sensor will produce a detectable current output in its I output coil 26 in response to a line fault to earth, will not produce a useful current in response to line-to-line faults, for example, between lines of a multiple phase circuit. Also, the configuration of the ground fault sensor of Figure 1 does not detect arc formation in the seine. With reference to Figure 2, a simplified device for detection of branch arc of differential current. A branch arc fault current is schematically indicated at 34. This apparatus and the method by which it operates are based on the principle that ^ P the current entering and leaving the intended 5 connection points of a cable will add zero except during a line-to-line or line-to-ground fault. A source 23, line impedance 25, impedance source 27, and load impedance 29 are indicated schematically. In the illustrated embodiment, the line driver 20 passes through the t < 3 current transformer sensors (TC) 30 and 32 at the source and load ends or limits of a given area. Preferably each of the current transformer sensors 30, 32 comprises a toroidal coil through which the line conductor passes. The connection wires or "pilot wires" 35, 37 of the current transformer sensor and 32 are directed so that their current flow is for one of the ends of an arc detector of the current adder 36 which will add these two currents (which flow in opposite directions so that a "differential" is detected. of current ") In the absence of the branch arc such as the arc 34, the currents should be equal so that the net output of the current differential sensor 36 will be zero.The arc detector 36 may also include elements to avoid response to "noise" signals, such as requiring the magnitude of the current differential, ie the currents summed, they exceed some pre-selected threshold value before producing an "off" signal or output. This output or disconnection signal can be used to disconnect a breaker ^ P circuit (not shown) or other circuit interruption device and / or to produce a perceptible indication for human of the existence of an arc fault. It should be noted that if the line conductor is derived, each lead on the line conductor 20 will be provided with an additional current transformer sensor such as the sensors 30 and 32. Thus, in Figure 2, for example, if there is another load at the end of the first branch 40 on the line conductor 20, then a similar current transformer 38 would be provided in the branch 40 to feed the differential current sensor 36 together with the current transformers 30 and 32. The apparatus and associated method described with reference to Figure 2 does not detect a serial arc within the zone. Also, Figure 2 illustrates a simple single fault line, so that in a three-phase circuit with a neutral conductor, the zone would employ a total of eight current transformers, every four at the source end and at the load end of the zone. Referring now to Figure 3, a circuit for the sequence voltage differential method is shown zero for serial arc detection. This method of Sense arc detection recognizes three principles: 1) The line-to-ground voltage at each point of a wire in the area between a source and a load is the same except for ^ P any voltage drop due to the load current through the line impedance in sequence of any induced voltage due to the inductive coupling between the individual conductors of the circuit. 2) The currents in the phase conductors will add zero anywhere on the zone. It can be shown that voltage drops and voltages Mutually induced leads also add up to zero for cable systems with similarly shaped phase conductors. 3) Current carrying conductors have similar electrical properties and similar mutual coupling between all phase conductors. In the circuit 50 of Figure 3, the voltages are summed in all phases of the circuit at each of the origin and load ends. An element to add estost voltages in the illustrative mode takes the form of a ™ plurality of coupled resistors 52 which are coupled together at one end in a common line with pilot wire 64 and have their opposite ends coupled to the respective opgene ends of the respective phase lines 54, 56, 58 and to the neutral line 60. A similar set of coupled resistors 62 has the first ends coupled together to a common line or pilot wire 66 and the second ends coupled respectively to the load ends of the phase lines 54, 56, 58 and the neutral line 60. The respective summed voltages which are obtained at the common ends of • the two groups of coupled resistors 52 and 62 can be known as zero sequence voltage. Preferably, resistors with relatively high value, coupled, such as 200k ohms are used. However, coupled capacitances can also be used to obtain zero sequence voltages of the phase conductors. This arrangement is such that the voltage on the respective pilot wires 64, 66 represent an average of the phase conductor voltages at that point, also known as the zero sequence voltage. In order to avoid mutually coupled noise sources, it is preferred to direct the respective lines or the pilot wires 64 and 66 in a common path with the phase conductors back to an arc detector 70. This eliminates the cycle of a turn between the pilot wire and the phase conductors which can pick up magnetic field fluctuations and wavelengths. ™ low frequency radio. The arc detector 70 comprises comparison elements for comparing the two zero sequence voltages, that is, the respective sums of the voltages in the pilot wires starting from the loading end and the originating end of the line. Under normal conditions, this arc detector zero sequence voltage or comparator circuit 70 will produce a difference signal corresponding to any difference between the two zero sequence voltages in pilot wires 64 and 66 If there are no arc-forming faults in the monitored phase lines, the difference should ideally be zero A difference in volts implies an arc voltage or an error due to unbalanced wire impedances The resultant detector signal is defined as a difference between the sum of the voltages of the source end of the zone and the sum of the voltages of the load end of the zone When a series arc fault occurs in one of the phase conductors, a sene voltage will appear in that phase conductor that does not match the other phase conductors. In this way the corresponding pilot wire 64 or 66 will pick up a voltage which can show that it is equal to the arc voltage divided by the zone impedance end number, for example, resistors 52 or 62, that is, in the circuit neutral plus three conductors illustrated, an arc voltage that appears as a square wave of 15 to 20 volts with random lengths of time will cause a zero sequence voltage picked upstream of the fault point, inducing for example, a square wave of 20 / 4 or 5 volts on the corresponding pilot wire 64 or 66 (or, in a three-phase system without picking up voltage in the neutral, 15/3 = 5 volts) The arc detector 70 will detect this difference square wave potential of 5 volts between the zero sequence sensors of origin point 52 and the end of the zone sensors 62 on the respective pilot wires 64 and 66. This will produce a non-zero difference signal indicating a failure of arc. In general, arc formation will not occur below a minimum voltage of approximately 11-12 volts, and, as a practical matter, usually does not occur below 15-20 volts. A small noise signal may occur due to differences in impedance between the load current wires and / or due to inductance and mutual capacitance of pilot wires unbalanced by the phase conductors. In this way, the arc fault detector 70 may also include elements to avoid response to these noise signals, such as requiring that the magnitude of a signal or difference between the zero sum voltages in wires 64 and 66 be above of some minimum value or predetermined threshold before producing an output or disconnect signal. This output or disconnection signal can be used to disconnect a circuit breaker (not shown) or other circuit interruption device, and / or to produce a perceptible indication to humans of the existence of an arc fault in the area that It is being monitored. It will be noted that in the case of a three-phase system, the coupled impedances or resistors 52 and 62 comprise coupled resistors connected in bifurcation.

In a practical application, a disconnect curve of the circuit of Figure 3 could include a threshold level of approximately 5 volts and a characteristic of ^ P reverse time disconnection. However, if the devices of switching are in the protected zone, additional disconnect control elements would be provided in the arc detector 70 which responds to the opening of any switching device in the zone to disable the disconnect signal. In this way, the control element of disconnection of the arc detector 70 would normally allow the I disconnection signal. Although the circuit of Figure 3 mainly detects arcs in series, the presence of an arc fault in the zone will also generate some differential signal of zero sequence, due to the increased current in the fault of ground in the phase conductor will cause a higher voltage drop in that phase conductor that does not match the other phase conductors. Even if all phase conductors had ground faults, it is unlikely that all would result in equal voltage drops. However, the phase to phase or bypass arcs will not generate a zero sequence differential signal. In accordance with the foregoing, the circuit of Figure 3 for serial arc detection could be made in combination with the circuit of Figure 2 for the detection of branch arc to provide a arc detection system capable of detecting both types of bows In addition, the arc detector 70 may include elements for compensating systems where the neutral conductor ^ has a much higher impedance or shares more 5 large currents than other leads. If the respective phase conductors are derived within the zone by branch circuits, the additional zero-sequence sensors (i.e. additional impedances such as the coupled resistors 52 or 62 which run back to the common lines or pilot wires) can be placed at all desired limit points or at any intermediate connection point for additional accuracy when signaling the placement of the arc fault. However, the added sensors are not required for loads with derivation energy within the zone as it happened for the differential current method as described above. The ground fault detector of Figure 1 may also be combined with both, or with any of the branch or bypass arc detection circuits of Figures 2 and 3 for the protection of arc and ground fault in a cable system, or in specific areas of a loading system. In this combined protection system, the ground fault sensor would generally respond to ground faults throughout the cable system without respecting the zones, while that zone arc fault detection circuits of Figures 2 and 3 would be employed in those areas where this protection is desired. Referring now to Figure 4, illustrated ^ P a circuit for the detection of differential phase 5 current arc, and is generally designated by the reference number 80. This method requires that each phase conductor, in the area of the same to be protected, consist of a pair of identical parallel insulated conductors, for example, conductors 82, 84, shown in Figure 4. Without However, pilot wires are not required for this method. In this way, the protected zone is defined as the degree or length of the conductors 82 and 84 between the respective points where again they are joined to a single conductor, for example, the respective entry and exit points 86, 88, in the example shown in Figure 4. The two conductors 82 and 84 are arranged to pass through the magnetic core 90 so that the current in the two conductors 82 and 84 travel in opposite directions to I through the core 90. The core 90 is provided with a coil 92 on which an output signal develops which corresponds to any difference in current between the two conductors 82 and 84. Since identical insulated parallel conductors are used, the output of the coil 92 should be essentially zero in absence of any failure of arc formation in the area. However, any arc in sene in the connections of one of the conductors in the zone will create or generate a circulating current that will pass through the point of union of origin, that is, the point at which the wires 82, 84 are coupled to the source end or input wire 86, and also through the arc detection detection core 90. This circulating current will be detected and a fault will be detected as a result of an output current from output coil 92. Line-to-line or line-to-ground faults will generate relatively large fault currents that are picked up directly by the magnetic arc detection core 90. As the above mentioned modes , an arc detector 95 can be coupled with an output coil 92 and this arc detector 95 can respond to any current in the output coil 92 to produce a cut-off signal in the event that this current exceeds some predetermined threshold value. This method can also be used to capture the active impedance of the integrity of the cables with the cables either energized or de-energized. An alternating current signal applied to the coil 92 of the magnetic core 90 can be used to interrogate the total circulation impedance of the phase conductors. An open lead or an unusual increase in resistance will be detected as a corresponding change in the alternating current signal in coil 92. Referring also to FIGS. 5-10, they can implement several different equivalent versions of the circuit for differential phase current detection shown in Figure 4. It will be recognized that other ^ P implementations may be possible using the principles 5 discussed hereinabove and illustrated in Figure 4, the specific embodiments being illustrated by way of example only. For example, in Figure 5, the two conductors 82 and 84 of the phase pass through the respective cycles of the -shaped core 100 which is provided with a similar pickup coil 92. It should be noted that the point at which the core segments cross in the form of eight do not touch in this mode. In Figure 6, the two conductors 82 and 84 intersect as they pass through the respective upper and lower segments of a core 110 whose shape is also shown in Figure 7. A similar output or sensing coil 92 is provided at a convenient point on the core 110.

I Additional schemes for crossed conductors of different types as they pass through a generally rectangular core 120 are shown in Figures 8-10. In the differential phase detection scheme described above, any derivation of energy or charge within the zone is also required to have a device additional end zone, that is, a transformer core additional . Two schemes for providing these cores in energy or load branches are illustrated respectively in Figures 11 and 12. In Figure 11 an eight-shaped core 122 is used, similar to that shown in Figure 5, while in Figure 12 , a generally rectangular core 124 similar to the core 120 of Figures 8-10 is used. Respective pairs of conductors 82, 84 enter the branch through the respective core 122, 124 and emerge from the branch as conductors 82A and 84A. These latter transformer cores 122 and 124 present significant impedance in the circulating currents that pass the torque conductors, but are not provided with coils such as the coils 92 illustrated and described above with respect to Figures 4-10. Instead, the purpose of the cores 122, 124 is to maintain any differential circulating current due to an arc fault when the two conductors are joined at a bypass point. Referring now to Figure 13, there is shown a circuit 125 for arc detection in a voltage drop. This circuit detects the same arc voltage, symbolized schematically in the circuit of Figure 13 as VMC. It is noted that this circuit detects arcs in senes instead of arcs of derivation or in parallel. These arcs generate direct current frequency components in the megahertz range and beyond. The arc voltage always opposes the current on the line. The arc voltage in sene therefore always adds to the voltage drop of the line so that an unusual increase in voltage drop will indicate an arc in ^ the driving path. In the circuit of Figure 13, a pilot wire 126 coupled near the charging end of the zone feeds an input of an arc detector 128, which may be a voltage detector, to detect or compare the voltage on the wire pilot 126 with the voltage at or near the origin end of the circuit that is fed to the detector 128 on a line 130. The total voltage drop seen by the detector 128 would then be the voltage drop of the line due to the nominal line impedance plus the arc voltage. The impedance voltage drop in the line should be subtracted from the total voltage drop in the detector 128 with in order to extract the arc voltage. The impedance compensation methods could include a set of inductors and resistors in series with the detector 128 which mimics the line impedance, a compensation algorithm of I equivalent software. Sequence voltage monitoring Zero as described above with reference to Figure 3 avoids the need for line impedance compensation. Referring now to Figure 14, a circuit 135 is shown for use in an energy loss method line (arc energy) arc detection. This method It monitors the loss of energy through a conductor and subtracts the energy due to the resistance. The arc energy relates more directly to the potential for the damage of the ™ equipment and other problems, other measurements 5 such as arc voltage or arc current only. The same pilot wires or connections 126 and 130 that in the circuit of Figure 13 supply the voltage drop of the line to an arc energy sensor type of arc detector circuit 132. A current transformer sensor 134 supplies the current in the line from the line's origin end to the arc energy sensor 132. In this way, the current by the total voltage drop can be calculated by the arc energy sensor 132 to determine the arc energy in the presence of a voltage of arc formation as indicated in Figure 14 similarly to the arc-forming voltage indication as discussed above in Figure 13. With this circuit 135 and approach, a reverse time disconnect curve could be used. , allowing components such as breakers and fuses had a higher arc operating power for a few milliseconds, while the conductors and connections would not be allowed too little arc energy. As with the circuits of Figures 2 and 3, the arc detectors used in circuits of Figures 13 and 14 they could be provided with threshold limits, so that the voltage drop or loss of energy read in the detector must exceed some threshold value before a disconnect signal occurs. Referring now to Figure 15, a circuit 139 for di / dt detection for arcs in series is shown. This circuit uses a current transformer sensor 140 having a primary line in line with the line and a secondary line coupled through a voltage sensor arc detector 142. The current sensor 140 can be a current sensor (core). air) mutual inductor to produce an output voltage that is proportional to the noise generated by any arc voltage. The output voltage of the sensor 140 is then grossly proportional to the arc voltage divided by the inductance in series with the arc. The bypass capacitance or relatively low inductance loads will allow relatively large di / dt arc signals. The shunt capacitance has the effect of reducing or resonating the load inductance at some frequencies. Highly inductive loads such as motors will produce only a relatively silent arc signal, whereas variable speed motor activations, which are mainly capacitive loads will allow relatively sound arc signals. Typically, however, the noise generated by the activation Variable speed motor is still louder than the arc signal. The arc voltage typically contains many high frequency noise components so that rapid fluctuations in the arc voltage result in current fluctuations. The magnitude of these current fluctuations depends on the frequency response of the line and the loads connected at any given moment. The di / dt sensor 140 collects all these fluctuations and can not distinguish di / dt generated by charge of the di / dt generated by arc. In this way, relatively noisy loads can induce a disconnect signal using a di / dt 140 sensor as shown in Figure 15. More, the high mductance loads attenuate di / dt, and the arcs that occur in circuits with very inductive loads are therefore difficult to detect with the di / dt sensor. In order to improve the di / dt level during arc faults with inductive loads, the Line Impedance Stabilization Networks (REIL) 150 and 152 are added to the circuit as shown in Figure 15. These RC networks are charged both the line end and the cable load to allow the tuning of the circuit and the load combination to a desired frequency response. By connecting a bypass filter of this type with approximately one microfarad and a resistance series of 10 to 30 ohms in each branch circuit at the load end it tends to masking the effects on di / dt due to different load energy factors, such as both resistive and inductive loads tend to produce similar ^ P arc fault responses di / dt. The frequency response The improved high achieved by this method allows relatively high-level di / dt signals to be monitored successfully during a series of arc faults with an inductive load or a resistive load. Zones in various types of electrical systems are can identify and define in various ways in order to achieve arc protection using one or more of the above methods or circuit for arc protection. In addition to the above methods, critical junctions (for example, junctions between wires, terminals, connectors, contacts, etc.) in an electrical system could be monitored separately, as indicated in Figure 16 and Figure 17 by an operably coupled sensor to monitor each junction, and with multiple sensors in this system connected with conductors I twisted pair. A joint that has such an arc voltage as the junction 160 in Figures 16 and 17 would typically develop a detectable voltage drop during the arcing event. The sensor would then respond to this voltage drop across the junction generating a corresponding signal which would be sent through the conductor of the twisted pair to a convenient detector circuit (not shown). In the sensor scheme shown in Figure 16, a double-wound toroid 164 has one of its windings 166 ^ operatively coupled through or in parallel with the junction 5 160, and a second winding 168 coupled in series with the twisted pair 170. This circuit also allows the bidirectional testing of the junction because the alternating current applied to the twisted pair 170 will result that the current is reduced at junction 160. In this way, any union that is J.0 open will result in an increase in measurable impedance in the twisted pair network. A completely open circuit in the junction will generate a large current in the primary coil 166, so that a current limiting impedance such as a capacitor 172 is preferably added in sequence with the primary coil 166. Alternatively, an overcurrent protection such as a fuse (not shown) could be used. In Figure 17, a similar circuit uses an optical sensor scheme to develop a signal in a network Twisted Pair ™ in response to an arcing voltage through a junction 160. For example, a light emitting diode 174 only requires some volts and milliampere level currents to operate. The light from a light emitting diode would be picked up directly by an optical fiber (not shown) over short distances or operate an optocoupler switch 176. The light emitting diode 174 and the optocoupler switch 176 can be packaged as an optoisolator 178. The light emitting diode can be protected from overcomers from a fully open junction 160 with a limiting capacitor ^ P of current 172 similarly to the circuit of Figure 16 5 described above. Multiplexing could be used with respect to the opto-insulators connected in parallel along the twisted pair network 170 to be able to individually identify a problem joint when arcing occurs. The characteristic threshold voltage of the light-emitting diode 174 could also be used to monitor the voltage differential across a busbar or wire or to determine excessive voltage drops due to arcing faults. It will be appreciated that the methods described above for The detection of arc formation in a joint can be applied through terminals, contacts, or at any point in a circuit where arc in series can occur in a relatively short length of circuits. In this way, the The term "union" as used herein is to give a broad meaning, consistent with the latest observations. Many other components and elements of electrical systems can lend themselves to arc fault detection using one or more of the above schemes herein. You can think of a connection board as a short section of busbar with many branches. A The arc in a connection board can start as an insulating flash or as a deterioration of the connection of the busbar. The flash type will cause high phase-phase fault currents ^ P and will disconnect the protection device over current if the fault is within the protection zone over instantaneous current. For multiple branch protection, a di / dt sensor can be used for arcs in senes with silent loads. The junctions in the path of the bus bar can have a lot of conflabilidad but occasionally one is ^ It can deteriorate and form an arch. Series arc formation also coincides with starting charges and high incoming currents that create peak vibrations and high contact points. For bus sections with few high current tees, a detection scheme of arc of zero sequence voltage (eg, Figure 3) with the zero sequence voltage being captured in each section te. Serial arcs on either side of the system will generate some di / dt noise levels, especially if I connects resistive or capacitive load. Small changes in the load capacitance and resistance can cause large swings in the magnitude of the di / dt signal. This effect can be visualized as a change in the impedance of the peak network from the point of view of the arc. The cycle impedance controls the high frequency current flow The changes in capacitance or resistance either on the line side or load of the arcing point changes the cycle impedance and will change the amount of the transmitted di / dt signal. The feeders of leads and cables include ^ P isolated wires in crossbars, cable tracks, and conduits. 5 The close proximity between different branches and phases couples the high frequency signals. Arc-forming faults are unlikely except when vibration damages the insulation or loosens the connections. Abrasions most likely initiate line-to-ground faults, which involve faults line by line. The arc-forming faults in the neutral conductor are the most difficult to find due to the low potential of neutral to ground. Ground fault current transformers would protect against most cable faults where the ground surface is near the arch. Differential current monitoring schemes (eg, Figure 2) provide a method for ground faults and phase-to-phase arc formation, using runs of pilot cables as long as the phase conductors. 20 A differential scheme of zero sequence voltage (e.g., Figure 3) can be used to define a zone arc detection failure detection zone. The detection zone is the conductive path in the seine between the voltage sensors at the beginning and at the end of the cable. The signal of monitored arc formation is independent of the charges in the circuits of the derivation. The arcs in sene in circuits derived outside the zone can not be detected with this method. Some ground faults can be detected. The ^ Phase-to-phase faults do not appear as zero sequence voltage differences 5 and require protection by overcurrent devices or by differential current monitoring (eg, Figure 2). The di / dt sensor can be an arc fault detection scheme for case cable systems specials or areas where the load noise is low and predictable. Avoiding false disconnections due to normal connection events that look like arcs in senes can be difficult, so these zones should not include switching operations. 15 The internal insulation fault of a transformer coil can cause arcing. Large transformers often have overpressure relays that respond to internal winding failures. Ground faults (the core or tank) can be detected as standard ground fault currents using differential relay. The motors have similar arc detection solutions of the transformers. One difference between the motors and the transformers is that the probability of arcs from phase to phase in the slots and turns extreme of the engines. Another is that virtually all three phase motors have neutral points without grounding to avoid third harmonic current. The faults V to ground are easily detected since zero sequence currents are present only during a fault. The motors have the characteristic that the rotor currents are induced, which oppose changes in the stator flux due to short back-to-back times. The vibration is higher and the decomposition of the insulation is often most likely. The di / dt signals generated by arcs are attenuated compared to the ambient noise levels. One technique for detecting phase-to-phase coil faults involves monitoring the differential current (e.g., Figure 2) in each phase winding. All motor winding terminals are carried out of the motor so that three current transformers monitor the difference between the incoming current and the current leaving in each phase winding. Any current escaped »Detects as a fault. 20 Motor control centers can be operated similarly to junction boards and bus sections. The ground fault detection would detect arcs of earth bypass but not arcs in seine. The short distance of the motor control cells makes attractive serial arc detection "pilot wire" (for example, Figure 3). In the motor control cells, the contacts and switches will generate arcs each time the devices are operated. The arcs in series of a couple of cycles ^ of duration are part of a normal contact operation and should be anticipated. Arc detection should be restricted from operation during normal commutation operations in the area. The restriction can be carried out by auxiliary contacts operating before the main contact switching action. Alternatively, you can define .0 separate zones for each component in the path of - current. The inverse characteristics of time to disconnection on the voltage signal can be used to allow a normal arcing time during the operation of each component. The differential method of zero sequence voltage of Figure 3 offers the advantages of detecting arcs in series by compensating the similar arc contact voltages in three phases and extending the protection zone through the connections with the power section of the bar ^ collector. 20 Motor drives tend to generate more di / dt noise than typical arcs. The detection of arcs in series with unfiltered conduits using di / dt is therefore difficult. The conduction current can be filtered by adding mductance to the sides of the line and the load of the conduction to improve the detection of arc type di / dt. He Zero sequence voltage method (eg, Figure 3) was found to distinguish the arc voltage without the need for inductor filtering. The arcs in senes appeared in the signal ^ P of zero sequence voltage without the high frequency noise 5 apparent in the di / dt signal. A similar arc detection scheme is possible between the driver and the motor but more induced noise will be found in the pilot wire. What has been illustrated and described herein are vain methods for detecting arc faults in vain applications. As described above, the detection of branch arc fault failure can be used by monitoring the differential current of each phase conductor in an area, where current transformers at the beginning and end of the zone add up to generate a signal from fault current. Ground fault detection by a single zero-sequence current transformer detects arc-to-ground faults but not line-to-line arcs or arcs in series. Serial arcs can be detected by monitoring the difference between the zero sequence voltage of the source with the zero sequence voltage of the load end of the zone. This method is independent of the load and origin impedances. An inverse time disconnect curve based on this zero sequence voltage difference is a reasonable series arc zone protection scheme. This method requires a defined zone beginning and ending and that is not can extend indefinitely to all connected charges. Serial arcs representing charges derived outside the zone are not detected by this method. However, the differential phase current detection can also be used to detect arcs in series, such as the voltage drop and energy loss methods described above. Arrays such as those shown in Figures 16 and 17 can be used to detect arc formation in senes in joints or through connectors or similar contacts. The di / dt method for arc detection is simple to implement; however, the arc signal has a high signal to noise ratio only for silent capacitive or resistive loads. Some interference of di / dt signals between the leads has been observed. Switching operations and solid state components also produce high frequency di / dt signals that could be mistakenly taken as failures.

Claims (59)

1. NOVELTY OF THE INVENTION Having described the foregoing invention, it is considered as a novelty and therefore the content of the following is declared as ^ P property. 5 CLAIMS 1. An arc fault zone detection system for detecting failures of arc in a defined area in the electrical circuit, comprising: a zero-sequence differential voltage arc detector for detecting faults in ^) Arcing in series and including: a voltage summing element to sum the voltages of all the phases of the circuit both at the origin and load end of said zone, and a comparator element to compare the sum of the voltages at one of the load and origin ends of each phase 15 and the sum of the voltage at the other load end and origin of each phase and to produce a difference signal corresponding to any difference between them.
2. 2. The system in accordance with that claimed in claim 1, and a differential voltage arc detector. 20 of zero sequence and further includes an arc-forming fault detecting element that responds to the magnitude of said difference signal by being above a predetermined threshold value to produce a disconnect signal.
3. 3 The system in accordance with what was claimed in the 25 claim 1, characterized in that the summing element of voltages comprises a plurality of coupled resistors, each resistor having a first end coupled with one of the load ends and origin in said area of each phase of said ^ P circuit; and wherein the comparison element has an input coupled to an end opposite the first end of each of the matched resistors coupled to the originating ends of each phase and a second input coupled to an opposite end of the first end of each of equalized resistors coupled to the load end of each phase. W
4. 4. The system according to claim 3, characterized in that the electric circuit is a line of three phases and where the matched resistors coupled with each of the origin and load ends are connected in bifurcation.
5. 5. The system according to claim 3 and further comprising a disconnect control element that responds to predetermined conditions in the zone to alternately enable and disable the disconnect signal.
6. 6. The system according to claim 1, characterized in that the disconnection control element responds to the opening of any switching device in the zone to disable the disconnection signal. 25
7. 7. The system in accordance with what was claimed in the claim 1 and that also includes compensation elements to provide compensation for a circuit where the neutral conductor has a much higher impedance than the ^ P phase conductors or where the neutral conductor shares 5 large streams from other branches.
8. 8. The system according to claim 1, further comprising a differential current arc detector.
9. 9. The system in accordance with what is claimed in the ^^ - u. claim 8 characterized in that the differential current arc detector comprises a current sensor for developing a current signal corresponding to the current at each of the load ends and origin of each phase in the area; and a coupled current adder element 15 with the current sensors to receive the current signals and sum the current signals from each of the source ends with the current signals in the opposite direction starting from the corresponding one of the load ends. ^
10. 10. The system in accordance with the claim in the 20 claim 9 and further includes a second arc-forming failure detecting element that responds to the magnitude of any of the summed current signals from the current summing element that is above a predetermined threshold value to produce a signal from 25 disconnection.
11. 11. The system according to claim 9 and further including an additional current sensor at each branch location within the zone, said summing element of currents also adds the current signals of the additional current sensors with the signals of current from the current sensors at the load end and the source end of the zone, respectively.
12. 12. The system according to claim 1 and further includes a differential phase current arc detector.
13. The system according to claim 12 wherein the differential phase current arc detector comprises a pair of parallel insulated conductors substantially identical for each zone of each conductor to be protected by the phase current detector. differential by means of which a protected zone is defined comprising the length of the parallel conductors between two points in which the two conductors are coupled to each other; and a transformer current sensor inductively coupled to each of the pair of parallel conductors, the transformer current sensor and the conductors respectively being configured and arranged so that the current induced in the current transformer sensor from one of the drivers are subtracted from the current induced in the transformer current sensor from the other of the conductors to produce a difference current output. ^ P
14. 14. The system in accordance with the claim in the 5 claim 13 and further includes another arc-forming fault detecting element that responds to the magnitude of any of the difference current outputs that exceed a predetermined threshold value to produce a disconnect signal.
15. 15. The system according to claim 9, further comprising a differential current arc detector.
16. 16. The system according to claim as claimed in claim 15 wherein the current arc detector of The differential phase comprises a pair of parallel insulated conductors substantially identical for each zone of each conductor which will be protected by the differential phase current detector whereby a protected area is defined »Which comprises the length of the parallel conductors between 20 two points in which the two conductors are coupled together; and a transformer current sensor inductively coupled to each of the pair of parallel conductors, the transformer current sensor and the conductors respectively being configured and arranged so that the 25 induced current in the current transformer sensor from one of the conductors is subtracted from the current induced in the transformer current sensor from the other of the conductors to produce a current output ^ P of difference.
17. 17. The system according to claim 16 and further comprising an arc-forming fault detecting element that responds to the magnitude of any of the difference current outputs that exceed a predetermined threshold value to produce a ^ ß disconnection signal.
18. 18. An arc fault zone detection system for detecting arc faults in a defined area of an electrical circuit, comprising: a differential phase current detector including a pair of insulated conductors in Parallel substantially identical parallels for each zone of each conductor that will be protected by the differential phase current detector whereby a protected zone is defined which comprises the length of the parallel conductors between ^^ two points at which the two drivers are coupled between 20 yes; a transformer current sensor inductively coupled with each of the pair of parallel conductors, the transformer current sensor and the conductors respectively being configured and arranged so that the current induced in the transformer sensor from current to 25 from one of the conductors is subtracted from the current induced in the current transformer sensor from the other of the conductors to produce a difference current output. ^ P
19. 19. The system in accordance with the claim in the 5 claim 18 and further includes an arc-forming fault detecting element that responds to the magnitude of any of the difference current outputs that exceed a predetermined threshold value to produce a disconnect signal. ^ LO
20. 20. The system according to claim claimed in claim 18 which further includes a differential current arc detector.
21. 21. The system according to claim as claimed in claim 20, characterized in that the arc detector of The differential current comprises a current sensor for developing a current signal corresponding to the current at each of the load ends and origin of each phase in the area; and a current adder element coupled with the current sensors to receive the current signals and sum the current signals from each of the source ends with the current signals in the opposite direction from the load ends.
22. 22. The system in accordance with that claimed in claim 21 and which also includes a second element 25 arc-forming failure detector that responds to the magnitude of the summed current signals of the current summing element which is above a predetermined threshold value to produce a disconnection signal. ^ P
23. 23. The system in accordance with the claim in the 5 claim 21 and furthermore includes an additional current sensor at each branch location within the zone, the current summing element also adds the current signals from the additional current sensors to the current signals from the sensors of the current. current at ^ LO the charge end and the origin end of the zone, respectively.
24. 24 An arc fault zone detector system comprising: a differential current arc detector for detecting fault arc formation conditions of 15 earth and line to line; and a differential voltage arc detector for detecting arc-forming faults in a signal.
25. 25. The system according to claim as claimed in claim 24 wherein the voltage arc detector Differential ™ comprises a zero sequence differential voltage arc detector.
26. 26. The system according to claim 24 and further comprising a differential phase current detector for detecting both arc-forming faults in series and in parallel. 25
27. 27. The system in accordance with what was claimed in the claim 26 wherein the differential voltage arc detector comprises a zero sequence differential voltage arc detector. ^ P
28. 28. The system in accordance with the claim in the 5 claim 24 and that also includes a ground fault detector.
29. 29. The system according to claim 28, wherein the differential voltage arc detector comprises a voltage arc detector. ^ .0 zero sequence differential.
30. 30. An arc zone detection system comprising a differential voltage arc detector for detecting arc faults in series and a differential phase current detector for detecting faults in the formation of an arc. 15 arc in sene as in parallel.
31. 31. The system according to claim 30 wherein the differential voltage arc detector comprises a voltage arc detector. ™ differential of zero sequence.
32. 32. An arc fault zone detector system comprising: a differential current arc detector for detecting conditions of ground fault arc and line to line; and a differential phase current detector for detecting arc-forming faults in line 25 and in parallel.
33. 33. An arc fault zone detection system for detecting arc faults in series in an area of an electrical circuit, comprising: ^ P voltage to capture the voltage in a conductor in line of 5 electrical circuit both at the source and load end of the zone; and arc detector elements coupled with the sensor element to produce a difference signal corresponding to any difference between the voltages at the source and load ends of the zone. ^^ >
34. 34. The system according to claim 31 and further including line impedance compensation elements to remove the effect of voltage drop caused by the line impedance from the difference signal.
35. 35. The system according to claim 31 further comprising a current transformer adjacent to the origin end of the line in the area, and an arc detection element that responds to the ^^ current transformer to produce a signal like 20 response to an arcing fault in series in the line conductor.
36. 36. The system according to claim as claimed in claim 35 and further comprising a line impedance stabilization network coupled to the source and load end of the zone.
37. 37. The system according to claim 36 wherein the line impedance stabilization network comprises an RC filter connected in ^ derivation. 5
38. 38. An arc fault zone detection system for detecting arc faults in a defined area of an electrical circuit, comprising: a current transformer operatively coupled to an electrical circuit line conductor to produce a current signal ^ LO corresponding to an electric current in the line conductor; an arc detector coupled to the current transformer to detect a change in current due to a failure of arcing in series on the line conductor; and a line impedance stabilization network 15 coupled with the ends of origin and line within the defined area of the line conductor.
39. 39. The system according to claim as claimed in claim 38, characterized in that the stabilization network ^ of the line impedance comprises a bypass RC filter 20 coupled with the ends of origin and load respectively of the line conductor within the zone.
40. 40. An arc fault zone detection system for detecting arc faults in a defined area of an electrical circuit, comprising: a sensor element of 25 voltage coupled with each of the source and load ends within the area of a line conductor of the electrical circuit; a current sensor coupled to the line conductor of the electrical circuit adjacent to the source end of ^ P the area; and an arc energy detector operatively 5 coupled with the voltage sensors and with the current sensor to detect any loss of energy through the line conductor in the area corresponding to a solid arc in the line within the zone.
41. 41. The system according to claim as claimed in claim characterized in that the arc energy sensor also includes elements that respond to the magnitude and duration in time of the arc energy captured to produce the disconnection signal when the magnitude and the length of time exceed certain threshold values. 15
42. 42. An arc fault zone detection system for detecting arc faults in a defined area of an electric circuit, comprising: a voltage summing element to sum the voltages of all phases of the circuit both at the extreme origin as cargo of said area; a 20 comparator element to compare the sum of the voltages of the phases in the other of the load and origin ends and the sum of the voltages of the phases in the other load and origin ends and to produce a difference signal corresponding to any difference between them; a 25 differential current arc detector; and an element ground fault sensor comprising a current transformer sensor operatively coupled with the line and neutral conductors of the electrical circuit to detect a ^^ difference in the current between them and produce a signal of 5 corresponding ground fault.
43. 43. The system according to claim 42, and further comprising an arc-forming fault detecting element responsive to the magnitude of the voltage difference signal that is above a value. The predetermined threshold for producing a disconnect signal and responsive to the magnitude of the ground fault signal that is above a predetermined threshold value to produce a disconnect signal
44. 44. A system for monitoring the arc fault in a junction of an electrical circuit, the system includes elements for monitoring a voltage differential across the junction and for producing a corresponding arc-forming fault signal. ^^
45. 45. The system in accordance with what was claimed in the 20 claim 44 wherein the element for monitoring comprises a two-winding toroid, the first winding being coupled through the joint and the second winding to produce the arc-forming failure signal.
46. 46. The system according to claim 25, characterized in that the secondary winding it is coupled in a monitoring network.
47. 47. The system according to claim 46, characterized in that the monitoring network ^ comprises a network for connecting a parallelity of the 5 secondary windings in series circuit configuration
48. 48. The system according to claim 44, characterized in that the detector comprises a light-emitting diode coupled through the joint to be monitored. ^ >
49. 49. The system according to claim 48, characterized in that the detector further includes an element that responds to light from the light emitting diode to produce an arcing fault signal.
50. 50. The system according to claim 15, characterized in that the detector also includes an optical switch that responds to the light of the light emitting diode to produce a corresponding arc-forming fault signal.
51. 51. The system according to claim 20, characterized in that the optical switch is coupled in a network to monitor a plurality of joints and includes elements to identify any of the plurality of joints in which the formation failure occurs. of arc in response to the corresponding arc-forming failure signal 25.
52. 52. The system according to claim 51, characterized in that the network includes a twisted pair of network conductors operatively coupled with a ^ * plurality of optical switches, each associated with 5 of the junctions to be monitored.
53. 53. The system according to claim 1 and further including a ground fault detector to detect a ground fault on either side in the electrical circuit. ^^
54. 54. The system according to claim 9 and also includes a ground fault detector to detect a ground fault on either side in the electrical circuit.
55. 55. The system in accordance with that claimed in claim 15 and which also includes a ground fault detector to detect a ground fault on either side of the electrical circuit.
56. 56. The system in accordance with what is claimed in claim 18 and which also includes a failure detector of 20 ground to detect a ground fault on either side of the electrical circuit.
57. 57. The system according to claim 33 and further including a ground fault detector to detect a ground fault on either side in 25 the electrical circuit.
58. 58. The system in accordance with that claimed in claim 38 and further includes a ground fault detector to detect a ground fault on either side in the electrical circuit.
59. 59. The system as claimed in claim 40 and further including a ground fault detector to detect a ground fault on either side in the electrical circuit.