Abstract:
A method for detecting arcs in a DC circuit, having a detection conductor, which is routed at least along a conductor of the DC circuit, which carries an electric voltage and is to be monitored. In this connection, a voltage signal that refers to a potential of the DC circuit is coupled into the detection conductor at an incoupling point via a series resistor, the voltage signal is coupled out again at an outcoupling point of the detection conductor, the voltage signal before the series resistor is compared to the voltage signal at the outcoupling point, and a conclusion is drawn that an arc has occurred when the voltage signal at the outcoupling point deviates from the voltage signal before the series resistor by more than a predetermined amount. This invention also relates to a device for carrying out the method. The method and the device permit a quick, reliable detection of an arc in a DC circuit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Field of the Invention 
         [0002]    This invention relates to a method for detecting arcs in a DC circuit, having a detection conductor, which is routed along a DC circuit conductor, which carries an electric voltage and is to be monitored, and also relates to a device for carrying out the method. 
         [0003]    Discussion of Related Art 
         [0004]    A method and device of this type are disclosed in German Patent Reference DE 101 32 750 A1. This reference describes a fuse array for an electric line in a motor vehicle. In this case, a twin conductor is routed along a conductor to be monitored or a wiring harness to be monitored. The two lines of the twin conductor are separated from each other by an insulation and are contacted by a different potential. A short circuit in the line/wiring harness to be monitored causes the insulation to melt and the conductors of the twin conductor to come into contact. As a result, current flows between the conductors, which is detected and evaluated for an arc detection. In this known embodiment, according to one exemplary embodiment, one conductor is contacted with a predetermined potential and the second conductor is contacted to ground. In another embodiment, the potential difference is created by a voltage divider, with a voltage being supplied via a first resistor to a first conductor of the twin conductor at the front end of the latter. Both of the twin conductors are connected to a second resistor at their ends. The front end of the second conductor is connected to ground via a third resistor. This known fuse array is designed so that the two conductors of the detection conductor must be routed in such a way along the electrical line to be monitored that they come into electrical contact in the event of a short circuit. 
         [0005]    German Patent Reference DE 10 2012 019 996 A1 describes a method for monitoring a line network for arcs. For this purpose, a sensor line is connected to ground via a first resistor and a ground line. A voltage increase in the sensor line is evaluated. This can take place when the ground line is interrupted by an arc or when, after an arc, an electrical contact with a voltage-carrying line of the line network to be monitored occurs. This known design is an active safety system that is currentless in the basic state and results in a current flow in the event of a fault. 
         [0006]    Today, the electrical system of passenger vehicles is operated with 12 V. By contrast, an electrical system of 42 V or 48 V offers significant advantages particularly in the form of a CO 2  reduction due to improved recuperation possibilities or in the form of a possible reduction in vehicle weight. Consequently, the voltages provided are below the contact protection range, which starts at 60 V and require special safety precautions. 
         [0007]    With increasing voltage, the risk of arc formation increases. An arc can occur between two components at different potentials, for example between a plus cable and a minus cable or also along the current path in the vicinity of elevated transmission resistances. Elevated transmission resistances are caused, for example, by corroded contact points or due to damaged electrical conductors. Whereas a voltage of 12 V is still below the necessary discharge voltage of an arc, for vehicles supplied with 42 V or 48 V, there is an increased risk of such an arc formation, which stabilizes at approximately 20 V and thus represents a significant fire risk for the vehicle and a corresponding hazard to people. In this case, fuses that are provided to monitor electrical consumers and their feed lines do not react or only react in a delayed fashion since the currents occurring in an arc between two components with a different potential are low in comparison to a direct short circuit and in an arc along the current path, can even be reduced relative to the regular operation of a connected consumer. 
         [0008]    The German Patent Reference DE 101 32 752 B4 discloses a method and device for protecting a conductor when an arc occurs. In this case, an insulated detection line is routed along the supply line to be monitored. If an arc occurs, the insulation of the detection line melts and a direct contact occurs between the detection line and the supply line. This produces a current flow in the detection line, which is used for detecting an arc. The disadvantage here is that a direct contact between the detection line and the supply line is required in order to produce the current flow that is characteristic of an arc. This occurs in the uncontrolled, hot region of an arc in which insulation material, when a corresponding generation of gas occurs, can abruptly catch fire. On the other hand, the supply line and the detection line can be separate from each other in the region of the arc, as a result of which the current flow does not occur or falls below a range that can be reliably assessed. In this case, the reference provides for the use of a special insulation material with a melting point between 130° and 180°, which flows away in a controlled fashion in the event of an arc. In the vicinity of an arc, however, temperatures of significantly greater than 1000° C. occur, which causes the insulation material to disintegrate. Another disadvantage lies in the fact that the system is an active safety system which is currentless during regular operation and in the event of a fault, conveys a detectable current. A defect in the safety system, for example due to corroded contacts of the detection line, can, in the event of a fault, result in the fact that despite the occurrence of an arc, it is not possible for current to flow in the detection line and the arc therefore remains undetected. In order to avoid this, complex and correspondingly expensive contacts are required. In order to comply with safety standards for an active safety circuit in the automotive sector, it is necessary to continuously check this circuit, for example by monitoring the resistance of the detection line. This is also very complex and is connected with corresponding costs. 
         [0009]    The German Patent Reference DE 101 50 377 A1 describes a method and device for short circuit detection in signal lines of a sensor. For this purpose, in addition to the sensor signal, a static reference potential is applied via a series resistor to the signal line to be monitored. A short circuit exists when the static potential in the signal line after the series resistor lies outside a predetermined range. 
         [0010]    The method can only be used for signal lines, which are operated essentially without power and in which the signal curve is detected and evaluated in a correspondingly high-impedance manner. Supply lines of a consumer in a DC circuit cannot be overlaid with a static reference potential since this immediately results in an altered power consumption of the consumer and, with a limited power of the reference voltage source, causes a change in the static potential to be monitored. 
         [0011]    German Patent Reference DE 199 35 439 A1 discloses a sensor line with at least two connector elements that are separated by an insulator. The insulator is composed of a material that has an electrical conductivity that can be changed by external influences such as a temperature increase. If the temperature in a region of the sensor line increases, for example due to a fire, the conductor elements are short-circuited by the insulator, which has been become conductive. This short circuit is detected and by it, a potential fire source is detected. One disadvantage of this method is the fact that a special plastic is required for the insulator, which is conductive at higher temperatures (T≧80° C.). Another disadvantage is that even this plastic disintegrates at the high temperatures that occur in an arc so that no conductive phase is produced. An arc detection is therefore not possible with the described sensor line. 
         [0012]    As has already been described in connection with German Patent Reference DE 101 32 752 B4, the device known from German Patent Reference DE 199 35 439 A1 disadvantageously constitutes an active safety system that is currentless during regular operation and in the event of a fault, carries a current that is to be detected. One defect in the safety system, for example due to corroded contacts of the detection line, can therefore also, in the event of a fault, result in the fact that despite the conductive connection between the conductor elements, no current flow in the detection line is possible and the fault will therefore remain undetected. The device therefore cannot be certified for use in the automotive sector. 
       SUMMARY OF THE INVENTION 
       [0013]    One object of this invention is to provide a method that permits a reliable, inexpensive detection of an arc in a DC circuit. Another object of this invention is to provide a corresponding device. 
         [0014]    The above and other objects of this invention relating to the method are attained if a voltage signal that refers to a potential of the DC circuit is coupled into the detection conductor at an incoupling point via a series resistor, the voltage signal is coupled out again at an outcoupling point of the detection conductor, the voltage signal before the series resistor is compared to the voltage signal at the outcoupling point, and the conclusion is drawn that an arc has occurred when the voltage signal at the outcoupling point deviates from the voltage signal before the series resistor by more than a predetermined amount. During regular operation, the detection conductor carries no electrical charge or carries only a slight electrical charge due to the high-impedance voltage measurement. Consequently, the voltage signals before and after the series resistor are at least almost the same. If an arc occurs, due to the very high temperatures accompanying it, this immediately results in an ionized and therefore conductive atmosphere in the vicinity of the arc. The combustion products of insulation materials, for example of a cable that is to be monitored, also contribute to this ionized atmosphere. This ionized atmosphere enables a flow of current from the detection conductor. When a current flow occurs, the voltage drop via the series resistor increases so that the voltage at the outcoupling point of the detection conductor decreases. This is detected by comparing the voltage signals before the series resistor and at the outcoupling point and as a result, the arc is detected. 
         [0015]    Through a suitable selection of the series resistor, even with a comparatively low current load of the detection conductor via the ionized atmosphere, a significant change in the voltage at the outcoupling point is produced so that it is possible to already detect an arc early in its creation. 
         [0016]    Another advantage of this invention is that the arc recognition is embodied in the form of a passive safety system. A voltageless state of the detection conductor is detected as an arc by the steps that are then provided. If such a voltageless state occurs due to another fault, for example due to corroded contacts or an interrupted detection conductor conductor, then these steps are also taken and the full safety is achieved. There is thus a continuously closed detection circuit, which is continuously checked for interruptions and in the event of an interruption, initiates performance of the provided safety steps. The method therefore makes it possible to meet the requirements with regard to fail safety of safety-relevant components that are applicable, for example, to motor vehicles. Fault rates of 10 −10  faults per year are achievable since the safe state is the switched off state. 
         [0017]    Another advantage of the method of this invention is that a direct contact between the detector conductor and another conductor of the DC circuit, which does not occur under unfavorable circumstances inside the arc, is not required. For the evaluation, it is sufficient if the detection conductor is routed through the ionized region of the arc. At the same time, however, a voltage change occurs in the detection conductor, even if such a direct contact is present so that in both cases, a reliable detection of an arc is enabled. 
         [0018]    So that in the event of an arc, the detection conductor is electrically charged in a reliable fashion, the voltage signal refers to a potential of the DC circuit. This can be the minus potential, but can also be the plus potential and as a result, can be the operating voltage of the DC circuit or a potential that lies between the minus potential and the plus potential. 
         [0019]    According to different embodiments of this invention, a DC voltage signal, an AC voltage signal, or a DC voltage signal that is overlaid with an AC voltage can be used as the voltage signal. A DC voltage signal can be easily and inexpensively produced in the DC voltage system to be monitored. By contrast, an AC voltage offers the advantage of a reliable detection. Under the indefinite conditions of an arc, arbitrary DC voltages from ground to the voltage of the voltage signal or of the supply voltage of the DC circuit can occur in the detection conductor, which complicates the task of fault detection based on a DC voltage signal. An AC voltage, however, can be clearly distinguished from the possibly overlaid DC voltages and can be reliably evaluated. In this case, sinusoidal voltages can be provided, but any other AC voltages such as sawtooth voltages, delta voltages, and square wave voltages can also be provided. The selection can be carried out in a phase-selective and/or frequency-selective fashion in order to reliably exclude interference voltages. 
         [0020]    According to one embodiment of this invention, the power supply of the DC circuit can be interrupted when an arc is detected. This results in the immediate extinguishing of the arc, which makes it possible to avoid more significant damage. 
         [0021]    One object of this invention relating to the device is achieved if the detection conductor is connected at an incoupling point via a series resistor to a signal source that outputs a voltage signal, the voltage signal of the signal source refers to a potential of the DC circuit to the monitored, the voltage signal output by the voltage source and a voltage signal output at an outcoupling point of the detection conductor are supplied to a comparator, and the device is associated with a processing device that detects an arc if the voltage signal, which is present at the outcoupling point, deviates from the voltage signal, which is output by the signal source, by more than a predetermined amount. The device thus makes it possible to carry out the above-described method in which a current flow that is enabled by the ionized atmosphere near or in the vicinity of an arc, starting from the detection conductor, results in a change in the voltage signal at the outcoupling point while the voltage signal before the series resistor remains constant. The comparison of the voltage signals can be carried out by a difference measurement in the comparator. Alternatively, the voltage signals before the series resistor and at the outcoupling point can also be detected separately and then compared. The processing device can be provided in the form of an electronic circuit or else a microprocessor can be provided as the processing device, which detects a voltage difference determined by the comparator and then decides whether or not an arc is occurring. As a decision criterion for the presence of an arc, it is also possible to select the fact that the voltage signal is no longer present at the outcoupling point and that the predetermined amount therefore corresponds to the amount of the voltage signal. 
         [0022]    In order to enable a current flow through the ionized atmosphere of an arc between the detection conductor and a conductor of the DC circuit, the voltage signal of the signal source must refer to a potential of the DC voltage circuit to be monitored. 
         [0023]    According to one embodiment of this invention, the detection conductor is embodied as a closed conductor loop with the incoupling point and the outcoupling point being located at the ends of the conductor loop. In order to monitor, for example, a cable carrying the conductor of the DC circuit, starting from the incoupling point, the detection conductor can extend with a feed along the cable and then back along the cable via a return to the incoupling point at the beginning of the cable. This arrangement makes it possible to place the incoupling point and the outcoupling point at one end of the cable, which permits a simple connection to an electronic circuit having the signal source, the series resistor, and the comparator. In this case, both the feed and the return are positioned along the conductor to be monitored, in or on the cable, making it possible for at least a part of the detection conductor to reliably be positioned or situated near or in the vicinity of an occurring arc. The voltage signal of the signal source is coupled via the series resistor at the incoupling point of the detection conductor. At the end of the detection conductor, the voltage signal is picked off at the outcoupling point and supplied to the comparator. If an arc occurs, then the ions occurring in the arc distort the voltage signal so that this can be clearly detected in the voltage signal that is present at the end of the detection conductor. 
         [0024]    In order to be able to monitor a conductor of the DC circuit over its entire length, at contact points of the DC circuit to be monitored, the detection conductor can likewise be embodied as a contact and/or the contact of the detection conductor can be embodied as an interlock circuit with a delay device, in particular a lagging contact or a leading contact. The detection conductor can therefore be integrated into a plug of a DC circuit, for example in the form of an additional plug contact. It is thus also possible to monitor contact points of the DC circuit such as the above-mentioned plug for an arc that has formed therein. If the contact of the detection conductor is embodied as an interlock circuit with a delay device, then it is possible to increase the safety standard for the contact point. In an interlock circuit of this kind, it is first necessary to disengage the contact of the detection conductor, for example by unscrewing, before the main plug of the DC circuit can be opened. Before the plug is disconnected, the DC circuit is interrupted. If this is step forgotten, then the opening of the detection conductor results in the DC circuit being switched off. This makes sense, for example, for electric connections to trailers of commercial vehicles since this reliably switches off the power supply before the main plug is disconnected. 
         [0025]    In order to be able to detect arcs in the entire DC circuit, it is possible for the detection conductor to be routed through consumers or alongside consumers of the DC circuit and/or for the detection conductor to be routed through cables or alongside cables of the DC circuit. In this case, the detection conductor is advantageously laid inside a housing, for example, of the consumer because the atmosphere inside the housing is immediately ionized in the event of an arc, thus permitting a very quick detection of the arc. In order to obtain a large monitoring region, the detection conductor can be embodied with a correspondingly large area near or in the vicinity of consumers, for example in the form of a sensor plate. Possible consumers to be monitored include air conditioner compressors, alternators, starters, drive electronics, or power steering units. When monitoring consumers for arcs, it is advantageous that these are mostly high-quality and correspondingly expensive components whose complete destruction can be avoided with prompt detection of an arc and a corresponding switching off of the power supply. By routing the detection conductor through or alongside cables of the DC circuit, it is possible to monitor them form arcs. 
         [0026]    The series resistor and the resistance that is produced inside the arc between the detection conductor and a reference potential combine to produce a voltage divider. Through a suitable selection of the series resistor, the voltage divider can be embodied so that when the expected resistances inside the arc occur, a maximum change in the voltage signal at the outcoupling point occurs. This can be achieved if the series resistor lies in a range between 10 ohm and 10,000 ohm (10 k ohm), for example between 100 ohm and 10,000 ohm, preferably in a range between 300 ohm and 3000 ohm. With a series resistor of for example 1000 ohm, it is possible to already detect arcs as they are being created, while the gas that is still only slightly ionized by comparison and thus a correspondingly high resistance is present, with the aid of the disrupting voltage signal at the outcoupling point. In vehicles like buses, there are relatively large cable lengths to monitor. In order to achieve a high power at the detection conductor, it can be advantageous, for example, to work with a series resistor in the range from 10 ohm to 50 ohm. 
         [0027]    It is possible for the signal source to be embodied as an AC voltage source, a DC voltage source, or an AC voltage source with an overlaid DC voltage offset. The use of an AC voltage source offers one advantage of a simple differentiation from DC voltages, which can be transmitted from the DC circuit to the detection conductor with the occurrence of an arc. DC voltage sources, however, are simpler and less expensive to produce in the DC circuit environment that is to be monitored. 
         [0028]    In order to be able to avoid more significant damage with the occurrence of an arc, it is possible for the processing device to be designed to react to the detection of an arc by tripping a circuit breaker, which interrupts the power supply of the DC circuit. To interrupt the power supply, heavy-duty circuit breakers can be used, which for example disconnect a battery or a generator of a motor vehicle from the DC circuit. They are designed to reliably switch, even under short-circuit conditions with powerful currents, without an arc occurring in the circuit breakers themselves. Heavy-duty circuit breakers can be provided in the form of heavy-duty disconnect relays, electronic circuit breakers, or pyrotechnic fuses. The heavy-duty circuit breaker can be positioned directly on or in the battery or on or in the generator. 
         [0029]    According to different embodiments of this invention, the detection conductor is embodied as insulated in at least some areas and/or the detection conductor is embodied without insulation in at least some areas. An insulated detection conductor can be placed in direct contact without the danger of an inadvertent short circuit with the DC circuit conductor that is to be monitored. If an arc occurs, the insulation is destroyed and the ionized and thus conductive atmosphere forms. With a bare detection conductor, it is not necessary for an insulation to burn first, thus resulting in a very quick reaction speed upon occurrence of an arc. Advantageously, a part of the detection conductor, for example the feed of a detection conductor embodied as a conductor loop, can be insulated and another part of the detection conductor, for example its return, can be bare so that the two advantages can be combined. With a bare detection conductor, care must be taken that it does not come into contact with other voltage-carrying components or with ground during normal, arc-free operation. In one embodiment, the detection conductor can be routed in a tube together with a cable that is to be monitored. 
         [0030]    Bare detection conductors are also advantageous particularly inside consumers. If an arc occurs inside the housing of a consumer, ionized and therefore conductive gas is immediately present in the entire interior. If bare detection conductor is used, then it is not necessary for an insulation of the detection conductor to first burn, so that it reacts very quickly to the ionized atmosphere that has formed. The use of a bare detection conductor for monitoring consumers is also advantageous because temperatures high enough to burn an insulation of the detection conductor are not present throughout the entire housing. By contrast with an insulated detection conductor, a bare detection conductor reacts even if it is not routed through a region of a very high temperatures inside the housing of the consumer, as long as the atmosphere in the housing is sufficiently ionized. 
         [0031]    Corresponding to another embodiment of this invention, it is possible for consumers of the DC circuit to be connected to a vehicle ground on one side and for the supply voltage to be conveyed to the consumers via plus lines that are monitored by detection conductors. Without the arc monitoring according to this invention, consumers in motor vehicles equipped, for example, with 42 V or 48 V electrical systems are electrically supplied via separate plus lines and minus lines. The use of a shared vehicle ground as the minus pole, as is customary in 12 V electrical systems, is not provided due to the significant risk of arc formation between voltage-carrying lines and ground. With the arc detection according to the invention and interruption of the power supply of the DC circuit when an arc is detected, it is once again possible to provide a shared vehicle ground without an increased risk of arc formation and thus the triggering of a vehicle fire. By this measure, it is possible to significantly reduce both the costs for the wiring harness of a motor vehicle and the weight of the motor vehicle. 
         [0032]    The method and device can preferably be used to monitor a DC circuit in a voltage range between 20 V and 60 V, for example between 24 V and 60 V, or to monitor a DC circuit in a motor vehicle in a voltage range between 20 V and 60 V, for example 24 V and 60 V, preferably in a voltage range between 40 V and 50 V. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    This invention is explained in greater detail in view of an exemplary embodiment shown in the drawings, wherein: 
           [0034]      FIG. 1  shows a schematic, perspective front view of a section taken through a bipolar cable with a detection conductor; 
           [0035]      FIG. 2  is a schematic view of an electronic assembly for detecting arcs in a cable and a consumer of a DC circuit; 
           [0036]      FIG. 3  shows the cable from  FIG. 1  with signal curves for comparison, during regular operation of the DC circuit without arcs; 
           [0037]      FIG. 4  shows the cable from  FIG. 1  with the signal curves for comparison from  FIG. 3 , when an arc is occurring; 
           [0038]      FIG. 5  shows an electronic assembly for detecting arcs, with an integrated safety circuit for monitoring the detection conductor; 
           [0039]      FIG. 6  shows a cable pair with a monopolar plus cable and a monopolar minus cable; 
           [0040]      FIG. 7  shows a monopolar cable (plus line) with a detection conductor alongside the plus line; and 
           [0041]      FIG. 8  shows another embodiment for a circuit with an electronic assembly. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0042]      FIG. 1  shows a schematic, perspective front view of a section through a bipolar cable  10  with a detection conductor  13 . A plus line  11  and a minus line  12  of a DC circuit are routed inside the insulated casing  14 . In addition to the plus line  11  and the minus line  12 , a feed  13 . 1  and a return  13 . 2  of the detection conductor  13  are located inside the insulated casing  14 . As shown in  FIGS. 2 and 5 , the feed  13 . 1  and the return  13 . 2  are connected to each other at the end of the cable  10  so that the detection conductor  13  constitutes conductor loop. 
         [0043]    The cable  10  is part of a 48 V electrical system of a motor vehicle and is used to supply electrical energy to an electrical consumer  17  shown in  FIG. 2 . In an alternative embodiment, the detection conductor  13  can be routed outside the casing  14  or it can extend around the plus line  11  and the minus line  14 , for example in a helical form, or can be embodied in the form of a net or a screen. The detection conductor  13  can also be conveyed in a tube around the cable  10  or around the plus line  11  or the minus line  12 . In order to improve the interference distance, it is also possible for the detection conductor  13  to be embodied in the form of a twisted double line. It is also possible to provide two detection conductors  13  and thus to achieve a redundant system. 
         [0044]    The detection conductor  13  is used for detecting arcs of the kind that can occur, for example, along a current path along the plus line  11  or the minus line  12  at faults with increased electrical resistance or between the plus line  11  and the minus line  12 . Such arcs burn in a stable fashion starting at voltages of 20 V and can cause serious damage to the vehicle. With the change-over from the currently customary 12 V electrical system, with a voltage that lies under the necessary discharge voltage for arcs, to 42 V or 48 V electrical systems, the risk of such arcs forming rises considerably. If an arc occurs, for example at a corroded contact point of the plus line  11  or minus line  12  with an elevated electric resistance, then the current flow through the plus line  11  or minus line  12  generally decreases so that fuses provided in order to protect the cable  10  do not react. If arcs occur between the plus line  11  and the minus line  12 , the current flow is in fact significantly greater, but it still lies in a range that can lead to a delayed reaction of the fuse. 
         [0045]    If an arc occurs, then the air near or in the vicinity becomes ionized and therefore conductive. Insulation material situated near or in the vicinity of the arc is thermally disintegrated and contributes to the formation of an ionized atmosphere. 
         [0046]    In order for arcs to be detected by the detection conductor  13 , it is necessary for the detection conductor  13  to be routed close to the plus line  11  and the minus line  12  so that when an arc occurs, the conductor is routed through an ionized region produced by the arc. This can, for example, be achieved by the above-mentioned embodiments of the cable  10 . 
         [0047]      FIG. 2  is a schematic depiction of an electronic assembly  20  for detecting arcs in a cable  10  and a consumer  17  of a DC circuit. 
         [0048]    The electronic assembly  20  includes a signal source  22 , a series resistor  24 , and a comparator  23 . The signal source  22  is connected to the series resistor  24  and via a signal line  25  to the comparator  23 . The series resistor  24  is connected to an incoupling point  21 . 1  at a loose end of the feed  13 . 1  of the detection conductor  13  that has already been introduced in connection with  FIG. 1 . The return  13 . 2  of the detection conductor  13  is connected to the comparator  23  via an outcoupling point  21 . 2 . The feed  13 . 1  and the return  13 . 2  of the detection conductor  13  are routed together with the plus line  11  and the minus line  12  in the cable  10  that has likewise already been shown in  FIG. 1 . In this case, the plus line  11  is connected to a supply voltage  16  on the input side and the minus line  12  is connected to a minus potential  18  of a power supply that is not shown. The electrical consumer  17  is connected between the plus line  11  and the minus line  12  and is supplied with electrical energy via the cable  10 . In the immediate vicinity of or near the electrical consumer  17 , a loop  13 . 3  of the detection conductor  13  is routed, which connects the feed  13 . 1  and the return  13 . 2  of the detection conductor  13  so that the detection conductor  13  forms a closed conductor loop. Along the cable  10 , a first contact point  15 . 1  and a second contact point  15 . 2  are embodied in the form of plug-in contacts. The contact points  15 . 1 ,  15 . 2  form contacts for the plus line  11  and the minus line  12  and also for the feed  13 . 1  and the return  13 . 2  of the detection conductor  13 . 
         [0049]    The signal source  22  and the comparator  23  refer to a shared reference potential  26  with the DC circuit, in the present case to the minus potential  18  of the DC circuit. Alternatively, the supply voltage  16  or an arbitrary reference voltage of the DC circuit can be provided as the reference potential  26 . 
         [0050]    The signal source  22  feeds a voltage signal into the feed  13 . 1  of the detection conductor  13  via series resistor  24  at the incoupling point  21 . 1 . The voltage signal is conveyed via the contact points  15 . 1 ,  15 . 2  to the loop  13 . 3  and from there, via the return  13 . 2  and the contact points  15 . 1 ,  15 . 2  back to the outcoupling point  21 . 2 . The comparator  23  carries out a high-impedance measurement of the voltage signals that are present at a measuring point MP1  27 . 1  before the series resistor  24  and at a measuring point MP 2  27 . 2  at the end of the detection conductor  13  and compares them. If no arc is present, then the conductor loop of the detection conductor  13  is only charged by the measurement current of the comparator  23 . The voltage drop via the series resistor  24  is correspondingly negligible so that before the series resistor  24  and at the outcoupling point  21 . 2 , the same voltage signal is present, with low tolerances. The comparator  24  recognizes that the voltage signals are the same and then a processing device, not shown, for example in the form of a microprocessor, decides that no arc is present near or in the vicinity of the cable  10 , the contact points  15 . 1 ,  15 . 2 , or the consumer  17 . If an arc does occur, however, the atmosphere near or in the vicinity of the arc becomes ionized and thus conductive. The detection conductor  13  is positioned so that it is routed through this ionized region. Because of the shared reference potential  26  of the signal source  22  and the DC circuit, a current flows from the detection conductor  13  into the DC circuit. As a result, the voltage drop via the series resistor  24  increases so that the voltage signal at the outcoupling point  21 . 2  decreases relative to the voltage signal before the series resistor  24 . This is determined by the comparator  23 . If the difference between the voltage signals is greater than a predetermined permissible amount, then the processing device detects an arc. 
         [0051]    Preferably, an AC voltage is provided as the voltage signal because it can be clearly distinguished from existing DC voltages of the kind that can be transmitted, for example, from the DC circuit to the detection line in the event of an arc. The series resistor  24  is chosen so that with the expected current load of the detection conductor  13  in an arc, a higher voltage drop at the series resistor  24  and thus a greater voltage difference between the voltage signal before the series resistor  24  and the voltage signal at the outcoupling point  21 . 1  is produced. Advantageously, a series resistor  24  in the range between 300 ohm and 3000 ohm is provided. As an arc is being created, when an ionization of the atmosphere is comparatively slight, a series resistor  24  of this kind already yields a significant voltage drop and consequently a clearly detectable change in the voltage signal at the outcoupling point  21 . 1  and thus at the measuring point MP2  27 . 2 . With a corresponding choice of the series resistor  24 , the voltage signal in the detection conductor  13  completely collapses, which is clearly detectable by the comparator  23 . 
         [0052]    The detection conductor  13  in the exemplary embodiment is routed together with the plus line  11  and the minus line  13  in the cable  10 . In this case, the detection conductor  13  is likewise connected by contacts at the contact points  15 . 1 ,  15 . 2 . The loop  13 . 3  is routed directly on or in the consumer  17 . In this case, the detection conductor  13  can be embodied with a large area in the vicinity of the consumer  17  by being embodied in the form of a sensor plate. In this way, it is possible to detect arcs along the cable  10 , in the contact points  15 . 1 ,  15 . 1 , and in the consumer  17 . 
         [0053]    If an arc is detected, then the power supply of the DC circuit is interrupted. To that end, the DC circuit is provided with a heavy-duty circuit breaker, which for example disconnects a battery and/or a generator from the DC circuit. Heavy-duty circuit breakers can be provided in the form of relays, electronic circuit breakers, or pyrotechnic fuses. In this case, the heavy-duty circuit breaker can be positioned directly on or in the battery or on or in the generator. 
         [0054]    The device shown is a passive safety system. Passive safety systems are characterized by the fact that the currentless state is the safe state. If a fault occurs, which causes the expected currents not to flow, then such a passive safety system switches to fault mode. In the present case, the safety system detects an arc and switches off the supply voltage  16  of the DC circuit if a fault occurs in the detection conductor  13  that changes the voltage signal at the outcoupling point  21 . 2 . The system therefore reverts to the safe state in any case. There is a closed circuit of the detection conductor  13 , which is continuously checked for interruptions during operation and which switches off in the event of an interruption in full safety mode. This offers significant advantages as compared to an active safety system, which is currentless in the basic state and results in a current flow in the event of a fault. In active systems of this kind, it is necessary to continuously and thus labor-intensively monitor the power connections and the transition resistances of the contacts of the sensor system. By the present passive safety system, it is possible to achieve very low fault rates in the range of 10 −10  faults per year. This enables a certification in accordance with the applicable requirements for automobile safety circuits. 
         [0055]    According to an embodiment that is not shown in the drawings, with flying connections, for example in electrical connections in semi-trailers, the detection conductor  13  can be embodied as an interlock circuit. With interlock circuits of this kind, it is first necessary for a sensor line, in the present case the detection conductor  13 , to be interrupted before a main connector of the DC circuit can be disconnected. This allows the DC circuit to be reliably switched into a currentless state before the main connector is disconnected. 
         [0056]      FIG. 3  shows the cable  10  already shown in  FIG. 1  with signal curves  30 ,  31  for comparison, during regular operation of the DC circuit without arcs. In this case, the signal curve MP1  30  corresponds to the output signal of the signal source  22  shown in  FIG. 2  at the measuring point MP1  27 . 1  before the series resistor  24 . The signal curve MP2  31  is present at the measuring point MP2  27 . 2  and corresponds to the voltage signal, which occurs at the outcoupling point  21 . 2  at the end of the return  13 . 2  of the detection conductor  13 . 
         [0057]    As the output signal of the signal source  22 , an AC voltage is provided, with a frequency in a range between for example 20 and 50 Hz. In order to keep the interference voltage difference in the event of interferences of the kind that can be introduced by the plus line  11  as large as possible, the highest possible amplitude of the AC voltage is selected. In the present embodiment, the amplitude of the AC voltage is above the voltage of the DC circuit of 48 V. 
         [0058]    The voltage signal of the signal source  22  with the signal curve MP1  30 , starting from the measuring point MP1  27 . 1 , is coupled into the feed  13 . 1  of the detection conductor  13  via the series resistor  24  and the incoupling point  21 . 1 , is conveyed by this feed in accordance with  FIG. 2  along the cable  10  and the loop  13 . 3  to the return  13 . 2 , is coupled out of the detection conductor  13  at the outcoupling point  21 . 2 , and is conveyed on to the measuring point MP2  27 . 2 . The comparator  23  compares the signal curve MP1  30  at the measuring point MP1  27 . 1  to the signal curve MP2  31  at the measuring point MP2  27 . 2 . In the present case, no arc is present along the course of the detection conductor  13  so that the detection conductor  13  is not electrically charged. The signal curve MP2  31  at the end of the detection conductor  13  therefore corresponds in its amplitude, but also in its phase and frequency, to the signal curve MP1  30  at the measuring point MP1  27 . 1  before the series resistor  24 . Then a processing device, not shown, decides that no arc is present. 
         [0059]      FIG. 4  shows the cable  10  shown in  FIG. 1  with the signal curves  30 ,  31  for comparison from  FIG. 3 , when an arc is occurring. If an arc occurs, for example inside the cable  10  or at the consumer  17  shown in  FIG. 2 , then a current flows from the detection conductor  13  to the DC circuit. The detection conductor  13  is thus electrically charged so that a drop occurs in the voltage signal output by the signal source  22  via the series resistor  24 . In the present exemplary embodiment, the detection conductor  13  is so powerfully charged by the short circuit through the ionized atmosphere of the arc that the voltage signal at the end of the detection conductor  13  completely collapses in accordance with the signal curve MP2  31 . The comparator  23  shown in  FIG. 2  measures the difference between the signal curve MP1  30  before the series resistor  24  and the signal curve MP2  31  at the end of the detection conductor  13  and based on the detected difference, the processing device, not shown, detects an arc in the DC circuit. 
         [0060]      FIG. 5  shows the electronic assembly  20  for detecting arcs shown in  FIG. 2 , with an integrated safety circuit  40  for the monitoring and maintenance of the detection conductor  13 . Components that have already been shown in  FIG. 2  are provided with the same reference numerals. To simplify the depiction, the components of the DC circuit are not shown, unlike in  FIG. 2 . For example, the DC circuit is part of a 48 V electrical system of a motor vehicle. 
         [0061]    The electronic assembly  20  also includes a maintenance and diagnosis unit  40 . Starting from a power supply  41 , this unit includes a first relay  42 , a resistance  43 , and a second relay  45 . 
         [0062]    The power supply  41  can be connected via the first relay  42  to the resistance  43 , whose opposite end is connected via the incoupling point  21 . 1  to the feed  13 . 1  of the detection conductor  13 . The end of the detection conductor  13  is connected via the outcoupling point  21 . 2  to the second relay  45 , which when switched produces a connection to ground  19 . A measuring point  44 . 1 ,  44 . 2  is provided at the beginning and end of the detection conductor  13 . 
         [0063]    The maintenance and diagnosis unit  40  is used for performing maintenance on contacts of the arc detection device, particularly in order to prevent high transition resistances due to corrosion. Such high transition resistances can, for example, occur at the contacts of the detection conductor  13  located at the contact points  15 . 1 ,  15 . 2 . After the DC circuit is switched off, for example after the engine of a motor vehicle is switched off, the relays  42 ,  45  are closed for a predetermined length of time, for example for  30 s. In this time, a regeneration current flows from the power supply  16  via the first relay  42 , the feed  13 . 1 , the loop  13 . 3 , and the return  13 . 2  of the detection conductor  13  with the contact points  15 . 1 ,  15 . 2  provided therein, and via the second relay  45  to ground  19 . The regeneration current in this case is set, for example, to 1 ampere through an appropriate choice of the power supply  41  and the resistance  43 . Such an intermittently applied regeneration current reliably prevents a creeping corrosion of the contacts. In this case, the transition resistances at the measuring points  44 . 1 ,  44 . 2  are tested over the entire length of the detection conductor  13 , for example at greater than 500 mΩ. If higher transition resistances are nevertheless present, a corresponding fault message is generated and the repair shop or a driver receives a maintenance notice in advance. Safety is nonetheless assured with the present passive safety system because an arc continues to be detected even in the presence of elevated transition resistances. High transition resistances can only lead to an erroneous arc detection, which can be avoided by the maintenance and diagnosis unit  40 . 
         [0064]      FIG. 6  shows a cable pair in an embodiment variant of this invention, with a monopolar plus cable  50  and a monopolar minus cable  51 . The plus line  11  and the feed  13 . 1  of the detection conductor  13  are routed inside the casing  14  of the plus cable  50 . The minus line  12  and the return  13 . 2  of the detection conductor  13  are routed inside the casing  14  of the minus cable  50 . The feed  13 . 1  and the return  13 . 2  at the end of the plus cable  50  and minus cable  51  are connected to a loop  13 . 3  shown in  FIGS. 2 and 5  to form a closed conductor loop. 
         [0065]    In motor vehicles with 42 V or 48 V electrical systems that are currently in the planning stages, both the positive and the negative voltage are often conveyed to the consumers via insulated lines (such as plus lines  11 ) and minus lines  12  and the use of a shared vehicle ground is omitted. The reason behind this is the risk of the plus cable  50  possibly being damaged, for example against sharp-edged parts of the body, which can result in an immediate short circuit with the risk of arc production. This risk is significantly reduced by the separate supply of positive and negative voltage via separate plus lines  11  and minus lines  12 . In comparison to the bipolar cables  10  shown in  FIGS. 1, 3, and 5 , in which both the plus line  11  and the minus line  12  are routed together, the danger of a short circuit can be further reduced in this case through the use of separate monopolar plus cables  50  and monopolar minus cables  51 . With the cable pair shown, it is also possible to monitor such monopolar plus cables  50  and minus cable  51  for arcs, thus further increasing the operational reliability. 
         [0066]    If the motor vehicle is equipped with an arc detection according to the invention, with a shutoff of the DC circuit when an arc is detected, then even in a 42 V electrical system or a 48 V electrical system, the body can be used in an economically advantageous fashion as the minus pole (as a rule). As a result, it is only necessary to route a plus cable  50  to the consumer  17  and to monitor it with a detection conductor  13 , while the negative connection of the consumer  17  is connected to the vehicle ground, as shown in  FIG. 8 . If, as described above, an arc occurs between the plus line  11  and the vehicle ground, then the detection conductor detects this immediately and the power supply of the DC circuit is immediately interrupted, thus ensuring safety. Eliminating the minus line  12  makes it possible to significantly reduce the costs for the electrical wiring of the motor vehicle. It is also possible to reduce the weight of the motor vehicle. In a modification of the plus cable  50  shown in  FIG. 6 , the detection conductor  13  is routed, preferably together with its feed  13 . 1  and return  13 . 2  that are connected at one end via a loop  13 . 3 , alongside or in the plus cable  50 , as shown in  FIG. 7 . As a result, there is a closed conductor loop or closed loop network. 
         [0067]    In the embodiment with the electronic assembly  20  shown in  FIG. 8 , the vehicle ground or the body forms or constitutes the minus pole. In this case, a battery  60  powers two consumers  90 ,  91 , for example. The return current flows via the vehicle ground. With return flows via the vehicle ground, however, the way that the current will flow locally is not defined. For example if the consumer is screwed to the vehicle ground with two screws in order to produce the minus contact and these screws are not tightened, then it is not possible to predict which of these two screws is the one where arcs will occur. Basically, with a conductor that has a large area such as the vehicle frame or the body, one can no longer stumble upon the position of a potential arc by a detection line. There remains, however, the problem of the series arc along the ground connection. In this case, an assembly like the one shown in  FIG. 8  is advantageous. For each consumer  90 ,  91  with the stub lines  80  and  81 , which if possible extend into the consumers  90 ,  91 , a measuring device  70  performs a comparison to make sure that all of the consumers have the same ground relative to the battery  60 . Since the arc threshold lies at roughly 18 volts, it is sufficient to set the response threshold at ground differences of greater than 5 to 10 volts. This is very easy to demonstrate and is highly effective. The electronic assembly  20  is preferably closely connected to the battery. 
         [0068]    In the exemplary embodiment according to  FIG. 8 , the plus lines  11  and  11 ′ can be embodied as shown in  FIG. 7 . This achieves a defined closed loop network and avoids the need for distributor boxes for the supply voltage. In this assembly, the plus line has a protection against all types of arcs, namely circuit-breaking arcs and short-circuit arcs. The ground line via the vehicle frame is only protected against circuit-breaking arcs, but this is sufficient. 
         [0069]    When switching off the supply voltage (for example of 48 V) by circuit breakers such as relays and/or pyrotechnic fuses, it should be noted that in addition to the battery, it is also necessary to disconnect the generator or alternator. A high safety standard is advantageously achieved with the electronic assembly  20  in connection with the evaluation and shut-off electronics by providing two microprocessors that monitor each other. 
         [0070]    Many high-current connections in a 48 volt supply voltage are screw-connected. In this case, it is advantageous to embody the contacting of the closely adjacent detection line so that the detection conductor is first disconnected before the screw can be actuated. This achieves the fact that such a screw cannot be undone when under load. With plug contacts, lagging detection contacts are advantageous in the same way during the plugging so that contacts cannot be opened or closed when under load. 
         [0071]    Luxury busses often have 24-volt cable lengths that are often more than 10 km long. These 24 volt cables form or constitute a very real danger source. A frequent mishap in busses is that a fire in the engine compartment causes the insulation of power cables in the engine compartment to melt, these lines come into contact with the vehicle frame, and arcs cause the bus to be engulfed in a sea of flames. In this regard, it is necessary to bear in mind that the technical voltage of a bus when its engine is running amounts to 28 volts and for comparison, 30 volts are used in arc welding, for example when welding together armor plates. 
         [0072]    One solution in busses, as is also being considered in passenger car concepts at this time, is to embody only 1 to 2% of the relevant supply lines with a 24-, 36-, or 48-volt nominal voltage in busses and thus to provide a protection by the measures explained above. In such busses, the remainder of the bus can be operated at 12 volts and with 2 or 3 distributed 12-volt backup batteries. A starting configuration with 24 volts for the relevant supply lines and 12 volts for the remainder of the bus is advantageous because all of these components already exist.