Patent Publication Number: US-2005128661-A1

Title: Method and device for protecting a conductor when an electric arc is produced

Description:
BACKGROUND AND SUMMARY OF THE INVENTION  
      The invention relates to a method for protecting a conductor when an electric arc occurs, in particular for protection of a 42V vehicle power supply system conductor in a motor vehicle. It also relates to an apparatus for carrying out the method.  
      As a result of the increase in the vehicle power supply system voltage in motor vehicles from the previously normal 12V to 42V now, there is a risk of electric arcs being formed as a consequence of contact between a line which is carrying a correspondingly high potential and either a further line which is carrying a comparatively low potential or the vehicle bodywork, which is normally at ground potential. If an electric arc is formed, then its high temperature furthermore results in the risk of a stable operating point being formed for the electric arc, wherein the arc thus burns continuously. This may result in considerable damage in the vehicle.  
      Since electric arcs can occur only above an operating voltage of about 16V, because the present-day vehicle power supply system voltage is only 12V, no specific measures have been provided until now for protection against electric arcs in a motor vehicle power supply system. In fact, only fuse links are used at the moment in order to interrupt live supply lines in the event of a short circuit in a motor vehicle, and their tripping times are defined by the so-called melting integral (I 2 ·t).  
      The use of fuse links in a motor vehicle with an operating voltage above 16V, in particular greater than or equal to 36V, does not, however, offer sufficient protection for a situation in which contact between a 36V line or 42V line and ground does not result in a “hard” short-circuit as a result of a direct contact, but rather the formation of an electric arc—even if this is intermittent. As a consequence of the electric are formation, the current is considerably less than the short-circuit current without an arc, and is thus considerably reduced.  
      For example, in the case of an operating voltage of 36V and a vehicle battery having an internal resistance of 30 mΩ, and with an additional resistance in the short-circuit path of 1 mΩ, the resultant short-circuit current is  1160 A. This short-circuit current is more than five times the rated current of a conventional  200 A fuse link, so that this makes it possible to ensure that the fuse link will blow quickly. When, in contrast, an arc occurs in the region of the short-circuit point, then a voltage of, for example, 26V is dropped across this arc. This results in the voltage across the short-circuit being reduced to 10V, and the short-circuit current being reduced to approximately  320 A. However, since a fuse link does not blow until a current of  270 A at the earliest, a short-circuit current such as this means that it cannot be assumed that the fuse will blow quickly. In fact, in this case, it can be expected that it will take several minutes for the fuse to blow, during which period, however, the arc can result in considerable damage to the vehicle.  
      U.S. Pat. No. 5,541,803 discloses an electrical safety device which can be used to detect an overtemperature or mechanical damage. This is done by the use of a sensor conductor with an uninterrupted loop. The sensor wire is arranged between a part (which is supplied with power) of an electrical component to be monitored and the outer surface of the components, for example a supply line. Furthermore, a reference conductor is provided and is used to detect the ambient temperature, by using resistance differences between this reference conductor and the sensor conductor to detect any overtemperature. The sensor conductor and the reference conductor are also embedded in the insulation of the supply line to be monitored, in which case the sensor conductor may also have its own insulation, although this is not the same as the insulation of the supply line to be monitored.  
      EP-A-0 617 498 also discloses a safety device for protection of cables and devices against faults caused by fire. Electrical arcs caused, for example, by wear, and conductivity resulting from it, can lead to permanent carbonization of the insulation and, following this, to ignition of the insulation. In order to prevent this, a monitoring wire is wound around the conductor. This monitoring wire is used as an electrical heat sink in order to reduce the temperature and is connected to a current limiting circuit, which acts provided that the amount of heat which is developed is not yet excessive. The monitoring wire has a flexible insulating coating which is in turn surrounded by the insulation of the conductor to be monitored.  
      The invention is based on the object of specifying a method by which an arc is identified and countermeasures are initiated. A further aim is to specify an apparatus which is particularly suitable for carrying out the method for protection of a conductor when an arc occurs, in particular for protection of a 42V vehicle power supply system conductor in a motor vehicle.  
      With regard to the method, the stated object is achieved according to the invention. The invention provides for a sensor line to be routed along at least part of a live supply line. A detection current, which flows via the sensor line as a consequence of an arc, is then used preferably to interrupt the current flow via the supply line, or at least to reduce it, in that an insulation material which surrounds the detection line becomes thinly viscous at a defined melting temperature at the point at which the arc occurs, and flows away, so that a reliable coupling is quickly formed between the detection line and the supply line.  
      The invention is in this case based on the idea that an arc can be identified when a coupling to a further line can be formed at the relevant point as a consequence of the arc, which coupling can be used only for arc detection but not for carrying the normal operating current of a load which is connected to the relevant supply line. A coupling such as this to an additional detection line then occurs either to the potential of the arc voltage, to the potential of the supply line, or to ground.  
      The coupling can in turn be produced by using the locally greatly increased temperature of the appropriate point that is known to occur in the event of an arc, in order to melt insulation which is located between the detection line and the supply line, which is normally part of a cable. This insulation will then melt as a consequence of the high temperature at this point when an arc occurs, and will in practice be split open locally. This will result in a reliable contact, in particular between the supply line and the detection line, with the consequence that a current flow is produced in the detection line, and this can be identified as the occurrence of an arc.  
      The corresponding detection current itself, or a voltage which is produced as a consequence of it, is then advantageously used in order to initiate the protection measure. At least a reduction in the current in the supply line is envisaged as a protection measure. Expediently, however, the connection of the supply line to the current source, and thus the current flow which is passed through it, are interrupted by means of a switch which is preferably arranged in the supply line itself and is driven directly by the detection current or indirectly via the voltage which is preferably measured on the detection line. This may be achieved by using a pyrotechnic isolating switch which is triggered by a detection current, or an electromechanical switch, for example a relay, or a semiconductor switch.  
      With regard to the apparatus, the stated object is achieved according to the invention by having at least one detection line, at least part of which is arranged along a supply line which can be connected to a current source, and having a protection device, which is connected in the connection between the current source and the supply line and is driven by a detection current. The detection current flows via the detection line as a consequence of an arc and drives the protection device in order to reduce the current flow via the supply line. The detection line is surrounded by insulation material, which is thinly viscous at a defined melting temperature, and can flow away. Advantageous refinements and developments are the subject matter of the dependent claims.  
      The sensing or detection line which is provided for identification and thus for detection of an arc expediently itself has insulation and may, for example, be twisted as an insulated braided cable harness into the cable which contains the supply line. In this case, the detection or additional line may be twisted with the supply cable as an internal or external braided cable harness. In this case, the detection line is arranged within the supply cable and is expediently arranged spirally along the supply line, so that the detection line surrounds the supply line at least in places so that, in practice, an arc can be identified over the entire circumference of all sides of the supply line.  
      If the twist spiral has a relatively long lay length, the expediently insulated additional or detection line may also be twisted into place retrospectively. In this case, it is expedient to reduce the pitch of the spiral by means of a short lay length. The additional detection line may also be in the form of a single, varnished copper wire. This can then be incorporated into the insulation of the supply cable during an extrusion step. As a consequence, the complexity is considerably less than that of a screened cable, owing to the lack of insulation. An individual detection line or two or more detection lines, may also be arranged or fastened externally on the cable insulation.  
      The insulation material of the additional detection line need not be the same as the insulation material of the supply line to be protected. An insulation material, for example PP, with a defined melting point in the range between preferably 130° C. and 180° C., is advantageously used for the additional detection line. The insulation material should in this case not be viscofluid, but should be capable of flowing away as well as possible in order to make it possible to ensure a reliable coupling and contact. The supply line may also be in the form of a flat conductor or ribbon conductor. When using film technology such as this, in which the live flat conductor is embedded as a supply conductor in insulating film, the flat conductor, which acts as the supply conductor, is at least partially surrounded by the sensing or detection line. This may likewise be a thin sheet or else a varnished copper wire. Two such detection lines may also run in the ribbon conductor, arranged at a distance from the actual supply line. A single detection line may entirely surround the supply line or else may be arranged only on one side along it. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Exemplary embodiments of the invention will be explained in more detail in the following text with reference to a drawing in which, in each case illustrated in a highly simplified form:  
       FIG. 1  shows a detail of a motor vehicle power supply system, illustrated in the form of a block diagram, with a circuit breaker which is connected directly to a detection line,  
       FIG. 2  shows in an illustration corresponding to  FIG. 1 , a circuit breaker which is connected to the detection line via a voltage meter,  
       FIGS. 3   a  to  3   d  show section illustrations of various embodiments of a detection line which is incorporated in or fitted to a supply cable, and  
       FIG. 4  shows a supply line which is in the form of a ribbon conductor with an adjacent detection line. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
      Mutually corresponding parts are provided with the same reference symbols in all of the figures.  
      As is shown in  FIGS. 1 and 2 , a vehicle power supply system cable  2  in a motor vehicle power supply system  1  has a supply line  6  which is surrounded by insulation  4  and which is connected to a current source in the form of a battery  8 , in order to form a main circuit. The main circuit is in this case closed via the supply line  6 , which is connected to the positive pole (+) of the battery  8 , and ground  10 , to which the negative pole (−) of the battery  8  is connected. The cable network, part of which is illustrated, is, in particular, part of a 42V vehicle power supply system for a motor vehicle. A vehicle power supply system such as this is normally constructed hierarchically and the power is distributed between various loads, for example a turn indicator relay or a headlight with a dimming facility.  
      A circuit breaker  12  is provided in a supply line  6  between the battery  8  and a line or cable section (which is under consideration here and is illustrated in an enlarged form) of the supply line  6 . The supply line  6  leads to a load (which is not illustrated) and via this to ground  10 . In the exemplary embodiment, the circuit breaker  12  is a pyrotechnic isolating switch  12 , which is connected in the supply line  6  and has an initiator  14  which is connected via a connection  16  on the one hand to the positive pole (+) of the battery  8  and on the other hand to a sensing or detection line  18 . Instead of being connected to the positive pole (+) of the battery  8 , the connection  16  may also be connected, in a manner which is not illustrated in any more detail, to a comparable terminal connection in a controller in the motor vehicle.  
      The detection line  18  is surrounded by insulation  20 , which is preferably composed of a material with a defined melting point in the range between 130° C. and 180° C. The detection (or sensor line)  18  is routed along the supply line  6 , at least in places. The arrangement of the detection line  18  with its insulation  20  in or on a supply cable with a braided, preferably twisted, supply line  6  is illustrated in  FIG. 3 , while  FIG. 4  shows an embodiment of the supply cable  6  with an integrated detection line  18  using film technology. The detection line  18  preferably surrounds the supply line  6  on all sides, at least in places.  
      In the embodiment shown in  FIG. 2 , the detection line  18  is connected to ground  10  via a pull-down resistor  22 . The resistor  22  is connected in parallel with a voltage meter  24 , whose measurement signal, which is derived from the measured voltage UD, is supplied to a tripping or drive apparatus  26 . The initiator  14  of the isolating switch  12  may, in turn, be connected to the tripping apparatus  26 . In this embodiment as shown in  FIG. 2 , however, a semiconductor switch or an electromagnetic switch  12 , for example a relay, is shown instead of the pyrotechnic isolating switch. In order to operate this by the use of a control signal S which is produced by the tripping or drive apparatus  26 , it is then in turn connected to the detection line  18  directly on the drive side as shown in  FIG. 1 , or indirectly via the voltage measurement devices  22  to  26 , as shown in  FIG. 2 .  
      In the cable embodiments shown in  FIGS. 3   a  to  3   c , the detection line  18  is twisted with individual cores or conductors  28  of the supply line  6 . Independently of any twisting or the degree of twisting, the individual conductors  28  of the detection line may be incorporated into the cable or conductor insulation  4  of the supply line  6 , running centrally or off-center, depending on the supply cable which is used and depending on the best way of manufacture or production, respectively. The supply line  6 , which comprises the individual conductors  28 , together with the insulation  4  which surrounds or encloses it, will be referred to in the following text as the supply cable.  
      In this case, the detection line  18  may itself once again be twisted as an insulated additional line  18 , with its insulation  20 , as an insulated braided cable harness into the supply cable  6 , in which case an internal arrangement as shown in  FIG. 3   a  or else an external arrangement as shown in  FIG. 3   b  may be advantageous with regard to the detection sensitivity and/or production-engineering aspects. The insulated detection line  18  may also be integrated in the supply line  6  in such a way that, within the conductor or core assembly, it assumes the position of an individual core  28  of the supply line  6 . The essential feature in this case is that the detection line  18 , including its insulation  20 , is arranged as close as possible to at least one individual single core  28  of the supply line  6 , with the aim of arranging the detection line  18  as far as possible over the entire length of the supply line  6  and/or in length sections which are, as far as possible, indefinitely short, and/or at the same time on as many sides of the supply cable as possible.  
       FIG. 3   d  shows an alternative embodiment where four detection lines are arranged so as to be distributed uniformly around the circumference of the cable insulation  4  of the supply cable. Instead of the four detection lines  18  illustrated in the exemplary embodiment, it is also possible to arrange a greater or lesser number of detection lines  18  distributed around the circumference of the supply cable  4 , which carries the supply line  6 .  
      In the ribbon embodiment that is illustrated in  FIG. 4 , the supply line  6  is in the form of a flat conductor, which is arranged in an insulating film  4 ′. The detection line  18 ′ also runs within this insulating film  4 ′. In this case, the detection line  18 ′ may likewise be a flat conductor in the form of a thin flat or ribbon conductor, or else may be in the form of a varnished copper wire. Furthermore, a detection line  18 ′ (or two or more detection lines  18 ′) may be arranged on each of the two sides of the supply line  6 , which is in the form of a flat conductor. In this case, the distances d 1  and d 2  from the supply line  6  may be the same or may differ. Furthermore, the supply line  6  may be entirely or partially surrounded by each of the two detection lines  18 ′. In addition, the (or each) detection line  18 ′ may completely surround the supply line  6 , which acts as the main conductor, or else may be arranged only along one side of it.  
      In the situation where an arc L occurs as a consequence of a line defect on the supply line  6 , a greatly increased temperature occurs at the corresponding point. As a consequence of this, the insulation  20  on the detection line  18  splits open, and a coupling is produced at the relevant melting point either to the potential of the arc voltage or to the potential of the supply line  18 , or to ground  10 . This is identified by the detection line  18  through which this contact via the arc L results in a detection current ID flowing. This is used to drive the switch  12  or to activate the initiator  14 , and in consequence to trip the isolating switch  12 , so that the supply line  6 , and thus the current flow of the main or supply current Iv flowing through it, are interrupted. If an electronic semiconductor switch is used, it is also possible just to reduce the current Iv flowing via the supply line  6 , which results in the arc L being quenched.  
      In the embodiment that is illustrated in  FIG. 2 , the arc L is identified via a voltage measurement. In this case, the voltage UD on the detection or sensing line  18  is determined, and a corresponding tripping signal S is transmitted to the switch  12  via the tripping device  26  in order to operate it, that is to say to open it. Since, normally, that is to say when the vehicle power supply system and hence the supply line  6  are being operated correctly, without any faults, there is no potential on the detection line  18 , which is kept at ground potential via the pull-down resistor  22 , any connection or coupling in the event of an arc L and any current flow ID associated with this via the resistor  22  is detected by means of the voltage meter  24  and, consequently, the switch  12  is opened. If an electronic semiconductor switch  12  is used, this is actuated appropriately, for example only in order to reduce the current Iv flowing via the supply line  6 , completely, or to maintain a minimum current I v  via the supply line  6 .