Abstract:
An arc fault detector includes a shunt resistor deployed in a protected circuit, an arc discriminator sensing voltages across the shunt resistor and producing an arc indication deduced from current variations associated with parallel and series arc faults, a signal transformer buffering the arc detection signal and producing a pulse, a switch transient detector sensing a voltage differential across load switches and producing a pulsed switch transient signal when the voltage differential across load switches exceeds a reference value, a line interrupter, such as a static relay, a switch controller including logic gates generating a trip signal based on predetermined criteria, and a manual switch for resetting the line interrupter. An embodiment comprises a voltage sensing coil enveloping a toroidal core to detect current variations in conductors passing through the core.

Description:
PRIORITY  
       [0001]     This application is a continuation of, and claims benefit of priority from, co-pending U.S. non-provisional patent application Ser. No. 10/831,733, which was filed Apr. 23, 2004, and which is hereby incorporated by reference. This application also claims benefit of priority from U.S. provisional patent application Ser. No. 60/465,461, filed Apr. 24, 2003, which is also hereby incorporated by reference. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates generally to the protection of electrical circuits, and, more specifically to the art of electrical arc detection and prevention.  
       BACKGROUND OF THE INVENTION  
       [0003]     A need exists within the automotive industry to increase the electrical power capability for future vehicles. For example, future vehicle concepts are being studied requiring increased voltage levels in direct current (DC) systems to as high as 42 volts, approximately three times greater than conventional 14 volt systems. The driving forces contributing toward this change are the need to reduce fuel consumption and the introduction of additional electrical features. New power networks must accommodate the increased energy demand of comfort and security devices as well as the electrical needs of major systems such as braking, electric power steering and suspension systems.  
         [0004]     The introduction of a system voltage higher than approximately 20 volts causes considerable component and system changes and has the potential to significantly impact system reliability and safety. For example, one significant impact of the increase in system voltage to levels such as the envisioned forty-two volt, direct-current network is the need to address the increased potential for arcing (shorting over a finite gap) from and within the electrical distribution systems and components. One particularly vulnerable component of electrical systems are the wiring harnesses, wherein arc faults may be encountered as a result of cut, pinched or chaffed wiring. Accordingly, a substantial need exists to protect electrical distribution systems, and particularly wire harnesses, from unwanted arc faults.  
         [0005]     In the instance of a wire being cut or broken under an electrical load, an arc may be drawn between both ends. Such an arc is unwanted and unplanned for, and its extinction is uncertain. Therefore, severe damage may occur if the arc is sustained. This type of arc fault is called a series arc fault, as the arc is in series to the load. Hot unplugs due to vibrating loose connections fall into the same series arc fault category. Series arc faults cannot typically be cleared by fuses or circuit breakers.  
         [0006]     Arc faults in parallel to the load are identified as parallel arc faults. An example of parallel arc faults can be damaged wires drawing an arc to a ground potential, such as a chassis of an automobile. The insulation jacket of such wires might be broken due to aging or shaved, chaffed or pinched cable jackets. This type of arc fault is usually created by a temporary short circuit. The arc fault current, however, may thermally over load and damage contacts within the circuit due to low contact force resulting in melting and evaporating contact material followed by more arcing. The arc fault current, limited by the circuit impedance and the arc voltage, can be significantly lower than the trip current of the protection device such as a fuse or circuit breaker, so that the fault is cleared late depending on the time or current characteristics or in some cases not at all.  
         [0007]     Many patents disclose arc fault detection systems and methods for alternating current (AC) applications. However, fewer arc detection devices and methods are disclosed for direct current (DC) applications.  
         [0008]     Consequently, there remains a need in the art for arc detection and protection systems and methods for DC circuits capable of rapidly detecting both parallel and series arcs. It would be beneficial to have a system and method capable of distinguishing unwanted and unplanned arcs from expected transient arcs such as those caused by the opening of a load switch. It would be further beneficial to utilize arc detection components such as sensors that are small so that they can be incorporated in devices such as electrical connectors, junction blocks, relays, circuit breakers, and the like. It would also be desirable to have a systems and methods continuously monitoring for arcing conditions rather than periodically sampling. It would further be desirable to have an arc detection and protection system that uses low cost components without requiring the use of microprocessors or complex algorithms.  
       SUMMARY OF THE INVENTION  
       [0009]     The methods and apparatus of the present invention address many of the shortcomings of the prior art. The present invention provides a system and method of detecting an arc fault by detecting a current change indicative of an arc fault. An exemplary embodiment of the invention is capable of distinguishing an arc fault from slow current transients caused by load variations and low frequency commutation ripple current of DC motors under normal operating conditions. An exemplary embodiment of the invention includes a switch transient detector which detects voltage differential across load switches and generates a switch transient signal which is used to prevent a trip signal from being generated. This eliminates a source for nuisance tripping of a circuit interrupter and helps to avoid the need for costly microprocessors and complex algorithms to distinguish switch transients.  
         [0010]     In an exemplary embodiment, a current shunt resistor is coupled at the input side of a protected load as part of a system to detect a current change indicative of an arc fault. In an exemplary embodiment, the current shunt resistor is small enough to be deployed in devices such as smart electrical connectors and smart junction blocks.  
         [0011]     In another exemplary embodiment, a pickup coil wound on a toroidal or UI type magnetic core is coupled at the input side of a protected load to detect a current change indicative of an arc fault. Consequently, the invention is capable of being deployed at only an input side or only a load side without a need for sensors, wires, and other components to be deployed on both a load and input side.  
         [0012]     A first preferred embodiment of an arc fault detector according to the present invention includes a current shunt resistor coupled in series with a circuit and an arc discriminator including an amplifier sensing a first voltage on a first side of the current shunt resistor, sensing a second voltage on a second side of the current shunt resistor, and producing a first signal proportional to a current flow through the current shunt resistor. The arc discriminator further includes a change detector receiving the first signal as input and producing an arc fault detection signal as output when the change detector detects a change in the first signal indicative of a presence of an arc fault in the circuit.  
         [0013]     In a preferred embodiment the change detector includes a series arc detector and a parallel arc detector.  
         [0014]     Another preferred embodiment of an arc fault detector according to the present invention includes a current monitor detecting a rate of change in electrical current in said circuit and producing a first signal indicative of said rate of change. The arc fault detector further includes a signal isolator electrically coupled in series with the circuit monitor filtering the first signal to substantially eliminate signals outside a selected frequency range, the signal isolator producing a filtered signal (i.e., isolating a desired signal) representing changes in the electrical current within the selected frequency range. The arc fault detector further includes an arc indicator producing an arc detection signal when a voltage level of the filtered signal exceeds at least one threshold.  
         [0015]     In an exemplary embodiment, the current monitor is toroidal coil wrapped around a magnetic or UI core and the arc indicator is a Schmitt Trigger.  
         [0016]     A preferred method for detecting an arc fault in a circuit in accordance with present invention includes coupling a current shunt resistor in the circuit, monitoring a voltage differential across the current shunt resistor, detecting a change in the voltage differential, comparing the change to at least one threshold, and generating a signal when the change exceeds the at least one threshold.  
         [0017]     A second preferred method for detecting an arc fault in a circuit in accordance with present invention includes providing a coil wrapped around a toroidal core, passing at least one conductor of the system through the center of the toroidal core, detecting a voltage induced in the coil by a change in current flowing through the at least one conductor, filtering the voltage so as to eliminate signals outside a selected frequency range and to produce a filtered or isolated signal, comparing the filtered (i.e., isolated) signal to at least one threshold; and generating an arc detection signal when said filtered signal exceeds said at least one threshold.  
         [0018]     These and other features and advantages of the present invention will become apparent from the following brief description of the drawings, detailed description, and appended drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The above-mentioned features of the present invention can be more clearly understood from the following detailed description considered in conjunction with the following drawings, in which like numerals represent like elements and in which:  
         [0020]      FIG. 1  is a simplified block diagram of a system incorporating a first exemplary embodiment of an arc fault detector in accordance with the present invention;  
         [0021]      FIG. 2  is a circuit diagram of a system incorporating the first exemplary embodiment of an arc fault detector in accordance with the present invention;  
         [0022]      FIG. 3  is a circuit diagram illustrating an aspect of a fault detector in accordance with the present invention;  
         [0023]      FIG. 4  is a circuit diagram illustrating another aspect of a fault detector in accordance with the present invention;  
         [0024]      FIG. 5  is a simplified block diagram of a system incorporating a second exemplary embodiment of an arc fault detector in accordance with the present invention;  
         [0025]      FIG. 6  is a circuit diagram of a system incorporating the second exemplary embodiment of an arc fault detector in accordance with the present invention;  
         [0026]      FIG. 7  is a simplified fragmentary view illustrating an aspect of the present invention;  
         [0027]      FIG. 8  is a flow diagram of an exemplary method in accordance with the present invention; and  
         [0028]      FIG. 9  is a flow diagram of another exemplary method in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     As discussed above, the methods and systems of the present invention provide improved arc fault detection capable of detecting series and parallel arcs in a DC circuit using low cost electronics without a need for microprocessors or sophisticated algorithms.  
         [0030]     Referring first to  FIGS. 1-4 ,  FIG. 1  is a simplified diagram illustrating a first exemplary embodiment of an arc fault detector  20  of the present invention. As shown in  FIG. 1 , arc fault detector  20  provides protection to a circuit  22 . Circuit  22  includes a direct current power source or battery  24  providing power to a load  26  via a conductor  28 . A load switch  30  is disposed in conductor  28 .  
         [0031]     Arc fault detector  20  includes a current shunt resistor  100 , an arc discriminator  200 , an arc fault signal generator or signal transformer  300 , a switch transient detector  400 , a manual switch  500 , a switch controller  600 , and a line interrupter  700 .  
         [0032]     Current shunt resistor  100  is coupled to circuit  22  in series with conductor  28 . Arc discriminator  200  detects a voltage differential across current shunt resistor  100 , produces a signal proportional to a current flow through the current shunt resistor  100 , and detects a change in the signal to produce an arc fault detection signal when a series arc fault or parallel arc fault is detected.  
         [0033]     Signal transformer  300  generates a pulse in response to detection of an arc fault. The pulse width extends for the period of time that an arc fault is detected.  
         [0034]     Switch transient detector  400  detects a voltage differential across load switch  30  and outputs a pulsed switch transient detection signal when the voltage differential across load switch  30  exceeds a reference value. The reference value is set at a level indicating an opening of load switch  30 . As later described, switch transient detector  400  may include inputs to detect voltage differential across multiple load switches and one or more logic gates to output a switch transient detection signal if a voltage differential across any one of the switches exceeds a reference value.  
         [0035]     Manual switch  500  is used to manually close and open line interrupter  700 .  
         [0036]     Switch controller  600  senses the pulse output from signal transformer  300 , switch transient detection signal from switch transient detector  400 , and voltage provided by battery  24  via manual switch  500 . In a preferred embodiment, switch controller  600  uses logic gates to output a trip signal controlling line interrupter  700 .  
         [0037]     Referring now to  FIG. 2 , arc fault detector  20  provides protection to a circuit  22 . Circuit  22  includes direct current power source or battery  24  providing power to loads  26   a ,  26   b  via conductors  28 ,  28   a ,  28   b . A load switch  30   a ,  30   b  is disposed in each of conductors  28   a ,  28   b , respectively.  
         [0038]     Shunt resistor  100  is coupled in series with circuit  22  and also in series with conductor  28 . Arc discriminator  200  senses a voltage v a , v b  at each side of current shunt resistor  100 .  
         [0039]     As shown in  FIG. 3 , arc discriminator  200  includes a current shunt amplifier  202  and change detector  205 . Current shunt amplifier  202  senses voltages v a , v b  from each side of current shunt resistor  100  at input terminals  3  (+ve) and  4  (−ve), respectively. It is assumed that v a  is more positive than v b  as current flows from v a  to v b  in current shunt resistor  100 . Current shunt amplifier  202  outputs a voltage signal to a change detector  205 . The voltage signal is proportional to the current flowing through current shunt resistor  100 . Change detector  205  includes output resistor  204  electrically coupled to a pin  1  of current shunt amplifier  202 . Change detector  205  further includes a series arc fault detector  206  and a parallel arc fault detector  208  each feeding an output signal to an OR Gate  210  as further described below.  
         [0040]     Current shunt amplifier  202  is coupled to an input of a first buffer  212 . First buffer  212  has a first output electrically coupled to a first delay circuit  213  and a second output electrically coupled to a first reference circuit  215 . First delay circuit  213  provides a delayed signal as an output to a first input of a first comparator  224 . First reference circuit  215  provides a reference signal as an output to a second input of first comparator  224 . Series arc fault detector  206  includes first delay circuit  213 , first reference circuit  215 , and first comparator  224 .  
         [0041]     First delay circuit  213  includes a low pass signal isolator (R-C circuit)  214  including a resistor  216  and a capacitor  218 . First delay circuit  213  further includes a second splitter or buffer  220  and a resistor  222 . First reference circuit  215  includes a resistor  225  electrically coupled to an input of an amplifier  226 . First comparator  224  is configured to output a series arc detection signal only when a voltage at the output of first delay circuit  213  is greater than a voltage at the output of first reference circuit  215 . This condition occurs when there is a decrease in the voltage signal provided by the output of current shunt amplifier  202  which causes the voltage in first reference circuit  215  to decrease. The time delay provided by low pass signal isolator  214  delays a decrease in voltage in first delay circuit  213  enabling the voltage in first delay circuit  213  to exceed the voltage in first reference circuit  215 . Note that first reference circuit  215  includes amplifier  226  which provides a gain. First comparator  224  outputs to a first input of OR Gate  210 . OR Gate  210  is configured to output an arc fault detection signal when the said series arc fault detection signal is detected at the first input.  
         [0042]     Parallel arc fault detector  208  includes a second delay circuit  217  which originates from a second output of second buffer  220 , a second reference circuit  219  which originates from the second output of first buffer  212 , and a second comparator  252 .  
         [0043]     Second delay circuit  217  includes a resistor  253  electrically coupled to an input of an amplifier  254 . Second reference circuit  219  includes a resistor  250 .  
         [0044]     Second delay circuit  217  provides a delayed signal as an output to a second input of second comparator  252 . Second reference circuit  219  provides a reference signal as an output to a first input of second comparator  252 .  
         [0045]     Second comparator  252  is configured to output a parallel arc detection signal only when a voltage at the output of delay circuit  217  is less than a voltage at the output of reference circuit  219 . This condition occurs when there is an increase in the voltage signal provided by the output of current shunt amplifier  202  which causes the voltage in reference circuit  219  to increase. The time delay provided by low pass signal isolator  214  delays an increase in voltage in delay circuit  217  enabling the voltage in reference circuit  219  to exceed the voltage in delay circuit  217 . Note that delay circuit  217  includes amplifier  254  which provides a gain.  
         [0046]     Second comparator  252  outputs to a second input of OR Gate  210 . OR Gate  210  is configured to output an arc fault detection signal when the said parallel arc fault detection signal is detected at the second input.  
         [0047]     In a series arc condition the current flowing through current shunt resistor  100  goes down compared to a normal condition. The output signal from current shunt differential amplifier  202  is buffered with splitter or buffer  212  and the buffered voltage at a first output  8  is fed to a low pass signal isolator  214  including a resistor  216  and a capacitor  218  to introduce a time delay into the signal and to signal isolator low frequency ripple due to motor loads. The output of low pass signal isolator  214  is buffered by buffer  220  and fed to an input resistor  222  electrically coupled to the +ve input  9  of a comparator  224 . The amplified signal of buffer  212  is fed to input resistor  225  of buffer  226  electrically coupled to the −ve  8  input of comparator  224 . Comparator  224  compares voltage input at pin  8  with time delayed voltage input at pin  9  to detect a change in voltage that exceeds a gain provided by amplifier  226 . The amplifier gain is selected such that comparator  224  does not produce output voltage under normal conditions. In an exemplary embodiment, a gain of 1.1 to 1.2 is selected.  
         [0048]     When a series arc occurs, the current suddenly drops initially and current shunt amplifier  202  produces a voltage proportional to the arc current at location “A” and also at location “B”. The magnitudes of the voltages at these points are less than the voltages under normal load before an arc fault. Low pass signal isolator  214  filters the voltage transient caused by the arc current momentary and the buffered output voltage at location “C” is the same as before the arc fault which is greater than the voltage at location “B”. This condition allows comparator  224  to produce output voltage which is fed to input  2  of OR Gate  210 .  
         [0049]     In the case of a parallel arc fault condition, the current flowing through current shunt resistor  100  goes up compared to a normal load condition. The buffered output of current shunt amplifier  202  at location “A” is electrically coupled to input resistor  250  electrically coupled to the +ve input  5  of a comparator  252 . The buffered and time delayed signal isolator output at location “C” is fed to amplifier  254  to produce output at location “D” which is slightly less than at location “A”. This signal is fed to input resistor  256  electrically coupled to the −ve input  4  of comparator  252 . Under normal load conditions or under series arc fault condition comparator  252  does not produce an output voltage. When a parallel arc occurs, the +ve input  5  of second comparator  252  is greater than the −ve input.  4 . This is because low pass signal isolator  214  filters the voltage due to the initial current transient of the arc. This introduces a time delay. Second comparator  252  outputs a parallel arc detection signal to a second input of OR Gate  210  when a voltage at the output of delay circuit  217  is less than a voltage at the output of reference circuit  219 .  
         [0050]     When either a series or parallel arc occurs, OR Gate  210  produces an output signal at pin  1  indicating an arc fault which is sensed by signal transformer  300 . Referring back to  FIG. 2 , signal transformer  300  includes Schmitt Trigger  310  and J-K Flip-Flop  312 . The output signal from OR Gate  210  is received by input  5  of Schmitt Trigger  310 . Schmitt Trigger  310  output at pin  8  is fed to a clock input pin  3  of J-K Flip Flop  312 . Under fault J-K Flip Flop  312  is set with the rising edge of the input clock at pin  3 , a complementary output of the Q at pin  2  of J-K Flip Flop  312  goes from High to Low during fault. Under normal conditions pin  2  is High. The output signal from pin  2  is fed to an input  11  of an OR Gate  602  included in Switch Controller  600 .  
         [0051]     Referring to  FIGS. 2 and 4 , switch transient detector  400  includes a switch transition detection circuit  402   a ,  402   b ,  402   c  for each switch being monitored. As shown on  FIG. 4 , switch transition detection circuit  402   a  senses a voltage v s1 , v s2  on each side of load switch  30   a . The voltages v s1 , v s2  are each fed through a respective signal conditioner  404 ,  406  fed to a summing amplifier  408 . A voltage differential is output by summing amplifier  408  and fed to a comparator  410  where the differential voltage is compared to a reference voltage with a reference value preferably set between 6 and 10 volts. Switch transition circuits  402   b ,  402   c  are structured and function similar to switch transition circuit  402   a . The output of the respective comparators  410   a ,  410   b ,  410   c  associated with each monitored switch  30   a ,  30   b ,  700  is fed to an OR Gate  412 . Under normal conditions, the voltages differential across each of the switch terminals  30   a ,  30   b ,  700  and the output of the switch transient detector  400  are Low. A switch  30   a ,  30   b ,  700  opening under load creates an arc across the switch, which causes voltage drop across the switch. Under this condition, the output of switch transient detector  400  goes High. The outputs from each of the switch transition detection circuit  402   a ,  402   b ,  402   c  are fed to the input pins  2 ,  3 , and  4  of OR Gate  412 . The output of OR Gate  412  is fed to an input  10  of Or Gate  602 .  
         [0052]     Manual switch  500  enables line interrupter  700  to be manually reset. Manual switch  500  functions to provide a mechanism to manually close and open line interrupter  700 . Line interrupter  700  functions to disconnect an arc fault from power source  24 . Manual switch  500  is a single pole double throw switch with a first pole grounded. A second pole is electrically coupled to a 15V supply derived from power supplied-from 48V battery  24  by using linear regulators  502   a ,  502   b . Two regulators  502   a ,  502   b  are cascaded to reduce power dissipation in the individual devices. The second pole is electrically coupled to a pin  9  of an AND Gate  604  included in Switch Controller  600 . The second pole is also ground through capacitor  504  and resistor  506 . Under normal operation, manual switch  500  is closed and the input  9  to AND Gate  604  is High while the input pin  10  of AND Gate  604  is also High. Therefore, the output of AND Gate  604  is also High under normal operation thereby driving an n-p-n transistor  606  via a buffer  608 . The collector of transistor  606  is electrically coupled to 48V power supply (battery  24 ) via a 15K resistor  610 . The collector output is electrically coupled to line interrupter  700 . Preferably, line interrupter  700  is a static relay configured to close with zero input signal and open with 48V input signal. Under normal operation, line interrupter  700  is closed.  
         [0053]     Under an arc fault condition, output pin  8  of Schmitt Trigger  310  feeds a signal to input pin  3  of Flip Flop  312  thereby setting Flip Flop  312 . Consequently, output pin  2  of Flip Flop  312  goes low which is fed to input pin  11  of OR Gate  602 . Since all of the inputs ( 9 ,  10 ,  11 ,  12 ) of OR Gate  602  are Low, the output at pin  13  is Low. This Low output is fed to input pin  10  of AND Gate  604 . The signal from manual switch  500  is fed to input pin  9  of AND Gate  604 . The output  13  of AND Gate  604  is buffered by buffer  608  and fed to base of transistor  606  to turn OFF. This causes the line interrupter  700  to trip.  
         [0054]     Once line interrupter  700  trips, J-K Flip Flop  312  is in a set condition where pin  2  is Low. By opening and closing, manual switch  500  disconnects the 15V power supply and connects to input pin  1  of Schmitt Trigger  310  via capacitor  504  and an amplifier  508 . This produces a voltage spike across resistor  506  since R-C (resistor  506 -capacitor  504 ) combination acts as a differentiator. This pulse is fed to input pin  1  of Schmitt Trigger  310 . Output pin  2  is electrically coupled to input pin  3  producing a pulse of finite pulse width at output pin  4  of Schmitt Trigger  310 . This pulse width depends on the R-C time constant of resistor  506  and capacitor  504 . The output of pin  4  of Schmitt Trigger  310  is electrically coupled to reset pin  4  of J-K Flip Flop  312  and also to the input pin  9  of OR Gate  602 . The J-K Flip Flop  312  resets and thus makes the output pin  2  High, which is electrically coupled to the input pin  11  of OR Gate  602 . The output  13  of OR Gate  602  is High and fed to input pin  10  of AND Gate  604 . Since two inputs ( 9  and  10 ) of AND Gate  604  are High it output at pin  13  is High driving the collector of transistor  606  to Low. This Low signal to the input of line interrupter  700  closes line interrupter  700 . Line interrupter  700  can be manually opened by connecting manual switch  500  to ground, causing the output  13  of AND Gate  604  to go Low, thus tripping line interrupter  700 . Line interrupter  700  can be closed manually by connecting manual switch  500  to 15V power supply.  
         [0055]     Referring now to  FIGS. 5-6 ,  FIGS. 5 and 6  illustrate a second exemplary embodiment of an arc fault detector according to the present invention.  
         [0056]      FIG. 5  is a simplified diagram illustrating a second exemplary embodiment of an arc fault detector  1020  of the present invention. As shown on  FIG. 5 , arc fault detector  1020  provides protection to a circuit  1022 . Circuit  1022  includes a direct current power source or battery  1024  providing power to a load  1026  via a conductor  1028 . A load switch  1030  is disposed in conductor  1028 .  
         [0057]     Arc fault detector  1020  includes a current monitor  1100  detecting a rate of change in electrical current in circuit  1022  and producing a first signal indicative of said rate of change. Arc fault detector  1020  further includes a signal isolator  1110  electrically coupled in series with the current monitor  1100 . The signal isolator functioning to signal isolator the first signal to substantially eliminate signals outside a selected frequency range, the signal isolator  1110  producing a filtered signal representing changes in the electrical current within the selected frequency range. Arc fault detector  1020  also includes an arc indicator (preferably Schmitt Trigger  310 ) producing an arc detection signal when a voltage level of the filtered signal exceeds at least one threshold. The threshold is preferably established by Schmitt Trigger  310 . Another device, such as amplifier  1108 , or a comparator may also be used to provide a threshold. Another device such as a monostable vibrator may also be used  
         [0058]     Arc fault detector  1020  includes current monitor  1100  and an arc determining circuit  290 . Current monitor  1100  includes a coil  1101  wrapped around a toroidal core or UI core  1102 . Arc determining circuit  290  includes an attenuator or first resistor  1103 , a second resistor  1104 , a zener diode  1106 , an operational amplifier  1108 , and a low pass signal isolator  1110 . Low pass signal isolator  1110  includes a resistor  1112  and a capacitor  1114 . Arc fault detector  1020  further includes a signal transformer  300 , a switch transient detector  400 , a manual switch  500 , a switch controller  600 , and a line interrupter  700 .  
         [0059]     Circuit  1022  passes through the center of toroidal core  1102  inducing a voltage pulse in coil  1101  due to a current change caused by a series or parallel arc fault. Attenuator or resistor  1103  is selected so that the voltage level of the filtered signal output to Schmitt Trigger  310  when generated by operation of a motor commutator is lower than the threshold voltage needed to be provided to Schmitt Trigger  310  to enable an output pulse to be generated.  
         [0060]     Resistor  1104  provides a path to ground for the voltage pulse. The voltage is provided to operational amplifier  1108  via zener diode  1106  which clamps the voltage to 12V to protect downstream components in the event the induced voltage is too high.  
         [0061]     Signal transformer  300 , switch transient detector  400 , manual switch  500 , switch controller  600 , and line interrupter  700  of arc fault detector  1020  are configured and operate as described in the exemplary embodiment of arc fault detector  20  above.  
         [0062]      FIG. 7  is a fragmentary simplified diagram illustrating how coil  1501  wrapped around a toroidal core  1502  is used as part of an arc fault detector  1520  protecting a zone  1600 .  
         [0063]     As shown on  FIG. 7 , arc fault detector  1520  provides protection to a circuit  1522  and zone  1600 . Circuit  1522  includes a direct current power source or battery  1524  providing power to loads  1526   a ,  1526   b  via respective conductors  1528   a ,  1528   b . A line interrupter  700  is disposed in circuit  1522 . Both conductors  1528   a ,  1528   b  pass through the center of toroidal core  1502 . When a current changes due to arc fault in either conductor  1528   a ,  1528   b , a voltage is induced in coil  1501 . An arc determining circuit, signal transformer, switch transient detector, manual switch, switch controller, and line interrupter  700  may be incorporated in arc fault detector  1520  as described above.  
         [0064]     As shown on  FIG. 8 , a first preferred method  2000  for detecting an arc fault in a circuit in accordance with present invention includes providing a shunt resistor in the circuit  2002 , monitoring a voltage differential across the shunt resistor  2004 , detecting a change in the voltage differential  2006 , comparing the change to at least one threshold  2008 , and generating a signal when the change exceeds the at least one threshold  2010 .  
         [0065]     As shown on  FIG. 9 , a second preferred method  3000  of detecting an arc fault in a system includes providing a coil wrapped around a toroidal core  3002 ; passing at least one conductor of the system through the center of the toroidal core  3004 ; detecting a voltage induced in the coil by a change in current flowing through the at least one conductor  3006 ; comparing the voltage to at least one threshold  3008 ; and generating a signal when the voltage exceeds the at least one threshold  3010 .  
         [0066]     The preferred embodiments shown and described herein are provided merely by way of example and are not intended to limit the scope of the invention in any way. Preferred dimensions, ratios, materials and construction techniques are illustrative only and are not necessarily required to practice the invention. It is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments herein. Further modifications and alterations may occur to others upon reading and understanding the specification.  
         [0067]     For example, in an exemplary embodiment, line interrupter  700  is preferably embodied as a static relay. Line interrupter  700  may also be an electromechanical switch, a thyristor, an intelligent switch, or the like. Schmitt Trigger  310  and J-K Flip Flop  312  may be replaced by other devices that provide pulsed signals known to those skilled in the art such as monostable multivibrators, bistable multivibrators, timers, latches, or the like.