Patent Publication Number: US-2015060423-A1

Title: Contact welding detection system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims the benefit of priority from earlier Japanese Patent Applications No. 2013-175878 filed Aug. 27, 2013 and No. 2014-38856 filed Feb. 28, 2014, the descriptions of which are incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a contact welding detection system for detecting the presence or absence of welding of a pair of contacts in a relay provided along a current path between a direct current (DC) power source and a power-supply unit. 
     2. Related Art 
     A known component provided between a DC power source and a power-supply unit, such as a power converter, to connect and disconnect the DC power source to and from the power-supply unit may include a relay formed of an electromagnetic coil and a pair of contacts (hereinafter also referred to as a contact pair) to be opened or closed in response to whether or not the electromagnetic coil is in an energized state. Since Joule heat or arc heat or the like may be produced upon transitions of the electromagnetic coil between being energized or not, the pair of contacts in the relay tend to be hot and may possibly be welded to each other. Hence, several contact welding detection systems have been developed to detect such welding of the contact pair in the relay. 
     For example, a ground fault circuit interrupter disclosed in Japanese Patent Application Laid-Open Publication No. 2012-152071 includes two relays provided to connect and disconnect an alternating current (AC) power line at both ends of the AC power line, and is configured to individually control the two relays and determine whether or not a voltage detection unit detects an AC voltage when the two relays are controlled such that only one of the two relays is in a passing state. The ground fault circuit interrupter can thereby detect the presence or absence of welding of a pair of contacts in the other one of the two relays. 
     The ground fault circuit interrupter set forth above, however, has disadvantages that an unexpected current may flow through the power-supply unit when one of the two relays is placed in the passing state in the presence of welding of the pair of contacts in the other relay, and particularly, when the power-supply unit includes a capacitor or a coil or the like, an expected inrush current may flow through the power-supply unit. 
     Further, providing a separate circuit for detecting contact welding in the relays, as provided in the ground fault circuit interrupter set forth above, may increase the number of components. 
     In consideration of the foregoing, exemplary embodiments of the present invention are directed to providing a contact welding detection system for detecting the presence or absence of welding of a pair of contacts in a relay without controlling the relay in a passing state. 
     SUMMARY 
     In accordance with an exemplary embodiment of the present invention, there is provided a contact welding detection system including: a main circuit including a direct current (DC) power source, a power-supply unit electrically connected to the DC power source, and at least one main circuit relay electrically connected between the DC power source and the power-supply unit; a signal generation unit electrically connected to the main circuit and configured to generate an alternating current (AC) signal; and a measurement unit configured to measure an electrical characteristic value on a signal wiring between the signal generation unit and the main circuit. 
     The system further includes: a welding determination unit configured to determine presence or absence of contact welding in the at least one main circuit relay on the basis of the electrical characteristic value acquired from the measurement unit; and a first forced ground-contacting unit electrically connected between the main circuit and a conductive member connected to ground. The first forced ground-contacting unit includes a first welding testing switch for switching on and off a current flowing from the main circuit to the conductive member, and is electrically connected to the main circuit at a connection point that is located on an opposite side of the at least one main circuit relay to a connection point at which the signal generation unit is electrically connected to the main circuit. 
     As above, the contact welding detection system includes the signal generation unit electrically connected to the main circuit through the signal wiring, the measurement unit configured to measure the electrical characteristic value on the signal wiring, the main circuit relay, and the first forced ground-contacting unit electrically connected to the main circuit at the connection point located on an opposite side of the main circuit relay to the connection point at which the signal generation unit is electrically connected to the main circuit. The contact welding detection system is configured such that the main circuit and the conductive member are connected to each other by turning on and off the welding testing switch, and the electrical characteristic value acquired in the measurement unit may change in response to the presence or absence of contact welding in the main circuit relay while the main circuit and the conductive member are connected to each other. 
     In addition, in the contact welding detection system as above, the welding determination unit can determine the presence or absence of contact welding in the main circuit relay on the basis of the electrical characteristic value acquired in the measurement unit. As such, the contact welding detection system can determine the presence or absence of contact welding in the main circuit relay by switching on and off the first welding testing switch without controlling the main circuit relay in the (current) passing state. 
     The signal generation unit, the measurement unit and the welding determination unit can cooperatively serve as a leak detection system to test for the presence or absence of the electrical leakage from the main circuit. The signal generation unit, the measurement unit and the welding determination unit are components shared by the circuitry for detecting the presence or absence of contact welding in the main circuit relay and the circuitry for detecting the presence or absence of electrical leakage from the main circuit. This can lead to reduction in total number of components of the system. 
     The contact welding detection system configured as above is capable of detecting the presence or absence of contact welding in the main circuit relay without controlling the main circuit relay in the passing state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic circuit diagram of a contact welding detection system in accordance with a first embodiment of the present invention; 
         FIG. 2  is a schematic circuit diagram showing a current path of an AC signal in the absence of contact welding in a main circuit relay in accordance with the first embodiment; 
         FIG. 3  shows a graph of a waveform of an AC signal acquired from a measurement unit in the absence of contact welding in the main circuit relay in accordance with the first embodiment; 
         FIG. 4  is a schematic circuit diagram showing a current path of the AC signal in the presence of contact welding in the main circuit relay in accordance with the first embodiment; 
         FIG. 5  shows a graph of a waveform of the AC signal acquired from the measurement unit in the presence of contact welding in the main circuit relay in accordance with the first embodiment; 
         FIG. 6A  shows a schematic circuit diagram of a contact welding detection system in accordance with a second embodiment of the present invention; 
         FIG. 6B  is a schematic circuit diagram showing a current path of an AC signal in the presence of contact welding in a main circuit relay in accordance with the second embodiment; 
         FIG. 7  shows a schematic circuit diagram of a contact welding detection system in accordance with a third embodiment of the present invention; 
         FIG. 8  is a schematic circuit diagram showing a current path of an AC signal with an operation verification switch in a passing state in accordance with the third embodiment; 
         FIG. 9  is a schematic circuit diagram showing a current path of the AC signal in the presence of contact welding in a positive relay in accordance with the third embodiment; 
         FIG. 10  is a schematic circuit diagram showing a current path of the AC signal in the presence of contact welding in a negative relay in accordance with the third embodiment; 
         FIG. 11  is a partially sectional front view of a solenoid arrangement used for main circuit relays in accordance with the third embodiment; 
         FIG. 12  is a schematic circuit diagram of a contact welding detection system in accordance with a fourth embodiment of the present invention; 
         FIG. 13  is a schematic circuit diagram of a contact welding detection system in accordance with a fifth embodiment of the present invention; 
         FIG. 14  is a schematic circuit diagram of a contact welding detection system in accordance with a sixth embodiment of the present invention; 
         FIG. 15  is a flowchart of welding detection for main circuit relays of the contact welding detection system in accordance with the sixth embodiment; 
         FIG. 16  is a schematic circuit diagram of a contact welding detection system in accordance with a seventh embodiment of the present invention; 
         FIG. 17  is a flowchart of welding detection for charging circuit relays of the contact welding detection system in accordance with the seventh embodiment; 
         FIG. 18  is a schematic circuit diagram of a contact welding detection system in accordance with an eighth embodiment of the present invention; 
         FIG. 19  is a flowchart of welding detection for charging circuit relays of the contact welding detection system in accordance with the eighth embodiment; 
         FIG. 20  is a schematic circuit diagram of a contact welding detection system in accordance with a ninth embodiment of the present invention; 
         FIG. 21  is a sectional front view of a solenoid arrangement used for charging circuit relays where both contact pairs are in a blocking state in accordance with the ninth embodiment; 
         FIG. 22  is a sectional front view of the solenoid arrangement used for charging circuit relays where one of the contact pairs is in a passing state in accordance with the ninth embodiment; 
         FIG. 23  is a sectional front view of the solenoid arrangement used for charging circuit relays where both contact pairs are in a passing state in accordance with the ninth embodiment; 
         FIG. 24  is a flowchart of welding detection for charging circuit relays of the contact welding detection system in accordance with the ninth embodiment; 
         FIG. 25  is a schematic circuit diagram of the contact welding detection system during welding detection for a first contact pair in accordance with the ninth embodiment; 
         FIG. 26  is a schematic circuit diagram of the contact welding detection system during welding detection for a second contact pair in accordance with the ninth embodiment; 
         FIG. 27  shows a schematic circuit diagram of a contact welding detection system in accordance with a tenth embodiment of the present invention; 
         FIG. 28  shows a schematic circuit diagram of a contact welding detection system in accordance with an eleventh embodiment of the present invention; 
         FIG. 29  is a schematic circuit diagram showing a current path of an AC signal in the presence of contact welding in main circuit relays in accordance with the eleventh embodiment; 
         FIG. 30  is a schematic circuit diagram of a contact welding detection system in accordance with a twelfth embodiment of the present invention; and 
         FIG. 31  is a schematic circuit diagram of a contact welding detection system in accordance with a thirteenth embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments 
     The present invention will be described more fully hereinafter with reference to the accompanying drawings. Like numbers refer to like elements throughout. 
     First Embodiment 
     There will now be explained a contact welding detection system in accordance with a first embodiment of the present intention with reference to  FIGS. 1-5 . As shown in  FIG. 1 , the contact welding detection system  1  includes a main circuit  11 , a signal generation unit  3 , a measurement unit  4 , a welding determination unit  5 , and a first forced ground-contacting unit  6 . The main circuit  11  includes a direct current (DC) power source  12 , a power-supply unit  13  electrically connected to the DC power source  12 , and a main circuit relay  2  electrically connected between the DC power source  12  and the power-supply unit  13 . 
     The signal generation unit  3  is electrically connected to the main circuit  11  and configured to generate an alternate current (AC) signal (see  FIGS. 3 and 5 ). The measurement unit  4  is configured to measure an electrical characteristic value on a signal wiring  30  between the signal generation unit  3  and the main circuit  11 . The welding determination unit  5  is configured to determine the presence or absence of welding of a contact pair in the main circuit relay  2  on the basis of the electrical characteristic value acquired from the measurement unit  4 . 
     The first forced ground-contacting unit  6  is provided between the conductive member  61  (e.g., a vehicle-body ground) and the main circuit  11  and electrically connected to the main circuit  11  at a connection point B that is located on an opposite side of the main circuit relay  2  to a connection point A at which the signal generation unit  3  is electrically connected to the main circuit  11 . The first forced ground-contacting unit  6  includes a first welding testing switch  62  for switching on and off a current flowing from the main circuit  11  to the conductive member  61 . 
     Examples of the first welding testing switch  62  may include, but are not limited to, a relay, and a semiconductor switch, such as a metal-oxide semiconductor field-effect transistor (MOSFET) or an insulated gate bipolar transistor (IGBT) or the like. Use of the semiconductor switch may reduce power required to control the first welding testing switch  62  as compared to the relay. In addition, the semiconductor switch has a less operational time as compared to the relay, which may reduce a time period required to detect the presence or absence of contact welding. 
     In the main circuit  11 , as shown in  FIG. 1 , a positive terminal of the DC power source  12  and the power-supply unit  13  are electrically connected to each other through a positive wiring  14 , and a negative terminal of the DC power source  12  and the power-supply unit  13  are electrically connected to each other through a negative wiring  15 . In the present embodiment, the power-supply unit  13  includes semiconductor switches (not shown) for switching on and off the current within the power-supply unit  13  to form a power converter, such as a DC-DC converter or an inverter. 
     The main circuit relay  2  is provided in the negative wiring  15 . The main circuit relay  2  includes a stationary contact part  22  and a movable contact part  23  and is configured such that the movable contact part  23  is movable into and out of contact with the stationary contact part  22  by a magneto-motive force of an electromagnetic coil  71 . 
     The signal generation unit  3  is electrically connected to a connection point A along the negative wiring  15  between the DC power source  12  and the main circuit relay  2  through the signal wiring  30 . A capacitor  31  is provided in the signal wiring  30  to isolate the signal generation unit  3  from the direct-current (DC) voltage of the DC power source  12 . In the present embodiment, the signal generation unit  3  is configured to generate a square wave having an amplitude of 5V. In some other embodiments, the signal generation unit  3  may be configured to generate a sine wave, a triangular wave or the like as a continuous wave, or a mono-pulse wave. 
     The measurement unit  4  is electrically connected to a connection point C along the signal wiring  30  between the signal generation unit  3  and the capacitor  31 . As with the signal generation unit  3 , this allows the measurement unit  4  to be isolated from the direct current of the DC power source  12 . 
     In the present embodiment, the measurement unit  4  is configured to measure a voltage value at the connection point C as an electrical characteristic value. The voltage value measured at the connection point C will be equivalent to a voltage value measured at the connection point A. 
     The welding determination unit  5  is electrically connected to the measurement unit  4  to receive an alternating current (AC) signal measured by the measurement unit  4 . The welding determination unit  5  is configured to determine the presence or absence of welding of the contact pair  21  in the main circuit relay  2  on the basis of a voltage value of the received AC signal. 
     The first forced ground-contacting unit  6  electrically connects the main circuit  11  and the conductive member  61  via the first welding testing switch  62 . The connection point B between the first forced ground-contacting unit  6  and the main circuit  11  is positioned along the negative wiring  15  between the main circuit relay  2  and the power-supply unit  13 . 
     In addition, the first forced ground-contacting unit  6  includes a resistor  63  electrically connected in series with the first welding testing switch  62 . In the present embodiment, the first welding testing switch  62  and the resistor  63  are arranged in this order along a direction from the main circuit  11  to the resistor  63 . Alternatively, the resistor  63  and the first welding testing switch  62  may be arranged in this order along the same direction. 
     There will now be explained the operations of the contact welding detection system  1 . Testing for the presence of welding of the contact pair  21  in the main circuit relay  2  is conducted by controlling energization of the electromagnetic coil  71  so that the movable contact part  23  moves out of contact with the stationary contact part  22  and controlling the semiconductor switches in the power-supply unit  13  to be off so that no current flows through the power-supply unit  13 , and then placing the first welding testing switch  62  in a passing state. 
     In the absence of welding of the contact pair  21  in the main circuit relay  2 , the main circuit relay  2  will be in a (current) blocking state in which the stationary contact part  22  and the movable contact part  23  are out of contact with each other. Hence, the AC signal generated in the signal generation unit  3  will not pass through the main circuit  11 , but will pass to the measurement unit  4  through a connection point C along a current path P 1  as shown in  FIG. 2 . Further, as shown in  FIG. 3 , the voltage value measured in the measurement unit  4  will become comparable to an amplitude of the AC signal as generated in the signal generation unit  3 . 
     In the presence of welding of the contact pair  21  in the main circuit relay  2 , the main circuit relay  2  will be left in a passing state even after controlling energization of the electromagnetic coil  71  so that the movable contact part  23  moves out of contact with the stationary contact part  22 , in which state the stationary contact part  22  and the movable contact part  23  are in contact with each other. The AC signal generated in the signal generation unit  3  will then pass to the conductive member  61  through the connection point C, the connection point A, the main circuit relay  2 , the connection point B, and the first forced ground-contacting unit  6  in this order along a current path P 2  as shown in  FIG. 4 . The voltage value measured in the measurement unit  4  will become less than an amplitude of the AC signal as generated in the signal generation unit  3 . For example, in the present embodiment, as shown in  FIG. 5 , the voltage value measured in the measurement unit  4  becomes about 3V due to a voltage drop across the resistor  63 . 
     As above, the contact welding detection system  1  is configured such that the voltage value measured in the measurement unit  4  (hereinafter also referred to as the measured voltage value) changes in response to the presence or absence of welding of the contact pair  21  in the main circuit relay  2 . The welding determination unit  5  compares the measured voltage value with a predetermined threshold T (see  FIGS. 3 and 5 ). When the measured voltage value is equal to or greater than the predetermined threshold T (see  FIG. 3 ), then the welding determination unit  5  determines that the pair of the stationary contact part  22  and the movable contact part  23  are not welded to each other. When the measured voltage value is less than the predetermined threshold T (see  FIG. 5 ), then the welding determination unit  5  determines that the pair of the stationary contact part  22  and the movable contact part  23  are welded to each other. 
     In addition, the contact welding detection system  1  can serve as a leak detection system for detecting the presence or absence of the electrical leakage from the main circuit  11  by generating the AC signal while keeping the welding testing switch  62  in the blocking state. In the presence of the electrical leakage from the main circuit  11 , the impedance of the main circuit  11  will be lowered as compared to in the absence of the electrical leakage from the main circuit  11 . Accordingly, in the presence of the electrical leakage from the main circuit  11 , the voltage value of the AC signal measured in the measurement unit  4  decreases as compared to in the absence of the electrical leakage. As such, in the contact welding detection system  1  configured as above, the welding determination unit  5  may determine not only the presence or absence of welding of the contact pair  21 , but also the presence or absence of the electrical leakage from the main circuit  11 . 
     There will now be explained advantages of the present embodiment. The contact welding detection system  1  includes the signal generation unit  3  electrically connected to the main circuit  11  through the signal wiring  30 , the measurement unit  4  configured to measure the electrical characteristic value (voltage value) on the signal wiring  30 , the main circuit relay  2 , and the first forced ground-contacting unit  6  electrically connected to the main circuit  11  at the connection point B that is located on an opposite side of the main circuit relay  2  to the connection point A. The welding determination unit  5  can determine the presence or absence of welding of the contact pair  21  on the basis of the electrical characteristic value acquired by the measurement unit  4 . As such, the contact welding detection system  1  can determine the presence or absence of welding of the contact pair  21  in the main circuit relay  2  by switching on and off the first welding testing switch  62  without controlling the main circuit relay  2  to be in the passing state. 
     In addition, the signal generation unit  3 , the measurement unit  4  and the welding determination unit  5  can cooperatively serve as the leak detection system to test for the presence or absence of the electrical leakage from the main circuit  11 . That is, the signal generation unit  3 , the measurement unit  4  and the welding determination unit  5  are components shared by the circuitry for detecting the presence or absence of welding of the contact pair  21  and the circuitry for detecting the presence or absence of electrical leakage from the main circuit  11 . This can lead to reduction in total number of components of the system  1 . 
     The first forced ground-contacting unit  6  includes a series connection of the first welding testing switch  62  and the resistor  63 , which can reduce a power consumption required to detect the presence or absence of welding of the contact pair in the main circuit relay  2 . 
     As above, the contact welding detection system  1  is capable of detecting the presence or absence of welding of the contact pair in the main circuit relay  2  without controlling the relay  2  to be in the passing state. 
     Alternatively, in the present embodiment, the connection point A between the main circuit  11  and the signal generation unit  3  may be provided along the positive wiring  14 . Additionally or alternatively, the position of the connection point A between the main circuit  11  and the signal generation unit  3  and the position of the connection point B between the first forced ground-contacting unit  6  and the main circuit  11  may be exchanged. 
     Second Embodiment 
       FIG. 6A  shows a contact welding detection system  200  in accordance with a second embodiment of the present invention, which is similar in configuration to the contact welding detection system  1  of the first embodiment except that the main circuit relay  2  is provided in the positive wiring  14  and the first forced ground-contacting unit  6  is electrically connected to the positive wiring  14 . More specifically, the first forced ground-contacting unit  6  is electrically connected to the main circuit  11  at a connection point B′ located along the positive wiring  14  and between the main circuit relay  2  and the power-supply unit  13 , and the signal generation unit  3  is electrically connected to the main circuit  11  at the connection point A located along the negative wiring  15 . In  FIGS. 6A and 6B , elements having similar functions as in the first embodiment are assigned the same numbers, except where specified otherwise. 
     In the presence of welding of the contact pair  21  in the main circuit relay  2 , the AC signal generated in the signal generation unit  3  will pass to the conductive member  61  through the connection point C, the connection point A, the DC power source  12 , the main circuit relay  2 , the connection point B′ and the first forced ground-contacting unit  6  in this order along a current path P 2 ′ as shown in  FIG. 6B . Therefore, as in the first embodiment, the voltage value measured in the measurement unit  4  will change in response to the presence or absence of welding of the contact pair  21  in the main circuit relay  2 . This allows the welding determination unit  5  to determine the presence or absence of welding of the contact pair  21  in the main circuit relay  2  on the basis of the measured voltage value. The same advantages as those indicated above for the first embodiment also apply to the second embodiment. 
     Alternatively, in the present embodiment, the connection point A between the main circuit  11  and the signal generation unit  3  may be provided along the positive wiring  14  and between the DC power source  12  and the main circuit relay  2 . Additionally or alternatively, the position of the connection point A between the main circuit  11  and the signal generation unit  3  and the position of the connection point B′ between the first forced ground-contacting unit  6  and the main circuit  11  may be exchanged. 
     Third Embodiment 
     There will now be explained a contact welding detection system  300  in accordance with a third embodiment of the present invention with reference to  FIGS. 7-10 . The contact welding detection system  300  includes two main circuit relays  2 : a positive relay  2   b  provided in the positive wiring  14  and a negative relay  2   a  provided in the negative wiring  15 . As shown in  FIG. 7 , the contact welding detection system  300  of the third embodiment is configured such that a movable contact part  23   b  of the positive relay  2   b  and a movable contact part  23   a  of the negative relay  2   a  are movable into and out of contact with their respective stationary contact parts  22   b  and  22   a  by a magneto-motive force of an electromagnetic coil  710  shared by the positive relay  2   b  and the negative relay  2   a . The negative stationary contact part  22   a  and the negative movable contact part  23   a  forms a negative contact pair  21   a  of the negative relay  2   a , and the positive stationary contact part  22   b  and the positive movable contact part  23   b  forms a positive contact pair  21   b  of the positive relay  2   b.    
     The signal generation unit  3  is electrically connected to the main circuit  11  at the connection point A that is located between the DC power source  12  and the negative relay  2   a  through the signal wiring  30 . The first forced ground-contacting unit  6  is electrically connected to the main circuit  11  at the connection point B that is located between the negative relay  2   a  and the power-supply unit  13 . 
     In the present embodiment, the signal generation unit  3  includes a series connection of a signal source  32  that generates a square wave and a resistor  33 . The resistor  33  is directly connected to the capacitor  31  provided in the signal wiring  30 . 
     In the present embodiment, the first forced ground-contacting unit  6  includes the first welding testing switch  62 , the resistor  63 , and a capacitor  64 . One terminal  631  of the resistor  63  is electrically connected not only to the first welding testing switch  62 , but also to the signal wiring  30  at the connection point C that is located between the signal generation unit  3  and the measurement unit  4  through the operation verification switch  51 , so that, as shown in  FIG. 8 , when the operation verification switch  51  is in a passing state, the AC signal generated in the signal generation unit  3  can pass to the resistor  63  along a current path P 3  without passing through the main circuit  11 . The other terminal  632  of the resistor  63  is electrically connected to the conductive member  61  through the capacitor  64 , which allows the conductive member  61  to be electrically isolated from the main circuit  11 . 
     The operation verification switch  51  is electrically connected to the welding determination unit  5  to provide to the welding determination unit  5  information about whether the operation verification switch  51  is in the passing state or in the blocking state. 
     The contact welding detection system  1  includes a capacitor  16 . One terminal of the capacitor  16  is electrically connected to the positive wiring  14  between the positive relay  2   b  and the power-supply unit  13 , and another terminal of the capacitor  16  is electrically connected to the negative wiring  15  between the negative relay  2   a  and the power-supply unit  13 , more specifically, between the connection point B for the first forced ground-contacting unit  6  and the power supply unit  13 . Alternatively, the other terminal of the capacitor  16  may be electrically connected to the negative wiring  15  between the negative relay  2   a  and the connection point B for the first forced ground-contacting unit  6 . 
     There will now be explained the operations of the contact welding detection system  300 . Testing for the presence or absence of welding of the respective contact pairs  21  ( 21   a  and  21   b ) in the main circuit relays  2  (the negative relay  2   a  and the positive relay  2   b ) is conducted by controlling the energization of the shared electromagnetic coil  710  so that the movable contact part  23   a  moves out of contact with the stationary contact part  22   a  and the movable contact part  23   b  moves out of contact with the stationary contact part  22   b  in the positive relay  2   b , and then placing the first welding testing switch  62  in the passing state. 
     In the absence of welding of the positive contact pair  21   b  and welding of the negative contact pair  21   a , the positive relay  2   b  and the negative relay  2   a  are both placed in the blocking state, where the stationary contact part  22   b  and the movable contact part  23   b  of the positive relay  2   b  are out of contact with each other and the stationary contact part  22   a  and the movable contact part  23   a  of the negative relay  2   a  are out of contact with each other. Hence, as in the first embodiment, the AC signal generated in the signal generation unit  3  will not pass through the main circuit  11 , and the voltage value measured in the measurement unit  4  will therefore become comparable to an amplitude of the AC signal as generated in the signal generation unit  3 . 
     In the presence of either or both of welding of the positive contact pair  21   b  and welding of the negative contact pair  21   a , either or both of the positive relay  2   b  and the negative relay  2   a  will be left in the passing state even after controlling the energization of the shared electromagnetic coil  710  so that the stationary contact part  22   b  and the movable contact part  23   b  of the positive relay  2   b  are placed out of contact with each other and the stationary contact part  22   a  and the movable contact part  23   a  of the negative relay  2   a  are placed out of contact with each other. For example, in the presence of welding of the positive contact pair  21   b  only, the AC signal generated in the signal generation unit  3  will pass to the conductive member  61  through the connection point C, the connection point A, the DC power source  12 , the positive relay  2   b , the capacitor  16 , the connection point B, and the first forced ground-contacting unit  6  in this order along a current path P 4  as shown in  FIG. 9 . In the presence of welding of the negative contact pair  21   a  only, the AC signal generated in the signal generation unit  3  will pass to the conductive member  61  through the connection point C, the connection point A, the negative relay  2   a , the connection point B, and the first forced ground-contacting unit  6  in this order along a current path P 5  as shown in  FIG. 10 . Hence, in either one of the cases set forth above, the voltage value measured in the measurement unit  4  will become less than an amplitude of the AC signal as generated in the signal generation unit  3 . 
     In the present embodiment, the contact welding detection system  300  is configured to verify the operation of the contact welding detection system  300  by placing the operation verification switch  51  in the passing state. That is, as shown in  FIG. 8 , the signal generation unit  3  and the resistor  63  are short-circuited by switching the operation verification switch  51  from the blocking state to the passing state, which allows the AC signal generated in the signal generation unit  3  to pass to the conductive member  61  through the connection point C, the operation verification switch  51 , the resistor  63  in this order along a current path P 3 . The voltage value measured in the measurement unit  4  will become almost equal to the voltage value as measured in the presence of either or both of welding of the positive contact pair  21   b  and welding of the negative contact pair  21   a . Hence, whether or not the welding determination unit  5  is operating correctly can be verified by comparing information about whether the operation verification switch  51  is in the passing state or in the blocking state with the determination of the presence or absence of contact-pair welding made in the welding determination unit  5 . This can enhance reliability of the contact welding detection system. 
     The contact welding detection system  300  of the present embodiment is similar in configuration to the contact welding detection system  1  of the first embodiment, except that the contact welding detection system  300  can further function as a leak detection system for determining locations where electrical leakages are occurring. The contact welding detection system  300  functions as the leak detection system as follows. First, the AC signal is produced in the signal generation unit  3  with both the positive relay  2   b  and the negative relay  2   a  kept in the blocking state. In the absence of welding of the positive contact pair  21   b  and welding of the negative contact pair  21   a , the AC signal produced in the signal generation unit  3  cannot reach the power-supply unit  13  side portion of the main circuit  11  relative to the negative and positive relays  2   a  and  2   b , which allows the contact welding detection system  300  to determine the presence or absence of electrical leakage in the DC power source  12  side portion of the main circuit  11  relative to the negative and positive relays  2   a  and  2   b.    
     Subsequently, the AC signal is produced in the signal generation unit  3  with the negative and positive relays  2   a  and  2   b  both kept in the passing state and the power-supply unit  13  kept in a current non-flow state, which allows the contact welding detection system  300  to determine the presence or absence of electrical leakage along at least one of the positive wiring  14  and the negative wiring  15 . Thereafter, the AC signal is produced in the signal generation unit  3  with the negative and positive relays  2   a  and  2   b  both kept in the passing state and the power-supply unit  13  kept in a current flow state, which allows the contact welding detection system  300  to determine the presence or absence of electrical leakage in the power-supply unit  13 . In  FIGS. 7-10 , elements having similar functions as in the first embodiment are assigned the same numbers, except where specified otherwise. 
     Several advantages of the present embodiment will now be explained. The contact welding detection system  300  includes the positive relay  2   b  between the positive terminal of the DC power source  12  and the power-supply unit  13  and the negative relay  2   a  between the negative terminal of the DC power source  12  and the power-supply unit  13 . This can lead to more reliable operation for isolation between the DC power source  12  and the power-supply unit  13 . 
     In addition, in the contact welding detection system  300 , the signal generation unit  3  is electrically connected to the main circuit  11  at the connection point A between the DC power source  12  and one of the main circuit relays  2 . This, as described above, allows for switching between portions of the main circuits  11  for which the presence or absence of the electrical leakage is detected, thereby leading to enhanced performance of the leak detection system. 
     In addition, in the contact welding detection system  300 , the signal generation unit  3  and the measurement unit  4  are electrically connected to the resistor  63  through the operation verification switch  51 . This allows whether or not the welding determination unit  5  is operating correctly to be verified, thereby enhancing reliability of the contact welding detection system  300 . 
     In addition, in the contact welding detection system  300 , the movable contact part  23   b  of the positive relay  2   b  and the movable contact part  23   a  of the negative relay  2   a  are movable into and out of contact with their respective stationary contact parts  22   b  and  22   a  by a magneto-motive force of an electromagnetic coil  710  shared by the positive relay  2   b  and the negative relay  2   a . This can lead to reduction of power consumption in the main circuit relays  2 . 
     The contact welding detection system  300  includes the capacitor  16  electrically connected between the positive wiring  14  and the negative wiring  15 , where one terminal of the capacitor  16  is electrically connected to the positive wiring  14  between the positive relay  2   b  and the power-supply unit  13  and the other terminal of the capacitor  16  is electrically connected to the negative wiring  15  between the negative relay  2   a  and the power-supply unit  13 . With this configuration, in the presence of welding of the positive contact pair  21   b  only, the AC signal generated in the signal generation unit  3  can pass to the conductive member  61  through the current path P 4  not including the power supply unit  13 , as shown in  FIG. 9 . Therefore, even when the power-supply unit  13  is in the current non-flow state in which no current flows through the power-supply unit  13 , the contact welding detection system  300  can detect the presence or absence of welding of the positive contact pair  21   b , as well as the presence or absence of welding of the negative contact pair  21   a.    
     In the present embodiment, for example, a solenoid arrangement  7  as shown in  FIG. 11  may be used in the main circuit relay  2  including the positive relay  2   b  and the negative relay  2   a . The solenoid arrangement  7  will now be explained with reference to  FIG. 11 . In  FIG. 11 , elements having similar functions as in the first embodiment are assigned the same numbers, except where specified otherwise. 
     As shown in  FIG. 11 , the solenoid arrangement  7  includes the shared electromagnetic coil  710 , a stationary core  72  forming a portion of a magnetic circuit in which a magnetic flux is formed, two plungers  73  (a first plunger  73   a  and a second plunger  73   b ) that are oppositely located on both sides of the electromagnetic coil  710  in a direction of a winding axis of the electromagnetic coil  710 . The plungers  73   a  and  73   b  are respectively movable toward or away from the stationary core in response to energization or de-energization of the electromagnetic coil  710 . 
     The magnetic circuit includes a magnetoresistive portion  74  resistive to the magnetic flux formed in the magnetic circuit. The first and second plungers  73   a ,  73   b  are configured to be attracted to the stationary core  72  in response to energization of the electromagnetic coil  710  and then kept in the attracted state while the electromagnetic coil  710  is in a energized state. 
     The stationary core  72  is formed of a plurality of portions including a center core  721  that extends through the inside of the electromagnetic coil  710 , two side cores  724  that are located radially outwardly of the electromagnetic coil  710  and oppositely to each other, top and bottom cores  722  and  723  magnetically connecting the side cores  724  and the first and second plungers  73   a  and  73   b . The stationary core  72  further includes an intermediate core  725  located between the bottom core  723  and the electromagnetic coil  710  along the direction of the winding axis of the electromagnetic coil  710  to magnetically connect the side cores  724  and the center core  721 . 
     In the following, the direction of the winding axis of the electromagnetic coil  710  may be referred to as a “Z-direction”, and a direction in which the side cores  724  oppose each other may be referred to as an “X-direction”. 
     The first plunger  73   a  includes a substantially disc-shaped magnetic member  732  and a protrusion  733  that protrudes outwardly from the center of the magnetic member  732  in the Z-direction. The protrusion  733  is made of a resin and connected to the movable contact part  23   a  of the negative circuit relay  2   a  (but not shown in  FIG. 11 ). The plunger  73   b  is similar in configuration to the plunger  73   a.    
     More specifically, when the first plunger  73   a  is attracted toward the center core  721 , the movable contact part  23   a  of the negative relay  2   a  moves in contact with the stationary contact part  22   a  of the negative relay  2   a  in conjunction of movement of the first plunger  73   a . The movable contact part  23   a  of the negative relay  2   a  is kept in contact with the stationary contact part  22   a  of the negative relay  2   a  while the first plunger  73   a  is attracted to the center core  721 . Similarly, when the second plunger  73   b  is attracted toward the center core  721 , the movable contact part  23   b  of the positive relay  2   b  moves in contact with the stationary contact part  22   b  of the positive relay  2   b  in conjunction of movement of the second plunger  73   b . The movable contact part  23   b  of the positive relay  2   b  is kept in contact with the stationary contact part  22   b  of the positive relay  2   b  while the second plunger  73   b  is attracted to the center core  721 . 
     A plunger biasing member  731  is provided between each of the first and second plungers  73   a ,  73   b  and the center core  721  to bias the plunger away from the center core  721 . The plunger biasing member  731  may be formed of a coiled spring. 
     In addition, the intermediate core  725  has a magnetoresistive portion  74 . In the present embodiment, the magnetoresistive portion  74  may be a gap (e.g., an air gap) for severing a portion of the stationary core  72  (particularly a portion of the intermediate core  725 ) along a direction of a flux path. Alternatively, the magnetoresistive portion  74  may be a smaller cross-sectional area portion, an cross-sectional area of which perpendicular to the direction of the flux path is less than cross-sectional areas of the other portions of the stationary core  72 , or may be a lower-permeability material portion made of a resin or the like. Use of such a lower-permeability material portion can increase rigidity of the stationary core  72  as compared to use of the air gap for the magnetoresistive portion  74 . 
     In the present embodiment, the magnetoresistive portion  74  is configured such that the reluctance caused by the magnetoresistive portion  74  is lower than the reluctance cased by an air gap formed when the first and second plungers  73  are spaced apart from the center core  721 . 
     The operation of the solenoid arrangement  7  will now be explained. The electromagnetic coil  710  is energized when none of the first and second plungers  73   a  and  73   b  are attracted to the center core  721  (see  FIG. 11 ). In the present embodiment, there will be formed first and second closed flux paths. The first closed path includes the center core  721 , the first plunger  73   a  and the intermediate core  725 , and the second closed flux path includes the first plunger  73   a , the second plunger  73   b  and the center core  721 . since the first closed flux path has a lower reluctance than the second closed flux path, a magnetic flux will be formed first in the first closed flux path. Accordingly, the first plunger  73   a  will first be attracted to the center core  721 . 
     When the first plunger  73   a  is attracted to the center core  721 , the reluctance of the second closed flux path including the first and second plungers  73   a ,  73   b  and the center core  721  will be reduced. Further, the magnetoresistive portion  74  formed in the intermediate core  725  will lead to a limited amount of magnetic flux formed in the first closed flux path, thereby leading to formation of a sufficient magnetic flux in the second closed flux path to allow the second plunger  23   b  to be subsequently attracted to the center core  721 . 
     A sufficient amount of magnetic flux will thus be formed in each of the first and second flux paths, thereby allowing each of the first and second plungers  23   a  and  23   b  to be kept attracted to the center core  721  while the electromagnetic coil  710  is energized. 
     The solenoid arrangement  7  is configured such that a plurality of main circuit relays  2  are kept in the blocking state through the energization of the shared and single electromagnetic coil  710  and without using a plurality of electromagnetic coils  710 , which can lead to reduction of power consumption of the system. 
     In addition, the magnetic circuit includes the magnetoresistive portion  74 , which facilitates placing the plurality of main circuit relays  2  in the blocking state through energization of the electromagnetic coil  710 . That is, proper positioning of the magnetoresistive portion  74  may lead to formation of magnetic fluxes produced by the electromagnetic coil  710  along a plurality of closed flux paths respectively including the plurality of plungers  73 . This can effectively implement keeping the plurality of main circuit relays  2  in the blocking state through energization of the single electromagnetic coil  710 . 
     This configuration may reduce variations in magnitude of magnetic flux formed along the two closed flux paths. This allows the two plungers  73  to be attracted in a stable manner without increasing the current flowing through the electromagnetic coil  710 , thereby reducing power consumption required to keep the plurality of main circuit relays  2  in the blocking state. 
     Particularly, this configuration can significantly reduce the power consumption required to keep the plurality of plungers  73  (two plungers in the present embodiment) attracted to the stationary core  72  for a long time. More specifically, use of the solenoid arrangement  7  for the plurality of main circuit relays  2  electrically connected between the DC power source  12  and the power-supply unit  13  as in the present embodiment requires the two plungers  73  to be continuously attracted to the stationary core  72  to keep the main circuit relays  2  in the passing state. Hence, use of the single electromagnetic coil  710  to keep the main circuit relays  2  in the passing state may significantly reduce the power consumption as compared with use of a plurality of electromagnetic coils to keep the plurality of main circuit relays  2  in the passing state. 
     In addition, the solenoid arrangement  7  of the present embodiment including the single electromagnetic coil  710  can facilitate reducing manufacturing costs and downsizing of the system. 
     The solenoid arrangement  7  set forth above is configured such that the magnetoresistive portion  74  is a gap severing a portion of the stationary core  72  along a direction of a flux path. Alternatively, the solenoid arrangement  7  may be configured such that the magnetoresistive portion  74  is a smaller cross-sectional area portion which has a cross-sectional area perpendicular to the direction of the flux path less than cross-sectional areas of the other portions of the stationary core  72 . Use of the smaller cross-sectional area portion for the magnetoresistive portion  74  may lead to a more complicated magnetic design as compared to use of the gap for the magnetoresistive portion  74 . This is because attracting the plurality of plungers  73  to the stationary core  72  by using the single electromagnetic coil  710  and then keeping the plurality of plungers  73  attracted to the stationary core  72  will require magnetic saturation of the closed flux path including the previously attracted plunger  73  to the stationary core  72 . 
     The reluctance of the smaller cross-sectional area portion is low at the beginning of the attraction of the plungers  73  to the stationary core  72 . Near completion of the attraction of the plungers  73  to the stationary core  72 , a gap between the plungers  73  and the center core  721  is reduced. The reluctance of the entire closed flux path is thereby reduced, and then the magnetic flux density at the smaller cross-sectional area portion is increased. Hence, the magnetic saturation of the closed flux path passing through the previously attracted plunger  73  is required to increase the reluctance of the smaller cross-sectional area portion. The reluctance of the magnetic circuit suitable to keep the plurality of the plungers  73  attracted to the single electromagnetic coil  710  requires proper positioning of the magnetic saturation region. However, since the B-H curve of the electromagnetic coil  710  varies with individuals, the magnetic design has to be made taking into account such a variation. 
     Use of the gap for the magnetoresistive portion  74  as in the present embodiment has advantages that it is easy to design a constant reluctance value by using a distance and an area of the gap. 
     In addition, the solenoid arrangement  7  is configured such that when the two plungers  73   a  and  73   b  are attracted toward the stationary core  72 , the direction along which the plunger  73   a  is attracted to the stationary core  72  and the direction along which the plunger  73   b  is attracted to the stationary core  72  are opposite each other. The solenoid arrangement  7  configured as above can prevent both the plungers  73   a  and  73   b  from being displaced toward the stationary core  72  even when the solenoid arrangement  7  is subject to vibration while in use. For example, when the solenoid arrangement  7  is subject to vibration in the Z-direction and the first plunger  73   a  is thereby displaced toward the stationary core  72 , the second plunger  73   b  will be displaced away from the stationary core  72 . This can prevent both the two plungers  73   a  and  73   b  from being displaced toward the stationary core  72  and can therefore prevent the main circuit relays  2  from being placed in the passing state. This allows for more reliable operation for isolation between the DC power source  12  and the power-supply unit  13 . 
     The solenoid arrangement  7  set forth above is merely one example. Alternatively, the main circuit relays  2  may be used that include a solenoid arrangement other than the solenoid arrangement  7  set forth above. 
     Fourth Embodiment 
     There will now be explained a contact welding detection system  400  in accordance with a fourth embodiment of the present invention with reference to  FIG. 12 . The contact welding detection system  400  includes the negative relay  2   a , the positive relay  2   b , and a series connection of a precharge relay  2   c  and a resistor  17 . The series connection of the precharge relay  2   c  and the resistor  17  is electrically connected in parallel with the positive relay  2   b . The positive relay  2   b , the negative relay  2   a  and the precharge relay  2   c  are configured to be individually switched between the passing state and the blocking state in response to energization or de-energization of their respective electromagnetic coils  71 . The contact welding detection system  400  includes a capacitor  16 . One terminal of the capacitor  16  is electrically connected to the positive wiring  14  between the positive relay  2   b  and the power-supply unit  13 , and another terminal of the capacitor  16  is electrically connected to the negative wiring  15  between the negative relay  2   a  and the power-supply unit  13 . In  FIG. 12 , elements having similar functions as in the first embodiment are assigned the same numbers, except where specified otherwise. 
     The contact welding detection system  400  is able to detect the presence or absence of contact welding in each of the positive relay  2   b , the negative relay  2   a  and the precharge relay  2   c.    
     In addition, the contact welding detection system  400  is able to test for the presence or absence of contact welding in the three main circuit relays  2  (the positive relay  2   b  the negative relay  2   a  and the precharge relay  2   c ) through switching of the first welding testing switch  62 , which can lead to reduction in welding testing time. That is, without the first forced ground-contacting unit  6 , it would be needed to test for the presence or absence of contact welding in the negative relay  2   a  and the precharge relay  2   c  with the positive relay  2   b  kept in the passing state, and then test for the presence or absence of contact welding in the positive relay  2   b  with the negative relay  2   a  or the precharge relay  2   c  kept in the passing state, which would lead to a longer testing time to accomplish the welding test for the three main circuit relays  2  (the positive relay  2   b  the negative relay  2   a  and the precharge relay  2   c ). 
     In a configuration such that the three main circuit relays  2  (the positive relay  2   b  the negative relay  2   a  and the precharge relay  2   c ) can be individually driven by their respective electromagnetic coils  71 , welding test for the three main circuit relays  2  can somehow be conducted by individually controlling the their respective electromagnetic coils  71  as described in the introductory part of the present application. However, without the first forced ground-contacting unit  6 , it would take a longer time to accomplish the welding test for the three main circuit relays  2 . The contact welding detection system  400  of the fourth embodiment is able to test for the presence or absence of contact welding in a plurality of main circuit relays  2  at once through switching of the first welding testing switch  62 . Therefore, for example, the contact welding detection system  400  can prevent a user of a vehicle mounting therein the system from feeling uncomfortable when the welding test is conducted upon start of the vehicle. 
     Fifth Embodiment 
     There will now be explained a contact welding detection system  500  in accordance with a fifth embodiment with reference to  FIG. 13 . As shown in  FIG. 13 , the first forced ground-contacting unit  6  includes a series connection of the first welding testing switch  62  and a capacitor  65 . Particularly, in the present embodiment, the capacitor  65  is electrically connected between the first welding testing switch  62  and the main circuit  11 . In  FIG. 13 , elements having similar functions as in the third embodiment are assigned the same numbers, except where specified otherwise. 
     In the present embodiment, electrical isolation between the conductive member  61  and the main circuit  11  can be ensured this can prevent a voltage of the main circuit  11  from being applied to the conductive member  61 , such as a vehicle body or the like. In addition, in the present embodiment, the capacitor  65  is electrically connected between the first welding testing switch  62  and the main circuit  11 . This can protect the first welding testing switch  62  from a high-voltage of the main circuit  11 . 
     Besides these advantages, the present embodiment can provide similar advantages as those indicated above for the third embodiment. 
     Sixth Embodiment 
     There will now be explained a contact welding detection system  600  in accordance with a sixth embodiment with reference  FIGS. 14 and 15 . In the contact welding detection system  600 , the main circuit  11  includes, as in the fourth embodiment, three main circuit relays  2  (the positive relay  2   b , the negative relay  2   a  and the precharge relay  2   c ), and the first forced ground-contacting unit  6  includes, as in the fifth embodiment, the capacitor  65  electrically connected between the first welding testing switch  62  and the main circuit  11 . In  FIG. 14 , elements having similar functions as in the fourth or fifth embodiment are assigned the same numbers, except where specified otherwise. 
     There will now be explained the operations of the contact welding detection system  600  with reference to a flowchart of  FIG. 15 . Testing for the presence or absence of contact welding in the main circuit relays  2  (the negative relay  2   a , the positive relay  2   b , and the precharge relay  2   c ) is conducted by controlling the energization of their respective electromagnetic coils  71  so that all the main circuit relays  2  are placed in the blocking state in step S 1 . In addition, the power-supply unit  13  is placed in a current non-flow state by controlling the switching elements therein, and the operation verification switch  51  is also placed in the blocking state. 
     In this state, in step S 2 , the first welding testing switch  62  is placed in the passing state. Thereafter, in step S 3 , the AC signal in the form of the square wave of a predetermined amplitude (voltage) is generated in the signal generation unit  3 . The voltage value of this signal may hereinafter be referred to as a generated voltage value Vg. Step S 2  may be preceded by step S 3 . 
     Subsequently, in step S 4 , a voltage value is measured in measurement unit  4 . The voltage value measured in the measurement unit  4  (which may hereinafter be referred to as a measured voltage value Vm) may be comparable to the generated voltage value Vg, or may be less than the generated voltage value Vg. 
     In the absence of contact welding in the main circuit relays  2 , even when the welding testing switch  62  is turned on, no current will pass to the conductive member  61  through a circuit from the capacitor  31  to the conductive member  61  through the main circuit  11  and the first forced ground-contacting unit  6  in this order as the main circuit is interrupted by the main circuit relays. That is, the impedance of this circuit is so high that the voltage at the connection point C, that is, the measured voltage value Vm remains almost at the generated voltage value Vg. 
     However, in the presence of contact welding in at least one of the main circuit relays  2 , the AC current generated in the signal generation unit  3  can flow through the same circuit from the capacitor  31  to the conductive member  61  through the main circuit  11  and the first forced ground-contacting unit  6 . Hence, the AC voltage measured at the connection point C, that is, the measured voltage value Vm, will be divided depending on the resistor  33  and the impedance of the same circuit from the capacitor  31  to the conductive member  61  through the main circuit  11  and the first forced ground-contacting unit  6 . Therefore, the measured voltage value Vm will be lowered as compared to in the absence of contact welding in the main circuit relays  2 . 
     In step S 5 , the welding determination unit  5  determines the presence or absence of contact welding in the main contact pairs on the basis of such differences in measured voltage value Vm. That is, if the measured voltage value Vm is equal to or greater than a threshold that is predetermined taking account of the generated voltage value Vg (see  FIGS. 3 and 5 ), then it is determined in step S 6  that no contact pairs in the main circuit relays  2  are welded. If the measured voltage value Vm is less than the predetermined threshold, then it is determined in step S 7  that at least one of the main circuit relays  2  has suffered welded contacts. 
     It should be noted that the operation flows of the contact welding detection systems  100  to  500  are substantially similar to the operation flow of the contact welding detection system  600 . 
     The contact welding detection system  600  of the present embodiment can provide similar advantages as in the contact welding detection systems  500  and  400 . 
     Seventh Embodiment 
     There will now be explained a contact welding detection system  700  in accordance with a seventh embodiment with reference to  FIGS. 16 and 17 . The contact welding detection system  700  includes a charging circuit  110  and second forced ground-contacting units  60   a  and  60   b , as well as the main circuit  11  and the first forced ground-contacting unit  6 . 
     The charging circuit  110  includes a charger  130  for charging the DC power source  12 , and two charging circuit relays  20  electrically connected between the DC power source  12  and the charger  130 . The second forced ground-contacting units  60   a  and  60   b  are electrically connected between the conductive member  61  and the charging circuit  110 . 
     The second forced ground-contacting units  60   a  and  60   b  respectively include second welding testing switches  620   a  and  620   b  for switching on and off the current flowing from the charging circuit  110  to the conductive member  61 . The second welding testing switches  620   a  and  620   b  are respectively electrically connected to connection points D 1  and D 2  on the charging circuit  110 , which points are located on the opposite side of the charging circuit relays  20  to the connection point A for the signal generation unit  3 . 
     The charging circuit  110  includes, as the charging circuit relays  20 , a positive charging relay  2   e  disposed in the positive charging line  140  and electrically connected between the positive terminal of the DC power source  12  and the charger  130 , and a negative charging relay  2   d  disposed in the negative charging line  150  and electrically connected between the negative terminal of the DC power source  12  and the charger  130 . In addition, the second forced ground-contacting unit  60   b  is electrically connected between the positive charging line  140  and the conductive member  61 , and the second forced ground-contacting unit  60   a  is electrically connected between the negative charging line  150  and the conductive member  61 . 
     The second forced ground-contacting units  60   a  and  60   b  respectively include capacitors  650   a  and  650   b  that are respectively electrically connected in series with the second welding testing switches  620   a  and  620   b . The capacitor  650   a  is electrically connected between the second welding testing switch  620   a  and the charging circuit  110 , and the capacitor  650   b  is electrically connected between the second welding testing switch  620   b  and the charging circuit  110 . 
     The charging circuit  110  is electrically connected to the DC power source  12  in parallel with the main circuit  11 . The charging circuit  110  is electrically connected to the signal generation unit  3  at the connection point A that is also the connection point A between the main circuit  11  and the signal generation unit  3 . 
     In  FIG. 16 , elements having similar functions as in the fifth embodiment are assigned the same numbers, except where specified otherwise. 
     The contact welding detection system  700  of the present embodiment is capable of detecting the presence or absence of contact welding not only in the main circuit relays  2  (the negative relay  2   a  and the positive relay  2   b ), but also in the charging circuit relays  2   d  and  2   e.    
     Testing for the presence or absence of contact welding in the charging circuit relays  20  may be followed by testing for the presence or absence of contact welding in the main circuit relays  2 . 
     Testing for the presence or absence of contact welding in the main circuit relays  2  (the negative relay  2   a  and the positive relay  2   b ) is conducted by controlling the energization of their shared electromagnetic coil  710   a  so that both the negative relay  2   a  and the positive relay  2   b  are placed in the blocking state. Further, the power-supply unit  13  is placed in a current non-flow state by controlling the switching elements therein. Still further, the two second welding testing switches  620   a  and  620   b  are placed in the blocking state, and the charger  130  is also placed in a current non-flow state. 
     In this state, the contact welding detection system  700  tests for the presence or absence of contact welding in the main circuit relays  2  according to a flowchart that is similar to the flowchart (including steps S 2 -S 7 ) of the sixth embodiment as shown in  FIG. 15 . 
     Testing for the presence or absence of contact welding in the charging circuit relays  20  (the negative charging relay  2   d  and the positive charging relay  2   e ) is conducted by controlling the energization of their respective electromagnetic coils  710   b  so that both the negative charging relay  2   d  and the positive charging relay  2   e  are placed in the blocking state in step T 1  as shown in  FIG. 17 . In addition, the charging circuit  130  and the power-supply unit  13  are both placed in a current non-flow state. 
     In this state, in step T 2 , the second welding testing switch  620   a  electrically connected to the negative charging line  150  is placed in the passing state. Thereafter, in step T 3 , the AC signal in the form of the square wave of a predetermined amplitude (the generated voltage value Vg) is generated in the signal generation unit  3 . 
     The welding determination unit  5  then compares the measured voltage value Win that is measured in the measurement unit  4  with the predetermined threshold T, thereby determining the presence or absence of contact welding in the negative charging relay  2   d  in steps T 4 , T 5 , T 6 , T 7  in a similar manner as described in the sixth embodiment with reference to the flowchart of  FIG. 15 . 
     Similarly, testing for the presence or absence of contact welding in the positive charging relay  2   e  is conducted by controlling the energization of the electromagnetic coil  710   b  so that both the negative charging relay  2   d  and the positive charging relay  2   e  are placed in the blocking state in step T 1 . In this state, in step T 12 , the second welding testing switch  620   b  electrically connected to the positive charging line  140  is placed in the passing state. Thereafter, in step T 13 , the AC signal in the form of the square wave of a predetermined amplitude (the generated voltage value Vg) is generated in the signal generation unit  3 . The welding determination unit  5  then compares the measured voltage value Vm that is measured in the measurement unit  4  with the predetermined threshold T, thereby determining the presence or absence of contact welding in the positive charging relay  2   e  in steps T 14 , T 15 , T 16 , T 17 . 
     In the present embodiment, as above, testing for the presence or absence of contact welding in the negative charging relay  2   d  is followed by testing for the presence or absence of contact welding in the positive charging relay  2   e . Alternatively, testing for the presence or absence of contact welding in the negative charging relay  2   d  may be preceded by testing for the presence or absence of contact welding in the positive charging relay  2   e.    
     As above, the contact welding detection system  700  is capable of detecting the presence or absence of contact welding not only in the main circuit relays  2  ( 2   a  and  2   b ), but also in the charging circuit relays  2   d  and  2   e . The contact welding detection system  700  may be applied to a plug-in hybrid vehicle or an electrical vehicle or the like. 
     Besides these, the contact welding detection system  700  of the present embodiment can provide similar advantages as in the contact welding detection systems  500  of the fifth embodiment. 
     Eighth Embodiment 
     There will now be explained a contact welding detection system  800  in accordance with a seventh embodiment with reference to  FIGS. 18 and 19 . As shown in  FIG. 18 , the contact welding detection system  800  is similar to the contact welding detection system  700  of the seventh embodiment, except that the two second forced ground-contacting units  60   a  and  60   b  share a second welding testing switch  620  that is electrically connected to the conductive member  61 . 
     The second forced ground-contacting unit  60   a  includes the capacitor  650   a  between the shared second welding testing switch  620  and the negative charging line  150 , and the second forced ground-contacting unit  60   b  includes the capacitor  650   b  between the shared second welding testing switch  620  and the positive charging line  140 . 
     That is, the wiring from the charging circuit  110  side terminal of the second welding testing switch  620  bifurcates to form two wirings. One of the two wirings is electrically connected to the positive charging line  140  through the capacitor  650   b , and the other one of the two wirings is electrically connected to the negative charging line  150  through the capacitor  650   a.    
     In  FIG. 18 , elements having similar functions as in the seventh embodiment are assigned the same numbers, except where specified otherwise. 
     Testing for the presence or absence of contact welding in the charging circuit relays  20  (the negative charging relay  2   d  and the positive charging relay  2   e ) is conducted by controlling the energization of the shared electromagnetic coil  710   b  so that both the negative charging relay  2   d  and the positive charging relay  2   e  are placed in the blocking state in step T 21  as shown in  FIG. 19 . This step T 21  is similar to step T 1  in the seventh embodiment. 
     In this state, in step T 22 , the shared second welding testing switch  620  is placed in the passing state. Thereafter, in step T 23 , the AC signal in the form of the square wave of a predetermined amplitude (the generated voltage value Vg) is generated in the signal generation unit  3 . 
     The welding determination unit  5  then compares the measured voltage value Vm that is measured in the measurement unit  4  with the predetermined threshold T, thereby determining the presence or absence of contact welding in at least one of the negative charging relay  2   d  and the positive charging relay  2   e  in steps T 24 , T 25 , T 26 , T 27  in a similar manner as described in the sixth embodiment with reference to the flowchart of  FIG. 15 . 
     In the present embodiment, it can be determined the presence or absence of contact welding in at least one of the negative charging relay  2   d  and the positive charging relay  2   e.    
     In addition, only one second welding testing switch  620  is required for providing the contact welding detection system  800 , which leads to simplification of the system with respect to the second forced ground-contacting units, and thus to an intended low-cost system. 
     Besides these, the contact welding detection system  800  of the present embodiment can provide similar advantages as in the contact welding detection systems  1  of the first embodiment. 
     Ninth Embodiment 
     There will now be explained a contact welding detection system  900  in accordance with a ninth embodiment with reference to  FIGS. 20-26 . As shown in  FIG. 20 , only one second forced ground-contacting unit  60  is electrically connected only to the negative charging line  150  that is one of the positive charging line  140  and the negative charging line  150 . In the present embodiment, the second forced ground-contacting unit  60  is electrically connected between the negative charging line  150  and the conductive member  61 . 
     The contact welding detection system  900  further includes a voltage sensor  160  electrically connected between the positive charging line  140  and the negative charging line  150 . The voltage sensor  160  is electrically connected to the negative charging line  150  at a connection point that is located between the charger  130  and a connection point D 1  at which the second forced ground-contacting unit  60  is electrically connected to the negative charging line  150 . 
     In the present embodiment, two charging circuit relays  20  (the negative charging relay  2   d  and the positive charging relay  2   e ) share two electromagnetic coils  71   d  and  71   e  that form a solenoid arrangement  70 . The solenoid arrangement  70  includes two contact pairs (first and second contact pairs  21   d  and  21   e ) as in the solenoid arrangement  7  of the third embodiment (see  FIG. 11 ). As a difference from the solenoid arrangement  7  of the third embodiment, the solenoid arrangement  70  of the present embodiment is able to keep only the first contact pair  21   d  in the passing state, which will be described later. Conversely, the solenoid arrangement  70  of the present embodiment is unable to keep only the second contact pair  21   e  in the passing state. In the solenoid arrangement  70 , the first contact pair  21   d  forms the negative charging relay  2   d  and the second contact pair  21   e  forms the positive charging relay  2   e.    
     As shown in  FIG. 21 , the solenoid arrangement  70  includes the first electromagnetic coil  71   d  with relatively high magneto motive force and the second electromagnetic coil  71   e  with magneto motive force lower than the first electromagnetic coil  71   d . The first electromagnetic coil  71   d  and the second electromagnetic coil  71   e  are coaxially-arranged and have a center core  721  shared by and disposed inside these coils  71   e  and  71   d . The solenoid arrangement  70  includes a mid core  726  between the first electromagnetic coil  71   d  and the second electromagnetic coil  71   e  as a portion of the stationary core  72 . The mid core  726  includes a magnetic saturation section to bring the magnetic flux originating from the first electromagnetic coil  71   d  to saturation. Besides these, the solenoid arrangement  70  is similar to the solenoid arrangement  7  of the third embodiment (see  FIG. 11 ). In  FIGS. 21-23 , elements having similar functions as in the third embodiment are assigned the same numbers, except where specified otherwise. 
     In the solenoid arrangement  70 , when the first electromagnetic coil  71   d  and the second electromagnetic coil  71   e  are both de-energized, the movable contact parts  23   d  and  23   e  are respectively urged away from the stationary contact parts  22   d  and  22   e  by the first plunger  73   d  and the second plunger  73   e . Both the first contact pair  21   d  and the second contact pair  21   e  are thereby placed in the blocking state. In this state, the first electromagnetic coil  71   d  and the second electromagnetic coil  71   e  are both energized, where current conduction directions for the first electromagnetic coil  71   d  and the second electromagnetic coil  71   e  are opposite each other as shown in  FIG. 22 . 
     The energization of the first electromagnetic coil  71   d  will lead to formation of a magnetic flux φ along a closed flux path through the first plunger  73   d . The first plunger  73   d  will thereby be attracted to the center core  721 , which allows the first contact pair  21   d  to be placed in the passing state. 
     Without energization of the second electromagnetic coil  71   e , saturation of the magnetic flux φ caused by the energization of the first electromagnetic coil  71   d  at the mid core  726  would lead to formation of a magnetic flux along a closed flux path through the second plunger  73   e.    
     In the present embodiment, as above, the second electromagnetic coil  71   e  is energized with its current conduction direction opposite the current conduction direction of the first electromagnetic coil  71   d . The magnetic flux φ caused by the energization of the first electromagnetic coil  71   d  will thereby be compensated for by the magnetic flux caused by the energization of the second electromagnetic coil  71   e  along the closed flux path through the second plunger  73   e . This will prevent formation of the magnetic flux along the closed path through the second plunger  73   e . Hence, the second plunger  73   e  will not be attracted to the center core  721 , and the second contact pair  21   e  will be kept in the blocking state. That is, the first contact pair  21   d  in the negative charging relay  2   d  is placed in the passing state with the second contact pair  21   e  kept in the blocking state (see  FIG. 26 ). 
     While only the first plunger  73   d  is attracted to the center core  721 , the second electromagnetic coil  71   e  is de-energized with the first electromagnetic coil  71   d  kept in the energized state as shown in  FIG. 23 . A magnetic flux φ from the first electromagnetic coil  71   d  will be formed also along a closed flux path through the second plunger  73   e , which allows the second plunger  73   e  to also be attracted to the center core  721 . Both the negative charging relay  2   d  in the first contact pair  21   d  and the second contact pair  21   e  in the positive charging relay  2   e  are thereby allowed to be placed in the passing state. Besides these, the solenoid arrangement  70  is similar in configuration as in the seventh embodiment. In  FIGS. 20-26 , elements having similar functions as in the seventh embodiment are assigned the same numbers, except where specified otherwise. 
     In the present embodiment, testing for the presence or absence of contact welding in the charging circuit relays  20  (the negative charging relay  2   d  and the positive charging relay  2   e ) is conducted by controlling the energization of the shared electromagnetic coils  71   d  and  71   e  so that both the negative charging relay  2   d  and the positive charging relay  2   e  are placed in the blocking state in step T 31  as shown in a flowchart of  FIG. 24 . More specifically, in step T 31 , the first electromagnetic coil  71   d  and the second electromagnetic coil  71   e  are both de-energized, and both the negative charging relay  2   d  and the positive charging relay  2   e  are thereby placed in the blocking state as shown in  FIG. 21 . 
     Subsequently, as shown in  FIG. 25 , the second welding testing switch  620  is placed in the passing state in step T 32 , and then in step T 33 , the AC signal in the form of the square wave of a predetermined amplitude (the generated voltage value Vg) is generated in the signal generation unit  3 . 
     The welding determination unit  5  then compares the measured voltage value Vm that is measured in the measurement unit  4  with the predetermined threshold T, thereby determining the presence or absence of contact welding in the negative charging relay  2   d  in steps T 34 , T 35 , T 36 , T 37  in a similar manner as described in the sixth embodiment with reference to the flowchart of  FIG. 15 . In  FIG. 25 , a current path indicated by the dashed arrow P 6  is a current path through which the AC signal passes in the presence of contact welding in the negative charging relay  2   d.    
     In addition, while the first electromagnetic coil  71   d  and the second electromagnetic coil  71   e  are both de-energized in the solenoid arrangement  70 , the first electromagnetic coil  71   d  and the second electromagnetic coil  71   e  are both energized with their current conduction directions opposite each other as shown in  FIG. 22 . The solenoid arrangement  70  is thereby controlled such that only the first contact pair  21   d  in the negative charging relay  2   d  is placed in the passing state as shown in  FIGS. 22 and 26  in step T 38 . 
     In this state, the voltage value Vn measured by the voltage sensor  160  is compared with a threshold V 0  that is set taking into account a voltage value Vf of the DC power source  12  in step T 39 . For example, the threshold V 0  may be set slightly less than the voltage value Vf of the DC power source  12  such that the voltage value Vn may be above or below the threshold V 0  in response to the presence or absence of contact welding in the positive charging relay  2   e.    
     The welding determination unit  5  compares the voltage value Vn with the threshold V 0 , and when the voltage value Vn becomes less than the threshold V 0 , determines that contact welding is present in the positive charging relay  2   e . When the voltage value Vn is comparable to the threshold V 0 , the welding determination unit  5  determines that contact welding is absent in the positive charging relay  2   e . In  FIG. 26 , a current path indicated by the dashed arrow P 7  is a current path through which the AC signal passes in the presence of contact welding in the positive charging relay  2   e.    
     With the contact welding detection system  900  of the present embodiment, a power consumption of the solenoid arrangement  70  can be reduced. Besides these, the contact welding detection system  900  can provide similar advantages as in the seventh embodiment. 
     Tenth Embodiment 
     There will now be explained a contact welding detection system  1000  in accordance with a tenth embodiment with reference to  FIG. 27 . As shown in  FIG. 27 , the first forced ground-contacting unit  6  and the second forced ground-contacting unit  60  share a single testing switch  66  as the first welding testing switch  62  and the second welding testing switch  620 , respectively. One terminal of the shared testing switch  66  is electrically connected to the conductive member  61  through the resistor  63 . Another terminal of the shared testing switch  66  is branched into three wirings. One of the three wirings is electrically connected to the main circuit  11  through the capacitor  65 . The other two wirings are respectively electrically connected to the positive charging line  140  and the negative charging line  150  of the charging circuit  110  through their respective capacitors  650 . 
     Besides these, the contact welding detection system  1000  is similar in configuration as in the seventh embodiment. In  FIG. 27 , elements having similar functions as in the seventh embodiment are assigned the same numbers, except where specified otherwise. 
     In the present embodiment, testing for the presence or absence of contact welding in the main circuit relays  2  and testing for the presence or absence of contact welding in charging circuit relays  20  are to be conducted by using the single shared testing switch  66 . First, the two main circuit relays  2  and the two charging circuit relays  20  are all placed in the blocking state by controlling energization of their respective electromagnetic coils. Thereafter, the AC signal in the form of the square wave of a predetermined amplitude is generated in the signal generation unit  3 . When the voltage value measured in the measurement unit  4  is less than the threshold, it is determined that contact welding is present in at least one of the two main circuit relays  2  and the two charging circuit relays  20 . 
     The present embodiment is advantageous, particularly when the two main circuit relays  2  and the two charging circuit relays  20  are constructed as a single relay unit  200 . In the present embodiment, the number of components can reduced, which may lead to a simplified contact welding detection system. Besides these, the contact welding detection system  1000  is similar in configuration as in the seventh embodiment. 
     Eleventh Embodiment 
       FIG. 28  shows a contact welding detection system  1100  in accordance with an eleventh embodiment of the present invention, which is similar in configuration to the contact welding detection system  200  of the second embodiment (see  FIGS. 6A and 6B ) except that the first forced ground-contacting unit  6  further includes a capacitor  67  electrically connected in series with the first welding testing switch  62 . One terminal of the capacitor  67  is electrically connected to the positive wiring  14  at the connection point B′, and another terminal of the capacitor  67  is electrically connected to the first welding testing switch  62 , as shown in  FIG. 28 . 
     In  FIG. 28 , elements having similar functions as in the second embodiment are assigned the same numbers, except where specified otherwise. 
     As in the second embodiment, in the presence of welding of the contact pair  21  in the main circuit relay  2 , the AC signal generated in the signal generation unit  3  will pass to the conductive member  61  through the connection point C, the connection point A, the DC power source  12 , the main circuit relay  2 , the connection point B′, and the first forced ground-contacting unit  6  in this order along a current path P 2 ′ as shown in  FIG. 29 . Also in the present embodiment, the voltage value measured in the measurement unit  4  will change in response to the presence or absence of welding of the contact pair  21  in the main circuit relay  2 . This allows the welding determination unit  5  to determine the presence or absence of welding of the contact pair  21  in the main circuit relay  2  on the basis of the measured voltage value. The same advantages as those indicated above for the second embodiment also apply to the second embodiment. 
     Twelfth Embodiment 
       FIG. 30  shows a contact welding detection system  1200  in accordance with a twelfth embodiment of the present invention, which is similar in configuration to the contact welding detection system  1  of the first embodiment except that the first forced ground-contacting unit  6  is electrically connected between the power-supply unit  13  and the conductive member  61  and the first forced ground-contacting unit  6  further includes a capacitor  68  electrically connected in series with the first welding testing switch  62 . Further, as in the third embodiment, the main circuit  11  includes two main circuit relays  2 : a positive relay  2   b  provided in the positive wiring  14  and a negative relay  2   a  provided in the negative wiring  15 . 
     In  FIG. 30 , elements having similar functions as in the first and third embodiments are assigned the same numbers, except where specified otherwise. 
     In the presence of contact welding in at least one of the two main circuit relays  2 , the AC signal generated in the signal generation unit  3  will pass to the conductive member  61  through the power-supply unit  13  and the first forced ground-contacting unit  6 . The same advantages as those indicated above for the first embodiment also apply to the present embodiment. 
     Thirteenth Embodiment 
       FIG. 31  shows a contact welding detection system  1300  in accordance with a thirteenth embodiment of the present invention, which is similar in configuration to the contact welding detection system  1200  of the twelfth embodiment except that the first forced ground-contacting unit  6  consists of the capacitor  67 . 
     In  FIG. 31 , elements having similar functions as in the first and third embodiments are assigned the same numbers, except where specified otherwise. 
     In the presence of contact welding in at least one of the two main circuit relays  2 , the AC signal generated in the signal generation unit  3  will pass to the conductive member  61  through the first forced ground-contacting unit  6 , that is, through the capacitor  67 . The same advantages as those indicated above for the first embodiment also apply to the present embodiment. 
     It should be noted that such a configuration is available for the hybrid vehicles and the electrical vehicles as they have a floating capacitance between the power-supply unit  13  (e.g., an inverter or a DC-DC converter) and the conductive member  61  (e.g., a vehicle-body ground). Use of such a floating capacitance allows for testing for the presence or absence of relay contact welding without adding the capacitor  67  as in the twelfth embodiment. In the present embodiment, the floating capacitance can provide the first forced ground-contacting unit  6 , which may lead to reduced costs. 
     In the first to thirteenth embodiments, the first forced ground-contacting unit  6  and the second forced ground-contacting unit  60  are provided with the resistors  63  and  630 . Alternatively, these resistors may be removed. Even without such resistors, testing for the presence or absence of contact welding may be conducted. 
     In the first to thirteenth embodiments, the measurement unit  4  is configured to measure the voltage value at the connection point C along the signal wiring  30 . Alternatively, the measurement unit  4  may be configured to measure a current value. 
     In the first to thirteenth embodiments, the measurement unit  4  is configured to directly measure the electrical characteristic value on the signal wiring  30 . Alternatively, the measurement unit  4  may be configured to indirectly measure the electrical characteristic value on the signal wiring  30  via an electronic component such as a resistor or the like. Contact welding detection systems configured such that the electrical characteristic value changes in response to the presence or absence of contact welding in the main circuit relays  2  and others can provide similar advantages as in the first to tenth embodiments. 
     In the first to thirteenth embodiments, the first forced ground-contacting unit  6  and the second forced ground-contacting unit  60  are provided with the capacitors  65  and  650 . Alternatively, these capacitors may be equivalent capacitors having a floating capacitance between the main circuit  1  and the conductive member  61 . 
     Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.