Abstract:
A tester for verifying the integrity of insulation in a branch circuit of a power distribution system. Two test circuits are included; the first providing an insulation test; and a second, a shared/grounded neutral test. In the insulation test, a 500-volt ac output limited to 5 milliamps is selectively applied to the pairs of wires of the branch circuit. If an output current of greater than 3 milliamps is recorded, an insulation failure is noted and the operator proceeds to the second test which applies a pulsed 3 volt, 1 ampere current-limited voltage across the suspected leads. One of the suspected leads is monitored with a portable ammeter to detect any pulse current.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to testing of electrical distribution circuits, and particularly to a tester that will verify the existence of wiring problems such as an insulation failure or grounded/shared neutrals. 
     2. Related Art 
     The common type of circuit breaker used for residential, commercial and light industrial applications has an electromechanical thermal magnetic trip device to provide an instantaneous trip in response to a short circuit and a delayed trip in response to persistent overcurrent conditions. Some circuit breakers of this type include ground fault protection, which trips the circuit breaker in response to a line-to-ground fault and, in some cases, a neutral-to-ground fault. Ground fault protection is provided by an electronic circuit which is set to trip at about 4 to 6 milliamps of ground fault current for people protection, and at about 30 milliamps for equipment protection. It is known to incorporate a test circuit in the circuit breaker, which tests at least portions of the electronic ground fault test circuit. It is also known to test for proper wiring connections. Test circuits for this purpose are commercially available. One such circuit is described in U.S. Pat. No. 6,072,317, assigned to the assignee of this application. 
     More recently, interest has arisen in providing protection against arc faults. Arc faults are intermittent, high impedance faults which can be caused, for instance, by worn insulation, loose connections, broken conductors and the like. Arc faults can occur in the permanent wiring, at a receptacle or, more likely, in the wiring of loads or extension cords plugged into the receptacle. Because of their intermittent and high impedance nature, they do not generate currents of sufficient instantaneous magnitude or sufficient average current to trigger the thermal-magnet trip device which provides the short circuit and overcurrent protection within a circuit breaker. 
     Arc fault detectors are generally of two types. One type responds to the random high frequency content of the current waveform generated by an arc. The other basic type of arc fault detector responds to the step increase in current occurring as the arc is repetitively and randomly struck. Examples of arc fault detectors of the latter type are disclosed in U.S. Pat. Nos. 5,224,006 and 5,691,869. Built in test circuits have also been proposed for such arc fault detectors. U.S. Pat. No. 5,459,630 discloses several forms of built in test circuits for such arc fault detectors. In one embodiment, in which the arc fault detector utilizes a coil to sense current, the test circuit adds a capacitor which forms, with the impedance of the coil, an oscillator generating wave form with an amplitude which simulates the rapid rise of a step change in current produced by an arc. In another embodiment, the user must repetitively close a switch, which connects a resistor between the line conductor and neutral to again generate large amplitude pulses. 
     While arc fault and ground fault circuit breakers will trip on ground or arcing fault conditions, they do not necessarily indicate where the fault is in a real installation. One difficulty is that the circuit breaker containing the detectors is located at a load center together with the circuit breakers for other circuits in the installation. However, the fault condition can occur anywhere downstream. Also, there may be some loads that cause nuisance tripping when a fault current does not exist. 
     There is a need, therefore, for improved test circuits for electrical distribution systems that can verify the integrity of branch wiring. 
     There is also a need for verifying the proper operation of an arc fault or ground fault circuit that has responded to a fault current condition, especially faults which are remote from the detectors, to assure the problem is within the circuit and not within the detector. Additionally, there is a further need for such a testing circuit that will assist in identifying the location of the fault. Furthermore, there is need for such testers which are flexible, simple and economical. 
     SUMMARY OF THE INVENTION 
     These needs and others are satisfied by this invention, which is directed to a tester that has two modes of operation, an insulation failure test mode and a shared/grounded neutral test mode. In the insulation failure test mode, a relatively large AC voltage source supplying a relatively small current, in the order of approximately 3 to 5 milliamps, is selectively applied to a pair of wires in the circuit under test. The magnitude of the voltage source that is applied is substantially greater than the line voltage normally applied to the circuit, but less than the voltage rating of the wiring insulation. A current meter monitors the leakage current flowing in the test circuit. If the leakage current is greater than approximately 3 milliamps, then an insulation failure exists. 
     In the shared/grounded neutral test mode, a pulsed low voltage source is applied across the ground and neutral conductors of the circuit under test. A portable ammeter then monitors any current flowing through either the neutral or ground conductors, starting at a location relatively near the voltage source, to detect the flow of a pulse current. If a pulse current is detected, the portable ammeter is moved along the conductor in a direction away from the source. The point at which the pulse current vanishes will identify the location of the fault. Both tests are conducted with the main power to the circuit and any load disconnected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawing in which: 
     FIG. 1 is a circuit diagram of a preferred embodiment of the tester of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The tester  10  of this invention illustrated in FIG. 1 is intended to be applied to a branch circuit of an electrical power distribution system, having a line conductor  86 , a neutral conductor  88  and ground conductor  90 . Typically, the branch circuit is protected by a circuit breaker mounted in a load center that provides overcurrent protection. The load center normally houses additional circuit breakers providing protection for additional branch circuits. The neutral conductor  88  and the ground conductor  90  are connected to earth-ground on the utility side of the load center. Typically, the load center is located in a basement or utility room and the branch circuit extends through the walls of the structure to provide electric power to a portion of the structure. The tester of this invention is intended to verify the integrity of the insulation electrically isolating the conductors  86 ,  88  and  90  from each other, so that degraded insulation can be identified. 
     The tester of this invention is referred to generally by reference character  10 . The test circuit  10  is connected to the line  20 , neutral  22  and ground  24  terminals of the power source  18 . The line branch  20  of the tester  10  is connected serially through an on/off switch  16 . A light emitting diode is connected between the line  20  and neutral conductor  22  just downstream of the on/off switch  16  to provide an indication when the tester is switched to the “on” position by closing the switch  16 . Preferably, the light emitting diode  26 , which serves this purpose, is green to conform to normal conventions. The load side of the tester  10  is connected through line, neutral and ground leads, respectively,  56 ,  58  and  60 , which preferably are correspondingly color-coded black, white and green. The line neutral and ground leads respectfully terminate in terminals  62 ,  64  and  66  which are connected to the corresponding line, neutral and ground conductors  86 ,  88  and  90  of the branch circuit. 
     The tester  10  basically comprises two separate testing circuits  12  and  14  to respectively provide an insulation test and a shared/grounded neutral test. Mode switch  28  connects the line wire  20  to either the insulation test circuit at terminal  30  or the shared/grounded neutral test circuit at terminal  32 . When the mode switch  28  is connected to terminal  30 , the primary winding  38  of the step-up transformer  36  is connected between the line and neutral connectors  20  and  22  respectively through the test button switch  34 , which is in a normally open position. A light emitting diode  68  is connected in parallel with the primary winding  38  on the neutral side of the test switch  34  so that when the test switch is closed, the light emitting diode  68  turns on and indicates that the testing circuit  12  is energized. Preferably, the light emitting diode  68  is a color different than the light emitting diode  26 , for example, red. The primary/secondary winding ratio of the step-up transformer  36  is chosen to desirably provide a substantially higher voltage on the secondary side  40  that is below the voltage rating of the branch circuit wiring  86 ,  88  and  90 . Home wiring is normally rated for 600 volts ac. In this preferred embodiment, for a normal 120-volt household source  18 , the secondary is designed to provide an output voltage of approximately 500 volts. The output of the secondary side  40  of the step-up transformer  36  is connected through a series arrangement of a 0 to 5 milliampere ammeter  54 , 10K ohm resister  44  and two serially connected 0.047 microfared capacitors  46  and  48  to output switches  50  and  52 . The output switch  50  can be moved from an open position to connect to either the line output test lead  56  or the neutral output test lead  58 . Similarly, the output switch  52  can be moved from its open position to either connect the neutral output test lead  58  or the grounded output test lead  60 . The branch circuit conductors  86 ,  88  and  90  can be respectively connected to the output test lead terminals  62 ,  64  and  90 . Thus, by movement of the switches  50  and  52 , the test circuit  12  can be positioned to place the output of the secondary side  40  of the step-up transformer  36  across the line-to-neutral, line-to-ground and neutral-to-ground connections to the branch circuit conductors. Preferably, the line output test lead  56 , the neutral output test lead  58  and the ground output test lead  60  are color coded, respectively black, white and green for ease of connection to the correspondingly colored conductors of the branch circuit. 
     Desirably, the output current of the secondary side  40  of the step-up transformer  36  is current limited to approximately 5 milliamps by the series arrangement of the resister  44  with the capacitors  46  and  48 , so that damage is avoided if there is a short in the branch circuit. 250 feet of household romex  2  wire with a ground will have a capacitive current at 500 volts of less than 1 milliampere. If the leakage current between the branch circuit conductors is greater than 3 milliamperes, then an insulation failure exists. Therefore, the meter scale of the ammeter  54  need be only in the range of 0 to approximately 5 milliamps. To perform the test, the branch circuit has to be isolated from the utility side of the load center. Preferably, this is done by removing the corresponding circuit breaker so that any abnormality in the breaker circuit does not influence the test. Removal of the breaker also provides ready access to the branch circuit conductors  86 ,  88  and  90 . To perform the insulation test after the tester is connected to the source  18  through terminals  20 ,  22  and  24  and to the branch circuit conductors through terminal  62 ,  64  and  66 , the switches  50  and  52  are properly positioned to place the output of the secondary side  40  of the step-up transformer  36  across line  86  and neutral  88  in the first test; line  86  and ground  90  in the second test; and neutral  88  and ground  90  in the third test. All positions should have a current reading less than 1 milliampere. A current reading of greater than 3 milliamperes is indicative of an insulation failure. When all three insulation tests are complete, the mode switch  28  is switched from the insulation test position  30  to terminal  32  to connect the shared/grounded neutral test circuit  14 . 
     The test circuit  14  includes a pulse generator  74  which is connected between the line  20  and neutral  22  inputs of the source  18  by way of the mode switch  28  and on/off switch  16 . The pulse generator  74  is in turn connected in parallel with the primary winding  78  of a step-down transformer  76 . The secondary winding  80  of the step-down transformer  76  is connected across the ground output lead  60  and the neutral output lead  58  through a normally closed relay contacts  72  and 3 ohm resister  82 . The winding ratio of the primary  78  to secondary  80  windings of the step-down transformer  76  is designed such that a substantially lower output voltage is applied across the neutral and ground output test leads  58  and  60  than is applied across the pulse circuit  72  and primary winding  78 . For example, with an input voltage of 120 volts, the output of the step-down transformer  76  at its secondary winding  80  would be approximately 3 volts. A fuse link  84  is connected in series with the secondary winding circuit for safety purposes in case a 120 volt is erroneously placed across the neutral and ground conductors  88  and  90 . Thus, with the on/off switch  16  in the closed position and the mode switch  28  connected to terminal  32 , a 3 volt, line frequency signal is pulsed approximately 1 second on and 1 second off, with a maximum voltage level of approximately 3 volts and current level of 1 ampere is applied across the neutral and ground output test leads  58  and  60 . As an added precaution, the activation circuit of relay  72  is connected between the neutral wire  22  and the line  20  side of the test button  34  so when the mode switch  28  is moved to connect with terminal  30 , a relay  72  is opened to open circuit the secondary side  80  of the step-down transformer  76 . 
     When the white and green output test leads  58  and  60  are connected to the secondary side of the step-down transformer  76  with the mode switch  28  in a shared/grounded neutral test position, using a clipon portable ammeter connected to the white lead, the operator conducting the test then looks for an ammeter reading showing a corresponding pulsing current indicating a shared/grounded neutral fault. The operator can then move the clipon ammeter take-up coil down the corresponding branch circuit conductor in a direction away from the test circuit until the pulse current is no longer detected. The position at which the pulse current is no longer detected should be adjacent to the shared or grounded neutral fault. 
     In a new installation, each branch circuit should be tested before a breaker for the tested branch is inserted. Only after a successful test result is achieved should the breaker be inserted. In an old installation, if an arc fault breaker trips, it is recommended that the breaker be removed from the circuit before the test is conducted in that branch circuit. First measure the voltage on the line, neutral and ground conductors with reference to ground. If all voltages are less than 3 volts, then proceed with the testing steps set forth above for new installations. If any of the voltages are more than 3 volts, a circuit to circuit fault is present that has to be corrected before the test can proceed. The foregoing voltage measurement for old installations can be made using a simple Voltmeter Type 72-6173, manufactured by TENMA. 
     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. For example, while specific circuit values for a number of the components have been identified, other combination of values may be employed without departing from the scope of the overall teachings of this specification. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.