Abstract:
A system includes a hot wire and a neutral wire configured to establish a closed circuit between a power source and a load. The system further includes first and second transformers as well as a sensor. The first current transformer is coupled to the hot wire and is configured to introduce a first test current, with a first polarity, into the hot wire. The second current transformer is coupled to the neutral wire and configured to substantially simultaneously introduce a second test current into the neutral wire. The second test current has the same polarity as the first test current. The sensor is configured to sense an asymmetry between the first and second test currents and is further configured to cause interruption of the closed circuit upon sensing the asymmetry.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/233,690, filed Sep. 28, 2015, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure generally relates to ground fault circuits and, more particularly, to automating the testing of ground fault circuits. 
       BACKGROUND OF THE DISCLOSURE 
       [0003]    Ground fault circuit interrupters (GFCIs) are designed to protect individuals from electrical shock by interrupting the operation of circuit that is connected to a load. In a typical GFCI circuit configuration, a three-wire configuration is used to establish a circuit with the load including a hot wire that delivers a current to the load, a neutral wire that returns the current, and a ground wire. In standard operation, the current on the hot wire will match the current on the neutral wire. However, a difference in current between the two wires indicates that an abnormal diversion of current from the hot wire is occurring. That diverted current has a return path to ground rather than through the neutral, i.e., a ground fault has occurred. Ground faults are an indication of undesired, faulty and/or unsafe use of equipment, as properly designed equipment will always use the neutral as a return path. Thus, upon the occurrence of a ground fault, the GFCI in the circuit will break the circuit to prevent shock to someone coming in contact with a hot wire. Ground fault is generally below 5 mA for human protection, but heat tracing operates on the same principle and used for equipment protection at 30 mA. 
         [0004]    Important in the maintenance of GFCIs is the periodic testing of the GFCI; the NFPA (National Fire Protection Association) prescribes monthly testing. In a residential setting, a GFCI may be easily tested by activating the “test” switch on a GFCI outlet then returned to normal operation by activating the “reset” switch. However, in industrial situations, most particularly in Class I, Division 1 or 2 environments, where GFCIs are provided for improved safety, periodic testing of the GFCI is not as easily performed. Often the GFCI is housed within an explosion proof enclosure that does not provide easy access to its contents. Alternatively, explosion proof ground fault control stations may be provided to enable testing of the GFCI, with each station adding cost to the electrical control system. 
         [0005]    By way of background, Class I locations are those in which flammable vapors and gases may be present. Class I, Division 1 locations are those in which ignitable concentrations of hazards exists under normal operation conditions and/or where hazard is caused by frequent maintenance or repair work or frequent equipment failure. Class I, Division 2 locations are those in which ignitable concentrations of hazards are handled, processed or used, but which are normally in closed containers or closed systems from which they can only escape through accidental rupture or breakdown of such containers or systems. 
         [0006]    For arc-generating electrical equipment to be allowed in a Class I, Division 1 or Division 2 location, it must be enclosed in an explosion proof housing. An explosion proof apparatus may be defined as an apparatus enclosed in a case that is capable of withstanding an explosion of a specified gas or a vapor that may occur within it and of preventing the ignition of a specified gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within, and that operates at such an external temperature that a surrounding flammable atmosphere will not be ignited thereby. Further, an explosion proof enclosure must be of adequate strength and be “flame-tight.” 
       SUMMARY 
       [0007]    A first aspect of the disclosure is directed to a system that includes a hot wire and a neutral wire configured to establish a closed circuit between a power source and a load. The system further includes first and second transformers as well as a sensor. The first current transformer is coupled to the hot wire and is configured to introduce a first test current, with a first polarity, into the hot wire. The second current transformer is coupled to the neutral wire and configured to substantially simultaneously introduce a second test current into the neutral wire. The second test current has the same polarity as the first test current. The sensor is configured to sense an asymmetry between the first and second test currents and is further configured to cause interruption of the closed circuit upon sensing the asymmetry. 
         [0008]    Another aspect of the disclosure is directed to a method. The method comprises: (1) introducing a first test current into a hot wire of a closed circuit, the hot wire coupled between a power source and a load; (2) introducing a second test current into a neutral wire of the closed circuit, the neutral wire coupled between the power source and the load; (3) sensing an asymmetry between the first and second test currents; and (4) opening the closed circuit in response to sensing the asymmetry between the first and second test currents. The second test current is introduced at substantially the same time as the first test current and has the same polarity as the first test current. 
         [0009]    Yet another aspect of the disclosure is directed to a system having a hot wire and a neutral that are configured to establish a closed circuit between a power source and a load. The system additionally includes a ground fault interrupt circuit as well as first and second current transformers. The ground fault interrupt circuit includes a sensor that is configured to continuously monitor the currents flowing in the hot wire and the neutral wire. The ground fault interrupt circuit is configured to open the closed circuit if the current in the hot and neutral wire are not substantially equal in magnitude. The first current transformer is coupled to the hot wire and is configured to introduce a first test current, having a first polarity, into the hot wire. The second current transformer is coupled to the neutral wire and is configured to, substantially simultaneously, introduce a second test current, having the same polarity as the first test current, into the neutral wire. The sensor of the ground fault interrupt circuit is additionally configured to detect an asymmetry in the waveforms of the first and second test currents. The ground fault interrupt circuit is additionally configured to open the closed circuit upon detection of the asymmetry. 
         [0010]    The above summary is not intended to describe each embodiment or every implementation. A more complete understanding will become apparent and appreciated by referring to the following detailed description and claims in conjunction with the accompanying drawings. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic of a standard GFCI (ground fault circuit interrupt) breaker. 
           [0012]      FIG. 2  is a front view of an explosion proof ground fault control station. 
           [0013]      FIG. 3  is a schematic of an automated ground fault interrupt tester according to various embodiments of the disclosure. 
           [0014]      FIG. 4  is a front view of panel board the may incorporate the automated ground fault interrupt tester according to various embodiments of the disclosure. 
       
    
    
       [0015]    The figures are not necessarily to scale. Like numbers used in the figures refer to like components. However, it will be understood that the use of a number to refer to a component in a given figure is not intended to limit the component in another figure labeled with the same number. 
       DETAILED DESCRIPTION 
       [0016]    The present disclosure is directed to a system and method for automating the testing of ground fault current interrupters (GFCIs). The system and method are particularly suited to industrial environments and, even more particularly so, to Class I, Division 1 or Division 2 environments, where GFCI test circuitry is maintained behind the door of an enclosure or within an accessible but costly ground fault control station. The system and method of the present disclosure may be implemented through the use of wireless controls that will be described in further detail below. 
         [0017]      FIG. 1  is a schematic of a standard GFCI circuit breaker  100  that one might find in an industrial environment. As shown, the GFCI circuit breaker  100  is coupled between a line voltage A/B and a load  102  that is wired using a standard three-wire configuration of a load power (“hot”) wire  104 , a neutral wire  106 , and groundwire  108  tied to ground  110 . In this example, a fourth wire, e.g., white wire  112 , is also provided to tie the neutral wire  106  to a panel neutral N, which is tied to ground  110 . The GFCI circuit breaker  100  includes a standard trip mechanism  114 , coupled to a trip solenoid  116 , to protect the load  102  from shorts and power surges. Further, within the GFCI circuit breaker  100  are the components that comprise the actual ground fault circuit interrupter. These components include a sensing coil  118 , coupled to electronic circuitry  120 , to monitor the currents passing through the neutral wire  106  and the “hot” wire  104 . 
         [0018]    The neutral wire  106  and the “hot” wire  104  are passed through the sensing coil  118  so that the currents in the two wires at any instant are traveling in opposite directions, give a net zero current in the sensing coil  118  if the two currents are exactly equal. Since a current-carrying wire produces an AC magnetic field external to the wire, a non-zero current would induce a voltage in the sensing coil  118 . The voltage sensed in the coil  118  is typically provided to electronic circuitry  120  where the voltage is rectified and applied to sensitive comparator to detect differences. If a difference is detected, the electronic circuitry  120  operates to activate a trip coil (not shown) which opens both the “hot” wire  104  and the neutral wire  106 . 
         [0019]    A test switch  122  is provided to test the ground fault circuit interrupter. Upon activating the switch, the “hot” wire  104  is coupled to ground through a resistive load as indicated by the dashed line incorporating the resistor R L . The test should cause the ground fault circuit interrupter to trip. The ground fault circuit interrupter may then be reset for normal use. 
         [0020]      FIG. 2  illustrates an example of an explosion proof ground fault control station  200  configured for testing of dual GFCI circuit breakers. As illustrated, the enclosure itself is a substantial structure  202  incorporating two pilot lights  204 , two operating handles  206 , with positions Off/Trip/On, and two “Push to Test” buttons  208 . The cost of such a structure and related controls is not insignificant and can add up quickly in application where numerous GFCI circuit breakers are needed. 
         [0021]    With the above standard GFCI in mind, the present disclosure provides a scheme for automating GFCI testing and for eliminating costly ground fault control stations. More specifically, the present disclosure is directed to a GFCI breaker circuit  300 , as illustrated in  FIG. 3 . As shown, the GFCI circuit breaker  300  is similar to the GFCI circuit breaker  100  of  FIG. 1  in that the GFCI circuit breaker  300  is coupled between a line voltage A/B and a load  302  that is wired using a standard three-wire configuration of a load power (“hot”) wire  304 , a neutral wire  306 , and groundwire  308  tied to ground  310 . In this example, a fourth wire, e.g., white wire  312 , is also provided to tie the neutral wire  306  to a panel neutral N, which is tied to ground  310 . The GFCI circuit breaker  300  includes a standard trip mechanism  314 , coupled to a trip solenoid  316 , to protect the load  302  from shorts and power surges. Further, within the GFCI circuit breaker  300  are the components that comprise the actual ground fault circuit interrupter. These components include a sensing coil  318 , coupled to electronic circuitry  320 , to monitor the currents passing through the neutral wire  306  and the “hot” wire  304 . 
         [0022]    The neutral wire  306  and the “hot” wire  304  are passed through the sensing coil  318  so that the currents in the two wires at any instant are traveling in opposite directions, give a net zero current in the sensing coil  318  of the two currents are exactly equal. Since a current-carrying wire produces an AC magnetic field external to the wire, a non-zero current would induce a voltage in the sensing coil  318 . The voltage sensed in the coil  318  is typically provided to electronic circuitry  320  where the voltage is rectified and applied to sensitive comparator to detect differences. If a difference is detected, the electronic circuitry  320  operates to activate a trip coil (not shown) which opens both the “hot” wire  104  and the neutral wire  306 . 
         [0023]    However, unique to the GFCI circuit breaker  300  is that the test switch has been eliminated and replaced with electronics  326 . The electronics  326  may be incorporated into the electronics  320  or may be distinct from the electronics  320 . In one example embodiment, the electronics  326  are contained within a circuit board that is mounted inside a closed enclosure and that has been configured with at least a wireless transceiver, e.g. RF, Bluetooth, etc., and a processor. The transceiver is configured to receive a signal, for example from a wireless user interface  328 , indicating which GFCI circuit breaker is to be tested and to provide this information to the processor to activate the test. Notably, the test is no longer performed by coupling the “hot” wire to ground through a resistive load. Rather, current transformers  324   a ,  324   b  are coupled to the neutral wire  306  and the “hot” wire  304 , respectively. 
         [0024]    During a test of the ground fault interrupter, the processor directs that impulses of the same polarity be provided into the current transformers  324   a ,  324   b . The impulses to the current transformers  324   a ,  324   b  will cause currents, e.g., in the 6-10 mA range, on the neutral wire  306  and the “hot” wire  304  that are flowing in the same direction, as indicated by the arrows on  FIG. 3 . The two currents manifest themselves as a DC shift to the load (6-10 mA will not impact the load) but present a completely asymmetrical current waveform to the sensing coil  318  causing the ground fault interrupter to trip immediately. 
         [0025]      FIG. 4  provides an example of the type of enclosure  400  that in which the automated ground fault tester of the present disclosure may be utilized. Enclosure  400  is an explosion proof circuit breaker panel board that includes a plurality of GFCI breaker circuits, each of which requires periodic maintenance testing. 
         [0026]    The automated ground fault test system and method described herein enable the removal of pieces of equipment from the enclosure assembly, which reduces cost and labor in production of the enclosure. Additionally, by automating the GFCI testing, the exact current at which the GFCI trips can be identified. Further, by using the current transformer approach, there is no additional wire required beyond the current carrying conductors and the test circuit itself could be integrated into an existing current measurement board. 
         [0027]    Systems, devices or methods disclosed herein may include one or more of the features, structures, methods, or combination thereof described herein. For example, a device or method may be implemented to include one or more of the features and/or processes above. It is intended that such device or method need not include all of the features and/or processes described herein, but may be implemented to include selected features and/or processes that provide useful structures and/or functionality. 
         [0028]    Various modifications and additions can be made to the disclosed embodiments discussed above. Accordingly, the scope of the present disclosure should not be limited by the particular embodiments described above, but should be defined only by the claims set forth below and equivalents thereof.