Abstract:
A sensing configuration for a ground fault circuit interrupter for devices having an internal or floating ground electrically isolated from an external ground. The sensor includes a conductor connected to the internal or floating ground so that EMF and leakage current are detected, thus preventing the ground fault circuit interrupter from tripping due to signals caused only by EMF and leakage current in the load circuit.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to detecting and interrupting ground faults in circuits having a grounded load. Specifically, a ground fault circuit interrupter (GFCI) is provided that differentiates between ground faults and leakage and electromagnetic interference (EMI) induced currents generated in the load circuit and returned to the source through ground. 
     BACKGROUND OF THE INVENTION 
     GFCI devices are intended to detect and interrupt potentially harmful ground fault currents. GFCI devices work well for this purpose unless electromagnetic interference or voltage transient leakages also set off the GFCI device. In these cases, the interference or transients cause a current sensor to detect in the phase lines a current that reaches or exceeds the trip setting of the ground fault interrupter, causing it to falsely activate in the absence of an actual fault. 
     In certain applications (such as aircraft devices and appliances) a secondary ground wire provides a path to ground for EMI or transient leakage currents. This path reduces the risk that such transients may pose to aircraft passengers and crew. The threshold for such a GFCI circuit has to be set so high to avoid false trips that a person in direct contact with such equipment could receive a fatal electrical shock. Trip voltages over 8 Milliamps are considered fatal. Typical transient voltages found in IFE applications exceed 20 Milliamps. 
     Underwriters&#39; Laboratories has studied the ground fault phenomenon and has issued UL 943, Category A as the standard for GFCI devices that protect human life for 50/60 cycle AC power systems. According to this standard, the minimum level for a dangerous ground fault current is 6 milliamps. The ability to distinguish leakage currents from ground fault currents in these applications is paramount. Electrical systems where individual equipment is required to have an attachment to system ground (such as aircraft devices and appliances) are inherently more difficult to monitor. 
     The concept of a ground fault circuit interrupter using a current transformer is not new. Such a transformer detects the current differential between the line and neutral wires. This detector may be used in both single phase and three phase AC power systems. Any leakage current, ground fault current or the sum thereof is interpreted as a ground fault occurrence. 
     U.S. Pat. No. 5,793,587 (Boteler) discloses a dual trip level ground fault interrupter having an equipment current transformer and a personnel current transformer. The personnel transformer is more sensitive to fault currents than the equipment transformer. An equipment ground conductor passes through the personal transformer but not through the equipment transformer. While the differential current transformer responds to any leakage current not returning on the ground conductor, the electrical circuit for the load is ungrounded. The Boteler configuration is thus inapplicable for devices having a grounded load (such as aircraft devices and appliances) for preventing electrical shock. 
     Thus, there exists in the art the need for a ground fault circuit interrupter that is sensitive to ground faults but nonresponsive to leakage and electromagnetic interference currents in systems having a grounded load. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus and method for detecting a ground fault in a grounded load circuit having a floating ground and powered through at least one line conductor and a neutral conductor. The system comprises a first load terminal for receiving a conductor connected to the floating ground, a second load terminal for receiving the neutral conductor, and a set of third load terminals for receiving at least one line conductor. A first source terminal is provided for receiving a conductor connected to absolute ground, a second source terminal is provided for receiving a conductor connected to the neutral of the power source, and a set of third source terminals are provided for receiving a set of source conductors connected to each source conductor. 
     A current sensor senses the current in a set of conductors that connect, respectively, the first load terminal and the first source terminal, the second load terminal and second source terminal, and the third load terminals and set of third source terminals. 
     The present invention may be particularly useful as a resettable ground fault responsive interrupt circuit for a 400 Hz aircraft electrical system. Such a system may comprise an AC input from the aircraft electrical system and a floating ground conductor for connection to the floating ground of a device. A current imbalance sensor is connected to the AC input and the floating ground conductor for generating an imbalance output signal in response to a ground fault in the aircraft electrical system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a preferred ground fault circuit interrupter of the present invention shown with select devices connected thereto 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A ground fault circuit interrupter (GFCI)  10  constructed in accordance with the present invention is illustrated in FIG.  1 . The GFCI  10  comprises a current sensor  12  having a plurality of conductors  14 ( a )- 14 ( e ) passing therethrough. Each conductor is connected to one of a plurality of first terminals  16 ( a )- 16 ( e ) and a respective one of a plurality of second terminals  18 ( a )- 18 ( e ). The preferred GFCI  10  also includes a first ground terminal  20  and a second ground terminal  22 . The ground terminals  20  and  22  are connected by a conductor  24 . 
     The current sensor  12  of the preferred embodiment comprises a toroid having a core  26  and winding  28  The winding  28  has first and second terminals. The terminals are input to an amplifier circuit  30  The amplifier circuit  30  has an output conductor connected to a logic circuit  32 . The logic circuit  32  provides an output to a drive circuit  34 . Two output terminals of the drive circuit  34  are connected to GFCI output terminals  36 ( a ) and  36 ( b ). The GFCI also preferably includes a test circuit  38 , discussed below. 
     Output terminal  18 ( a ) is configured for connection to a conductor  40  directly connected to system ground. Output terminal  18 ( b ) is configured for connection to the neutral conductor of a single or three phase power source. Output terminal  18 ( e ) is configured for connection to the line conductor of a single phase power source and (optionally) output terminals  18 ( c )- 18 ( e ) are configured for connection to the line conductors of a three phase power source. 
     Input terminal  20  is configured for connection to a conductor  42  connected directly to the case of a protected device  44 . Input terminal  16 ( a ) is configured for connection to a conductor connected to ground of the load circuit, i.e., of the protected device. Input terminal  16 ( b ) is configured for connection to the neutral conductor of a single or three phase load circuit. Input terminal  16 ( e ) is configured for connection to the line conductor of a single phase load circuit and (optionally) input terminals  16 ( c )- 16 ( e ) are configured for connection to the line conductors of a three phase load circuit. As illustrated, the line conductors (for either single or three phase) include a relay circuit  46  operating under control of drive circuit  34 . In an alternate embodiment, the relay circuit  46  is integrated in the GFCI  10 . 
     In the present embodiment, the relay circuit  46  operates under control of drive circuit  34  and is connected thereto at output terminal  36 ( a ). Drive circuit  34  is also preferably in communication with an external circuit breaker  48 . The drive circuit  34  provides a signal for tripping the circuit breaker  48  when the drive circuit opens the line conductor(s) in relay  46 , thereby providing an indication of the operational status of relay  46 . Drive circuit  34  is also preferably configured for receiving a signal from the circuit breaker  48  when it is reset, signaling the drive circuit  34  to reset line relay  46 . 
     A second circuit breaker  50  is also preferably provided for protecting the system from voltage overload. 
     The GFCI of the present invention is specifically configured for a protected device  44  (of the prior art) having an internal (load circuit) ground  52  isolated from an external (system) ground  54  Typically, the internal ground  52  will comprise a floating ground of the load circuit and the external ground  54  will comprise the casing or chassis of the protected device. The external ground  54  is further connected to equipment ground, i.e., absolute ground of the system (such as the fuselage of an aircraft). It should be noted that in this design of the prior art, the internal ground is electrically isolated from the external ground to prevent load circuit leakage current from passing or flowing to ground. Such currents will thus not generate hazardous voltages at the casing or chassis. Because leakage currents are detected by the current sensor of the present invention, such currents will not trip the GFCI. 
     The GFCI of the present invention is thus configured for detecting only an insulation failure or a short circuit from the load circuit to external ground. Because transient leakages pass through the current sensor, the sensor may be set to detect currents at 6 milliamps or lower, thus satisfying the Underwriters Laboratories standard, while greatly reducing false ground fault trips. 
     As is well known in the art, in response to sensing a current imbalance through the toroid, the toroid generates a voltage differential between its output conductors. The output of the toroid is preferably provided to an amplifier circuit  30  for conditioning the voltage signal for input to a logic circuit  32 . The logic circuit  32  provides an output signal for controlling drive circuit  34 . 
     In the preferred embodiment, the drive circuit  34  provides an output signal for controlling a circuit breaker (which may be a remote control circuit breaker)  46 . If a ground fault condition is detected, the circuit breaker  46  is activated and opens the conductive path between the line conductors (single or three phase) of the electric load and the source. Preferably, the drive circuit  34  also generates a multiple purpose status output signal for indicating the status of the circuit breaker  46 . A status output signal may be implemented on computerized power systems or on systems that require built-in test equipment (BITE) features. The drive circuit  34  may also generate a signal for input to a circuit breaker  48 , the status of which may indicate the status of the circuit breaker  46 . The drive circuit  34  is also preferably configured to reset the line circuit breaker  46  by resetting circuit breaker  48  The reset switch  48  may be connected to a plurality of GFCI  10 , so that only one reset switch need be activated for resetting a plurality of relays. An additional circuit breaker  50  is provided for providing overload protection for the line conductor(s). 
     The GFCI of the present invention also preferably includes a test circuit  38 . Test circuit  38  includes a pushbutton switch for selectively establishing a conductive path between a line conductor and system ground, for simulating a short circuit condition in the system. 
     In an alternate embodiment, the drive circuit  34  communicates with the line relay  46 , and the circuit breaker  48  via wireless communication. In this embodiment, the terminals  36 ( a ) and  36 ( b ) may be replaced by infrared emitters/receivers (driven by drive circuit  34 ) for establishing communication channels to line relay  46  and circuit breaker  48 , also configured for wireless communication, in accord with methods well known in the art. 
     The present invention has been described in the context of a preferred and alternate embodiments, as well as a system into which the invention may be incorporated. It is apparent to those skilled in this art, however, that modifications and variations to the disclosed embodiments can be made without departing from the spirit and scope of the invention. Accordingly, this invention is to be construed and limited only by the scope of the appended claims