Abstract:
A device for providing ground fault protection for one or more loads in an electrical wiring system. The device includes a pickup for sensing electrical characteristics associated with conductors supplying power to the one or more loads and generating a pick up signal when predefined electrical characteristics are sensed; a ground fault detector powered using conductors that are independent of the conductors supplying power to the one or more loads, said ground fault detector being configured to receive said pickup signal, detect ground faults from said pickup signal, and generate a trigger signal when a ground fault is detected, and a power disconnect responsive to said trigger signal such that when said trigger signal is generated power to the one or more loads is disconnected. A corresponding method is also provided.

Description:
This application claims priority pursuant to 35 U.S.C. 119(e) from provisional application number 60/142,717, filed on Jul. 7, 1999. 
    
    
     BACKGROUND 
     1. Field 
     The present application relates to ground fault circuit interrupters that provide ground fault protection for dimmer controlled wiring systems. More particularly, the present application relates to ground fault circuit interrupters that provide ground fault protection for one or more loads in an electrical wiring system when a current waveform to the loads is non-sinusoidal. 
     2. Description of the Related Art 
     Electrical wiring systems are typically included in residential, commercial and industrial environments where electrical power is supplied to various components in the system. Generally, such electrical wiring systems include phase and neutral (or return) conductors, which when properly connected, supply electrical power to the various components, e.g., loads, in the system. 
     In such electrical wiring systems certain current safety codes require the installation of circuit protection devices that trip when certain electrical based faults occur. One example of such a circuit protection device is a ground fault circuit interrupter (GFCI) that is responsive to the detection of ground faults. 
     Conventional ground fault circuit interrupters typically use a sense transformer, such as a differential transformer, to sense a difference current in the phase and neutral conductors that pass through the transformer. The difference current is transferred to a secondary winding of the differential transformer. Typically, the current at the secondary winding, known as the secondary current, is proportional to the difference current. Conventional GFCI devices also use a ground/neutral transformer to detect ground to neutral faults. 
     Generally, when detecting ground faults from a difference current, a sense amplifier converts the secondary current to a voltage level. This voltage level is compared to two window detector reference voltages, and if one of the compared voltages exceeds a designed threshold for a predetermined period of time, a trigger signal, representing a difference current ground fault, is generated. Generally, when detecting ground to neutral faults, the ground/neutral and sense transformers are coupled through external resistors and capacitors and a neutral wire ground loop, to form a positive feedback loop around the sense amplifier. The feedback loop causes the sense amplifier to oscillate at a frequency determined by the inductance of the secondary winding of the ground/neutral transformer and a capacitor. Typically, oscillation occurs at 8 KHz. If the amplifier oscillates for predefined period of time, the trigger signal, representing a ground to neutral fault, is generated. 
     The trigger signal is as a control of a gate of an SCR. When the trigger signal is present, the SCR is turned on and a relay solenoid is energized to open contacts between a line side and a load side of the GFCI device to cut off power supplied to the load side of the device. 
     Current GFCI devices are required in certain branch circuits in electrical wiring systems, such as bathrooms, kitchens and certain outdoor environments. Such branch circuits are typically supplied with AC power that has a continuous sinusoidal waveform so that conventional GFCI devices, which are designed to sense RMS or average ground fault currents, are capable of providing ground fault protection. 
     However, in other environments, such as theaters, movie sets and other entertainment settings or stages, designers are asked to develop electrical wiring systems which are more theatrical in nature, in environments where ground fault protection is desired. One such theatrical feature is to dim certain lighting fixtures in wet environments. In such systems, phase controlled dimmer devices are used to limit current supplied to the loads. The current supplied to such loads is also identified herein as phase dimmed current. In such electrical wiring systems where the AC current supplied to certain loads is phased dimmed, the waveform of the AC current supplied to the loads may no longer be sinusoidal. In the event the phase dimmed current is non-sinusoidal, conventional GFCI devices may become desensitized, so that their use in such electrical wiring systems is not recommended. 
     SUMMARY 
     The present application provides a ground fault protected phase controlled dimmer system, and a GFCI device that can be used in such phase controlled dimmer systems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present application are described herein with reference to the drawings in which similar elements are given similar reference characters, wherein: 
     FIG. 1 is an exemplary system block diagram for a ground fault protected phase controlled dimmer system according to the present application; 
     FIG. 2 is a system wiring diagram for the ground fault protected phase controlled dimmer system according to the present application; 
     FIG. 3 is a schematic diagram of circuitry capable of detecting ground faults. 
     FIG. 4 is a perspective view of a GFCI device capable of providing ground fault protection in the phase controlled dimmer system; 
     FIG. 5 is a perspective view, with parts separated, of the GFCI device of FIG. 4; 
     FIG. 6 is a side elevational view of the GFCI device of FIG. 4 with a portion of the device housing removed to illustrate a mechanism for opening and closing at least one conductive path; which is in an open position, and 
     FIG. 7 is a side elevational view similar to FIG. 6, illustrating the mechanism for opening and closing the at least one conductive path in a closed position. 
    
    
     DETAILED DESCRIPTION 
     Generally, the present application describes exemplary embodiments of a ground fault protected phase controlled dimmer system and a ground fault circuit-interruption device capable of being used in such dimmer systems. As with conventional ground fault circuit interrupting devices, the faults detected from difference current and ground to neutral faults are both encompassed in the term ground fault. 
     The dimmer systems may be used in various environments where it is desirable to control the AC supplied to one or more loads with a dimming device that outputs a phase dimmed current that is non-sinusoidal. The various environments include, for example, theaters, movie sets and other entertainment settings or stages. An example of a ground fault protected dimmer system that can be used in such environments is shown in FIGS. 1 and 2. 
     In FIG. 1, the ground fault protected phase controlled dimmer system  10  includes a dimmer device  12 , a GFCI device  14  and one or more loads  16 . In the configuration of FIG. 1, the phase input of the dimmer device  12  is connected to the phase conductor  18  of the AC supply and the output of the dimmer device  12  is electrically connected to the line side connection  14   a  of the GFCI device  14  via conductor  20 . The conductor  20  between the dimmer device  12  and the GFCI device carries the phase dimmed current. A corresponding neutral conductor  22  from the AC supply is also connected to the line side connection  14   a  of the GFCI device  14 . Conductors  28  and  30  connect the load side connection  14   b  to the one or more loads  16 . 
     As described, the GFCI device  14  generally has a line side connection  14   a  with phase and neutral connections, such as binding terminals, wire leads or other known connections capable of connection to electrical conductors. The line side connection connects the GFCI device  14  to the dimmer device. The GFCI device has a load side connection  14   b  with phase and neutral connections, such as binding terminals, wire leads or other known connections capable of connection to electrical conductors. The load side connection connects the GFCI device to the one or more loads. The GFCI device  14  also includes a control connection  14   c  with phase and neutral connections, such as binding terminals, wire leads or other known connections capable of connection to electrical conductors. The control connection  14   c  connects the GFCI device to an AC supply that is independent of the AC supplied to the one or more loads. 
     Referring to FIG. 2, the GFCI device  14  also includes a conductive path  32  between the line and load phase connections, and a conductive path  34  between the line and load neutral connections. A power disconnect  36  that is capable of opening and closing at least a portion of the conductive path is provided. In the embodiment of FIG. 2, the power disconnect  36  is capable of opening and closing the phase and neutral conductive paths. However, it is also contemplated that the power disconnect  36  can be configured to open one of the phase or neutral conductive paths. Preferably, the power disconnect  36  is a relay capable of switching high currents. Examples of other suitable power disconnects include solid state switches. 
     The GFCI device  14  also includes a pickup  38  and a ground fault detector  40 . The pickup  38  is used to monitor the phase dimmed current associated with the conductive paths for predefined electrical characteristics, and to generate a pickup signal when the predefined current characteristics occur. The ground fault detector  40  is electrically coupled to the pickup device  38  and is used to determine whether the pickup signal includes a ground fault. The AC supply from control connection  14   c  supplies power to the ground fault detector  40 . As seen in FIG. 2, to ensure that the AC power supplied to the ground fault detector  40  is sinusoidal, the AC supply is independent of the phase dimmed current supplied to the loads. 
     As noted, the pickup  38  generates a pickup signal when predefined electrical (e.g., current) conditions occur. In the exemplary schematic diagram of FIG. 3, the pickup  38  includes a differential transformer T 1  and a ground-neutral transformer T 2  connected to the ground fault detector  40 . In this embodiment, the pickup  38  generates a pickup signal when: 1) there is a difference in current flowing in the phase and neutral conductors of the conductive path passing through the differential transformer T 1  (a difference current ground fault); or 2) when there is current flowing from the neutral conductor to ground passing through the ground-neutral transformer T 2  (a ground to neutral ground fault). Preferably, the ground fault detector  40  uses a ground fault interrupter integrated circuit U 1  (GFI circuit), such as the RV4141 integrated circuit manufactured by Raytheon Semiconductor, that is capable of detecting low level ground fault conditions. Preferably, the GFI circuit U 1  has a level of ground fault signal integration that is determined by a capacitor that is external to the GFI circuit. 
     More particularly, to detect difference current ground faults, the differential transformer T 1  picks up differences in the current flowing in the phase and neutral conductive paths  32  and  34  passing through a center of the transformer. Such current differences cause a secondary current to flow in the secondary windings of the differential transformer T 1 , which is the pickup signal. The pickup signal is input to the GFI circuit U 1  which converts the secondary current to a voltage, usually using an operational amplifier, and outputs this voltage as a trigger signal. Preferably, the trigger signal is a pulse. Resistors R 6  and R 7  are used to set the trip level of the GFI circuit and, thus, the GFCI device  14 . 
     To limit or prevent high frequency noise from affecting the pickup signal input to the GFI circuit U 1 , a capacitor C 9  may be connected to across secondary windings of the differential transformer T 1 , which in combination with resistor R 7  forms an RC filter. 
     As noted, the pickup signal is usually provided as one input to the GFI circuit. This input is compared to a reference voltage, which is preferably one half the supply voltage for the GFI circuit U 1 . The reference voltage is generated using zener diodes, typically within the GFI circuit U 1 , and a capacitor C 6  stabilizes the reference voltage if the supply voltage fluctuates. 
     To ensure that the reference voltage does not change after the supply voltage is applied to the GFI circuit U 1 , capacitor C 6  is preferably set to a value that is smaller than the value of the capacitor C 4  across the GFI circuit supply voltage. Preferably, the value of capacitor C 4  is large, e.g., about 1 NF or greater, and provides a reservoir for the GFI circuit supply voltage: Capacitor C 5 , which is in parallel with capacitor C 4 , is a decoupling capacitor that prevents high frequency noise, in the supply voltage, from affecting the GFI circuit U 1 . 
     Ground to neutral ground faults are detected using the grounded neutral transformer T 2 , and capacitors C 7  and C 8  form the positive feedback loop as described hereinabove. 
     As previously described, AC current supplied to the ground fault detector  40  independent of the phase dimmed current supplied to the loads. Ferrite beads FB 1  and FB 2  provide high frequency filtering for the AC supplied to the ground fault detector  40 . Preferably, two LC filters formed by inductors L 1  and L 2  and capacitor C 2  provide further filtering of the AC supplied to the ground fault detector  40 . Capacitor C 2  in conjunction with resistor R 3  act as a snubber. A metal oxide varistor (MOV) MV 1  is used to protect the GFCI device  14  from external voltage spikes. The filtered AC is then is then used to power the ground fault detector  40  through a full wave bridge rectifier formed by diodes D 1 , D 2 , D 3  and D 4 . 
     On the DC side of the bridge rectifier, the rectified AC is used as the supply voltage for the GFI circuit U 1 , through voltage dropping resistor R 5 . In addition, the supply voltage is used as an input voltage to a driver circuit  42  used to control the power disconnect  36 . The supply voltage is dropped through the resistor network created by resistors R 1 , R 2  and R 4 . The voltage across resistor R 1  is used to turn on transistor Q 1  when the GFCI device  14  is first powered. 
     When the transistor Q 1  is turned on, the power disconnect  36  is energized (through electrical connections designated by the legends A and B) and the contacts between the line and load in the phase-dimmed circuit are closed. Preferably, a relay coil in the power disconnect is energized. Capacitor C 1  is used to attenuate voltage spikes that may be created when the relay coil is energized and de-energized. 
     In the embodiment of FIG. 3, the driver circuit also includes an SCR (SC 1 ) that turns on the transistor Q 1 . If a ground fault is detected, the GFI circuit U 1  outputs a trigger signal, which is connected to the gate of the SCR to turn on the SCR. Capacitor C 3  is used as a filter on the gate of the SCR. When the SCR is turned on, the voltage between resistors R 4  and R 2  drops down to a diode voltage of the SCR. As a result, the voltage across the base of transistor Q 1  drops causing the transistor to turn “off, thus turning” off current flow through the relay coil in the power disconnect  36 . With the relay coil de-energized, the contacts open and power is removed from the load on the dimmed circuit. In this way the GFCI device  14  interrupts power to the load when a ground fault is detected. 
     In the configuration of FIG. 3, the contacts in the power disconnect  36  are in a closed position when the independent AC supply is applied to the GFCI device  14 , and the contacts are in an open position when the GFCI device  14  loses power. As a result, if the independent AC power supplying the ground fault detector  40  of the GFCI device  14  is lost, the power disconnect  36  will open so that AC supplied to the one or more loads is disconnected. 
     The operation of the GFI device will now be described with reference to FIGS. 2 and 3. If the GFI circuit U 1  detects a ground fault from the pickup signal, the GFI circuit U 1  outputs a trigger signal (e.g., a pulse) to the gate of SCR, thus, turning the SCR “on”. When the SCR is turned “on”, the power disconnect  36  is energized causing, the conductive paths  32  and  34  to open. 
     To reset the GFCI device  14 , a reset actuator (e.g., a reset pushbutton switch) is activated so that a short is created across the SCR. Since the trigger signal is a pulse, shorting the SCR commutates the current through the SCR, thus, causing the SCR to turn “off”. Transistor Q 1  then turns “on” so that the power disconnect  36  is energized causing the contacts to close. It is noted. that if a ground fault is still present, the SCR would again be triggered so that the GFCI device  14  trips. 
     Preferably, the GFCI device  14  includes a test circuit, which tests the operating components of the device. The test circuit includes an actuator (e.g., a test pushbutton switch) and a conductor passing through the differential transformer T 1 . When the test button is activated, a test current (simulating a ground fault) flows through the conductor  44 . The level of the test current is determined by resistor R 8 , which, in this embodiment, generates a test current of about 8.3 mA (rms) that is capable of testing the GFCI device  14  with the dimmer device  12  set at half brightness or greater. Alternatively, the resistor R 8  can be set to a value that produces a peak current that is greater than 8.3 mA at a predefined phase angle. In such instances, a test current can be used to test the GFCI device  14  with the dimmer device  12  set to the predefined phase angle. 
     Referring again to FIG. 3, the GFCI device  14  can be provided with an indicator which when activated identifies a predefined condition. The indicator may be a neon light NE 1 , which in combination with current limiting resistor R 9  can be used to indicate power is supplied to the one or more loads  16 . However, the indicator can be used to indicate a variety of events. For example, if indication of relay position is desired, the neon light NE 1  and resistor R 9  can be placed in parallel with the relay coil of the power disconnect  36  so that the neon light will be “on” when the relay coil is turned “on”, and the neon light will be “off” when the relay coil is turned “off”. 
     Referring now to FIG. 4, the power disconnect  36 , ground fault detector  40  and pickup  38  are preferably enclosed in a housing  50  for installation in an electrical wiring system. The housing  50  can be configured and dimensioned to fit within a single gang recessed junction box commonly used in electrical wiring systems. However, the housing can also be configured to fit within a multiple gang recessed junction box, or as a stand alone wall mountable unit capable of being used in either interior or exterior environments. 
     In the embodiment of FIG. 4, the housing  50  is a two-part housing configured for installation in a single gang recessed junction box. The two-part housing includes a front cover  52  and a rear cover  54 , which permit easy assembly of the GFCI device  14 . As seen in FIGS. 4 and 5, the front cover  52  includes a pair of openings  56  and  58  through which test and reset actuators  60  and  62  extend. The front cover  52  also includes a plurality of circuit board mounts  64  for securing circuit board  68  to the front cover  52 . In this embodiment, the ground fault detector  40  and pickup  38  are located on the circuit board  68 , and the test and reset actuators  60  and  62  interact with or form part of the test and reset switches on the circuit board. The power disconnect  36  is secured to mounting arms  66  on front cover  52  and is positioned adjacent the circuit board  68 . The rear cover  54  is secured to the front cover to enclose the ground fault detector  40 , power disconnect  36  and the pickup  38 . 
     Finally, referring to FIG. 6, a side elevational view of the GFCI device of FIG. 4 with a portion of the rear cover  54  removed illustrates the power disconnect  36  in the open position. Likewise, referring to FIG. 7, a side elevational view of the GFCI device of FIG. 4 with a portion of the rear cover  54  removed illustrates the power disconnect  36  in the closed position. 
     It will be understood that various modifications can be made to the embodiments of the present invention herein without departing from the spirit and scope thereof. Therefore, the above description should not be construed as limiting the invention, but merely as preferred embodiments thereof. Those skilled in the art will envision other modifications within the scope and spirit of the invention as defined by the claims appended hereto.