Electricity: electrical systems and devices – Safety and protection of systems and devices – Ground fault protection
Reexamination Certificate
2000-07-06
2003-04-29
Vu, Bao Q. (Department: 2838)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Ground fault protection
C361S050000, C361S045000, C361S093100
Reexamination Certificate
active
06556395
ABSTRACT:
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
eutral 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
eutral 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
eutral 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.
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Bradley Roger M.
Chan David Y.
Germain Frantz
Leviton Manufacturing Co. Inc.
Sutton Paul J.
Vu Bao Q.
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