Electricity: electrical systems and devices – Safety and protection of systems and devices – Ground fault protection
Reexamination Certificate
2001-12-31
2004-02-24
Toatley, Jr., Gregory J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Ground fault protection
C361S104000
Reexamination Certificate
active
06697238
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a ground fault circuit interrupter (GFCI) device that has secondary test switch contacts to allow for removing power from the load. More particularly, the invention relates to a GFCI having secondary test switch contacts that operate to blow a fuse if actuation of the primary test switch fails to trip the GFCI.
BACKGROUND OF THE INVENTION
GFCI devices are designed to trip in response to the detection of a ground fault condition at an AC load. Generally, the ground fault condition results when a person comes into contact with the line side of the AC load and an earth ground at the same time, a situation which can result in serious injury. The GFCI device detects this condition by using a sensing transformer to detect an imbalance between the currents flowing in the line and neutral conductors of the AC supply, as will occur when some of the current on the line side is being diverted to ground. When such an imbalance is detected, a mechanically latched circuit breaker within the GFCI device is immediately tripped to an open condition, thereby opening both sides of the AC line and removing all power from the load. Many types of GFCI devices are capable of being tripped not only by contact between the line side of the AC load and ground, but also by a connection between the neutral side of the AC load and ground. The latter type of connection, which may result from a defective load or from improper wiring, is potentially dangerous because it can prevent a conventional GFCI device from tripping at the intended threshold level of different current when a line-to-ground fault occurs.
GFCI devices may be connected to fuse boxes or circuit breaker panels to provide central protection for the AC wiring throughout a commercial or residential structure. More commonly, however, GFCI devices are incorporated into electrical receptacles that are designed for installation at various locations within a building. A typical receptacle configuration is shown, for example, in U.S. Pat. No. 4,568,997, to Bienwald et al., the entire content of which is incorporated herein by reference. This type of receptacle includes test and reset pushbuttons and a lamp or light-emitting diode (LED) which indicates that the circuit is operating normally. When a ground fault occurs in the protected circuit, or when the test button is depressed, the GFCI device trips and an internal circuit breaker opens both sides of the AC line. The tripping of the circuit breaker causes the reset button to pop out and the LED to be extinguished, providing a visual indication that a ground fault has occurred. In order to reset the GFCI device, the reset button is depressed in order to close and latch the circuit breaker, and this also causes the LED to illuminate once again.
In addition to ground fault detection/protection, protection from miswiring is also needed. Specifically, GFCI receptacles of the type described above may be erroneously connected with the incoming AC source conductors being tied directly to the load or feedthrough terminals of the receptacle rather than to the source terminals. Because of the nature of the internal wiring of the GFCI receptacle, this miswiring condition is not easily detected. AC power will still be present at the receptacle outlets, making it appear that the receptacle is operating normally. If the test push button is depressed, the latching mechanism within the GFCI receptacle will be released and the reset push button will pop out, again making it appear that the GFCI receptacle is operating normally and providing the desired ground fault protection. In reality, however, no such protection is being provided because the AC source has been wired directly to the receptacle outlets without passing through the internal circuit breaker of the GFCI device.
Furthermore, a user may not know the state the GFCI is in without having to operate the test and reset buttons. Therefore a visual indication should be provided to indicate to a user the different states the GFCI is in.
Additionally, the safety function of GFCI devices depends upon power being prevented from reaching the receptacle when a trip condition occurs. A potentially unsafe condition occurs if the test button is pressed and the GFCI fails to trip. Therefore, the need exists for a GFCI device with a fail safe system to ensure that when the test button is pressed and the GFCI device fails to trip, the failed condition of the GFCI devices is indicated to the user in some manner.
SUMMARY OF THE INVENTION
The above and other objectives are substantially achieved by a system and method employing a ground fault circuit interrupter (GFCI) in accordance with the principles of the present invention. The method and GFCI include an interrupt module for breaking a plurality of conductive paths between source and load terminals of an AC receptacle in response to an imbalance of current flow in the paths. A test circuit is connected between the conductive paths, where the test circuit enables intentional generation of the imbalance of current flow. A lockout circuit places the GFCI in a lockout state when operation of the test circuit fails to break the conductive paths such that the lockout circuit breaks one of the conductive paths on a source side of the interrupt module and prevents restoration of the broken conductive path.
In another aspect of the invention, the lockout circuit includes a fusing mechanism and a secondary test switch. The fusing mechanism is connected in parallel with a first conductive path, while the secondary test switch is connected between the fuse and a rectifier bridge causing a short circuit when the primary switch fails to trip the GFCI.
Another aspect of the invention is that the GFCI cannot be powered if it is reverse wired. Specifically, if the GFCI is powered from the load side, the GFCI will not work because the electronics to operate the GFCI are on the line side.
Another aspect of the invention is that no power is provided to the face of the GFCI if the GFCI is reverse wired. Face plate contacts are connected to a set of relay contacts which when open prevent power from reaching the faceplate.
Another aspect of the present invention is that the visual indications are provided to alert a user to the status of the GFCI. For example, the user is made aware that the GFCI is operating as a receptacle without GFCI protection.
In one embodiment, a test switch comprises a primary test switch for allowing manual testing of the trip mechanism of a GFCI device, and a secondary test switch contact is activated after the primary test switch contact fails to trip the GFCI. The secondary test switch contact causes a fuse to be blown which can disable the GFCI device and/or remove power from the load in the event that the manual test is unsuccessful, or extinguishes a visual indicator.
Further in accordance with the present invention, a system and method for protecting an AC receptacle from ground faults is provided. The system and method include the step of enabling intentional generation of an imbalance of current flow in a plurality of conductive paths between source and load terminals of the receptacle. One of the conductive paths is broken when the intentionally generated imbalance of current flow fails to cause an interrupt module comprising relays to break the conductive paths. The method further provides for preventing switch restoration of the conductive path when the intentionally generated imbalance of current flow fails to cause the interrupt module to break the conductive paths such that the receptacle is placed in a lockout state.
Another object of the present invention is to provide a fail safe method and apparatus to remove power from a GFCI device.
REFERENCES:
patent: 4104581 (1978-08-01), Arkosy
patent: 4574324 (1986-03-01), Packard
patent: 4816957 (1989-03-01), Irwin
patent: 4833564 (1989-05-01), Pardue et al.
patent: 5136457 (1992-08-01), Durivage
patent: 5477412 (1995-12-01), Neiger et al.
patent: 5661623 (1997-08-01), McDonald et al.
patent
Bonilla Nelson
DeBartolo, Jr. Joseph V.
'Hubbell Incorporated
Buczynski Joseph J.
Goodman Alfred N.
Holmes John E.
Ritz F
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