Electricity: electrical systems and devices – Safety and protection of systems and devices – Ground fault protection
Reexamination Certificate
1996-04-12
2002-04-30
Huynh, Kim (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Ground fault protection
C361S047000, C361S094000, C361S093600
Reexamination Certificate
active
06381113
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a leakage current protection device, such as a ground fault circuit interrupter (GFCI), and more specifically, this invention relates to a leakage current protection device adapted to function effectively over a wide range of operating conditions both domestically and internationally.
2. Description of the Prior Art
GFCIs protect against undesired current paths to ground that may create hazardous conditions. A common form of such a GFCI includes a differential amplifier having a core with opposed primary windings, one primary winding having current from the power line lead passing through it, while the other primary winding has return current from the neutral lead passing through it. These primary windings produce magnetic fluxes in the core that flow in opposite directions, known as “bucking.” If all of the power line current going to the load returns through the neutral lead, then the fluxes will be equal and will cancel out one another. However, if some of the load current is drawn off through an undesired path to ground, the bucking fluxes will not cancel out and there will be a resulting flux flow in the core.
A secondary winding is also located on the magnetic core. The resulting flux flow when an imbalance occurs in the currents flowing through the primary winding will induce a signal in the secondary winding. The signal induced in the secondary winding is conveyed to the control circuitry of the GFCI to open the power line lead, thus preventing the development of a dangerous condition.
This type of GFCI has been utilized for some time, and various modifications and improvements have been made from time to time to meet particular conditions. For example, in applicant's U.S. Pat. No. 4,598,331, an arrangement is disclosed in which the power line lead is opened if an open neutral or an open ground lead is detected. However, there are other situations in which it would be preferable to maintain GFCI protection even if the neutral lead is broken.
Around the world the applications for GFCI's involve a wide variety of conditions. For example, in the United States it has been decided that to insure personal protection a ground fault current in excess of 6 milliamperes cannot be permitted. However, in other countries the permissible ground fault current may be as high as 30 milliamperes. Accordingly, a GFCI for use in all international situations must be able to provide protection against ground fault currents in the range of 6-30 milliamperes.
Also, not all countries utilize the 60 hertz frequency of AC power that is utilized in the United States. Therefore, a GFCI for international applications must be able to provide protection for a frequency range of 50-60 hertz. Further, the GFCI, for maximum flexibility in application should be able to handle both single and polyphase input power, with either balanced or unbalanced phase loading, and input line to neutral voltages ranging from 70-264 volts (AC). All of these features should be achievable with load current capabilities of up to 100 amperes or more.
In some situations the magnetic circuitry of the GFCI must be able to respond to pulsating DC requirements. GFCIs in the art do not presently meet this requirement in a satisfactory manner.
Another problem that arises is in connection with multiple GFCIs connected to a power line. If one GFCI is being tested by simulating a ground fault circuit for that GFCI, the other GFCIs in the series may detect the test as a ground fault and be actuated in response thereto. A similar situation may occur when a GFCI having an alternate lead to ground is actuated and arcing of the contact opening the neutral lead maintains current flow through the neutral lead until after there is current flow through the alternate path to the ground lead. This is also detected by other GFCIs as a ground fault, and they may be undesirably actuated.
In still other situations, it may be desirable to have a choice between a circuit breaker and a power contactor to open the power line lead or leads. Prior art devices do not provide such a feature.
SUMMARY OF THE INVENTION
The present invention provides a GFCI which meets the requirement of being able to function under a wide range of operating conditions to prevent dangerous ground fault currents from occurring. The basic GFCI operation is that disclosed in connection with the prior art devices, with a number of additional features.
Also, it should be recognized that this description, although directed to a GFCI, may be equally applicable to other types of leakage current protection devices, such as an appliance leakage current interrupter (ALCI), an equipment leakage current interrupter (ELCI), or an immersion detection circuit interrupter (IDCI).
With respect to being able to continue providing GFCI protection even when a neutral lead is open, control means in the form of unidirectional current devices, such as diodes, are utilized to direct the GFCI actuating current to ground, when the neutral lead is open. In normal operation, code requirements preclude a current return path to ground. However, in the case of an open neutral, a very short pulse of current to ground may be utilized to actuate the GFCI without creating hazardous or dangerous conditions. An important aspect of the control means is that the diodes permitting current flow to ground have a higher forward voltage drop than the diodes permitting current flow to neutral. This is necessary in order to preclude current flow to ground during normal operation.
This approach may be utilized with either a single phase or polyphase arrangement, as illustrated in the preferred embodiments disclosed herein. In one of the preferred embodiments, provision is made to permit the GFCI to be used with either a single phase or a polyphase system without modification or adjustment of the device.
In order to permit the magnetic circuit of the differential transformer to respond to pulsating DC requirements, means are utilized to provide a leading power factor for the circuit. A preferred embodiment to achieve this leading power factor is to attach a suitable capacitor across a secondary winding of the differential transformer. Limitation of peak voltage on the secondary winding may be achieved by connecting a pair of clamping diodes in opposite directions across the winding in parallel with the leading power factor capacitor.
In order to protect other GPCIs from being actuated unnecessarily, a time delay means is utilized to prevent current flow to the ground lead until current flow has been discontinued in the neutral lead. The time delay of the time delay circuit is sufficiently long to permit the discontinuance of arcing as the neutral lead is opened, before permitting current flow to the ground lead. This time delay circuit is also useful in preventing undesired actuation of other GFCIs when a test circuit is utilized to provide a simulated ground fault to one of the GFCIs.
This test circuit may be formed by placing a supplemental secondary winding on the core of the differential transformer. When the test circuit is closed, such as by a manually actuated switch, current flow through the supplemental secondary winding will create a flux flow that simulates the existence of a ground fault current. Energization of the test circuit may be achieved either directly from the line lead or from a regulated output of the GFCI control circuitry. To have these GFCIs respond to various levels of permitted ground fault currents, as determined in different nations, an adjusting means may be utilized to determine the trip level of the ground fault current. At the present time, trip levels of 6, 10 and 30 milliamperes would seem to suffice, although more or less can be provided as required or desired. The adjusting means may be provided by a replaceable or variable resistor in the GFCI control circuitry.
By utilization of a regulator circuit, the GFCI may be adapted to work over a wide range of input voltages, such as 70 to 264 v
Gealow Jon Carl
Huynh Kim
Technology Research Corporation
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