Electricity: electrical systems and devices – Safety and protection of systems and devices – Ground fault protection
Reexamination Certificate
2000-08-17
2002-10-29
Huynh, Kim (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Ground fault protection
C361S093200, C340S650000
Reexamination Certificate
active
06473281
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to circuit breakers. More specifically, this invention relates to trip units for circuit breakers having switchable protection or annunciation for various fault conditions.
Circuit breakers are designed for opening a circuit upon the detection of a fault condition. A trip unit monitors current sensors and outputs a trip signal when a fault condition is detected. Some trip units monitor the current for overload or over-current conditions but have no ground fault sensing capability. Other trip units are available for opening a circuit when an over-current condition or ground fault condition exists. In addition, electronic circuit breakers having sensors to determine when these conditions exist sometimes include annunciation capability, so that an engineer or electrician can determine what fault condition arose to cause the circuit breaker to open the circuit.
However, in certain situations it is desirable or even necessary to provide annunciation that a ground fault condition exists, but without opening the circuit. For example, in the case of critical life-saving equipment in hospitals and in the case of fire pumps, it is required by the National Electric Code that ground fault conditions be detected but for annunciation only, not for breaking the circuit. The National Electric Code also permits annunciation-only in the case of industrial processes, where shutting down a manufacturing process due to a ground fault in one sub-system can cause expensive down time.
Accordingly, there remains a need in the art for a circuit breaker implementing a ground-fault sensing circuit that annunciates a ground fault condition but does not open the power circuit in the case of a ground-fault condition.
BRIEF SUMMARY OF THE INVENTION
The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a trip unit for annunciating a ground fault condition including a current sensor generating a current signal indicative of current in a power circuit, an analog to digital converter converting said signal into digital form, and a controller monitoring the current signal and sensing whether an over current condition exists and whether a ground fault condition exists. In the case of an over-current condition, the controller causes a trip signal to be output resulting in the power circuit being opened. In the case of a ground fault condition, the controller causes an annunciation output signal but does not generate a trip signal and therefore does not cause the power circuit to be opened.
REFERENCES:
patent: 4319298 (1982-03-01), Davis et al.
patent: 5258885 (1993-11-01), Hattori et al.
patent: 5933306 (1999-08-01), Santos et al.
patent: 6167329 (2000-12-01), Engel et al.
GE Electrical Distribution & Control; Application Engineering Information;Ground-Fault Protection for Solidly Grounded Low-Voltage Systems, 1991.
DET-061 Data Sheet;Spectra RMS Molded-Case Circuit Breakers with Microversatrip Plus and Microversatrip PM Trip Units, 1994.
General Electric Company
Huynh Kim
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