Electricity: electrical systems and devices – Safety and protection of systems and devices – Ground fault protection
Reexamination Certificate
1997-03-07
2001-02-27
Leja, Ronald W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Ground fault protection
Reexamination Certificate
active
06195241
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the protection of electric circuits and, more particularly, to the detection of hazardous arcing faults typically ignored by conventional circuit interrupters.
BACKGROUND OF THE INVENTION
The electrical systems in residential, commercial and industrial applications usually include a panelboard for receiving electrical power from a utility source. The power is then routed through overcurrent protection devices to designated branch circuits supplying one or more loads. These overcurrent devices are typically circuit interrupters such as circuit breakers and fuses which are designed to interrupt the electrical current if the limits of the conductors supplying the loads are surpassed. Interruption of the circuit reduces the risk of injury or the potential of property damage from a resulting fire.
Circuit breakers are a preferred type of circuit interrupter because a resetting mechanism allows their reuse. Typically, circuit breakers interrupt an electric circuit due to a disconnect or trip condition such as a current overload or ground fault. The current overload condition results when a current exceeds the continuous rating of the breaker for a time interval determined by the trip current. The ground fault trip condition is created by an imbalance of currents flowing between a line conductor and a neutral conductor such as a grounded conductor, a person causing a current path to ground, or an arcing fault to ground.
Arcing faults are commonly defined as current through ionized gas between two ends of a broken conductor, between two conductors supplying a load, or between a conductor and ground. A major problem results from arcing faults which do not cause the circuit breaker to trip. Arcing fault current levels may be reduced by branch or load impedance to a level below the trip curve settings of the circuit breaker. In addition, an arcing fault which does not contact a grounded conductor or person will not trip a ground fault protected circuit.
There are many conditions that may cause an arcing fault. For example, corroded, worn or aged wiring or insulation, loose connections, wiring damaged by nails or staples through the insulation, and electrical stress caused by repeated overloading, lightning strikes, etc. These faults may damage the conductor insulation and reach an unacceptable temperature. Arcing faults can cause fire if combustible materials are in close proximity.
There are also many conditions that may cause a “false” arcing fault. For example, the occurrence of an arcing fault event in one branch circuit of an electrical distribution system often causes a false arcing fault signal in another branch circuit as a series path is created between the branch circuits through a load center. As a result, circuit interrupters in more than one branch circuit are erroneously tripped. Another example is a noisy load such an arc welder, electric drill, etc. producing a high frequency disturbance in the electrical circuit which appears to be an arcing fault.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an arc fault detection system and method which reliably detects arc fault conditions ignored by conventional circuit interrupters.
Another object of the invention is to provide an arc fault detection system which can be conveniently retrofitted into existing residential, commercial and industrial facilities at minimal cost.
A further object of the invention is to provide an arc fault detection system and method which distinguishes between hazardous arc faults and normal operation of equipment or household appliances, as well as noisy loads, using the electrical circuit.
Still another object of the invention is to provide an arc fault detection system which electrically isolates multiple branch circuits in an electrical distribution system.
Other and further objects and advantages of the invention will be apparent to those skilled in the art from the present specification taken with the accompanying drawings and appended claims.
In accordance with the present invention, the foregoing objectives are realized by providing a system for detecting arcing faults in an electrical distribution system by monitoring the rate of change of electrical current in the line conductor and producing a signal representing the rate of change, producing a pulse each time the rate-of-change signal exceeds a selected threshold, filtering the rate-of-change signal and/or the pulses to substantially eliminate a signal or pulse representing changes in the electrical current outside a selected frequency range, monitoring the remaining pulses to detect when the number of pulses that occur within a selected time interval exceeds a predetermined threshold, and generating an arc-fault-detection signal in response to the occurrence of a number of pulses exceeding the threshold within the selected time interval.
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Brooks Stanley J.
Dickens James W.
Strader Walt
Golden Larry I.
Irfan Kareem M.
Leja Ronald W.
Squares D Company
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