Arc fault detector method

Details Arc fault detector method Arc fault detector method

1998-11-23

2002-10-01

Metjahic, Safet (Department: 2858)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

For fault location

C324S520000

Reexamination Certificate

active

06459273

ABSTRACT:

TECHNICAL FIELD
The present invention relates, in general, to arc fault detectors and, more specifically, to sputtering arc fault detectors using high pass filtering techniques.
BACKGROUND OF THE INVENTION
There are various conditions that may cause an arc fault. Corroded, worn or aged wiring or insulation, worn power cords, old wall outlets with insufficient contact pressure, electrical stress from repeated overloading, etc., may result in an arc fault. These conditions may damage the insulation of the wiring and create excessive heating temperatures. Arc faults may result in a fire depending on various conditions, such as if combustible materials are in close proximity.
There are also various conditions that may result in a false arc fault. For example, the occurrence of an arc fault in one branch circuit of a power distribution system may generate a false arc in another branch circuit. As a result, circuit breakers in more than one branch circuit may erroneously trip. Another example is a relatively noisy load such as an electric drill creating a high frequency disturbance in the circuit, which may appear to be an arc fault.
There are two types of arc faults that may occur in a home. A first type is a high-energy arc that may be related to high current faults; a second type is a low current arc that may be related to persistent momentary contact of electrical conductors. The first type may result from inadvertent connection between a line conductor and neutral conductor or a line conductor and ground. The first type may draw current that is above the rated capacity of the circuit, arcing as the contacts are physically joined.
The other type of arc fault, the momentary contact of electrical conductors, may be considered more problematic. Since the current in the arc may be limited to less than the trip rating of an associated circuit breaker, such arcs may become persistent without observation and may result in certain conditions. Contact arcs may be caused by springs in switches that become worn which, in turn, may reduce the forces that hold electrical contacts together. As the electrical contacts heat and cool down, the conductors may touch and separate repeatedly, thereby possibly creating arcs known “as sputtering arcs.”
Contact arcs or sputtering arcs may also be observed in contacts which are made from different materials. For example, aluminum wiring which contacts copper wiring may oxidize at the contact points. In this case a non-conductive layer may build up over time between the contact points and arcing may result. Sputtering arcs may also be observed in extension cords having insufficient current carrying capacity. As the plug is heated by resistance heating, insulating materials around the contacts may eventually melt and flow into the contact area, preventing proper contact from being made. The current in the conductors may produce magnetic repulsion forces which may push the conductors apart, resulting in an arc. The arc may be extinguished as the current passes through zero. Mechanical or electro-static forces may bring the conductors back into contact, and the cycle may be repeated.
It is believed that various circuit breakers are not specifically designed to guard against sputtering arcs. Special purpose detectors have been designed to detect sputtering arcs and, when detected, trip the circuit breakers. Some detectors are believed to depend on the sputtering arcs exceeding a predetermined current or voltage threshold before tripping the circuit breaker; other detectors are believed to depend on the sputtering arcs having a specific high frequency signature. Still other detectors are believed to depend on the sputtering arcs producing a wideband high frequency noise ranging from 10 kHz to 1 GHz while the arc is conducting current. These detectors may require that no noise be produced while the arc is not conducting current, i.e., during the gaps between arc conduction. These detectors use various processing techniques to analyze the repetitive patterns in the noise.
It is believed that a need still exists for a sputtering arc fault detector that does not depend on the frequency content of the noise generated by the arc or on relatively involved analysis techniques to determine the arc signature characteristics.
SUMMARY OF THE INVENTION
To meet this and other needs, and in view of its purposes, the present invention provides a fault detector that monitors rapid transition of current flow in conjunction with current magnitude. In one embodiment, a sputtering arc fault detector is provided for a system having an electrical conductor carrying current to a load. The sputtering arc fault detector includes a current monitor coupled to the conductor for generating a variable signal responsive to behavior of the current in the conductor. A level detector is coupled to the monitor and generates a first pulse when the variable signal exceeds a first level. A step detector is coupled to the monitor and is responsive to rapid step increases of the variable signal. The step detector generates a second pulse when the variable signal exceeds a second level. An arc verifier, which is coupled to the level detector and the step detector, combines the first and second pulses, and generates a fault signal when the combined pulses exceed a third level. Furthermore, the arc verifier includes a transistor having first and second terminals that define a principal conduction path and a third terminal that controls conduction through the path. The first terminal is coupled to the level detector, the third terminal is coupled to the step detector, and the second terminal provides an output signal which represents a combination of the first and second pulses. A capacitor is coupled to the second terminal, and charges in response to the combined pulses. A comparator is coupled to receive and compare the charge to the third level. The fault signal is generated by the comparator when the charge exceeds the third level. The capacitor charges at a first rate and discharges at a second rate which is slower than the first rate. The comparator includes a hysteresis circuit which holds the fault signal when the charge exceeds the third level.
Another object of the invention is to provide a sputtering arc fault detector for a system having an electrical conductor carrying current to a load, said sputtering arc fault detector comprising: a current monitor coupled to the conductor for generating a variable signal responsive to behavior of said current in said conductor; a level detector coupled to said monitor and generating a first pulse when said variable signal exceeds a first level; a step detector coupled to said monitor and responsive to rapid step increases of said variable signal, said step detector generating a second pulse when said variable signal exceeds a second level; and an arc verifier coupled to said level detector and said step detector for combining said first pulse and said second pulse, said arc verifier generating a fault signal when said combined pulses exceed a third level.
Another object of the invention is to provide the sputtering arc fault detector described above wherein said arc verifier includes a transistor having first and second terminals that define a principal conduction path and a third terminal that controls conduction through said path, said first terminal being coupled to said level detector, said third terminal being coupled to said step detector, and said second terminal providing an output signal which represents a combination of said first and second pulses, a capacitor coupled to said second terminal, said capacitor charging responsive to said combined pulses, and a comparator coupled to receive and compare said charge to said third level, wherein said fault signal is generated by said comparator when said charge exceeds said third level.
Another object of the invention is to provide the sputtering arc fault detector described above wherein said capacitor charges at a first rate and discharges at a second rate which is slower than said first rate.
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