Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
2001-01-17
2004-08-24
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C702S057000, C702S059000, C361S045000
Reexamination Certificate
active
06782329
ABSTRACT:
FIELD OF THE INVENTION
This invention is directed generally to the detection of electrical faults and more particularly to the detection of arcing faults in aircraft wiring and the like
BACKGROUND OF THE INVENTION
Aircraft power systems have historically differed from ground based power systems in several ways. 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 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.
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. A ground fault trip condition is created by an imbalance of currents flowing between a line conductor and a neutral conductor which could be caused by a leakage current or an arcing fault to ground.
Arcing faults are commonly defined as current through ionized gas between two ends of a broken conductor or at a faulty contact or connector, between two conductors supplying a load, or between a conductor and ground. However, arcing faults may not cause a conventional 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 protector.
There are many conditions that may cause an arcing fault. For example, corroded, worn or aged wiring, connectors, contacts or insulation, loose connections, damaged wiring, and electrical stress caused by repeated overloading, lightning strikes, etc. These faults may damage the conductor insulation and cause the conductor to reach an unacceptable temperature.
The need for arc detection in aircraft has become increasingly clear. For example, wire arcing may be a factor in some aircraft fires. Past responses to aircraft fires have been to increase the flame retardant properties of wiring and other interior components of aircraft. Standard overcurrent devices used in circuit breakers respond to the heating effect of current in a resistive wire to “thermal trip” the breaker, but these do not respond to the sputtering arc currents which cause intense arc heating and fire.
We propose a better approach—to stop the arc when it happens rather than wait for a fire to start or for a circuit breaker to thermal trip.
Until recently, such arc detection capability has not been available in circuit breakers or relays. Arc detection has been available for 60 Hz residential, commercial or industrial systems, but has not heretofore been resolved for aircraft wiring systems. In addition, most aircraft circuits do not have the neutral return conductor found in 60 Hz systems. This prevents the use of differential detection of ground faults on most aircraft branch circuits. A standard aircraft circuit breaker contains bimetals and/or magnetic solenoids which provide an inverse time response to current. Arcing fault detection is not provided by these devices. Aircraft arc detection is not possible using arc detectors designed for 60 Hz circuits for several reasons. For example, 60 Hz arc detectors partly respond to ground fault which is not possible on standard aircraft branch circuits. Also, the methods used at 60 Hz cannot be automatically extended to cover a power frequency range as high as 400 Hz, typically used in aircraft.
Advancements in electrical circuit protection introduced by the residential and commercial industries have been slow finding their way into aerospace applications. Ground Fault Circuit Interrupters (GFCI) for personnel protection have been available in the home since the early 1970's. Under ideal conditions, GFCI can detect phase to ground arcs as low as six milliamps, but cannot detect series arcs or improve line to neutral fault trip times.
Arc Fault detection technologies are a new and exciting innovation in circuit protection in the U.S. We have found that Arc Fault Circuit Interrupters (AFCI) can be designed to detect a series or parallel arc, as well as line to neutral arcs by “listening” for the unique signatures which arcs generate. We have found that AFCI can detect arc currents well below the trip curves of today's Mil-Spec aircraft circuit breakers. This enhanced detection capability may provide improved protection from arcing conditions onboard aircraft.
An arc fault circuit interrupter is a device intended to provide protection from the effects of arc faults by recognizing characteristics unique to arcing and by functioning to de-energize the circuit when an arc fault is detected.
Aircraft circuit breakers have historically been the best available protection for aerospace wiring. Today's design standards are based on technologies that are up to 40 years old. In aircraft/military type breakers, the protection is provided in two ways. Short circuit currents operate a magnetic trip latch, while overload currents operate either a bimetal trip latch or hydraulic damped magnetic plunger. The “instantaneous trip” is the high current magnetic trip action found on some but not all aircraft breakers. The time to trip during an overload is determined by the time it takes to heat a bimetal to the temperature that delatches the breaker. The more current that heats the bimetal, the shorter the time it takes to trip the breaker. A hydraulic-magnetic style of breaker contains a magnetic slug sealed in fluid which moves to a trip position in response to the square of the current. These circuit interruption devices are selected by aircraft design engineers to protect the aircraft wiring from overheating or melting. During arcing faults these currents are often small, short in duration and well below the overcurrent-time protection curve designed into these breakers.
Arcing in a faulted AC circuit usually occurs sporadically in each half cycle of the voltage waveform. The complex arcing event causes sputtering arcs that vary the current from normal load patterns. The precurser to the arc may be a high resistance connection leading to a “glowing contact” and then a series arc, or a carbon track leading to line-to-line or parallel arcing. In a home circuit breaker equipped with Ground Fault Circuit Interrupter (GFCI), a carbon or moisture track can be detected early if the short is to ground. In many aircraft circuits, the neutral conductor is not available to complete the necessary ground fault detection circuit and GFCI protection is not possible. With the introduction of AFCI breakers, protection of arcing shorts from line-to-line, not involving ground, can also be detected and interrupted.
UL approved AFCI circuit breakers for residental use are available commercially. These are now in the NEC and will be required in home bedroom circuits 2002. Since the electrical loads in residential circuits can vary widely, they will be designed to allow for almost an infinite combination of electrical loads. Their AFCI programming is combined with GFCI as well as magnetic and thermal overload components. They are designed to form fit and function in place of standard residential circuit breakers.
There are two types of arcing faults in aircraft electrical circuits and wiring: Parallel and Series.
Parallel arcing occurs when there is an arc between two wires or wire-to-frame and the current is limited by the impedance of the voltage source, the wire, and the arc. When the fault is solidly connected and the arc voltage low, the normal aircraft breaker trips v
Golden Larry I.
Hoff Marc S.
Square D Company
West Jeffrey R
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