Electrical fires are often caused by loose connections, broken
wires, or shorted wires in the power distribution system. Arcing at the point of faulty
contact may cause sparks that can jump several inches to ignite
combustible material. Furthermore, heat generated by loose contacts
can deteriorate the insulation thus exposing the wires and resulting
in intermittent short circuits between conductors.
There are two types of arcing in electrical circuits: Parallel and Series.
Parallel arcing occurs when there is a direct short between two power wires and the current is limited only by the resistance of the wire in the distribution circuit. Usually the magnitude of the parallel arc current is much higher than the breaker rating. Even though it only flows intermittently, the average current may be sufficient to eventually trip a conventional breaker due to heating of the bimetal strip or the peak current may be large enough to trigger the magnetic sensor. This makes the thermal/magnetic breaker reasonably effective in protecting against parallel arcing when the peak current is a few hundred amps. Unfortunately, the parallel arc current can be limited if the circuit is implemented with wire that is too small for the application. Extension cords often aggravate the situation by introducing additional resistance in the circuit so the average value of the arc current is below the thermal breaker's rating and the peak current is below the sensitivity of the magnetic sensor.
Parallel arcing is generally more hazardous than series arcing because the energy in the sparks is much higher and the hot metal that is ejected is more likely to come in contact with combustible material. Since parallel arcing usually results in peak currents well above the handle rating of the breaker, electronic circuits can detect it easily and trip the breaker in a fraction of a second.
Series arcing is caused by a loose connection in series with the load circuit. Series arc current is limited to a moderate value by the resistance of the device that is connected to the circuit, such as an appliance or lighting system. The amount of energy in the sparks from series arcing is less than in the case of parallel arcing but only a few amps are enough to be a fire hazard. Series arcing is particularly insidious because the arc current remains well below the rating of the thermal breaker and the magnetic sensor will never respond to such low amplitude currents. Since the peak current is never greater than the steady-state load current, series arcing is much more difficult to detect than parallel arcing.
The detection of series arcing is complicated by natural occurences of arcing when switches are activated or appliances are plugged into the socket with the power turned on. These short-term arcing situations are generally not hazardous and should not cause nuisance tripping.
When the normal load current being supplied by the circuit is below the breaker rating, circuit breakers in common use today do not react to an intermittent condition. For example, if a hair dryer normally draws 10 amps but the wall outlet has a loose screw terminal so it makes contact only half the time, the average current is 5 amps and the thermal circuit breaker thinks the current is still well below its rating of 15 or 20 amps. In this case the magnitude of the arc current is limited by the heating element in the hair dryer and the maximum value is never more than 10 amps.
New arc fault circuit interrupters (AFCI's) have the ability to detect the distinctive difference between normal current and intermittent arc current. In 1994 an insurance company survey of 660 fires of electrical origin indicated that more than one-third of them were due to arcing conditions. By opening the circuit when a hazardous condition is sensed, electrical safety can be significantly increased. AFCI's in compact and economical packages have been made possible by advances in integrated electronic components.
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