An arc fault is a high power discharge of electricity between two or more conductors. This discharge translates into heat, which can break down the wire's insulation and possibly trigger an electrical fire. These arc faults can range in power from a few amps up to thousands of amps high and are highly variable in terms of strength and duration. Common causes of arc faults include faulty connections due to corrosion and faulty initial installation.
Arc fault Wikipedia
Two types of wiring protection are standard thermal breakers and arc fault circuit breakers. Thermal breakers require an overload condition long enough that a heating element in the breaker trips the breaker off. In contrast, arc fault circuit breakers use magnetic or other means to detect increases in current draw much more quickly. Without such protection, visually detecting arc faults in defective wiring is very difficult, as the arc fault occurs in a very small area. A problem with arc fault circuit breaker is they are more likely to produce false positives due to normal circuit behaviors appearing to be arc faults. For instance, lightning strikes on the outside of an aircraft mimic arc faults in their voltage and current profiles. Research has been able to largely eliminate such false positives, however, providing the ability to quickly identify and locate repairs that need to be done.
In simple wiring systems visual inspection can lead to finding the fault location, but in complex wiring systems, for instance aircraft wiring, devices such as a time-domain reflectometer are helpful, even on live wires.