Tracking and ARC resistance of materials

Arc resistance by the common usage definition refers to the ability of an insulator to sustain surface discharges, at currents of 10 ma and higher, without electrical or structural damage. Here moisture usually is not a significant factor among the effects, or causes, of these discharges, or arcs. In the case of tracking, on the other hand, the damaging discharges are usually less than 10 ma, and are initiated and sustained by the presence of moisture and dust. Moisture also causes tracking failures along the interior of certain insulators, as well as surface tracks. The life-time of insulation that has both low track and arc resistance, e.g., phenolic resin, can be of the order of months under tracking conditions or may take less than one second of arcing before failure occurs. In contrast to corona effects on insulation, which are due to chemical reaction of the active products of the corona discharges at currents less than 1 ma, the effects of both arcing and tracking discharges are essentially thermal, i.e., they involve high-temperature reactions such as decomposition or combustion.