Flashover failures from wet-wire arcing and tracking

Flashover failure occurs in a wire bundle as a high-energy surge between a bare conductor delivering power and a ground-plane (e.g., metal framework or wire shielding) as the conductive path across adjacent wires approaches zero resistance. It happens as a violent blazing flashover in which the copper of the bare wire melts and splatters - causing melting and burn-through of some of the adjacent wires in the bundle. Thus, it could produce the loss of a number of circuits at once. Under service stress conditions the process leading to a flashover could begin as the wire insulation initially deteriorates by cracking or chaffing and the surfaces of adjacent wires in the bundles become contaminated due to salt spray, mist, fog and high humidity. Under these conditions, typical of Naval aircraft service, arcing and tracking initiate and eventually localized spots extend into a continuous carbonaceous path, bridging the wires from the fault to ground. At some point the wetting is no longer necessary to sustain the current, and the flashover strikes. This phenomena is known to develop on the surfaces of polymeric materials that readily carbonize when pyrolized and the susceptibility to fail by this mechanism is characterized in standard arcing and tracking tests. The phenyl-containing polymeric materials fall into this class. Laboratory tests have demonstrated that flashover will occur on wires insulated with a hi-phenyl polyimide composite and also on wires insulated with a polyphenyl ketone polymer; whereas, two types of a polytetrafluoroethylene insulation did not carbonize or flashover in the test. These results are in agreement with other references on arcing and tracking studies.