Metal-to-water discharges on insulation surfaces — A differential wet tracking test

Tracking across the wet surface of insulators usually occurs by the advance of a tree-like conducting track growth, extending from one electrode (conductor) to another. These track growths occur by a succession of short tracks extending one after another in a random manner. Each track segment is added by the effect of short discharges occurring between the metal electrode or the tip of an extended carbon track and the edge of a receding surface water film. This film of water serves as a current limiting resistor and, therefore, the magnitude of its resistance, controls the discharge power intensity. These considerations apply to surface tracking as well as to internal tracking; with the exception that in the case of the latter the high conductivity region is located below the surface and tracks may be generated by localized heating effects. In general, internal tracking effects, analogous to surface tracking, follow the penetration of moisture into interlaminar spaces or along individual fibers or fiber bundles which are not bonded to the surrounding resin.