Growth of discharges on polluted insulation

The voltage required to maintain local discharges on polluted insulation may increase with increase in discharge length, and if this voltage exceeds the supply voltage, the discharges extinguish without causing a flashover. An analysis based on this mechanism has been made for the simplified case of an insulator having a constant surface resistance, rc, per centimetre of leakage path. This has shown that the flashover stress, Ec, is proportional to p0.43, where p is the resistivity of the pollution, and that the maximum leakage current which can flow when flashover is impossible is 233Ec¿1.31 amperes, Ec being in volts (peak) per centimetre. It has been found that these conclusions apply to conventional bushings subjected to wet-flashover tests, and to power insulators subjected to natural pollution under service conditions. Further confirmation of the analysis was obtained from experiments made by the authors and previous workers with artificial pollution, deposited so as to maintain rc constant along the surface. The agreement between this analysis and the experimental results of several investigators indicates that the mechanism discussed here controls flashover in many instances.