Electrical erosion breakdown

Erosion breakdown n. In an electrical-conductor insulation, deterioration caused by chemical attack of corrosive chemicals such as ozone and nitric acid that are formed by corona discharge from a high-voltage cable. This breakdown can occur even the most chemically polymers, such as fluorocarbons, after long exposure to the condition. 

In polluted conditions where moisture and solid pollutants collect and in the condition of electrical stress which obtain in HV applications, small leakage currents flow across the surface of the outer insulator. The currents cause a rise in temperature which in turn causes dry bands to form on the surface of the insulator. Small sparks then cross the dry band the temperature of the sparks is 2000-3000 C, and the surface of the insulator can reach 450-600 C. Such temperatures can easily cause degradation of polymers with the development of carbonaceous tracking which extends in dendritic fashion. Alternatively, erosion can occur, causing cratering and final breakdown of the insulator. The worst damage is caused by currents of less than 20 mA. See Fig. 26.4. 

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