Final breakdown current

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. 

SiPy treatments (Fig. 5.12a,b, left) produce a significant shift of the pitting forward-scans towards more noble potentials with respect to the bare substrate, this ennobling being more evident when multiple SiPy deposition is used. Also, the forward scan reveals that between the pitting potential and final breakdown, several zones of nearly constant current fluctuations are identified. That is, the forward scan is characterised by semi-passivation zones followed by a rise in the current at nearly... 

Assuming that the protection system consists of a combination of coating and cathodic protection, the required current for protection is multiplied by the coating breakdown factor, /j, which is the anticipated reduction in cathodic current due to the application of an electrically insulating coating. Total initial, average, and final current will be adjusted with the initial, mean, and final coating breakdown factors,and/jp, respectively [4], Total initial current ... 

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