Initiation of dry bands on polluted insulation

The loss of moisture and the initiation of dry-band formation on wet contaminated insulation has been studied by measuring the insulation resistance of the contaminated specimen as a function of the degree of contamination, relative humidity, applied voltage and time. Drying of the wet contamination consistently occurred when the power dissipated by the surface leakage currents became sufficient to raise the surface temperature by rather less than 1 deg C. The loss of moisture caused by a subsequent increase in the power dissipation is inevitably nonuniform and leads to the formation of a dry band.