Electrical breakdown of air between insulated conductors

Studies are reported of the influence of insulating surfaces on the breakdown of air using lightning impulse and direct voltages. Tests were made on air gaps of 3 to 22.5 mm between two conductors covered with XLPE insulation. Breakdown of such an air gap is in reality a partial breakdown of the air between the solid insulation. Pre-breakdown and breakdown of the air creates free charges which are deposited and accumulated on the XLPE insulation. These charges influence the electric field in the air gap when voltage is applied subsequently. The paper describes experiments which determine the polarity, magnitude and the location of charges created by impulse and direct voltages, and which illustrate their effects on (partial) breakdown of the air and on partial discharges along the surface of the solid insulation