Influence of the various parts of the switching impulse front on discharge development

The region of the front of a switching impulse on which the breakdown probability of a gap depends (the so-called active region) is determined by testing with a 4-m rod-plane gap stressed by bumped impulses. The experimental results show that the lower limit of the active region is rather a given voltage than a percentage of the amplitude of the impulse. The extent of this region depends on the form of the electrode, the gap clearance and the voltage level, and it is this which determines the breakdown probability corresponding to the particular impulse. Breakdown probability distributions in the form of an inverted S are explained in terms of the active region and its lower limit.