Time characteristics of fast pulsed flashover in vacuum

The results of measurements of surface flashover across epoxy resin filled with Al(OH)₃ between stainless steel electrodes are described. The test gap was subjected to an impulse voltage of 20 ns/20 μs. The flashovers were observed for various insulator shapes and insulator lengths. It is found that for the cylindrical shape, flashover voltage, delay time, and fall time are evidently in linear relation with the insulator length. In comparison with the results in the absence of an insulator for the same experimental conditions, for the same length, flashover voltage and fall time with an insulator are 43% and 67% of those without the insulator, respectively. The times t_d and t_f are in the microsecond and nanosecond ranges, respectively. It is postulated that time delay of flashover voltage occurs mainly at the initiation stage, while collapse of voltage takes place at the final stage of the flashover process. The shape of the insulator has a dominating influence on surface flashover. The positive angle cone and concave cylinder have a higher voltage holdoff capability than the negative angle cone and convex cylinder