Electrical breakdown time delay and breakdown propagation velocity in polypropylene under a highly non-uniform field condition

The electrical breakdown phenomenon in polypropylene for point-to-plane gap configuration has been investigated using a photo-optical current-measuring technique and a 760 ns rectangular high-voltage pulse generator. The waveform of prebreakdown current in polyethylene is essentially the same as that in liquid nitrogen. A linear relation to formative time delay and gap spacing is obtained for gap lengths greater than some critical value, indicating a constant propagation velocity in this region. The velocity is deduced to be 1.7 km s-1 for a positive point at 60 kV. This value is coincident with that of polyethylene, and close to that of a longitudinal wave in polypropylene at 193 K.