The flashover along the polythene surface of a coaxial cable has been studied under the condition of nanosecond impulse voltage. The average impulse flashover voltage for unit length E/sub 1/ along the insulation length (D) of 5-15 cm was measured and found to be 6-3.5 kV/cm. The delay time of the flashover is in the range of 500-700 ns. The velocity of the propagation of discharge was measured and found to be 2+or-0.5*10/sup 7/ cm/s at D=12 cm. In order to compare with the DC values, the DC average flashover voltage for unit length was measured and found to be 7.8-5.8 kV/cm for D=5-15 cm. The authors measured the voltage-time curves of the flashover along the polythene surface and a pressurized spark gap.
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