The impulse electric strength of polythene as a function of voltage rise time

The electric strength of polythene has been measured using impulse voltages of differing rise times. The mean value of electric strength increases from 5·6 MV cm −1 for a voltage rise time of 60 μs to 6·6 mV cm −1 when the voltage rise time is 80 μs. The dc strength is 6·4 mV cm −1 . The change in electric strength could be due to an internal polarization which reaches its final value in about 80 μs.