The electric strength of Perspex as a function of voltage rise time

The electric strength of Perspex was measured using impulse voltages reaching their maximum values in times ranging from 0·5 μs to 10 000 μs. For sphere-sphere electrodes the electric strength is 6·0 MV cm −1 at the shortest rise time, 13·1 MV cm −1 at 177 μs, and 8·4 MV cm −1 for slowly rising direct voltage. Similar trends were found for point-plane electrodes. The results are consistent with a rapid accumulation of negative space charge close to the cathode, and a positive space charge close to the anode.