The paper, which is based on E.R.A. Reports Ref. S/T92 and S/T105, summarizes the results of tests made with surge voltages of 1/50 microsec waveshape on a wide range of insulators and air-gaps over a period of 10 years. In determining 50% flashover voltages it was found that the range of voltages over which transition from no flashover to flashover on each of 10 applications of the surge occurred was somewhat variable but averaged about 15% for small and 7% for large insulators and gaps. The extent of this range, in conjunction with the finite number of surges (50????????100) generally employed, could account for the variations obtained in the values, corrected to standard atmospheric conditions, of the 50% flashover voltage of a given insulator. In all the systems tested the electric field between the electrodes was highly non-uniform, and a roughly common relationship was found to exist between v, the average value of the positive and negative 50% flashover voltages, and the length of the corresponding flashover path. The time-lag curves obtained in tests at higher voltages on systems with small polarity differences approximated to exponential curves decaying with a time-constant of 2.9 microsec to a voltage V and passing through 1.5 V at 2 microsec.
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