New method for the assessment of switching-impulse insulation strength

By examining the nature of the variate of the breakdown frequency, the conventional method of analysing data is shown to be incorrect, unless some breakdowns occur on the wavetail. A new method is devised for the derivation of the cumulative frequency distribution of breakdown voltage (or `time to breakdown¿), which is an estimate of the corresponding probability distribution. The new method extracts all the relevant information contained in the test data and gives more significant results, which are likely to be fairly reproducible. Practical experience indicates that the new method is particularly suited to the assessment of voltages of low breakdown probability, and its use should lead to simplifications of testing technique. It is shown that problems of insulation co-ordination can readily be solved on a probability basis by the use of the cumulative frequency distribution of time to breakdown.