Insulation Variability Its Influence in Determining Breakdown Voltages

The purpose of the paper is to describe a method of analysis for taking into account the effect of insulation variability in the determination of breakdown voltages and allowable operating stresses. The method is applicable to systems containing large areas of insulation, or large numbers of units of insulation in parallel. General equations are derived, giving the breakdown voltage and number of failures to be expected, in terms of two parameters; the mean breakdown voltage and the variability of the insulation, both of which are determined from tests made on samples. It is shown how the results may be applied to the special case of electric power cables, for comparing the relative merits of different cables, and for determining the safety factor required to take account of the inherent variability and consequent unreliability of the insulation. The necessity of considering the variability as well as the so-called strength of insulation is emphasized, and its importance brought out in terms of allowable operating stresses and failures. A graphical method of analysis is described, which makes use of probability paper, changing the probability scale from ``per cent'' to ``failures per hundred miles per year'' in the case of cables. Predictions of theory are compared with the results of practise, using results from experiments made to determine the effect of electrode area on breakdown voltage. Good agreement is shown between the two.