Particularities of air insulation behavior

In actual practice the withstand voltage level of air insulation is estimated by an extrapolation based on the 50% breakdown voltage, U50%, and the standard deviation, σ both obtained by laboratory tests. For this extrapolation to be possible it is assumed that the breakdown probability follows a gaussian distribution. It is observed however that in many cases this distribution deviates appreciably from a normal. Unexpected inconsistencies of the breakdown probability are also observed. This particular behavior of air insulation is studied in the present report and a physical explanation is attempted.