The statistics of electric breakdown due to physical imperfections

Extreme value statistics have in the last few years been applied to electrical breakdown of insulation with good results. It is considered timely to present a short review of the statistics and at the same time attempt to illustrate the physical reasons for the statistical behavior. It is a well-known experimental fact that physical imperfections such as voids or foreign particles can govern electric breakdown initiation in insulating materials. This is the case in both solids and liquids of commercial purity and also in gases at higher pressures and in vacuum where imperfections or impurities on the electrodes may govern breakdown initiation. Statistically the problem of electric breakdown under these conditions is similar, for instance, to testing of mechanical strength when physical imperfections govern the mechanical breakdown. The statistics used can best be described as a weak link theory since breakdown by definition occurs in the weakest spot.