Weibull statistics in short-term dielectric breakdown of thin polyethylene films

A Weibull statistical analysis of breakdown voltages of thin polyethylene-insulated power cable slices is performed on large populations. Computation of confidence intervals implies that the statistically correct description is a three-parameter Weibull distribution, i.e., with a nonzero location parameter. It is shown that a data set described by a two-parameter Weibull distribution contains additional statistical dispersion factors which may or may not yield information on the insulation itself. In other words, a zero location parameter, always results from inhomogeneities in the sampling. Comparative testing is used to discriminate between the various sources of inhomogeneity. When it is obtained under carefully controlled experimental conditions, the location parameter value can be considered a true quality factor of the system under test. The statistical analysis of data collected in routine breakdown tests provides a very sensitive tool to investigate small changes in electrical insulation when performed on extensive data sets