Breakdown statistics as a tool to investigate electrical aging

The statistics of short-term dielectric breakdown derived from a given test procedure is shown to be a very sensitive tool for investigating small changes in the dielectric properties of insulation. The most complete characterization encompasses all the parameters of the Weibull distribution, including the location parameter. When the sampling is very homogeneous and the other experimental parameters are well under control, the plot in a two-parameter Weibull representation exhibits a regular downward curvature. This indicates a threshold that is a true characteristic of the population. The samples studied were cross-linked polyethylene slices 200 μm thick and 40×40 mm² in section cut from the insulation of a power cable. The aging time was varied between 250 and 9000 hours. The two-parameter Weibull distribution of the 750-hours-aged population is displayed with its 90% confidence bounds