Breakdown probability and size effect in liquid helium

Breakdown statistics in liquid helium have been evaluated in order to correlate with size effect degradation, i.e. the area and the volume effect, respectively. Reprocessing large number of data sets yielded the popular two parameter Weibull distribution not being representative; more sophisticated functions, e.g. the Gompertz function had to be used in order to fit the data. A considerable amount of data were found to be not homogeneous, and a sum of 2-parameter Weibull subdistributions had to be defined. These results are discussed in detail. No formal straightforward correlation with the known size effect could be established. Special emphasis has been attributed to the low breakdown probability limit in order to meet engineering design needs. There seems to be a kind of threshold, but size degradation cannot be ruled out, and long term performance has still to be proven.