AbstractDesigning high-voltage insulation it is a main prohlem to determine the withstand voltage and the minimum life time. As a rule extrapolation hased on experimental mean values and variances (e.g. Three-Sigma Law) is used for determining the respective voltage level or life-time. For practical reasons only a limited numher of voltage tests may be carried out. Therefore the experimental results of small sample sizes are only the most probable inference of the actual values. Given a normal or transformed normal distribution of the measured values diagrams of the tolerance limits for the actual withstand voltage and minimum life-time have been developed using the non-central t-distribution. The application of this stochastic modelling technique permits a much preciser determination of the tolerance limits than the Kolmogorov-Smirnov goodness of fit test or conventional worst case design based on the central-t and Chi2-distribution.