The dielectric breakdown voltage of an insulating liquid is important as a measure of the liquid's ability to withstand electric stress. The parameters of the standard test methods from different countries are not similar. In the different standards there are used five or six single breakdowns in a glass or a plastic vessel with about 0.5 litter test oil content. The voltage raising up velocities are between 0.5 and 3 kV/s. After filling the test oil in the test cell until the start of the first breakdown test there is a pause between 3 to 10 minutes and between each breakdown there are breaks of 1 or 2 minutes. Also the form of electrodes are different, sphere formed (according VDE) or flat formed (ASTM). The mean value of these single breakdowns, which is a parameter for the oil quality, and the coefficient of variation were calculated. A weak point of the standard dielectric breakdown tests is that it is possible to reach values for the coefficient of variation up to 30% and higher. In this paper it will be shown where the problems are and some ways are given how to minimize the coefficient of variation.
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