The breakdown voltage is of importance as a measure of the suitability of an oil to withstand electric stress. Dry and clean oil exhibits an inherently high breakdown voltage. Free water and solid particles, the latter particularly in combination with high levels of dissolved water, tend to migrate to regions of high electric stress and reduce breakdown voltage dramatically. The measurement of breakdown voltage, therefore serves primarily to indicate the presence of contaminants such as water or conducting particles, one or more of which can be present when low breakdown voltage values are found by test. However, a high breakdown voltage does not indicate the absence of contaminants.