Dielectric Breakdown Voltage Testing - Part 7
The IEC method from Standard 60156 including values used in IEC standards for unused and in-service mineral oil uses electrodes that are similar geometrically to the VDE electrodes used in ASTM D1816. The spherical electrodes are spaced 2.5 mm apart, and the rate of voltage increase is 2,000 volts per second. The method in Standard 60156 allows the optional use of an impeller, operating in similar fashion to the one described for D1816, except that it operates at 250 to 300 rpm. The IEC method also allows use of a magnetic stirrer operating at a similar rate, if there is no significant chance that magnetic particles will be removed from the oil. The presence of magnetic particles would affect dielectric breakdown in the transformer, so removal of those particles by the stirrer during the analysis would yield unrepresentative values.