IEC standard (International) for insulating oil dielectric breakdown voltage testing
The International Electrotechnical Commission (IEC) defines only one method for oil breakdown dielectric voltage testing. This is IEC 60156 Insulating Liquids - Determination of the Breakdown Voltage at Power Frequencies - Test Method.
IEC 60156
IEC 60156 is an international standard that appears in many forms as IEC member national committees from various countries have adopted it. Examples are British Standard BS EN 60156 and German VDE 0370 part 5. IEC 60156 specifies the use of (either spherical, or) mushroom (shaped) electrodes the same those used in the ASTM D1816 standard. The IEC standard differs in a number of ways from D1816, but the main difference is the IEC standard allows the optional use of a stirring impeller, the use of a magnetic bead stirrer or even no stirring at all. The standard states that differences between tests with or without stirring have not been found to be statistically significant. The use of a magnetic stirrer is only permitted when there is no risk of removing magnetic particles from the oil sample under test. When oil is used as a coolant and therefore circulating it would be stirred during testing. For example oil from a transformer normally circulates if it is used as coolant, so an oil sample taken here would normally be stirred to ensure the best chance of detecting particle contamination. Oil from a circuit breaker is normally static in use so particles would naturally fall to the bottom where they are unlikely to cause a problem. So in static use applications, an oil sample would usually not be stirred.
The dielectric breakdown values from the IEC 60156 method are usually higher than those from the ASTM methods. Possibly this is in part because of the differences in voltage ramp up speed and electrode gap compared with D1816, and electrode shape compared with D877. (The IEC electrode shape provides a more uniform electric field). The result is that for well-maintained transformers the breakdown voltages may be higher than a 60 kV test instrument can reach.
This may not be a problem when evaluating new oil from a supplier or even for in-service oil, but often an actual breakdown voltage value is required. When testing to IEC 60156, therefore, the use of an instrument capable of applying a higher voltage is advisable. As with D1816, dissolved gas in the oil sample may reduce breakdown values but the effect is much less pronounced than with the IEC 60156 standard.
