IEC 60156
ISO 3016
ISO 3104
The breakdown voltage at power frequency is obviously of prime importance with respect to the
insulation ability of a liquid.
The pour point is the lowest temperature at which the oil flows. To avoid local over-heating in cold
start operations, it is recommended that the pour point should be 10 K below the IEC Lowest Cold
Start Energizing Temperature (LCSET), which is standardized at –30°C. The corresponding referring
pour point is consequently –40°C.
The insulating oil viscosity is also an important parameter to describe heat dissipation in the
transformer. At operating temperatures, the thinner the oil, the easier it flows. This characteristic
becomes important when considering non-forced circulation transformers.