Breakdown Voltage Analysis The magnitude of the low breakdown voltage is of ESSO Univold oil with an average of 43.46 kV/2.5 mm, while for the type of Diala B oil has an average of 59.88 mm kV/2.5 mm. The greater the breakdown voltage in the transformer oil, the better the oil serves as a coolant and insulation on a transformer. The magnitude of the breakdown voltage of the high Diala B oil (59.88 KV/2.5 mm) is suitable for voltages above equipment is needed more than 170 KV power transformers while as the breakdown voltage of ESSO Univold oil rather low voltage equipment suitable for less than 70 KV as