BDV stands for Breakdown Voltage. It is also called as dielectric strength value. It is the dielectric strength of transformer oil. This can indicates the presence of contaminants. If the value of BDV is higher then it indicates that the presence of contaminants is lower.

The minimum value of breakdown voltage is 30 kV-40 kV. It depends upon on the moisture present in transformer oil. If the moisture is higher then the breakdown voltage will be lower in transformer oil. It is an important parameter for transformer oil.

Oil testing machine to measure the breakdown voltage

What is Breakdown Voltage test in transformer oil?

Breakdown voltage is an important test for transformer oil. During the time of transformer operation, the oil will become degradation which may be the main cause for loss of function, decreasing the cooling capacity of the transformer, and etc. BDV test is used to test or check the quality of transformer oil. Depending upon the result of BDV test the oil will be refined or purified again. This test will be repeated in 6 months.

The procedure of Breakdown Voltage test:

Take a vessel of glass, plastic or transparent with 300-400ml of transformer oil for testing. And the temperature of the oil should be 27°C at the testing time.

  • Take two copper/brass/bronze or stainless electrodes of 12.5 mm-13mm size and dipped these electrodes into the vessel. The electrodes must be polished in a spherical shape. And the distance between two the electrodes should be 2.5mm.
  • Before the testing, the vessel may be disturbed due to the homogeneous distribution of impurities and air bubbles to escape.
  • Apply the alternating current voltage to the electrodes frequently from zero up to the appearance of breakdown, at the rate of 2kV/Sec.
  • After getting the breakdown voltage, the test kit will switch off automatically of the supply voltage of within 0.02 Sec.
  • The transformer oil testing device will measure the value of breakdown voltage (BDV).
  • The test will be repeated in 6 months in the same process. And remember that the test should conduct in a dry and clean place.


Breakdown voltage test is an important test for the transformer or transformer oil. It helps to detect the problem in transformer oil. By observing the voltage value of transformer oil we can decide that, is the oil useful for the transformer. When the transformer does not function properly then you can check the transformer oil by the help of BDV test.