Break down voltage that is the full form of BDV.

But break down of whom ?? Transformer oil.

The voltage at which transformer oil breaks down or simply start conducting the electric current.

We all know, transformer oil is used in the transformer for insulation and cooling of the transformer windings & core.

So it is necessary to check its insulation capability or known as dielectric strength.

If it does not have sufficient good Insulation or dielectric strength than what is the use of filling transformer with that oil.


Generally there is a standard test kit for measurement of the BDV value of transformer oil.

The standard test kit is nothing but a glass box in which there are two electrodes( 2.5-4 mm spacing between them) which are connected to a high voltage supply through variac (Auto transformer).

In this glass box , oil has been filled than voltage has been applied and slowly raised until there is spark.

Spark between electrodes is indication that transformer oil given path for current to jump from one electrode to another electrode.

That electricity flow , we see in the form of spark.

When BDV value comes out to be very less, suppose we have measured a transformer BDV and it comes out ..30 KV

Than what it means , transformer oil has some moisture because of which its result comes poor.

All we need to filter the transformer oil .

During oil filtration, transformer oil is heated and filtered out moisture and any suspended impurities are removed from the oil.

Code and Standards for BDV of insulating oil:

IEC code for finding out the Breakdown voltage of insulating oil is IEC 60156 that is widely accepted across the world.

The test method described above is used to find out insulation capability or dielectric strength of not only transformer oil but also for insulating oil in tap changers, reactors , capacitors, oil type current transformers and potential transformers/voltage transformers.

Results assessment:

How you can know the valves comes out from BDV test is good or bad?

As per IEC(International Electro technical commission)-60156, the following is considered as good BDV valve for new/fresh/ unused oil.

Equipment VoltageDesired BDV valve
?72.5 kV55 KV
>72.5 kV
?170 kV
Above 60 KV
Above 270 KVAbove 60 KV

Assessment of the results obtained of insulating oil taken from the equipment that are already in service.

Recommended BDV value of In-service oil:

Equipment voltageGood BDV valueFair BDV valuePoor BDV value
?72.5 kV>40 kV30 – 40 kV>30 kV
>72.5 kV
?170 kV
>50 kV40 – 50 kV>30 kV
270 kV>60 kV50 – 60 kV>50 kV