What is the acceptable BDV for transformer oil in service?

Acceptable BDV depends on voltage class, oil type, and the test method and electrode geometry used. Utilities usually set minimum in service BDV limits and action levels rather than one universal number. The BDV test itself, typically per IEC 60156, measures the oil’s ability to withstand AC stress without breakdown using standardized electrodes and a controlled voltage rise rate. Operators look at BDV together with moisture and particulate contamination, because low BDV often improves significantly after dehydration and fine filtration. Trending is often more valuable than a single result.