What is the acceptable breakdown voltage of transformer oil?

Acceptable BDV depends on the test method and whether the oil is new, processed, or in service. IEC 60156 is a common method for AC breakdown voltage of insulating liquids, using standard electrode geometry and controlled rise rate. Utilities set acceptance criteria in procurement specs and maintenance standards, often higher for new oil and lower but still controlled for in service oil after filtration. A BDV trend is often more useful than a single number, because moisture, particles, and gas bubbles can cause sudden drops and increased risk of internal discharge.