How does a transformer oil BDV testing kit measure dielectric strength?

A BDV kit measures the voltage at which oil breaks down electrically under controlled conditions. The tester uses a calibrated AC high voltage source connected to standardized electrodes immersed in an oil cup. Voltage rises at a defined rate until breakdown occurs, then the value is recorded, often over multiple consecutive tests to compute an average. Moisture and particles strongly reduce BDV, so good sampling and clean test vessels are essential. Results are used to decide whether oil needs filtration, dehydration, or replacement.