What does a breakdown voltage test of transformer oil measure?

A breakdown voltage test, often called BDV, measures the voltage at which an electrical arc forms through an oil sample under controlled conditions. It is a proxy for dielectric strength and sensitivity to moisture, particles, and contamination. Lower BDV typically indicates higher water content, suspended solids, or degraded oil that can reduce insulation margins. BDV is useful for trending and for acceptance testing of new or treated oil, but it should be interpreted alongside moisture, tan delta, and DGA for a complete view.