What does the breakdown voltage test of transformer oil indicate?

BDV indicates the oil’s ability to withstand electric stress without forming a conductive path between electrodes. A falling BDV is commonly associated with moisture, fibers, carbon, metal particles, or other contamination that triggers early discharge. BDV is a quick screening metric but not a complete health diagnosis, so it is paired with moisture content, acidity, dielectric dissipation factor, and DGA to determine whether filtration, drying, or deeper investigation is required.