What does a transformer oil breakdown test reveal about insulation strength?

A breakdown test, often BDV, reveals the oil’s ability to withstand an applied electric field without forming a conductive arc between electrodes. Low breakdown values usually point to contamination by water, particles, or degradation products that create weak points in the dielectric. It is a practical screening tool to validate oil handling quality after maintenance and to assess whether oil treatment is needed. However it does not directly measure paper insulation condition, so it is combined with DGA and furan testing for full insulation health assessment.