How does a transformer oil breakdown tester measure insulation strength?

A breakdown tester applies an increasing AC voltage across two electrodes immersed in a standardized oil sample until electrical breakdown occurs. The breakdown voltage is recorded, often repeated multiple times on the same sample to obtain an average and to reduce random variability. The test is highly sensitive to moisture, fibers, carbon particles, and bubbles, so sample handling and cleaning of the test cell matter a lot. IEC 60156 is a common reference method, specifying electrode geometry, gap, stirring conditions, and voltage rise rate.