How does a transformer oil dielectric strength tester work?

A dielectric strength tester applies increasing AC voltage across oil placed between standardized electrodes at a controlled gap. Voltage is raised at a defined rate until electrical breakdown occurs. The breakdown voltage value indicates the oil’s ability to withstand electric stress. Multiple tests are averaged for accuracy. Low results indicate moisture, contamination, or aging. This test is fundamental for assessing insulation safety margins before returning oil to service.