The DC hi-pot tester, though with a higher test voltage, is found with insulation weaknesses, but the insulation of electric appliances is mostly composite dielectrics, whose voltage is distributed by resistance under the effect of DC voltage. Therefore, the DC hi-pot test might not find out the weaknesses of the DC electric appliances under the AC electric field. On the contrary, the AC hi pot tester is in line with the electric conditions imposed on the electric appliance during its operation process. Meanwhile, the AC hi-pot tester is generally higher than the operating voltage. Therefore, after passing the test, the appliance can gain a larger safety margin, so this test has been a major method to guarantee operation safety.