In this study conducted testing to know the isolation prisoners on the generator bushing transformer PT. INDOENSIA POWER UJP PLTU Jawa Tengah 2 Adipala is still in good condition after the flashover. In this study the way to know the insulation resistance is still in good condition is to collect data and do testing the tangent delta at the research site. Further analyze whether the test results gained are well worth in accordance with the ANSI C 57.12.90 standards. In this study came the result that the testing of the tangent delta isolation prisoner on the transformer's generator bushing after the flashover was poorly priced under the standard. This is because of contamination in the isolation of bushing so that action is performed to get the best value of tangent delta. After the action is obtained the result of tangent delta with a value of 0.4984% for phase R, 0.4412% for phase S, and 0.4131% for phase T, where the value meets ANSI C standard 57.12.90 so that the equipment can work properly.