Lightning is a natural phenomenon that destroys a power line. According to the Sumatra load supplier and handling Center (Sumatera P3B), the intensity the number of lightning strikes on a 150 kV transmission line was very high in Indonesia, reaching 66%. As a result, the isokeraunik level on the 150 kV transmission line in that area is at 174 days/year, and the flashover occurrence rate in its ceramic insulators is 82% in the hilly area, 16 % in the rice fields, and 2% in the desserts/pastures. The investigation results showed correlations between the occurrence of back-flashover and contaminants of insulators. Furthermore, the results of the experiment show that the insulator contaminated with moss has a lower dielectric strength as the rate of breakdown voltage in the test voltage 78.728 kV is 20.54 kV in the test with a single plate, 30.511 kV with two plates, 56.328 kV with three plates, and at 196.85 kV for its 11 plates.