Relationship between polarity of surface leader and contact angle on insulator with backing electrode under AC voltage application

When AC high voltage is applied in air to a cylindrical insulator with a backing electrode the polarity of generated leaderlike surface discharges depends on the contact angle. The contact angle is the angle between the insulating surface and high voltage electrode on the insulator. In the range of the angle from 60 degrees to 120 degrees the negative surface leaders appear; in the range from 120 degrees to 180 degrees the positive surface leaders appear. No negative surface leaders appear in the range of the angle from 120 degrees to 180 degrees . The onset voltage of the positive leader is higher than that of the negative one. AC flashover voltage along the surface in air also shows dependence on the contact angle, which results from the difference in the onset voltage between the positive and the negative leaders. Impulse flashover voltage, on the other hand, shows no precise contact angle dependence.