The electric field distribution under lightning impulse voltage applied on the insulator in SF6 gas as well as the cause of flashover is anatomized with the methods of both calculation and experiment. In the case of lightning impulse voltage stress

adding a shielding case is an effective way to decrease the vertical component of electric field generated by the electrode configuration. The surface flashover level depends mainly upon two essential factors--the homogeneousdegree of electric field distribution and the insulator surface contaminated by the conductive particles in the ambient. For the partial field intensity increased by the particular electrode shape

it is useless adding the surface distance to increase the surface flashover level.