A partial discharge (PD) is an electric discharge that breaks through a small portion of the insulation between two conductors. A discharge is when electrical charge that has accumulated is suddenly released (like a spark). A partial discharge does not completely cross the insulation between the two conductors, only a small portion of it in one area. It may happen under the stress of high voltage. It is caused by a small opening (void) inside the insulating material. The insulating material can be either solid or fluid. The charge passing through this insulation is called electrical breakdown.