The effect of space-charge on the fracture toughness of Perspex

Tree initiation is believed to arise from the creation of small cracks by fatigue failure, caused by cyclical electrostatic forces induced by the alternating applied voltage, with tree growth resulting from microscopic explosions, within the dielectric due to localised intrinsic breakdown on the crack surfaces. The growth appears to follow the precepts of fracture mechanics as applied to crack propagation. It is a common observation that electrical trees can fully bridge the interelectrode spacing while still retaining voltage integrity. Breakdown does not occur until the channels of the tree have widened sufficiently to sustain an electrical discharge. On the assumption that channel widening is enhanced by the repulsive forces between charges trapped on opposite faces, this paper addresses the phenomenon by first establishing the magnitude of the forces between charged dielectric surfaces