Influence of electrode separation on electrical treeing in a glassy epoxy resin

The influence of electrode separation on electrical treeing in a glassy epoxy resin of needle-plane geometry has been studied under a fixed 50 Hz AC stress of 15 kV peak voltage. Growth of electrical trees is compared between samples of 1 mm, 2 mm, 4 mm and 6 mm electrode separation. The tree inception times and their distributions imply a change of mechanism for tree initiation when the electrode separation is above 4 mm. Subsequent tree growth was not affected by electrode separation. The trees were of the filamentary non-conductive type and did not feature a classical runaway to breakdown. Rather, tree propagation along the needle axis has been found to follow a rate independent of electrode separation. This implies that the filamentary tree growth is not dominated by modification of the electrical field in front of the tree tips, although such a conclusion is in conflict with existing treeing models. Breakdown does not occur rapidly after a filamentary tree fully crosses the dielectric, but is still primarily determined by the average electrical field across the sample.