Treeing in Polyethylene as a Prelude to Breakdown

Trees, or fine, branching, microscopic channels extending partly or even wholly through a wall of insulation, represent a characteristic form of latent damage. This phase of dielectric breakdown has been studied in polyethylene. The phenomenon is probably similar in other dielectrics, such as rubber, where it cannot be so readily seen. A method of observing the progressive stages of treeing and final punchout under the microscope is described. The similarity between the appearance in the final stage of needle breakdowns after preliminary treeing and that of the breakdown paths from ordinary tests with alternating, direct, or impulse voltage supports the view that in most breakdowns the eventual failure is preceded by treeing. It is probable that the time interval between the start of a tree and the final breakdown can vary from months to microseconds. At low over-all stress trees may start at points of high local overstress but eventually stop growing. This permits the growth of the other trees before breakdown, so that the presence of many trees in different stages of growth is most likely in insulation after prolonged life tests. At very high stress the first channel may traverse the wall before others form and the whole process from tree start to final punchout may require times only of the order of microseconds. Regardless of the time scale, the phenomenon seems to be basically similar, in that the tree comes before the punchout.