Treeing in polyethylene as a prelude to breakdown

This paper deals with latent damage that consists of “trees” formed by partial breakdown of defective or locally overstressed regions. This cannot be observed in opaque insulation like rubber, but the discovery of trees in polyethylene stimulated interest in this phase of study. Figure 1 shows trees in various stages in contaminated PE after 21 months at 68 volts per mil AC. They start from the conductor, from the outer surface, and from particles and fibres within the PE.