Tree initiation in polyethylene

Recently, a great variety of organic insulating materials has been, used as insulators in electric power cables. Since these devices are, being used at increasingly higher voltages and powers, the tree-like, breakdown phenomenon due to cracks and impurities in insulators, has become a serious problem. Plastic insulators containing cracks, and impurities have a greatly reduced lifetime. This is because the dielectric constant inside a cavity is lower than that on the outside, and the electric field tends to concentrate locally in the cavity. The, treeing phenomenon is found in thick materials and both the growth, and shape of trees depend on a corona streamer generated in narrow, channels of several microns in width. It is considered that the, growth of a tree forms by intrinsic and thermal breakdown due to, the high electric field at the tip of the tree caused by the growth of, a corona streamer¹.