Growth and forms of trees in polyethylene studied using intermittent applied voltages

Partial discharges in cavities or cracks in polyethylene power cables frequently cause dielectric breakdown, and an examination of the prebreakdown features reveals the occurrence of tree-like growths. To understand the growth processes of trees in polymer materials, we have investigated the nature of treeing and the modes of discharges in trees and artificial cavities in some thermoset and thermoplastic polymers.^1,2 The forms of trees generated by electric stress in polymers vary with the nature of the polymer material and the test conditions. Three types of trees are formed: tree-like, bush-like, and chestnut-like (with bur). We consider that the higher the pressure (in a tree) caused by gaseous decomposition products, the more the form of the tree changes from tree-like to bush-or chestnut-like. The present report describes the relation between the volume of gas generated in a tree and the growth and forms of the tree.