On the degradation of insulating materials withstanding electrical stress

An avalanche of physical and physico-chemical phenomena, often self-sustained, lead to the end of life of an insulating material. The collapse of the insulating role of a material is greatly due to the electrical field stress. The path leading from the catastrophic situation, the breakdown, to its origin is followed in the present report. A defect, preexisting or created by the in service stresses, gives rise to an ionization process in a volume bounded by insulating walls. With time the process does not remain identical according to the evolution both of the material and of the gas making sometimes difficult the selection of the electrical signal detected. The end of life will occur through micro-cracks in which new discharges are initiated activating the propagation of these structural failures. This change of structure of the insulating material when due to the existence of water molecules will lead to the same kind of final degradation. The polymeric insulating materials could contain antidotes but they lead only to a partial healing. From polymers as little complex as the polyethylene used in cables, assuming that there is no preexisting defect one endeavours to understand how a microstructure reacts with a local field or accumulated charges. The present material summarizes the thoughts of the author pertaining to the problems of ageing and dielectric breakdown in organic insulating materials, processes often given a concrete form through experiments.