Quantitative light emission as a probe of electrical treeing processes

Light emission during the initiation and early growth stages of electrical treeing in CT200 epoxy and unsaturated polyester resins was studied quantitatively in order to gain insight into the underlying physical mechanisms responsible for tree initiation and growth. The data were also used to critically assess the field limited space charge (FLSC) model of T. Baumann et al. (1987) as well as the Schottky and Fowler-Nordheim charge injection mechanisms. Three distinct types of light emission are observed as the applied voltage is increased. The first is associated with initial charge injection with no resulting defects, the second with tree initiation, and the third with the existence of large channels. No threshold field/voltage is found for charge injection into the material. Some evidence is found for a material-dependent threshold for tree initiation from short-term step ramp tests. However, the results reveal that this is not a true threshold and that the stressing time is also important. This has implications for the influence of the tree initiation phase on the time-to-failure statistics in service