THE EFFECT OF VOLTAGE AND MATERIAL AGE ON THE ELECTRICAL TREE GROWTH AND BREAKDOWN CHARACTERISTICS OF EPOXY-RESINS

Electrical tree growth (a long-term electrical breakdown process) has been investigated in Araldite CT200 and CT1200 epoxy resins as a funct ion of voltage and material age (defined as the time between manufactu re and testing of pin-plane samples). Reproducible and predictable ele ctrical tree growth was obtained for both CT200 and CT1200 epoxy resin s provided that (i) the essentially random tree initiation time is rem oved and (ii) the samples tested were of the same age. The tree growth and time to failure (defined as the time to breakdown from a pre-init iated 10 mu m tree) characteristics as a function of both voltage and sample age show large step changes at a critical voltage and critical age. In particular, the resin physical ageing has a large effect on th e tree growth behaviour, with the time to failure varying by three ord ers of magnitude over a time span of 3 years. Measurements of some of the physical properties (residual internal mechanical stress, surface refractive index, glass transition temperature and dielectric loss) of CT200 epoxy resin all indicate the occurrence of physical ageing of t he resin, with structural (network) relaxation as the most important a geing process. However, these measurements are unable to account for t he step change (critical age effect) found in the time to failure of t ree growth. The fractal nature of tree growth and its relationship wit h voltage and the long-term changes in the properties of the resin are briefly commented upon.