QUANTITATIVE MEASUREMENT OF LIGHT-EMISSION DURING THE EARLY STAGES OFELECTRICAL BREAKDOWN IN EPOXY AND UNSATURATED POLYESTER RESINS

Quantitative light emission studies of the initiation and early growth stages of electrical treeing in synthetic resins have been undertaken to gain insight into the underlying physical mechanisms responsible f or these processes. Mains-synchronous photon detection techniques coup led with an ultra-sensitive photomultiplier and large-area light colle ction optics were used to measure the low-level light emission from pi n-plane CT200 epoxy and polyester specimens subjected to 50 Hz AC step ramp electrical stress. Three types of light emission are observed co nsistently and correspond to (i) electroluminescence related to charge injection (Fowler-Nordheim or Schottky depending on the local electri c field), (ii) microdischarge activity with formation of microchannels and (iii) conventional partial discharge activity during tree growth. No material-dependent threshold voltage/field was found for electrolu minescence and hence charge injection in these materials.