Quantitative measurement of light emission during the early stages of electrical breakdown in epoxy and unsaturated polyester resins

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Abstract
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 for these processes. Mains-synchronous photon detection techniques coupled with an ultra-sensitive photomultiplier and large-area light collection optics were used to measure the low-level light emission from pin-plane CT200 epoxy and polyester specimens subjected to 50 Hz AC step ramp electrical stress. Three types of light emission are observed consistently and correspond to (i) electroluminescence related to charge injection (Fowler-Nordheim or Schottky depending on the local electric 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 electroluminescence and hence charge injection in these materials.