LONG-TERM LIGHT-EMISSION MEASUREMENT AND IMAGING DURING THE EARLY STAGES OF ELECTRICAL BREAKDOWN IN EPOXY-RESIN

An ultra-sensitive photomultiplier and large-area light collection opt ics were used with a high-sensitivity charge-coupled device camera to detect and image the long-term light emission behaviour during electri cal tree initiation in pin-plane CT200 epoxy resin samples under const ant alternating current voltage stress. The observed light emission is interpreted in terms of electroluminescence due to charge injection/e xtraction at the pin-tip followed by slow material degradation and eve ntual failure of the metal pin-resin interface. Interface failure is a prelude to formation of microchannels that result in tree initiation. High-resolution charge-coupled device imaging of the electroluminesce nce reveals that the maximum light emission occurs at the pin-tip and is still observable 10 mum into the resin. Under long-term continuous electrical stress, the light emission is found to spread principally a long the cone of the pin rather than into the resin.