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 optics were used with a high-sensitivity charge-coupled device camera to detect and image the long-term light emission behaviour during electrical tree initiation in pin-plane CT200 epoxy resin samples under constant alternating current voltage stress. The observed light emission is interpreted in terms of electroluminescence due to charge injection/extraction at the pin-tip followed by slow material degradation and eventual 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 electroluminescence reveals that the maximum light emission occurs at the pin-tip and is still observable 10 mu m into the resin. Under long-term continuous electrical stress, the light emission is found to spread principally along the cone of the pin rather than into the resin.