The effect of polarity and surge repetition rate on electroluminescence in epoxy

An experiment conducted to study the light emission from an electrically stressed transparent epoxy prior to the initiation of electrical trees is described. The effects of surge voltages from 0 to 15 kV, positive and negative polarities and repetition rate from 1 to 10000 p.p.s. on the light output were determined. It is shown that light pulses are emitted from transparent epoxy insulation, prior to the occurrence of a visible tree, during the application of 1-μs-duration surges. The location of the light pulses with respect to the surge pulse shape depends on the surge polarity. That is, charge carriers cause light emission (and presumably move) when the needle is negative with respect to the space charge region in the surrounding epoxy. An observation that the rate of light emission decreases with decreasing time between surges may imply that it takes a finite time for the space charge in the epoxy surrounding the needle tip to decay. Such repetition rate measurements may provide a measure of the detrapping times