Electroluminescence in epoxy insulation

Electroluminescence (EL) in transparent epoxy subjected to repetitive voltage surges has been investigated. The effect of surge voltage magnitude, repetition rate, and polarity on the magnitude and number of light pulses emitted was studied. Using a needle-plane electrode geometry, the light inception stress was determined to be ~200 kV/mm for both positive and negative voltage surges. As the peak voltage of a surge increased, the number of light pulses increased exponentially. More light pulses were emitted under negative surges than under positive ones, and more were detected on the leading edge than the trailing edge of a negative surge, whereas the reverse occurred under a positive surge. As the surge repetition rate was increased, the amount of EL per surge decreased. Finally, there was no change of the spectral composition of EL with either polarity or magnitude of the applied surge. The results suggest that the detected EL may be due to single photons. The observed EL response is attributed to the creation and depletion of trapped space charges