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The inception of ac treeing has been investigated in various cast epoxy resins, using a needle-plane test configuration. Mechanical stress due to shrinkage in the curing process is present at the needle tip, and in some cases causes micro-cracks. Lower tree inception voltages were observed in mechanically stressed specimens indicating that the micro-cracks act as weak points. In less stressed samples which have higher tree inception voltages light emission, probably electroluminescence, was observed at a voltage much lower than that of tree inception. In those cases, a small discolored area (about 10 μ) appeared prior to tree inception as the ac applied voltage was increased.
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Nakanishi et al. (1979) studied this question.
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