On the basis of experiments, a model is proposed for the initiation of electrical trees within voidless PMMA (polymethyl methacrylate) samples in a divergent field. Initiation of electrical trees under a DC applied voltage is due to the localized field intensity overcoming the intrinsic breakdown strength of dielectrics. However, for AC or pulsating voltages, as a result of accumulated charges, trees are initiated at lower voltages, and in general there is an incubation period before the trees appear. It is suggested that space charges play an important role in the initiation of trees under different conditions based on experiments of electron injection. It is concluded that all experimental results on tree initiation can be interpreted satisfactorily in accordance with the proposed model.
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