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The DMM life expression has been used to simulate dc electrical ageing in a 1 mm thick flat specimen of PET represented by a frozen distribution of parameter values centred on the characteristic values previously determined from 36 μm films. Simulations were performed for 40 MV/m, T =180 °C and 20 MV/m, T = 110 °C, and evaluated in terms of the generic ageing features of the life expression. In all cases it was found that only a few isolated sites of damage were produced during the major part of the ageing period, originating at sites of high energy concentration or damage susceptibility or both. At the end of the ageing period a rapid breakdown ensued from a region where the continual increase of energy concentration was possible. The breakdown structure took the form of a wormhole failure. Repeated simulations varying either the distribution of the damage susceptibilities of the sites or their energy concentrating ability, gave life distributions that showed a time threshold.