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Electrical tree growth has been investigated in free-from fibre and mechanically pre-stressed fibre-reinforced Araldite 5052 epoxy resin samples as a function of material age (defined as the time between manufacture and testing of pin-plane samples). Raman Spectroscopy has systematically assessed the tensile residual stress remaining in the cast-in fibres and by inference the compressive stress in the matrix. It has been found that the mechanical prestressing of electrical insulation, achieved by the casting in of fibres in tension into epoxy resin, could remain in the matrix but change with the sample age as a consequence of the absorption of water from the air. The mechanical pre-stressing does not only increase the tree growth times to failure but, most significantly there is a systematic increase in the tree growth times to failure due to the increase of tensile residual stress remaining in the cast-in fibres, as the sample age is increased. Based on the experimental data, a direct relationship between Raman band shift as well as residual stress of the mechanically pre-stressed fibre and electrical tree growth time to failure of the insulation has been found.