Mechanical prestressing improves electrical strength

There is a strong mechanical influence on the growth of electrical trees in electrical insulation resins. This is not to argue that electrical treeing is exclusively a mechanical phenomenon directly analogous to mechanical cracking. It is obviously an electrically driven process; no volts, no trees. Nevertheless, the mechanical dimension is of sufficient importance for its effects to be exploited to advantage using the prestressing technique. The effect that post-curing at 100/spl deg/C has on the development of prestress, and on the ability of the cured sample to retain its prestress as the temperature is raised, has both positive and negative aspects. On the down side, material cured at 100/spl deg/C requires a greater tension to be applied to the fibers during the casting process in order to achieve the same degree of prestress. On the other hand, the increase in the glass transition temperature from 65/spl deg/C to 113/spl deg/C permits the use of the composite at temperatures up to 80/spl deg/C before any significant loss of prestress occurs, as compared with 40/spl deg/C for material cured at room temperature. As an additional bonus, there is an enhancement of the mechanical strength resulting from the inclusion of cast-in fibers, which is important where the electrical insulation also acts as a structural member in the insulation system.