Moisture permeation in oversheaths and insulation

The initial part of the paper describes the moisture diffusion process in the materials used as a power cable sheath and insulation. The methods of calculating diffusion data are outlined and the importance of relating relative concentrations to the solubility coefficient is described. The relative merits of PVC and polyethylene sheaths are given. The figure of 200 parts per million of water has been used as a water content which limits the growth of water trees. The reasons for choosing this level are described. However, this work was done using conventional XLPE insulation and the complications of applying this result to tree retardant insulations are outlined. The role of water in cables is described using examples from service. The effect of service conditions on the role of water and long term life are described for cables with a polymeric sheath and for cables with a metallic barrier