Investigation of in-service degradation of a low voltage PVC cable

One of the most common insulating materials of low voltage cables is PVC, which is also used for jacket because PVC has outstanding resistance against environmental stresses. However, the mechanical stresses can injure the jacket and water easily penetrates inside the cable. Once this has happened, the metal belting layer will be oxidized and water will ingress into the core insulation because water swellable protective layer is not used in low voltage cables. Besides the thermal ageing, the water ingression into the material is the most important degradation mechanism of PVC insulation. In this study, mysterious and rapid degradation is investigated in a low voltage cable line. Operator has measured continuous and fast decreasing of insulation resistance since installation. The possible reasons of degradations are investigated by laboratory examination, which consists of structural analysis of cable, parallel electrical, mechanical and chemical measurements on core insulation. The results suggest that not the water penetration causes the degradation rather the not proper compound, which is resulted from manufacturing problems, is the reason of the poor insulation resistance values.