On the stability of the electrical resistivity and the contact resistance of electrically conductive modified silicone elastomers

Cable accessories for medium- and high voltage applications contain field grading and field zoning parts or layers. These parts often consist of electrically conductive modified silicone elastomers. The electrical conductivity is achieved by using selected and long-term tested carbon blacks. Conductive layers and parts are used to provide safety against exposure, inner shielding and an electrical contact to metallic parts of earth connections. The paper describes the measuring technology of the electrical resistivity of carbon black filled elastomers and of the resistance of electrical contacts between carbon black filled elastomers and metallic contact parts. Service relevant stress parameters such like current load of the elastomer, storage at elevated temperature and elongation of the rubber were systematically investigated with respect to their short- and long-term influence on the volume resistivity of silicone specimens. An adjustable model equipment is used to investigate contact processes between conductive silicone specimens and metallic contact partners. The contact status is evaluated by measuring the contact voltage in dependence of the time. Evaluated electrically conductive silicone elastomers are found to show a long term stable electrical conductivity, even at a current density that is by far higher than under service conditions. Neither the elongation of the specimens nor the storage at elevated temperature leads to significant changes of the resistivity. It is possible to achieve a reliable electrical contact between electrically conductive modified silicone rubber and metallic contact partners. Precautions are needed with respect to the design of the contact, the contact area and the contact force to ensure a safe long-term availability.