The performance of aged stress grading materials for use in electrical machines

Stress grading materials for rotating machines take the form of glass tape or epoxy paint which has been loaded with a semi-conducting filler, such as silicon carbide. The results from testing a large range of these stress grading materials showed that the surface resistivity could be characterised as a function of surface electrical stress by two constants which represented the resistance at zero stress and the degree of non-linearity. The present work relates the measurements made on samples in the laboratory to the performance found on production bars manufactured for service