Surface Conductivity Measurements on Large Polymer Samples

A system has been developed by the authors to attempt to measure the surface conductivity of a large high voltage gas-filled switch housing. The housing is a hollow cylinder with an internal diameter of 300 mm and has a wall thickness of 25 mm. Unlike the situation with thin films where it is possible to perform meaningful surface conductivity measurements by creating a well defined field parallel to the surface of the material, there is a problem when dealing with bulk samples in separating the current component that results from conductivity in the bulk of the specimen from the surface component which is of interest. In addition there are concerns associated with currents arising from transient volume polarisation effects close to the current sensing electrode. A custom designed measurement system was therefore developed and this paper reports on the results obtained using this system. The observed behaviour is complex with current transients observed that showed a double relaxation process, followed by an increase in the measured current, suggesting the development of a space charge within the sample arising either from a polarisation process or from the injection of space charge into the bulk or onto the surface of the sample. The implications of these processes on the measurement of surface conductivity in measurement systems for bulk samples are discussed.