The resistance of sheet insulation to surface discharges

Investigations show that a satisfactory classification of insulating materials with respect to their resistance to deterioration and breakdown by surface discharges in air is given by time-to-breakdown tests using simple rod and plate electrodes. It is necessary to circulate dry air over specimens to avoid the formation of semiconducting films, and comparison should be made only between specimens of similar thickness. The discharge resistance of most materials is reduced by increasing the ambient temperature or by applying mechanical strain. Results of tests on thin films and sheets of several polymers and on both glass and paper laminates of 1¿3 mm thickness are given. Recommendations for standard tests for comparing the discharge resistance of materials are given in the Appendices.