The service record of electric equipment depends largely upon the performance of its insulation. Insulation failures result from various combinations of such causes as progressive aging, moisture, mechanical damage, excess temperature, chemical action and abnormal voltage stresses. Commercial tests involving all these factors are impractical. It has long been the practice, however, to test new insulation at a voltage appreciably greater than its rated operating voltage. The dielectric test voltages should be chosen to result in good operating performance and satisfactory life. The establishment of suitable dielectric test specifications involves consideration of the over voltages to which equipment is subjected in service, a study of equipment strength and protective methods and a correlation of the dielectric test voltages and the performance of equipment in service.