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.

This report deals with dielectric tests on completed apparatus. Tests on materials and on components during the course of fabrication usually are left to the discretion of the manufacturer and are not a subject for standardization except as to method and technique. Impulse test values where specified in standards are based on the Basic Impulse Levels and thus are consistent for different equipment. Impulse test conditions and technique are outside the scope. Hence, this report deals mostly with conditions and test values associated with lowfrequency dielectric tests

A statement of the purpose of this report follows from this review of the general problem. It is:(1) to present a survey of the overvoltages encountered in service.(2) to review existing test values and practices in present standards(3) to propose guiding principles for the selection of dielectric test values (4) to investigate other types of testing to determine their latent usefulness and the desirability of standardization.