The basis for the nondestructive testing of insulation

DIELECTRIC strength is the property of insulation that is most important to the operating power companies. The factor of safety determined by the ratio of breakdown voltage to operating voltage must be maintained. Yet this breakdown voltage cannot be measured on a finished piece of insulation. Actual breakdown, except in the case of liquids, destroys the insulation. Overvoltage less than that required for breakdown has been found to injure the insulation and leave it with a reduced factor of safety. Non-destructive testing at overvoltage is feasible only if some electrical quantity, such as current, is measured and used as a sensitive indicator of incipient damage. The use of d-c overvoltage from a tube rectifier for cable testing is an example. Non-destructive testing of insulation, as currently practiced, is the measurement of certain electrical characteristics of the insulation, such as capacitance, dissipation factor,^∗ and leakage resistance, at a voltage not greater than the operating voltage. Ordinarily a relatively low voltage at the operating frequency is used, 100 volts to 10 kilovolts. Of these three characteristics, dissipation factor has come to be considered the most reliable indicator of the condition of insulation. The standard practice is to measure the dissipation factor of insulation at the operating frequency at stated intervals of time after installation. A progressive increase of dissipation factor with time is believed to indicate deterioration of the insulation. Certain limiting values are set depending on the type of insulation. When these limits are exceeded, the insulation is removed from service. Frequently, greater weight is placed upon continual change in the value of dissipation factor than upon its absolute magnitude.