Discussion on “voltage testing of cables” (Middleton and Dawes), Detroit, Mich., June 25, 1914. (see proceedings for June, 1914)

W. A. Del Mar: The factor of safety of an insulated wire or cable, being the ratio of the breakdown voltage to the normal operating voltage, cannot be determined experimentally without testing the insulation to destruction. It can, therefore, never be definitely known in practise, although susceptible of approximate predetermination in the absence of faults, by calculation based upon tests. If, however, we define the factor of assurance^∗ of an insulated wire or cable as the ratio of the test voltage to the normal operating voltage, we can provide an experimental basis for the comparison of voltage tests. Thus, if a cable for operation at 11,000 volts is tested at 25,000 volts, we have a factor of assurance of 2.27. If the breakdown voltage is 75,000 volts, the factor of safety is 6.82, and the ratio between the safety factor and the factor of assurance is 3.0.