Voltage testing of cables

In this country rubber compound, paper, and cambric are generally used for cable insulation. From the formula S = 0.868 V/d log10 D/d, the stress at any point in a homogeneous insulation may be determined. The minimum stress and the maximum allowable voltage occur when the conductor is 10/27 of the sheath diameter. The present irrational practise of testing cables should be standardized to conform to this formula or a modification of it. Over-stressing of the insulation is accompanied by a change of insulation resistance and electrostatic capacity. No one factor of safety is applicable to every cable system, but one must consider the conditions of operation as well. In testing the voltage may be applied: (1) by submersion; (2) between the conductor and metallic sheath; (3) between wires. The submersion test is the most severe. A sine wave is desirable for testing purposes, but rarely occurs in a commercial generator under these severe conditions of load. Reactance cannot always be used successfully to reduce the volt-ampere load on the generator. With a distored wave an a-c. voltmeter gives only a poor indication of the maximum voltage. The writers have devised an instrument based on the oscillograph principle, with which the maximum voltage may be determined, regardless of wave form.