A new method for the routine testing of alternating-current high-voltage paper-insulated cable

The usual methods of testing alternating-current high-voltage paper-insulated cables are based upon the ability of the insulating material to withstand excessive potentials in order to determine its condition. Accordingly, these methods are not suitable for routine tests imposed for the purpose of determining the degree of deterioration existing in any particular cable. In an effort to develop a routine test, which will serve to detect an impending fault, use has been made of the kenotron as the source of high-potential direct-current. By means of it a large volume of data has been secured concerning the input-current, for a cable, as a function of time after complete electrification at a constant high potential. These data when plotted, as curves, show by their shape the condition of the cable insulation. Curves showing a sharp decrease in the magnitude of input-current during the first minute and a gradual but persistent rate of decay for the succeeding six or seven minutes indicate that the insulation is in an acceptable condition from the operating point of view. Curves showing little, if any, decrease or a persistent tendency to increase during the time interval of the test indicate that the insulation has deteriorated to such a point that the cable may be expected to fail at an early date if retained in service. The degree of deterioration is indicated first: by the time which has elapsed since complete electrification before the increase occurrs, and second: by the sharpness of the upward trend of the curve in any instance. The value of this method of testing has been demonstrated by its actually having detected a considerable number of impending cable faults before they became a menace in operation. Additional refinements in the methods of measuring the input-current for different classes of cables may be desirable. Further investigations are being carried on to determine this necessity. Investigations of certain theoretical features are also being carried - n. These include, among other things, oscillographic studies of the input-current and voltage under test conditions.