Condenser cones for cable testing

The use of condenser cones for testing cable ends in air is considered in detail, a general description having been previously given [see Abstract 1934B00508]. For successful operation in air three conditions must be fulfilled as follows: (1) The length must be adequate to avoid external flashover. A table of cone length and flashover voltage is given. Cones may, if necessary, be connected in series. (2) Theradial thickness of insulation between the inner former and the outer foil of the cone must withstand the test voltage. Standard oil-impregnated cones, kept dry, and normally stored immersed in cable compound are perfectly satisfactory. (3) The filling compound must withstand the stress resulting from the test voltage. This point is most frequently the limiting factor and is considered in detail. Reduction of stress on the filling compound or oil may be accomplished by (a) building up the radial thickness of the solid insulation at the base of the cone, (b) by using compound of high permittivity and/or high power factor. The theory of the latter method is discussed. It is desirable to keep the viscosity high, and two compounds possessing high viscosity as well as high power factor have been developed, and their properties are described. The application of cones as guard rings for Schering bridge measurements is discussed. Finally, the use of cones for British and foreign standard tests is briefly considered.