GB334840A - Improvements in the manufacture of electric cables - Google Patents

Abstract

334,840. Smith, W. S., Garnett, H. J., and Dean, J. N. May 9, 1929. Insulating coverings for cables and conductors. -An electric cable, stated to have a low electrostatic capacity, is formed by applying to a conductor a layer of thermoplastic insulating material such as guttapercha, balata, rubber or rubber-bitumen mixtures, treated to develop a cellular structure The cellular structure may be developed by the inclusion of air or other gas under pressure in the thermoplastic material followed by the release of the pressure, or by the incorporation of a substance, such as ammonium carbonate or bicarbonate, which is resolved into a gas at moderate temperatures. In order to render the cable waterproof, a covering of commercial guttapercha or rubber may be applied over the cellular insulation. In an example, a cable is formed by spirally winding a tape of rubber on a copper core, a layer of normal guttapercha being then applied. In a further example, a mixture of guttapercha and finely powdered ammonium carbomate. is extruded upon the conductor at a temperature of 70‹ C. The die from which the mixture is extruded is heated to the full decomposition temperature of the ammonium carbonate-about 120‹ C. In order to allow expansion of the thermoplastic material, without decentralization of the conductor, the die is tapered and elongated. A covering of normal guttapercha completes the cable. In cases where a high breakdown resistance in a power cable is required, the core may be insulated with a cellular insulating material of the type in which the cells intercommunicate, the cells being filled with an impregnating compound having a, low dielectric constant, such as flexible bitumen compounds or mixtures of bitumen and wax. When normal rubber and gutta percha are used in the manufacture of the insulation, an undercoating of purified rubber or other material having a low leakance and low waterabsorbing power may be applied to the conductor. Specifications 307,390; 320,370, 322,208, [both in Class 70, Indiarubber &c.]; 329,275 and 331,454 are referred to.