ELEKTRISCHES KABEL FUER MITTLERE UND HOHE SPANNUNGEN

1516708 Cables INDUSTRIE PIRELLI SpA 23 Oct 1975 [27 Nov 1974] 43626/75 H1A An electric cable comprises one or more cores the or each of which comprises an insulating layer 13 extruded over a conductor 11, and a screening layer 14 extruded over and adhered face to face with the layer 13. The insulating layer 13 and screening layer 14 are each of synthetic polymeric material, that of the insulating layer 13 having been crosslinked by a chemical cross-linking agent that degrades the material of the layer 14, the surface of layer 14 that is in contact with layer 13 being degraded. The screening layer 14 includes an electrically conductive filler. During manufacture, the screening layer 14 is extruded on to the layer 13 as layer 13 is extruded on to the conductor. As described, an inner screening layer 12 is interposed between conductor 11 and layer 13, the conductor 11 being a plurality of wires (as shown) or a single wire. A single wire, as shown, may be enclosed in a tubular metal screen 15 and thermoplastics sheath 16, or a plurality of cores may be enclosed in a common screen 15 and sheath 16. The insulating layer 13 may be a cross-linked polyolefine, e.g. polyethylene, cross-linked copolymers of ethylene with at least one comonomer, e.g. ethylene-propylene, ethylenevinyl acetate, ethylene-ethylacrylate, or ethylene-propylenediene copolymers, or styrenebutadiene rubbers. Additives such as calcium carbonate, calcined kaolin or natural silicates, e.g. talc, may be added to modify the electrical characteristics. The cross-linking agent may be a mono- or poly-functional organic peroxide. Possible mono functional organic peroxides of the formula may be used, where R and R1 are the same or different and are each an alkyl or alkaryl radical. Suitable bifunctional peroxides are those of the formula: where R11 and R111 are the same or different alkyl radical, preferably t-butyl, and X is a divalent alkyl, alkaryl or alkyne radical, specific examples being given. The screening layer 14 is preferably a copolymer of isobutylene with isoprene (i.e. a butyl rubber) with carbon black as the conductive filter. Other materials, e.g. halogenated butyl rubbers or blends of such rubbers or butyl rubbers with other saturated or unsaturated rubber such as ethylene-propylenediene terpolymers, ethylene-propylene copolymers, or ethylene vinyl acetate, may be employed which can be cured in known manner. Additives to the butyl rubber can be employed to facilitate peeling, e.g. low