DE2527443

1507675 Metal sheathed heating cables PYROTENAX OF CANADA Ltd 20 June 1975 [21 June 1974] 27623/74 Heading H5H A metal sheathed mineral insulated cable has a resistive layer 4 embedded in mineral insulation 2 and contacting elongate supply conductors throughout their lengths, the resistive layer 4 consisting predominantly of resistive material without significant admixture of insulating material. As shown the sheath forms one conductor 3 and a wire I embedded in the insulation 2 forms the other conductor. Both conductors may be embedded in the insulation 2 (Fig. 3, not shown). The sheath 3 may have an external insulating coating or may be surrounded by a further layer of mineral insulation in a further metal sheath. The resistive layer 4 may be powdered carbon. A paper tape substrate impregnated or coated with graphite may be used to form layer 4, the tape being sandwiched between semiannular blocks of MgO insulation which are crushed to form insulation 2 during the sheath reduction stage of manufacture of the cable. The paper is carbonized during the annealing stage. A textile tape substrate e.g. glass fibre, coated with resistive material may be used, the glass fibre tape fragmenting during the reduction stage. The coating may be colloidal dispersions of graphite and molybdenum disulphide. Such a dispersion may be coated on semi-annular MgO blocks which are inserted in a metal sheath to contact a central conductor. The blocks are fragmented during the sheath reduction process, but the coating forms a resistive layer engaging the sheath and the central conductor.