GB653864A - Improvements in or relating to fire detecting devices and apparatus embodying such devices - Google Patents

Abstract

653,864. Powdered and granular resistances ; temperature variable resistances ; wire and ribbon resistances ; resistance measurement ; cables. POOLE, R., TREADWELL, R. J., and WOOSNAM, M. S. July 6, 1948, No. 18140. [Classes 36 and 37] [Also in Group XXI] A fire-detecting element in the form of a flexible cable and constituting an electrical resistance comprises a flexible conductive shell or sheath, for example a flexible tube 10; Fig. 1, separated from a flexible conductive core 11 by a filling 12 consisting wholly or mainly of one or more metallic oxides whose resistance /temperature characteristic is negative, for example barium oxide or manganese oxide or nickel oxide or a mixture of two or more of these oxides. Spacers 13, Fig. 2, of suitable insulating material such as glass, ceramic or magnesium oxide may be positioned at intervals along the core 11 which may be a rod, wire, strip or helix made for example of copper or copper-nickel alloy. In the manufacturing method illustrated in Fig. 3 the powdered oxide filling 12 is introduced loosely into the outer tube 10 and metallic magnesium spacers 14 are threaded on and spaced along the inner conductive core 11. The assembly is then drawn through a die or passed between rollers 15 which reduce the diameter of the tube 10 to the finished size and compact the filling 12 and the spacers. The latter in compacted form indicated at 13 are then converted to magnesium oxide by heat treatment. In use, Fig. 4, if a fire occurs in the vicinity of the cable 16 the resistance of the oxide filling will rapidly fall and the current which then flows from the D.C. supply 18 will operate relay 19 and consequently the fire alarm or extinguishing apparatus 17. If a high resistance core 11 is used the position of the occurrence of a fire in relation to the length of the cable can be determined by measuring the resistance between the tube 10 and one end of the core 11.