GB665870A - Improvements in or relating to fire or temperature rise detecting appliances - Google Patents

Abstract

665,870. Fire alarms. GRAVINER MANUFACTURINGCO.,Ltd. WILKINSONSWORD CO., Ltd., and FARMER, R. B. March 10, 1950. [April 8, 1949] No. 9574/49. Class 118 (i). [Also in Group XXXVI] In a fire detector cable, the negative temperature coefficient resistance material separating the cable conductors is a mixture comprising titanium dioxide and magnesium oxide. A preferred mixture is made up of 98 per cent commercial grade T 1 O 2 and 2 per cent MgO. In the cable shown, Figs. 1 and 2, the powdered oxide mixture c is compressed between an inner copper wire band an outer copper tube or capillary e. To facilitate securing and mounting in a terminal or circuit connector fitting, Fig. 1 (not shown), one end portion of the cable is not so reduced as the remainder or the cable proper which distributed in the area to be protected terminates in a moisture proofing cap p, Fig. 2, and has an outer covering in the form of a stainless steel or iron alloy tube f. The cable may be artificially aged by heating at a temperature between 200‹C. and 250‹C. for a period of about six hours with D.C. at 24 volts applied between the cable conductors and then resting the cable for not less than 15 hours at room temperature without any electric potential applied to the cable conductors. In use, Fig. 4, if a fire occurs in the vicinity of the cable the resistance of the oxide filling c quickly decreases and the current which then flows from the source s operates relay u whose contacts u<SP>1</SP> then close to light warning lamp t. Operation of relay u also opens relay contacts u<SP>2</SP> to place in. series with the relay coil a current limiting resistance v so that when the cable has cooled down the current flow cannot be of sufficient magnitude to cause local heating in a narrow track or path of the oxide filling c which would sustain the current and prevent resetting of relay u. In a modification, Fig..3 (not shown), the use of a high resistance, highly sensitive relay obviates the need for a separate current limiting resistance. The cable according to the invention is specifically designed for use as a fire or abnormal temperature rise detector in aircraft power plant installations and is permanent in the sense that it remains serviceable after repeated heating and cooling. The anatase form of T 1 O 2 is preferred. Specifications 619,797 and 653,864, [both in Group XXXVI], are referred to.