GB715317A - Improvements in and relating to fire and heat detectors - Google Patents

Abstract

715,317. Fire detector cables; temperature sensitive resistors. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Nov. I6, 1951, [Aug. 24, 1950.] No. 21003/50. Classes 36 and 37. [Also in Group XXXVIII] A temperature sensitive element for fire and heat detecting apparatus comprises a tubular conductor 4 and an inner conductor I, the intermediate space being filled with glass 2 and spacers 3 of quartz or other refractory material. The glass is in contact with both conductors and becomes conductive at a predetermined temperature. In manufacture, short tubular lengths of glass separated by the spacers are assembled between the conductors and heat and pressure are then applied to soften the glass and ensure intimate contact between the glass and the conductors. Fig. 4 shows an alarm circuit comprising the detector element 25 which is arranged to periodically reverse the polarity of the applied potential so that the glass material is not rendered unusable by polarization effects. The detector 25 is connected in series with a source of current, a reversing switch 22, and alarm relay 24 which controls contact 27 in series with warning lamp 26. If detector 25 reaches a predetermined temperature, the glass conducts sufficiently to operate relay 24 which is of the slow release type. Periodic current reversal is effected by means of a bimetallic switch having a heater 20. When the bimetallic switch operates, contact 28 opens to disconnect heater 20, and contact 2I closes to operate relay 23 which controls reversing switch 22. When the switch cools, contact 28 closes to repeat the cycle. Fig. 5 shows a modified circuit in which the polarity reversal occurs only while warning conditions are operative. Resistance 37 is connected across battery in series with relay 32. Detector 33 is normally connected across the upper half of resistance 37 via the upper "cold" contacts of bimetallic switch 3I. When, owing to a rise in temperature, 33 becomes conductive, relay 32 operates, to energize alarm 34 and heater 30 via contacts 36 and 35 respectively. When heated, bimetallic switch 3I operates so that heater 30 is disconnected and detector 33 is connected in reverse polarity across the lower half of resistance 37. On cooling, switch 3I restores to re-connect 33 across the upper half of resistor 37 and also to energize the heater 30, so that the cycle repeats so long as 33 is sufficiently conductive to operate relay 32, which is of the slow-release type. The detector may also be used in alternating current circuits, and may control means for reducing the temperature or extinguishing the fire. Reference is made to Specification 619,797 and U.S.A. Specification 2,477,348.