GB1320652A - Fire alarm systems - Google Patents

Abstract

1320652 Fire alarms NITTAN CO Ltd 19 April 1971 [31 March 1970 (2)] 25724/71 Headings G4H and G4N A fire detector for use in a fire alarm system includes means for sensing heat or smoke and an electromechanical filter which is put in a ready condition in response to the sensing of heat or smoke by the sensing means. In Fig. 2, a plurality of fire detectors 10-1, 10-2 ... are connected by common lines 1, 2, 3 to a central station 11. In a detector, e.g. 10-1, a smoke detector has ionization chambers 19, 20 respectively closed and open to the ambient air, detection of smoke increasing the conduction of FET 27. Sufficient conduction causes Zener diode 29 to render SCR 30 conducting thereby providing a D.C. short across lines 1, 2 and permitting a tuning-fork filter 18 to pass its characteristic frequency (different for each detector) from lines 1, 2 to line 3. The central station 11 has a relay 12 which responds to the D.C. shorting to energize a lamp 15 and audible alarm 16. A frequency generator 35 applies a continuously varying frequency to lines 1, 2, and when the characteristic frequency of an actuated detector is reached, a swing appears on a voltmeter 37 fed via a rectifier 36 from line 3. Modifications of the filter are shown, including one giving an audible alarm at the detector, driven by the tuning fork. In Fig. 7, a plurality of fire detectors 10-1 .... 10-n are connected by common lines 1, 2 to a central station 11, the far ends of the lines 1, 2 being linked via a highpass filter 42. In a detector e.g. 10-1, if a bimetallic plate 53 detects fire it rises from the dotted to the full-line position thereby closing contacts 56, 57 to D.C. shunt lines 1, 2, and raising block 59 to release tuning fork 32 forming a filter connected in line 1 and having a different pass frequency in each detector. At the central station 11, the D.C. shunt, via low-pass filter 43 and relay 12, energize lamp 15 and audible alarm 16. A generator 35 applies a varying frequency to lines 1, 2 and when the pass frequency of a released tuning fork is reached, this frequency is passed at the tuning fork and via a high-pass filter 44 to light a lamp 37 corresponding to the frequency currently being generated (and thus the detector). An audible alarm is also given at the detector, driven by the released tuning fork, at 62-65. A modification uses a separate generator for each required frequency, the frequencies being passed to line 1 via a chopper, any frequency received back being passed by one of a bank of band-pass filters to a respective rectifier and lamp to identify the detector. (For Figure see next column)