GB1338470A - Method and apparatus for avoiding false fire alarms - Google Patents

Abstract

1338470 Fire alarms; selective signalling PREUSSAG AG FEUERSCHUTZ 14 Oct 1971 [21 Oct 1970 (2)] 47859/71 Headings G4N and G4H In a fire alarm system utilizing variable impedence fire detectors connected in parallel across a line connected to a central station, the line current is repetitively interrupted, each interruption being initiated by a change in line current at least as great as that caused by one detector responding to a fire. A fire alarm is initiated only if, immediately after at least the majority of such interruptions the line current returns to a value which is at least as great as the change which occurs when one detector responds to a fire. False alarms are thereby minimized. As shown in Fig. 2, the current through a plurality of fire detectors (not shown) connected in parallel to a line 11, is monitored by an amplifier 14. The current is normally zero, but increases in the event of a fire. If the increase is sufficient, the output of amplifier 14 triggers a 5 ms monostable A1, the trailing edge of the output of which successively trigger an 80 ms monostable A2 and a 30 ms monostable A3. The line current is interrupted by switch 15 for the duration of the pulse from A3. Monostable A1 is inhibited by a memory 17 for the duration of monostables A1 ... A3 to prevent operation by spurious pulses. If the current in line 11 is still indicative of a fire, the above sequence is repeated. Each pulse from A1 steps a counter 16 and starts a slow timer A4, which re-sets the counter 16. An alarm is initiated after counter 16 has stepped to a predetermined count. Fig. 8 shows a system that avoids false alarms due to short circuits. The line current for amplifier 14 is differentiated at 211 and compared with a reference 231. If di/dt is within a given range, timers 38 and 39 are initiated to interrupt the line current at 15 and thus step a counter 37. If di/dt is excessive, an output at 36 re-sets the counter 37. Local fire detectors.-Figs. 5 and 6 (not shown) depict detectors which provide output signals on line 11 having a discrete rise time by the use of CR circuits in the input line of a switching transistor or output line of a thyristor, turned on in the event of fire. A Zener diode may also be used to shape the leading edge. Signalling location of fire.-Each local fire detector (as referred to above) may provide a fire-responsive signal having a different rise time. In Fig. 9 (not shown) the line current at the central station is differentiated and fed to a group of comparators respectively responsive to signals having rise times corresponding to those of the local detectors.