GB1355497A - Alarm system - Google Patents

Abstract

1355497 Alarm systems LIGNIN ENG CO Ltd 4 Feb 1972 [12 Nov 1970] 53833/70 Heading G4N In an alarm system having one or more detectors, false alarms are reduced by preventing switching from PASSIVE to ACTIVE mode except when an all-clear signal is being received from a detector. As shown in Fig. 3, the detector is a normally-closed switch, e.g. operated by opening a door. In the PASSIVE mode, thyristor Th3 is conductive and provides a high level input across R4 which is fed to a logic element LE. When an operating switch (not shown) or auto programmer is set to ACTIVE, Th2 is turned on, so that C1 turns off Th3 and transfers the feed to the logic element LE from the detector in series with a flow restrictor RR1 and linked terminals XX. When the door is opened, the detector contacts open, turning off Th2 and giving a reduced input to the logic element LE which initiates an alarm. If there is an open circuit in the detector circuit when switching to ACTIVE, Th2 cannot be turned on so that Th3 is maintained conductive, preventing an alarm signal being fed to LE. If there is a short-circuit in the detector circuit, Th1 is turned on, energizing relay RL1 and breaking the supply to Th2. The restrictor RR1 may alternatively be connected in parallel with a normally-open detector. Closure of the detector contacts turns on Th1 energizing RL1 which turns off Th2 to provide an alarm initiating signal. A compensating resistor R5 is inserted by SOL1 when required. In Fig. 2, closure of a circuit including programme unit U1, relays RL5, RL6 and operating switch OS energize a transfer element RL3 if the normally-closed detector is in the all-clear condition. When the detector is opened, due to an abnormal condition, logic element RL4 is released to initiate an alarm. RL3 cannot be energized unless the detector is in the all-clear condition. A voltage-sensitive relay RL1 prevents energizing of the transfer element RL3 in the event of a short-circuit across the detector circuit. A further relay RL2 and diode 24 enable operation with normally-open detectors. Modifications.-Reference is made to fluid systems in which the flow is restricted by means of an orifice at each detector.