(57) A fire alarm system comprises control panel 13, first and second two-wire alarm circuits
17,18 extending from the control panel 13 into respective zones, each alarm circuit
comprising a plurality of combined detector and sounder devices 10 connected in parallel
across the wires 11,12 of the circuit. Upon detection of smoke or heat in one of the
zones, the activated device 10 is arranged to draw a pulsed current from the control
panel 13, which can be detected to trigger an alarm condition in the control panel
13, whereupon the control panel 13 applies an alarm signal circuit to the alarm circuits
17,18 of both zones, so as to energise the sounders of all devices 10. The pulsed
current that is drawn by the devices 10 when they are activated by heat or smoke means
that any further change in current that is caused when more devices 10 are activated
as the fire spreads will be detected by the control panel 13, despite the heavy current
being drawn by the energised sounders.
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