World Patents Pending ©AirSense Technology Ltd. 1999
ISSUE 2.4
Page 7
T E C H N I C A L
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M A N U A L
response time to fire may be rapid enough to detect flame within a few milliseconds,
most fire situations will progress through a relatively long smouldering period, and
generally light sensing detectors are either not sensitive to this stage or are inefficient
at detecting it.
There are many variations on these principles, they are also used with varying degrees
of technical expertise in order to suit different markets and applications. In general the
sensitivity and reliability of a detector, of whatever principle, can be improved with an
increase in cost. The large majority of detector designs are ‘point’ type, for either
consumer or commercial use, where, due to the highly competitive nature of the
market, costs have to be kept to the minimum. In the great majority of cases, detectors
are operated from a remote control unit, and one control unit will usually have many
detectors connected to it. The control unit provides the power necessary for them to
operate and itself operates fire sounders, automatic signalling to a remote centre (e.g.,
the Fire Brigade), fire extinguishing systems, etc. It normally indicates the signals it is
receiving from the detectors by means of lamps but the use of special PC programmes
to display more comprehensive information, is becoming more common.
This is the name given to the type of detector normally found in most buildings
distributed on the ceilings. A point detector depends upon normal air currents and
convection to carry the smoke, products of combustion or heat to it. For this reason,
the volume covered by the detector is limited. Where large volumes need to be covered,
more detectors are used and in very large volumes a great many detectors may be
required to provide complete coverage. There needs to be more than one tier of
detectors for very high volumes.
In the earlier days of such detection, all these detectors were wired in parallel on a single
pair of wires from a control panel. The wires provided electrical power to the detector
and also carried the alarm signal. The cost of the wires makes a significant contribution
to the total cost of the installation. Since the system must continue running during a
mains power failure, back-up batteries are required to take over at such times. The
capacity of the batteries will depend upon how much power each detector needs to run
it. In order to keep the size and cost of batteries low, point detectors are designed to
draw very low currents.
Point Detectors
