World Patents Pending ©AirSense Technology Ltd. 1999
ISSUE 2.4
Page 8
T E C H N I C A L
.
M A N U A L
Any detector on the pair of wires which signals an alarm is signalled back to the control
panel. This type of control panel is unable to say which of the many detectors had
operated, so the signalling is only ‘zonal’. Later developments use an addressable system
whereby the detector is given an coded unique identification which forms part of the
signal back to the control panel. The control panel can identify which of the detectors
(or other alarm device) on a given pair of wires is signalling. The detector can also signal
back more information than a simple alarm state, for instance; it can signal back that it
has a fault or pre-alarm condition. This has the effect of allowing the ‘pinpointing’ of
which detector has given the alarm and it also allows rapid identification of certain faults
in a detector.
Since a large number of detectors is often needed in a point detection system, the cost
of each unit must be kept within tolerable limits. In order to do this the design is kept
reasonably simple and the performance is consequently comparatively modest com-
pared to that possible with modern technology. In addition to this, National Standards
always demand a limit to maximum sensitivity of these devices in an effort to reduce
unwanted alarms. Point detectors are usually set to give an alarm at a smoke density
corresponding to between 2% and 4% obscuration per metre. Although they are
capable of signalling a fault condition, they will not signal the occurrence of every
possible type of fault.
Although these detectors often provide quite adequate protection, the probability of
them giving an unwanted alarm could, in some cases, be lessened by a more costly
design, and the sensitivity could be increased if cost were no object. Their general
performance reliability, although good, may not be as high as that required by a person
wishing to protect a very valuable property, and a higher sensitivity would certainly give
an earlier warning. Where very valuable property is to be protected by point detectors,
or where certain fire extinguishing systems are to be automatically released, two
different types of point detector are commonly used in a ‘coincidence’ (or ‘double
knock’) configuration. This configuration requires an alarm to be indicated by both of
the detectors before it is accepted. Since two detectors of the same principle would be
equally susceptible to the same source of unwanted alarm, usually they are of different
operating principles, e.g. a heat detector and an ionisation detector. The probability
that such a configuration will give an unwanted alarm is very much less than with a single
detector. Normally detectors are specially selected to suit the hazards in the volume they
