4
GENERAL INFORMATION
Description of the refl ective beam detector
The intelligent reflective beam detector is a
single unit comprising a transmitter, a receiver
and control electronics. In traditional beam
detectors these three elements are supplied as
separate units.
The trans mit ter projects a cone shaped beam of
modulated infra-red light to a refl ector (prisms).
The refl ector returns the beam to the detector
where the receiver measures the amount of light
received and converts it to a signal for processing
in the control electronics.
The refl ective beam detector is mounted so that
the beam will project ap prox i mate ly 0.3m to
0.6m below and parallel to the roof or ceiling
level (assuming that there are no obstructions)
at distances up to 50 or 100m, depending on the
model of detector. The maximum lateral detec-
tion range is 7.5m either side of the beam.
The refl ective beam detector is available in two
versions: one, part no 55000-268, is for use at
distances up to 50m, and requires one refl ector.
Part no 55000-273 is for distances between 50m
and 100m and requires four refl ectors arranged
as a square.
The refl ective beam detector is loop-powered
and incorporates a bi-directional short circuit
isolator which switches the negative line in the
event of a short circuit.
How does a beam detector work?
In normal operating conditions the strength of the
light received is almost the same as the strength
of the light emitted. When smoke is present in the
beam path the light registered by the receiver
is reduced by an amount dependent on the
density of the smoke.
In the event of the smoke obscuring the light by
a pre-selected minimum level for a period of
8 to 10 seconds a fi re signal is generated. The
detection level can be set to 25%, 35% or 50%
to suit different environments, examples of which
are given in the table in the section 'Obscuration
Level Setting' on page 7.
If the infra-red beam is obscured rapidly by 90% or
more for approximately 10 seconds a fault condi-
tion is signalled. This condition normally indicates
an object being placed in the beam path. It
might also indicate loss of the refl ector.
Automatic reset
The refl ective beam detector automatically re-
sets once an alarm or fault condition is no longer
present. A normal status value is returned after
30 seconds following an alarm and 3 seconds
following a fault.
Drift compensation
Over a period of time the light registered by the
receiver might be reduced by factors such as dirt
building up on the lenses of the detector.
The intelligent refl ective beam detector com-
pensates for this automatically in order to reduce
the likelihood of nuisance alarms. At the limit of
compensation the beam detector transmits a
fault signal. In the event of a fi re being detected
when the beam detector has reached its com-
pensation limit, the fi re signal will override the
fault signal.
Ensure that the design and
installation is in accord-
ance with the relevant
national or local fi re protec-
tion standard or installation
code of practice
15m
over 15m
7.5m
7.5m
0.5m
min
beam to wall
distance=
0.5m to 7.5m
max = 40m
Fig 1 Positioning detectors under fl at ceilings—
exampl
es of two building widths
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