Applications; Coverage - Bosch D296 Application Manual

Projected-beam Smoke Detectors
Unlike spot‑type photoelectric smoke detectors , beam-type smoke detectors are generally
less sensitive to the color of smoke. A beam‑type smoke detector might be suited to
applications inappropriate for spot‑type detectors, such as applications where the anticipated
fire would produce black smoke. Beam‑type smoke detectors require visible smoke and might
not be as sensitive as ion detectors in some applications.
Beam‑type smoke detectors are sensitive to the cumulative obscuration presented by a smoke
field. A combination of smoke density and the linear distance of the smoke field create this
obscuration across the projected light beam . Cumulative obscuration is a measure of the
percentage of light blockage.
Since the sudden and total obscuration of the light beam is not a typical smoke signature, the
detector sees this as a trouble condition, not an alarm. This threshold is at a sensitivity level
that exceeds 90% to 95% total obscuration . This minimizes the possibility of an unwanted
alarm due to the blockage of the beam by a solid object such as a sign or ladder inadvertently
placed in the beam path.
Very small, slow changes in the quality of the light source are not typical of a smoke signature.
These changes can occur because of environmental conditions such as dust and dirt
accumulation on the transmitter , the receiver's optical assemblies, or both. Generally, the
automatic environmental compensation circuit compensates for these changes. When you
initially turn on the detector and run the setup program, it assumes the light signal level at
that time as a reference point for a normal condition. Over time, the quality of the light signal
degrades (perhaps due to dust), and the environmental compensation circuit compensates for
this change. The compensation rate is limited, ensuring the detector remains sensitive to slow
or smoldering fires. When the automatic environmental compensation circuit no longer
compensates for signal loss (as with an excessive accumulation of dirt), the detector signals a
trouble condition.
The receiver indicates a trouble condition if the beam strength increases more than 20%. An
incorrectly aligned transmitter and receiver or a partially blocked beam can cause a trouble
when the transmitter and receiver are installed.
3

Applications

3.1

Coverage

Generally, the D296/D297 detectors provide smoke detection in large areas with open or high
ceilings. The D296/D297 detector's coverage range is up to 350 ft (106 m). On a smooth flat
ceiling, you can place the beams up to 60 ft (18 m) apart, allowing one set of detectors
(transmitter and receiver ) to replace up to 24 spot‑type smoke detectors with 30 ft (9 m)
spacing . See the following figure:
Figure 3.1: Coverage Range
You can install beam‑type detectors at any angle needed to allow protection of the hazard
involved; such as, vertical beams through the open shaft area of a stairwell where there is a
clear vertical space inside the handrails.
Bosch Security System, Inc. Application Guide
Document Introduction | en
2013.02 | 02 | F.01U.500.951
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