Never mount a detector to a suspended support, such as a pipe or length of wood that is
supported at only one end. This type of mounting can create a pendulum effect that greatly
multiplies even very small movements at one end of the support. For example, a 2 in. (5.1 cm)
movement translates into a beam misalignment of more than 20 ft at 350 ft (6 m at 107 m).
Support mounting surfaces at opposite corners to reduce the pendulum effect.
Always select surfaces that are not subject to building movement. The automatic
compensation circuits eliminate most problems created by normal building expansions and
contractions. In some unusual circumstances, the walls and support structure might be
subject to more significant movement because of heavy equipment operation, such as cranes
anchored to the top of the walls.
When you are uncertain about mounting surface stability, measure the detector’s signal
voltage. This voltage must be between 3.8 VDC and 4.2 VDC. Beam misalignment can cause
this voltage to increase or decrease. The detector indicates a trouble condition when the
voltage increases to greater than ~4.8 VDC or decreases to less than ~2 VDC over a long
period. A trouble condition also occurs when voltage decreases to ~0.4 VDC for more than
20 sec. For how to take a voltage reading, see Reference voltage adjustment, page 24.
If you expect the mounting surface to move, relocate the detector to a stable surface or add
supports to the mounting surface to prevent movement.
Mounting the detector to a square or octagonal electrical mounting box of 4 in. (10 cm) can
bow the mounting plate. Bowing can occur because the mounting box screw tabs are below
flush with the top edges of the mounting box. Over tightening the mounting screws bows the
mounting plate inward at the bottom, causing the optics to aim low.
Irregularities in the flatness of the mounting box edge surfaces or at the screw tabs worsens
this condition. Only use mounting boxes with flat, regular surfaces and properly formed screw
tabs. Tightening the mounting screws to secure the mounting plate to 2 in. (5 cm) or 2 lb
(0.9 k) torque minimizes misalignment caused by mounting plate bowing.
Plan for the effects of stratification in cold environments
Air stratification might prevent smoke from reaching detectors mounted close to the ceiling.
Stratification occurs when smoke, rising because it is warmer than the surrounding air,
reaches a level where it is the same temperature as the surrounding air and does not rise to
the ceiling. In extremely cold environments such as unheated warehouses, smoke cools very
quickly and is less likely to rise to the ceiling. Add more detectors at lower mounting heights
to compensate.
Design for the expected fire load
When designing a fire alarm system, set detector sensitivity to respond to the proper smoke
obscuration and to reduce the chance of a false activation within the application. The total
obscuration of the infrared beam depends on the density of the smoke and width of the
smoke cloud along the beam path as illustrated in the following figure.
3.5
3.6
8
en | Notices
Long-range Beam Smoke Detectors
2014.01 | 06 | F.01U.068.899
Installation Instructions
Bosch Security Systems, Inc.