Use Correct Wire Gauge And Length; Provide A Stable Mounting Surface - Bosch D296 Installation Instructions Manual

D296/D297 | Installation Instructions |
3.3

Use Correct Wire Gauge and Length

Beam smoke detectors are often used to protect large
areas, requiring long wire runs to power the detectors
and signal alarm conditions. The voltage available at the
end of long wire runs might not be sufficient to power
the detector, especially when the system is running on
battery backup. Use the correct wire gauge when
installation detectors.
Refer to Table 1 for the proper wire size, wire length,
and the number of transmitter and receiver pairs.
Table 1: Wire Gauge and Length
Wire Length Wire Size
14 AWG
(1.8 mm)
500 ft (152 m) 20 pairs
1000 ft (304 m) 10 pairs
2500 ft (762 m) 4 pairs
5000 ft (1524 m) 2 pairs
3.4

Provide a Stable Mounting Surface

Always select stable mounting surfaces.
Beam smoke detectors depend on the projected beam
measurement to sense smoke. Trouble or alarm
conditions can be caused by shifts in beam alignment
when the transmitter or receiver moves.
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 D296's and D297's 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.
4
3.0 Installation Considerations
16 AWG 18 AWG
(1.5 mm) (1.2 mm)
13 pairs 8 pairs
6 pairs 4 pairs
3 pairs 2 pairs
1 pair 1 pair
When you are uncertain about mounting surface
stability, measure the detector's signal voltage. This
voltage must be 4 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. Refer to Section 7.3 Reference Voltage
Calibration on page 9 for voltage reading.
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. Overtightening 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 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.
3.5 Stratification in Cold Environments
Effects
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.
3.6 Expected Fire Load
When designing a fire alarm system, allow for expected
fire loads within the application. For example, fires
caused by the ignition of flammable petroleum-based
liquids generally lead to a rapid buildup of heavy
smoke. When this type of fire is probable, use sensitivity
settings 0 or 1 for a 5-sec response time.
Bosch Security Systems, Inc. | 09/06 | 31344F