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DS240/DS241 Installation Instructions

P/N: 22066P Page 3

4.4 Provide a stable mounting surface

Beam smoke detectors depend on the measurement of the projected
beam to sense smoke. Therefore, shifts in the alignment of the beam
due to movement of the transmitter/receiver can cause trouble or
alarm conditions.

• Always select a stable mounting surface.

• Never mount the unit to a suspended support such as a pipe or

length of wood supported at only one end.

This type of mounting can create a “pendulum effect” which 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 over 20 ft at 350 ft (6 m at 107 m).

Mounting surfaces should be supported at opposite corners to reduce
the pendulum effect.

• Always select surfaces not subject to building movement.

The automatic compensation circuits of the DS240 and DS241 will
eliminate most problems created by normal building expansions
and contractions. In some unusual circumstances, the walls and
support structure may be subject to more significant movement due
to the operation of heavy equipment, such as cranes, anchored to the
top of the walls.

When in doubt about mounting surface stability, measure the signal
voltage of the detector. This voltage should be 4.0 VDC. Misalignment
of the beam can cause this voltage to increase or decrease. The
detector will indicate a trouble condition when the voltage increases
to more than 4.8 VDC, or when it decreases to less than 2.0 VDC over
a long period of time or to 0.4 VDC greater than 20 sec. For more
information on reading this voltage, see

Section 8.3 Reference

Voltage Calibration on page 6).

If movement of the mounting surface is suspected, relocate the
detector to a surface that does not move or add supports to the
mounting surface to prohibit movement.

• DO NOT over-tighten the mounting screws.

When the detector is mounted to an electrical mounting box
[4 in. (10 cm) square or octagonal], the mounting plate may bow as a
result of the mounting box screw tabs being 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
screw tabs will worsen this condition. Using only mounting boxes
with flat, regular surfaces and properly formed screw tabs, and
tightening the mounting screws to secure the mounting plate to
2 in./lb torque will minimize any misalignment caused by bowing of
the mounting plate.

4.5 Consider the effects of stratification in cold environments

In very cold environments such as unheated warehouses, smoke
may cool very quickly and is less likely to rise to the ceiling. Consider
additional units at lower mounting heights to help compensate for
this situation.

4.6 Consider the expected Fire load

When designing a fire alarm system, consider the expected fire load
within the application.

For instance, fires caused by the ignition of flammable petroleum-
based liquids usually result in a rapid buildup of heavy smoke. When
these types of fires are probable, use sensitivity settings 0 or 1 which
provide a response time of 5 sec.

5.0

Mounting

NOTE: This product is not designed for use in outdoor applications.

Install a 4 in. (10 cm) square or octagonal electrical box (not supplied)
to a rigid surface that will not be subject to movement or vibrations.

NOTE: If not mounting to an electrical box, be sure the surface will

not be subject to movement or vibrations. For example: do
not mount directly to a suspended ceiling without extra
support.

• Remove the receiver's access door (1 screw), then remove the

cover (4 screws). See

Figure 4.

Figure 4 - Access Door and Cover

• Remove the receiver’s backplate by removing the single

mounting screw from the top of the circuit board carrier plate then
separating the two plates (see