4.0Installation Considerations
The proper location and spacing of detectors is critical in a properly installed and operating fire alarm system. For best results, it is recommended that the detectors be located and spaced in accordance with the National Fire Protection Association (NFPA) Standard 72 "The National Fire Code.” This standard is available at a nominal cost from The National Fire Protection Association, Batterymarch Park, Quincy, MA, 02269.
In all installations, good engineering judgement should prevail. The following are some general considerations:
•Do not use mirrors. The detectors should be installed with a clear
•The beam path should be clear of moving objects.
•Set the sensitivity based on the distance between the transmitter and receiver. See Section 7.2 Sensitivity Adjustment on page 5.
•The detectors may be mounted directly to the ceiling or to side walls. Do not mount the units where the beam path may run parallel within 4 in. (10.2 cm) of a corner.
•For a sloped or peaked ceiling, the beam path should be located within 3 ft (1 m) of the ceiling’s peak.
•Stratification of air may hinder 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 will not rise to the ceiling. If stratification is considered a possibility, extra beams should be installed where stratification is expected.
•Avoid areas subject to normal smoke concentrations such as kitchens and garages.
•Do not install units where normal ambient temperatures are below
The following are some specific installation considerations:
4.1 Avoid Sources of Heat and Air Movement
The detectors should be located where the beam path will not pass near heating and cooling outlets.
•Don’t mount where hot or cold air can blow directly into the beam path.
•Heating, ventilating, and air conditioning systems can cause smoke to be blown away from the beam path.
- Smoke must accumulate in the beam path to be detected.
•Heaters mounted close to the beam path can cause the beam to become distorted.
You can test for beam distortion by monitoring the signal voltage. After setup, the signal voltage of the detector should read 4 VDC. Monitor the voltage and turn on all heating and cooling devices in the area. The signal voltage should not fluctuate more than 0.20 VDC. If it does, the detector should be moved to avoid these disturbances.
4.2 Avoid sources of bright light
Sunlight and light from extremely bright sources such as the exposed bulbs of high pressure sodium, mercury vapor, and metal halide lights can cause stray signals. The receiver should not be pointed toward any of these sources.
Sunlight: Avoid pointing the receiver directly at the rising or setting sun. If you are installing the unit where sunlight can’t be avoided, mount the receiver slightly higher than the transmitter and aimed down toward the transmitter. This should reduce this problem by causing the receiver to look below the horizon.
Bright Lights: Don’t mount the receiver where it is looking at exposed bulbs of high pressure sodium, mercury vapor, and metal halide lights. The illustration below shows where these exposed lights should be avoided.
Keep bright lights out of this area 4.5 ft. (1.4 m)
Receiver |
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| 14.5 ft. | |
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| (4.4 m) | |
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225 ft. (68.6 m) 
350 ft. (106.8 m)
Bare fluorescent lights may also pose a problem especially in long hallways where a series of lights run perpendicular to the beam. Incandescent lights are not a problem as long as they are not directly in the beam path.
4.3 Use the correct wire gauge and length
Beam smoke detectors are often used to protect very large areas. This means that the wire runs used to power the detectors and signal alarm conditions can be very long. The voltage available at the end of very long wire runs may not be sufficient to power the detector, especially when the system is running on battery backup. It is very important that the correct wire gauge is used for these installations.
Use the following chart to determine the proper wire size, wire length and the number of transmitter and receiver pairs that can be used.
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| Wire Size |
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Wire |
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Length | 14 AWG | 16 AWG | 18 AWG |
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| (1.8 mm) | (1.5 mm) | (1.2 mm) |
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500 ft (152 m) | 20 pairs | 13 pairs | 8 pairs |
1000 ft (304 m) | 10 pairs | 6 pairs | 4 pairs |
2500 ft (762 m) | 4 pairs | 3 pairs | 2 pairs |
5000 ft (1524 m) | 2 pairs | 1 pair | 1 pair |
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Page 2 | P/N: 22066P | © 2004 Bosch | DS240/DS241 Installation Instructions |
