Detectors or sensors can also be omitted from below open grid ceilings where all of the following conditions are met:
• The openings of the grid are at least 1/4 inch, or 6 millimeters in the smallest dimension.
• The thickness of the material does not exceed the smallest of the grid openings.
• The openings constitute at least 70% of the area of the ceiling material.
Detectors or sensors are usually required or recommended underneath open loading docks or platforms and their covers, and in
accessible under-floor areas in buildings without basements. Detectors or sensors may be omitted from combustible blind spaces when all
of the following conditions prevail:
1. The space is not accessible for storage purposes, it is protected against the entrance of unauthorized persons, and it is protected
against the accumulation of windblown debris.
2. The space contains no equipment or structures, such as steam pipes, electrical wiring, ducts, shafts, or conveyors that could
potentially ignite or conduct the spread of fire.
3. The floor over the space is tight.
4. Non flammable liquids are processed, handled, or stored on the floor above the space.
“Total coverage” as described in NFPA 72, is the definition of a complete fire detection system. In some of the specified areas of coverage,
such as attics, closets, under open loading docks or platforms, a heat detector may be more appropriate than a smoke detector. Give
careful consideration to the detector manufacturer’s instructions and the following recommendations in this guide.
When only one detector/sensor is required in a room or space, install the detector or sensor as close to the center of the ceiling as
possible. Central location of the detector or sensor is best for sensing smoke or fire in any part of the room. If a center location is not
possible, place it no closer than 4 inches, or 10 centimeters from the wall, or if it is listed for wall mounting, it can be mounted on the wall.
Wall mounted detectors or sensor should be located approximately 4 to 12 inches, or 10 to 30 centimeters from the ceiling to the top of
the detector, and at least 4inches, or 10 centimeters from any corner wall junction, see Figure 1.
Figure 1: Wall and ceiling mounted detector or sensor
When an air supply or an air return duct opening is present in a room or space, install the detectors or sensors in the path of the air flow
toward the return air duct opening, see Figure 2.
page 6 574-709 Rev. AP
4098 Detectors, Sensors, and Bases Application Manual
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