How Far Apart?

By Wayne D. Moore | Sep 15, 2007
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This month, we address smoke detector spacing, which is found in NFPA 72-2007 or the National Fire Alarm Code. It is important that your technicians know the spacing requirements for these devices, so they can discover problems on the drawings and prevent mistakes during the installation. The code provides a list of factors to be considered for smoke detector placement in section: The design shall account for the contribution of the following factors in predicting detector response to the anticipated fires to which the system is intended to respond:

(1) Ceiling shape and surface

(2) Ceiling height

(3) Configuration of contents in the protected area

(4) Combustion characteristics and probable equivalence ratio of the anticipated fires involving the fuel loads within the protected area

(5) Compartment ventilation

(6) Ambient temperature, pressure, altitude, humidity, and atmosphere

The easiest configuration to address for spot type smoke detectors is the standard 10-foot smooth ceiling with normal ambient temperatures. In this example, the code requires the following rules be followed: Spot-Type Smoke Detectors. (NFPA 72-2007) Spot-type smoke detectors shall be located on the ceiling not less than 100 mm (4 in.) from a sidewall to the near edge or, if on a sidewall, between 100 mm and 300 mm (4 in. and 12 in.) down from the ceiling to the top of the detector. To minimize dust contamination, smoke detectors, where installed under raised floors, shall be mounted only in an orientation for which they have been listed. On smooth ceilings, spacing for spot-type smoke detectors shall be in accordance with through In the absence of specific performance-based design criteria, smoke detectors shall be permitted to be located using 9.1 m (30 ft.) spacing. For smooth ceilings, all points on the ceiling shall have a detector within a distance equal to 0.7 times the selected spacing.

When the ceiling configuration changes from smooth to joisted, beamed or sloped ceilings, the requirements change. And additional spacing changes found in sections and are necessary for detectors installed on sloped and beamed sloped ceilings. Solid joists shall be considered equivalent to beams for smoke detector spacing guidelines. For level ceilings the following shall apply:

(1) For ceilings with beam depths of less than 10 percent of the ceiling height (0.1 H), smooth ceiling spacing shall be permitted.

(2) For ceilings with beam depths equal to or greater than 10 percent of the ceiling height (0.1 H) and beam spacing equal to or greater than 40 percent of the ceiling height (0.4 H), spot-type detectors shall be located on the ceiling in each beam pocket.

Waffle and beamed ceilings have always been a difficult application for smoke detection. As a result of research that is described in Annex A of the code, these spacing requirements were changed from the 2002 edition of NFPA 72. These requirements are new to this edition of NFPA 72 and should be carefully reviewed.

(3) For waffle or pan-type ceilings with beams or solid joists no greater than 600 mm (24 in.) deep and no greater than 3.66 m (12 ft.) center-to-center spacing, the following shall be permitted:

(a) Smooth ceiling spacing including those provisions permitted for irregular areas in, substituting “selected spacing” for “listed spacing”

(b) Location of spot-type smoke detectors on ceilings or on the bottom of beams

(4) For corridors 4.5 m (15 ft.) in width or less having ceiling beams or solid joists perpendicular to the corridor length, the following shall be permitted:

(a) Smooth ceiling spacing including those provisions permitted for irregular areas in, substituting “selected spacing” for “listed spacing”

(b) Location of spot-type smoke detectors on ceilings, sidewalls, or the bottom of beams or solid joists

(5) For rooms of 84 m2 (900 ft.2) area or less, only one smoke detector shall be required.

The code reference in item (4)(a) above is the section of the code that allows for expanded spacing of smoke detectors in corridors, essentially allowing a 41-foot spacing between smoke detectors for 10-foot-wide corridors.

Use this vignette to help your technicians learn more about the technical aspect of the fire alarm systems that they install every day. Training is the key to reliability and profits.

About The Author

MOORE, a licensed fire protection engineer, was a principal member and chair of NFPA 72, Chapter 24, NFPA 909 and NFPA 914. He is president of the Fire Protection Alliance in Jamestown, R.I. Reach him at [email protected]

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