In this issue, I pose the question, “Where do fire alarm detection device spacing requirements come from?” If you answered “NFPA 72” or the National Fire Alarm Code, you would be correct. But the real question is do your technicians understand the basic requirements of NFPA 72-2007 when they are installing these devices? 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 easiest configurations to address for spot-type heat detectors are the standard 10-foot smooth ceiling. The spacing for heat detectors in this case simply follows the listed spacing (either UL or FM, depending on whether or not the property is FM insured). In this example, the code requires the following rules be followed.
220.127.116.11 Smooth Ceiling Spacing. (NFPA 72-2007)
18.104.22.168.1 One of the following requirements shall apply:
(1) The distance between detectors shall not exceed their listed spacing, and there shall be detectors within a distance of one-half the listed spacing, measured at right angles from all walls or partitions extending upward to within the top 15 percent of the ceiling height.
(2) All points on the ceiling shall have a detector within a distance equal to 0.7 times the listed spacing (0.7S).
22.214.171.124.2 For irregularly shaped areas, the spacing between detectors shall be permitted to be greater than the listed spacing, provided the maximum spacing from a detector to the farthest point of a sidewall or corner within its zone of protection is not greater than 0.7 times the listed spacing.
When the ceiling configuration changes from smooth to joisted, beamed or sloped ceilings, then the requirements change:
126.96.36.199 Solid Joist Construction. The spacing of heat detectors, where measured at right angles to the solid joists, shall not exceed 50 percent of the smooth ceiling spacing permitted under 188.8.131.52.1 and 184.108.40.206.2.
220.127.116.11 Beam Construction.
18.104.22.168.1 A ceiling shall be treated as a smooth ceiling if the beams project no more than 100 mm (4 in.) below the ceiling.
22.214.171.124.2 Where the beams project more than 100 mm (4 in.) below the ceiling, the spacing of spot-type heat detectors at right angles to the direction of beam travel shall be not more than two-thirds of the smooth ceiling spacing permitted under 126.96.36.199.1 and 188.8.131.52.2.
184.108.40.206.3 Where the beams project more than 460 mm (18 in.) below the ceiling and are more than 2.4 m (8 ft.) on center, each bay formed by the beams shall be treated as a separate area.
220.127.116.11 Sloping Ceilings.
18.104.22.168.1 Peaked. A row of detectors shall first be spaced and located at or within 900 mm (3 ft.) of the peak of the ceiling, measured horizontally. The number and spacing of additional detectors, if any, shall be based on the horizontal projection of the ceiling in accordance with the type of ceiling construction.
22.214.171.124.2 Shed. Sloping ceilings shall have a row of detectors located on the ceiling within 900 mm (3 ft.) of the high side of the ceiling measured horizontally, spaced in accordance with the type of ceiling construction. Remaining detectors, if any, shall be located in the remaining area on the basis of the horizontal projection of the ceiling.
126.96.36.199.3 Roof Slope Less Than 30 Degrees. For a roof slope of less than 30 degrees, all detectors shall be spaced using the height at the peak. For a roof slope of greater than 30 degrees, the average slope height shall be used for all detectors other than those located in the peak.
High ceilings offer the biggest challenge to installing detection devices and also have a dynamic impact on the spacing requirements for heat detectors. The reduction in spacing for heat detectors installed on high ceilings is governed by the following requirements:
188.8.131.52 High Ceilings.
184.108.40.206.1 On ceilings 3 m to 9.1 m (10 ft. to 30 ft.) high, heat detector linear spacing shall be reduced in accordance with Table 220.127.116.11.1 prior to any additional reductions for beams, joists, or slope, where applicable.
Exception: Table 18.104.22.168.1 shall not apply to the following detectors, which rely on the integration effect:
(1) Line-type electrical conductivity detectors (see 22.214.171.124)
(2) Pneumatic rate-of-rise tubing heat detectors (see 126.96.36.199)
In these cases, the manufacturer’s published instructions shall be followed for appropriate alarm point and spacing. 188.8.131.52.2 The minimum spacing of heat detectors shall not be required to be less than 0.4 times the height of the ceiling. EC
MOORE, a licensed fire protection engineer, frequent speaker and an expert in the life safety field, is a co-editor of the current National Fire Alarm Code Handbook. Moore is a principal with Hughes Associates Inc. at the Warwick, R.I., office.