Height, Racks, & Noise
Fire alarm system challenges in industrial and storage occupancies.
NFPA Journal®, March/April 2009
Examine industrial and storage occupancies closely, and you’ll find many fire alarm system design and installation challenges. Because these challenges affect both the design and installation of detection devices and notification appliances, designers and contractors must be aware of the requirements in the 2007 edition of NFPA 72®, National Fire Alarm Code®, that apply to these occupancies.
Two common challenges are significant ceiling heights and ceiling obstructions, both of which may affect fire detector placement. When a fire alarm system design calls for heat detectors, for example, the designer must become familiar with the requirements of Section 220.127.116.11, which apply to heat detector spacing on high ceilings. Specifically, Section 18.104.22.168.1 states that "on ceilings 3 meters to 9.1 meters (10 feet to 30 feet) high, heat detector linear spacing shall be reduced in accordance with Table 22.214.171.124.1…prior to any additional reductions for beams, joists, or slope, where applicable."
The NFPA technical committee developed the requirements to allow designers to provide heat detector performance on high ceilings equivalent to that of detectors installed on a typical 10-foot (3-meter) ceiling. In the aforementioned table, which provides heat detector spacing modifications that take into account differing ceiling heights, 30 feet (9 meters) is the upper ceiling height limit. For higher ceilings, Section 126.96.36.199.2 states that "the minimum spacing of heat detectors shall not be required to be less than 0.4 times the height of the ceiling." Because the "width of the uniform temperature of the plume" impinging on the ceiling "is approximately 0.4 times the height above the fire…reducing spacing below this level will not increase response time."
Another issue is high-rack storage, which creates obstructions and long aisles that affect the location of manual fire alarm boxes and visible notification appliances. NFPA 72 requires that manual fire alarm boxes be conspicuous, unobstructed, and accessible. Additional manual fire alarm boxes must be provided so that the travel distance to the nearest alarm box is no more than "61 meters (200 feet) measured horizontally on the same floor." Although designers will locate a manual fire alarm box at each exit from the building, they must specify additional boxes in the aisles based on the calculated increased travel distance created by the obstruction of the racks. These racks can also obstruct the occupants’ view of the visible notification appliances. Annex Sections A.7.5.3 and A.188.8.131.52 offer guidance for placing visible notification appliances in these occupancies.
High ambient noise levels present another notification issue in industrial and storage occupancies, and designers must carefully plan the type and location of audible notification appliances to ensure audibility. Designers using voice alarm notification appliances must pay special attention to the intelligibility of the messages throughout the building.
For most of these occupancies, designers will use public mode signaling. NFPA 72 requires fire alarm systems to use visible notification appliances when the average ambient sound level exceeds 105 dBA. To ensure that everyone can hear the signals in the public mode, audible notification appliances must provide sound levels of "at least 15 dB above the average ambient sound level or 5 dB above the maximum sound level having a duration of at least 60 seconds, whichever is greater."
Based on these challenges, designers and contractors should proceed cautiously when designing and installing fire alarm systems in industrial and storage occupancies.
Wayne D. Moore, P.E., FSFPE, is a principal with Hughes Associates and immediate past chair of the NFPA 72 Technical Correlating Comittee.