NFPA Journal

Although NFPA 72®, National Fire Alarm and Signaling Code, is not meant to be a design guide, the code nevertheless does contain certain application information that can be valuable in assisting fire alarm system designers in meeting challenges presented by a variety of industrial occupancies.

In many instances, the conditions in industrial occupancies can include an array of challenges, including large open spaces, high ceilings, varying ambient noise conditions, hard surfaces, dusty environments, and varying temperature environments. For example, large open spaces combined with high ceilings are the most challenging for almost any kind of detection, and a fire must grow to a “detectable size.” The designer must convey this information to the owner to avoid any misunderstanding of the fire alarm system’s capabilities.

There is a wealth of application information available in both the annexes of the code and in the code handbook that can assist designers when addressing these challenges. For example, when designing heat detection for a large open space with 30-foot ceilings, designers can review section 17.6.3.5 and find the table that provides information on reducing heat detector spacing based on ceiling height. As the information in the annex states, the tables are constructed to provide detector performance on higher ceilings, up to 30 feet high, that is essentially equivalent to that which would exist with detectors on ceilings 10 feet high.

In NFPA 72, Annex A recommends that, in facilities with high ceilings, spot-type smoke detectors are not accessible for maintenance and testing. According to Annex A, a better alternative would be to use projected beam smoke detectors or, in some applications, air sampling–type smoke detectors. Designers are cautioned that NFPA 72 does not possess all the answers to the possible questions arising from applications in the built environment, and the information provided in the annex are just examples of what the technical committee thought would be most beneficial for users of the code.

Possibly the most important part of the design process occurs before any design begins, when owners establish their design goals. In many instances, owners do not understand that it will be almost always impossible to detect a small fire in a large space with a high ceiling. Early warning of small fires in normal 10-foot ceiling spaces are difficult. Early warning of a small fire in a large space with ceilings of 30 feet or more is generally not possible.

The reason for this is stated in Annex A, A.17.7.3.1: “A smoldering fire produces little, if any, plume and no appreciable ceiling jet. Far more time elapses between ignition and detection under this circumstance.” Alternatively, according to the annex, “A hot, energetic fire produces large plume velocities and temperatures and hot, fast ceiling jets. This minimizes the time it takes for the smoke to travel to the detector.”

The system designer for industrial occupancy fire alarm systems also needs to understand how difficult these environments are with respect to audibility and intelligibility of the notification appliances. Industrial occupancies, in addition to being large, high-ceilinged buildings, include numerous hard surfaces that can reduce the intelligibility of voice systems. Depending on the activity taking place in the building, such as storage or manufacturing, there will be high ambient noise levels that must be overcome through competent design. The same holds true for strobe notification appliances. There may be storage racks that could interfere with wall-mounted strobe appliances. In many cases, ceiling-mounted strobes will provide more uniform coverage in a facility with numerous storage racks.

An informed designer will know that every building presents its own design challenges that need to be addressed using all the tools available, including NFPA 72 requirements and sound fire protection engineering principles.

WAYNE D. MOORE is vice president at Jensen Hughes.