If you have ever installed fire alarm systems, you more than likely have run into issues that have made you question the design or installation drawings.
Many of these questions develop into installation blunders due to a lack of understanding of NFPA 72, National Fire Alarm and Signaling Code and NFPA 70, National Electrical Code. The blunders also reduce the reliability of these important life safety systems. The most common are smoke detector placement on high ceilings, protection of control equipment and fire alarm control unit (FACU) access for testing and maintenance.
First, let’s review two important sections of NFPA 72 and NFPA 70 with regards to smoke detector placement on very high ceilings. You will often begin an installation and find that the plans have spot-type smoke detectors on a 30-feet or higher ceiling. You may have questioned the placement with the designer and found that they used a table in NFPA 72 to determine the spacing and assured you the design was correct. The table they referenced was probably Table 220.127.116.11.1, Heat Detector Spacing Reduction Based on Ceiling Height, and they claimed this table also applied to smoke detector placement.
They were wrong. Table 18.104.22.168.1 was developed based on live fire test research only for heat detectors. If you carefully review NFPA 72 you will discover that the location of smoke detectors must be based on an evaluation of potential ambient sources of smoke, environmental issues such as moisture, dust or fumes and electrical or mechanical influences.
The sole purpose of considering the ambient and environmental conditions is to avoid false alarms. Any designer or installer of fire alarm systems should know that smoke detectors can be affected by electrical and mechanical influences, aerosols and particulate matter found in protected spaces.
As stated in Annex A for Chapter 17, “While it might not be possible to isolate environmental factors totally, an awareness of these factors during system layout and design favorably affects detector performance.”
You need to consider the fact that the higher the ceiling, the more difficult it will be to get smoke to the detector. Or stated another way, the fire in a space with high ceilings must be larger for a spot-type smoke detector to alarm. If early warning was the protection goal, it will not be met.
Another reason for delayed detection in high ceiling areas is smoke stratification. To minimize nuisance, or worse, no alarms, the effect of stratification below the ceiling must also be considered.
The code states, again in Annex A, “Stratification occurs when air containing smoke particles or gaseous combustion products is heated by smoldering or burning material and, becoming less dense than the surrounding cooler air, rises until it reaches a level at which there is no longer a difference in temperature between it and the surrounding air.” In other words, the smoke will not rise to the level where spot-type detectors are installed.
Of course, you may have already been involved with a high ceiling smoke detector application where you used either a beam smoke detector or an active air sampling smoke detector. Either of these devices may be acceptable for the installation; however, when you encounter high-ceiling environments you need first to ask the question what function is served by the smoke detector? A smoke detector located on a high ceiling certainly will not provide early warning of a smoldering fire. Accessibility for testing and maintenance is another issue that arises when you install a smoke detector on a high ceiling. I have witnessed contractors installing smoke detectors on high ceilings using a lift. Once that contractor is finished, he leaves, and the lift goes with him.
The code does offer some guidance in Annex A of Chapter 17, stating, “In high-ceiling areas, such as atriums, where spot type smoke detectors are not accessible for periodic maintenance and testing, projected beam–type or air sampling–type detectors should be considered where access can be provided.” Of course, the key phrase is “where access can be provided.”
The second most common mistake I find in fire alarm system installations is not complying with Section 10.4.5, which states in part, “in areas that are not continuously occupied, early warning fire detection shall be at the location of each control unit(s), notification appliance circuit power extender(s), and supervising station transmitting equipment to provide notification of fire at that location by one of the following means:
- An automatic smoke detector at the location of each control unit(s), notification appliance circuit power extender(s), and supervising station transmitting equipment.
- An automatic heat detector where ambient conditions prohibit installation of an automatic smoke detector.” The protected control units are those that provide notification of a fire to the occupants and responders. If you determine that the environment is not suitable for smoke detection, you should reconsider the FACU location as the environment may not be suitable for it, as well.
The 2019 edition of NFPA 72 has added a new requirement that exempts the detection required in Section 10.4.5 where a dedicated function FACU has been installed and these controls are not required to provide local or supervising station alarm notification signals.
And finally, throughout NFPA 72 you will find statements requiring that all equipment be installed in locations accessible to the authority having jurisdiction and for the purpose of maintenance and inspection.
NFPA 72 has imported definitions sections from NFPA 70 relative to accessibility such as those found below The numbers in brackets indicates where in NFPA 70 the definitions also appear:
“3.3.1 Accessible (as applied to equipment). Admitting close approach; not guarded by locked doors, elevation, or other effective means. [70:100]
3.3.2 Accessible (as applied to wiring methods). Capable of being removed or exposed without damaging the building structure or finish or not permanently closed in by the structure or finish of the building. [70:100]
3.3.3 Accessible, readily (readily accessible). Capable of being reached quickly for operation, renewal, or inspections without requiring those to whom ready access is requisite to take actions such as to use tools (other and keys), to climb over or under, to remove obstacles, or to resort to portable ladders, and so forth. [70:100]”
Access applies to all control equipment, including the FACU, remote power supplies and terminal boxes. Essentially to avoid noncompliance with access requirements, you should apply the above definitions to the entire fire alarm system installation.
The bottom line to any fire alarm system installation is to know the applicable codes and understand their requirements.