Choose Wisely: How Well Do You Understand Fire Alarm System Design

A good fire alarm system design becomes a function of how well you understand fire-protection principles as well as the requirements of NFPA 72, National Fire Alarm and Signaling Code, and NFPA 70, National Electrical Code (NEC). However, I have met too many fire alarm contractors who assume that, if they know the manufacturer’s installation requirements and have some familiarity with NFPA 72, that’s enough to design a fire alarm system. These contactors then tell the owner that the system “has been designed to code,” whatever that means.


As I have stated before, NFPA 72 is not a design manual. Surely a manufacturer’s installation manual does not qualify either. Why, then, do contractors and designers feel those two books are all they need to provide a complete fire alarm system design?


For example, do you know the differences between the various smoke-detection technologies and how to best apply them in a design? Generally speaking, contractors probably use photoelectric, spot-type smoke detectors. For most applications that need early warning, this type will perform appropriately. However, if some of the spaces a contractor intends to protect have high ceilings, a spot-type smoke detector of any technology will not provide early warning.


Contractors must understand how a fire grows. Smoke cools as it moves away from fire and the thermals necessary to lift the smoke to a ceiling-mounted detector diminishes. This reduces the chance that a smoke detector will provide early warning of a small fire. Since early warning is a goal of smoke detection, this lapse in understanding defeats its purpose.


In high-ceiling spaces, it is difficult to satisfy the early-warning goal. Generally, the fire must become larger before enough thermal lift can move the smoke to the detector. 


Furthermore, because of its relative inaccessibility, a spot-type smoke detector installed on a very high ceiling is less likely to receive needed service. As a result, such detectors are prone to locking into alarm, and resetting the fire alarm system control panel becomes impossible. At that point, someone will get on a lift to clean the detector or replace it. Unfortunately, the detector is usually replaced with another spot-type detector.


The alternative


Contractors should consider other types of smoke detection for high-ceiling spaces. For example, linear projected beam-type smoke detectors may perform much better than spot-type detectors. Actual ceiling heights and room configuration will determine if this type of detector better suits fire-protection goals and design needs. In the case where it makes sense to use linear projected beam-type detectors, contractors should also understand that the fire size at detection will still have to be larger. Contractors should consider this fact when “selling” this feature.


What about the environment where the smoke detectors are to be placed? Annex A of NFPA 72 2016 covers causes of smoke detector false alarms. Table A.17.7.1.9(a), Common Sources of Aerosols and Particulate Matter Moisture, provides guidance on the different ambient conditions that adversely affect smoke detector operation. These conditions include moisture, combustion products and fumes, atmospheric contaminants, and engine exhaust; it also occurs when a heating element experiences abnormal conditions.


Table A.17.7.1.9(b) covers sources of electrical and mechanical influences on smoke detectors, such as electrical noise, transients and airflow.


Smoke detection usually becomes an integral part of any fire alarm system design, and the design goals always include the requirement for the system to remain stable—that is, false-alarm-free. Therefore, it is imperative to stay aware of the above information while developing and documenting designs.


A lack of audibility in nonvoice systems and intelligibility in in-building fire emergency voice/alarm communications systems (EVACS) presents even more design issues that regularly rear their ugly heads during acceptance testing. Authorities having jurisdiction are aware of the necessity for audibility and intelligibility when they witness fire alarm systems acceptance tests.


First, Chapter 18 of NFPA 72 2016, spells out the audibility requirements. Chapter 18 requires contractors to clearly document the audible notification appliances’ sound-pressure levels. Contractors must make this information available for use during the system’s acceptance, calculate the sound pressure losses, and provide additional audible notification appliances when the calculations show the sound pressure level will fall below the level required by the code.


For example, when meeting the public-mode requirements for audible notification, contractors will need to design the notification-appliance system so that all occupants will hear the alarm signal. To ensure the occupants can clearly hear the audible public-mode signals, the code requires a sound level of at least 15 decibels (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—measured 5 feet above the floor in the area served by the system, using the A-weighted scale (dBA). When designing a notification system for a hospital, a contractor would design to the private-mode requirements. In other words, the staff needs to clearly hear the audible signal but at a reduced soundpressure level to avoid disturbing patients when an alarm sounds and they cannot evacuate.


Private-mode audible requirements must have a sound level of at least greater than 10 dB above the average ambient sound level or 5 dB above the maximum sound level having a duration of at least 60 seconds, measured 5 feet above the floor in the area served by the system, using the A-weighted scale.


In addition to the above requirements, for sleeping areas, the code states, “Where audible appliances are installed to provide signals for sleeping areas, they shall have a sound level 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 or a sound level of at least 75 dBA, whichever is greater, measured at the pillow level in the area required to be served by the system using the A-weighted scale (dBA).”


If barriers exist—such as doors, curtains or retractable partitions—between the notification appliance and the pillow, contractors must take the sound-pressure-level measurement with the barrier in place.


A requirement that became effective in 2010 requires the design to use listed low-frequency notification appliances to awaken occupants in sleeping areas.


When planning and designing an EVACS, contractors must determine all of the acoustically distinguishable spaces (ADSs), as defined in Chapter 18 of NFPA 72 2016. For each ADS where the code requires intelligibility, voice-communications systems shall reproduce prerecorded, synthesized or live (e.g., microphone, telephone handset, radio) messages with voice intelligibility. Also, contractors must identify each ADS that does not require voice intelligibility. The enforcing authority; governing laws, codes or standards; or other parts of NFPA 72 may require ADS assignment submission for review and approval.


All this covers only a small amount of the information contractors must know and understand to ensure a fire alarm system design meets all code requirements and good design practices. Contractors must not assume that knowing the manufacturers’ equipment performance and having a cursory knowledge of NFPA 72 will provide everything needed to develop a proper design. Making such assumptions could be costly.


About the Author

Wayne D. Moore

Fire/Life Safety Columnist

Wayne D. Moore, a licensed fire protection engineer, frequent speaker, writer and expert in the life safety field, has been a principal member and chair of NFPA 72, Chapter 24, as well as a former principal member of NFPA 909 and NFPA 914. He is the...

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