When a potential client calls on you to provide a fire alarm system, do you assume the design responsibility? If so, what resources do you use?
Most electrical contractors rely heavily on experience, which serves them well because they perform electrical- systems-installation work daily. However, when you perform work that you seldom do, such as fire alarm installation, it may be a mistake to rely on experience.
You should always know that more than one code will affect your design. You need the current building code adopted in the jurisdiction where you will perform the work; the current NFPA 72 2013, National Fire Alarm and Signaling Code; and, of course, NFPA 70 2014, National Electrical Code (NEC). You also need to know which editions apply in the jurisdiction.
The building code provides guidance on the type of fire alarm system required for the particular occupancy. NFPA 72 and NFPA 70 provide us with the system-installation requirements.
Using the latest edition of NFPA 72 ensures you have up-to-date technical information for your design. For this example, we will use NFPA 72 2013.
Let’s assume the building code requires a fire alarm system with smoke detectors in the corridors and notification appliances—flashing strobes and speakers—placed as required throughout. The building code also requires manual fire alarm boxes and automatic sprinkler system monitoring.
With that in mind, begin your design with the detection layout. Basic spacing information for smoke-detector placement is in NPFA 72 2013, Chapter 17. You need to know the building’s structural features, such as ceiling height and whether or not the ceiling has obstructions, the locations of HVAC supply and return vents, and the locations of smoke doors. If magnetic door releases hold the smoke doors open, you must consider the spaces with the doors closed as separate spaces. This last issue will affect your strobe and speaker layout. To ensure your smoke-detection design is correct and stable—with no false alarms created as a result of your work— evaluate the environment where you will locate each smoke detector.
NFPA 72 2013 requires manual fire alarm boxes at each exit on each floor with no more than a 200-foot travel distance from any point on the floor to reach a manual fire alarm box location.
Next, decide the location of all automatic fire sprinkler system control valves. The sprinkler contractor will normally install the valve position supervisory switches and waterflow switches. Determine the appropriate zoning to allow responding personnel to quickly locate an operating sprinkler system or a closed sprinkler control valve. So far, the design elements seem pretty straightforward.
When I review designs, I find most mistakes occur in the notification- appliance layout. These can prove costly.
The code requires the design to meet performance metrics for audibility, visibility and intelligibility, which are contained in NFPA 72 2013, Chapter 18.
Audibility requirements start with identifying the rooms and spaces that will and won’t have audible notification. Based on the building code, the design must provide for audible occupant notification in all occupiable areas. Section 220.127.116.11 states the actual audible levels the appliances must achieve: “To ensure that audible public mode signals are clearly heard … they shall have a sound level 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, measured 5 feet above the floor.”
You will not normally meet this performance requirement using notification appliances installed only in the corridors. Typically, a good designer will first place the notification appliances (in this case, speakers) to meet the performance requirement for audibility. Each speaker has multiple power taps to assist in determining the decibel level output needed to meet the performance metric.
You can ensure your design will meet the visibility requirements for the strobes by installing the appropriate candela-rated strobe, as outlined in Section 18.5 and Tables 18.104.22.168.1(a) and 22.214.171.124.1(b).
After you have placed your speakers and strobes on the design, you may then determine where to combine the audible and visible appliances into a single unit. Remember, no requirement specifies that all notification appliances must be combination units.
The intelligibility of the speakers represents a more complex consideration and requires you to review NFPA 72 2013, Section 18.4.10. In some cases, you will need special training and instrumentation to determine the speaker notification appliances’ intelligibility as installed in the particular spaces.
Finally, the design must provide for the connection of the entire system to a supervising station in order to meet the building code requirements for automatic sprinkler system monitoring.
When you design a fire alarm system, you are responsible for all the concepts above. You will rely on experience, understanding of fire alarm system design concepts, and a strict adherence to code requirements. Do not take this responsibility lightly; it will require you to maintain your proficiency in order to avoid mistakes that could affect the lives of those you intend to protect.