Too many people look at NFPA 72, National Fire Alarm and Signaling Code, as a design manual and feel using the book is all that is necessary when designing a fire alarm system. Others assume that if they know how to put together a certain manufacturer’s fire alarm system they have all the design tools they need. These alone won’t be enough.
If you plan on taking on the liability for the design of a fire alarm system, then it is important that you understand certain basic fire protection engineering principles. For example, you need to know that you cannot design a system in a vacuum. You must discuss the owner’s fire protection goals by asking pointed questions. And “just meeting code” or asking the authority having jurisdiction what he or she wants are not appropriate methods to approach a fire alarm system design.
You will also need scaled drawings of the building in question, preferably in AutoCAD, to start your design. However, your design should begin with a determination of what Class of circuits you intend to use for detection, notification and control and then a layout of all detection and notification devices and appliances based on valid fire protection goals.
Once you have decided on the class of circuits, first start with the layout of all smoke detection devices planned for the installation. Before moving forward, review the smoke detector placement to ensure the environments where they are located are not going to be sources of false alarms after the installation is online. Also ensure you have not located any spot-type smoke detector on a ceiling height that exceeds 15 feet. If you have areas with high ceilings, i.e., greater than 15 feet, choose another type of smoke detector that is better suited for the application or review the need for a smoke detector in that specific location. You should know that the higher the ceiling the longer it will take smoke to get to the detector, and the design will probably not meet the owner’s goals for early warning.
When you have assured yourself that the smoke detector placement is meeting the design intent and the devices are placed well, you can move on to heat detector and manual fire alarm box placement. NFPA 72 does give specific requirements for manual fire alarm boxes. A manual fire alarm box must be located at each exit from each floor and be spaced at a travel distance no further than 200 feet to a manual fire alarm box, i.e., no more than 400 feet apart. Heat detectors have environmental and ceiling height considerations that must be evaluated also. The heat detector placement is covered in NFPA 72, Section 18.104.22.168.1 and Table 22.214.171.124.1 Heat Detector Spacing Reduction Based on Ceiling Height.
After the detection is placed on the scaled drawings, you should layout the audible appliances. In a non-voice system, the typical audible appliance is the horn. The code requires that audible notification appliances “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 ft (1.5 m) above the floor in the area required to be served by the system using the A-weighted scale (dBA).” This requirement applies to all audible signals.
You can use the listed sound level output at 10 feet as a starting point for placement. Every time you double the distance from the measurement spot you will experience a 6 decibel (dB) loss. You can find more information in Annex A, Section A.18.4.4, and you will find design assistance regarding typical ambient sound levels in Table A.18.4.4,Average Ambient Sound Level According to Location.
There are two additional issues with audible appliances that you should be aware of. All audible notification appliances located in residential sleeping areas must provide signals for sleeping areas, that 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).” And audible appliances provided for the sleeping areas to awaken occupants shall produce a low frequency alarm signal that complies with the code.
After you have determined the approximate locations of your audible appliances you are ready to determine where visual appliances must be placed. Where a visual appliance is close to the location of an audible appliance, then use a combination horn/strobe. Otherwise do not fall into the trap of using all combination audible/visible appliances. This latter approach will drive your power requirements and the associated battery requirements to higher levels than needed.
The fire alarm control unit (FACU) should be in an appropriate environment and be protected by a smoke detector as required by the code. The FACU should be designed to accommodate all detection devices and notification appliances; additional power supplies may be located elsewhere in the building to accommodate notification appliance power requirements. The remote power supplies will also need smoke detectors located in the area of the power supplies.
It should be obvious that not all design issues have been covered here, but the intent was to introduce the basics to help ensure better designs and help guide you when you choose to design the fire alarm system.