How many times while you are designing or installing a fire alarm system do you get unwanted and wrong advice (or mandates) from the authority having jurisdiction (AHJ)? Did you ever wonder where this misinformation came from? Generally, the misinformation started from an alarm installer who was a “friend” of the AHJ, who is usually misinformed and often bases their information on hearsay. He rarely reads the National Fire Alarm and Signaling Code, NFPA 72.

Here are four common statements regarding installations that are clearly wrong, and you should be aware of the correct response when this happens to you.

All notification appliances must be combination audible and strobe units.

This statement is false. The code has different requirements for audible appliances versus strobes. Section 18.4.4.1* states, “To ensure that audible public mode signals are clearly heard, unless otherwise permitted by 18.4.4.2 through 18.4.4.5, 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 ft. (1.5 m) above the floor in the area required to be served by the system using the A-weighted scale (dBA).”

Information proved in the NFPA 72 Handbook states: “Audible appliance ratings, as measured by the manufacturer and qualified testing laboratories, are specified as a decibel rating at a predetermined distance, usually 10 ft. (3 m). The rule of thumb is that the output of an audible notification appliance is reduced by 6 dB if the distance between the appliance and the listener is doubled. Similarly, if the distance is cut in half, the SPL is increased by 6 dB. The accuracy of this rule depends on many intervening variables, particularly the acoustic properties of the materials in the listening space, such as ceiling materials and floor and wall coverings. The use of the appliance’s rating along with this rule allows system designers to estimate audible levels in occupiable spaces before a system is installed.”

Visual appliances are required to follow different rules. The code requires visual notification appliances to be installed in accordance with Table 18.5.5.7.1(a) for wall-mounted appliances or Table 18.5.5.7.1(b) for ceiling-mounted devices. These requirements are related to the candela output of the visual appliance. Additionally, the location of visual appliances installed in corridors is spelled out in Section 18.5.5.8.1: “The installation of visual notification appliances in corridors 20 ft. (6.1 m) or less in width shall be in accordance with the requirements of either 18.5.5.7 (spacing in rooms) or 18.5.5.8 (spacing in corridors).” Generally speaking, visual notification appliances are not less than 15 candela and must be located not more than 15 feet from the end of the corridor and separated from each other by no more than 100 feet.

You can lay out either the visual appliances or audible appliances first and then lay out the other. They only need to be combination units where the two appliances would be installed near each other. If you follow these guidelines, your costs will be less and your chance of being lower than the other bidders is greater.

Adding one smoke detector to a manual fire alarm system will provide early warning.

This is again false (unless the detector is Irish!). This statement is made only by those who do not understand how fires develop, how smoke moves and the environmental effects on that smoke movement. Unless the fire starts in the immediate area of the smoke detector, will it have a chance at detecting the fire quickly. Similarly, if smoke detectors are installed only in the corridors of a building, they will not detect a fire in the offices or apartments in the building until the smoke moves to the corridor. Smoke movement is dependent on the fire growth. The larger the fire, the more smoke moves in the area. It is not until the smoke gets to a detector will we have smoke detection. So one smoke detector will only provide a false sense of security.

If the plan shows spot-type smoke detectors located on a 50-foot ceiling, it will be okay to install them as shown.

This is a rookie mistake. The higher the ceiling, the longer it will take for smoke to get to the detector and the larger the fire will grow before detection. Additionally, smoke detectors on high ceilings rarely get annual maintenance and therefore will have a propensity to false alarm. Even when installed as directed, you will be called to explain the false alarm issue and will lose both money and reputation by not speaking up before the detectors were installed on the high ceiling.

Smoke detectors are required throughout the room where the fire alarm control unit is located.

Once again, this statement is not true. Section 10.4.5* states that “Unless otherwise permitted by 10.4.5.1 or 10.4.5.2, 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:

(1)  An automatic smoke detector at the location of each control unit(s), notification appliance circuit power extender(s), and supervising station transmitting equipment

(2)  An automatic heat detector where ambient conditions prohibit installation of an automatic smoke detector”

The Annex to 10.4.5 states, in part, “…Where total smoke detection coverage is not provided, the code intends that only one smoke detector is required at the control unit even when the area of the room would require more than one detector if installed according to the spacing rules in Chapter 17. The intent of selective coverage is to address the specific location of the equipment.” Also, if the environment is not suitable for a smoke detector, it may not be appropriate for a fire alarm control unit either.

These are just a few of the common misconceptions regarding fire alarm system design. Always question a verbal requirement by simply asking for the code section reference. You will be glad you did.