NFPA 72, National Fire Alarm and Signaling Code, is primarily concerned with life safety. The word “safety” is mentioned 277 times. NFPA 72 also references NFPA 101, Life Safety Code, and this code requires installers to follow NFPA 72 when designing and installing fire alarm systems.
Further examples of safety requirements in the code occur in Chapter 17. Users need to understand the application concepts outlined in this chapter to provide the safety expected of fire alarm systems.
Section 17.2 states, “Automatic and manual initiating devices shall contribute to life safety, fire protection, and property conservation by providing a reliable means to signal other equipment arranged to monitor the initiating devices and to initiate a response to those signals.”
For as long as I can remember, we have considered smoke detectors to be life safety devices, and most designers understand this concept. To provide the measure of life safety that a building’s owner (or at least the occupant) expects, it’s necessary to know that a smoke detector can detect a smoldering fire while also acknowledging what its limitations are in early detection.
Keep in mind that the building codes and Life Safety Code require the minimum number of smoke detectors. Often, they only require smoke detectors in the means of egress such as hallways and stairwells. These detectors will not detect a fire in rooms that open to the hallways; they will only detect fires in the means of egress. Depending on several factors that include, but are not limited to, ceiling height and air movement, they may only detect fires that have already gone beyond simply smoldering.
Early warning considerations
You and the owner should also discuss expectations related to early warning. An owner may expect any smoldering fire in any location in the building to be detected and an alarm sounded, including by transmitting that alarm to the fire department. Due to the limitations of smoke detectors, this expectation may be unrealistic.
If the owner does expect an early warning about a fire, you need to design and install the system with more than the minimum number of smoke detectors. A goal for early warning will require you to know more about the limitations of smoke detection and proper device placement to get closer.
Chapter 17, Section 17.7 of NFPA 72 contains the minimum requirements for smoke detector installation. The beginning of the section discusses the need to locate and space smoke detectors based on the anticipated smoke flows from the plume and ceiling jet produced. This requires identifying the possible combustibles to determine what type of fire should be expected.
It will also be necessary to determine any ambient airflows that could exist in the protected compartment by reviewing the HVAC system and its effect on smoke detection planned for the space.
Remember, for detection to take place, enough smoke must get into a detector’s sensing chamber to actuate it. Fully understanding this concept will allow you to place smoke detectors in the best locations.
Take these into consideration
Section 17.7.3.1.2 requires the design to account for the contribution of six factors in predicting detector response to the anticipated fires you want to be detected:
- Ceiling shape and surface
- Ceiling height
- Configuration of contents in the area
- Combustion characteristics and probable equivalence ratio of the anticipated fires involving the fuel loads in the protected area
- Compartment ventilation
- Ambient temperature, pressure, altitude, humidity, and atmosphere
If the intent is to protect against a specific hazard, install the detector closer to it and in a position where it can intercept the smoke.
Other important considerations include ceiling height and ceiling configuration.
In my opinion, ceiling height is the most important factor. Remember: smoke needs to get to the sensing chamber in the right concentration for it to operate. Intuitively, you can probably see that the higher the ceiling, the larger the fire must be to drive the smoke to the detector.
Chapter 17 provides guidance on ceiling height and configuration challenges.
Stay informed and use the code to help you do the job expected of you. An early warning might save lives.
Robert Kneschke / stock.adobe.com
About The Author
MOORE, a licensed fire protection engineer, was a principal member and chair of NFPA 72, Chapter 24, NFPA 909 and NFPA 914. He is president of the Fire Protection Alliance in Jamestown, R.I. Reach him at [email protected].
