What Is the Purpose of a Fire Alarm System?

There have been so many technological and code changes in the last 10–15 years that it makes sense to step back and discuss why we install fire alarm systems and how the components of a fire alarm system work? Unfortunately, it seems the industry continues to evaluate fire alarm system performance based on the historical data before modern technological advances were available.

Part of this data is based on the false alarms generated by systems, and part is generated by experience with smoke alarms that have nothing to do with fire alarm systems’ performance. The data from installed systems invariably points to smoke detector false alarms and almost always is generated from systems older than 10 years. And most of this data fails to focus on the root causes of the false alarm problem, which are a total lack of system maintenance and misapplication of smoke detectors. I will speak to those issues in future articles because knowledge of both issues will provide opportunities for you and profits to your bottom line.

False alarms aside, fire alarm systems are installed primarily for life safety, and we need smoke detectors to meet that early-warning, life-safety goal. Start with the most basic question: do you know how a smoke detector works? For a spot-type smoke detector, smoke must get to the sensing chamber at a level that correlates to the detector's alarm setting before it will alarm.

Do you know how many are needed in a space or building to reliably meet our life safety goal? Don’t start your answer to the second question with “the required amount of smoke detection is in the code” because you would first need to define which code you are referencing.

Some of you would state that NFPA 72, National Fire Alarm and Signaling Code, will give us the number needed in every space. You would be only half right. First, NFPA 72 does not require any smoke detectors to be installed in a building. Those requirements are found in the International Building Code or the Life Safety Code. In addition, when you refer to the latter two codes, you may find only corridor smoke detection is required.

NFPA 72 provides the installed spacing requirements for smoke detectors. Your answer is now we “only” need corridor smoke detection spaced in accordance with NFPA 72. Have we met our stated life safety goal? The short answer is no. As stated above, for a spot-type smoke detector to detect smoke, smoke must enter the sensing chamber at a level that will cause the smoke detector to activate. If we have an office building with only corridor smoke detection, do you think the smoke will get to the detector to provide early warning from a fire in an office with closed doors to the corridor? How long will it take for the smoke to get to the detector’s sensing chamber to produce an alarm? Even the most novice technician will more than likely guess that would be a long time, and that answer, although not measurable, would be right.

What else affects the ability of a spot type smoke detector to do its job? First and foremost is the ceiling height. Anything higher than a 12-foot ceiling will delay detection, and our warning will not happen until the fire gets larger. Additionally, the environment can wreak havoc with detection. If the room or space has high air movement, we will again have difficulty detecting a small fire.

If there is a requirement for smoke detection on one of your projects and the designer is showing spot-type smoke detectors on a 30-foot ceiling, is that a problem? The answer is a very loud yes! Linear beam and air sampling smoke detectors may work better in these difficult environments. NFPA 72 in its Annex A provides some guidance in this area.

Why is it important for you to know these basic tenets of detection? Your customer looks to you as the expert. You can take an order for a fire alarm system that complies with the building code, or you can take it a step further and explain why this “code-compliant” corridor smoke detection system may not meet your customer’s goal to provide life safety to their building occupants. If you can at least discuss why a remotely installed smoke detector will not meet their early-warning, life-safety goal, odds are your customer is going to look at you in a different light compared to the competition.

Next time, I will go into more detail regarding smoke detector spacing, misapplications and correct applications of these early warning devices.

About the Author

Wayne D. Moore

Fire/Life Safety Columnist

Wayne D. Moore, a licensed fire protection engineer, frequent speaker and expert in the life safety field, is a principal member and past chair of NFPA 72, Chapter 24. He is a vice president with Jensen Hughes at the Warwick, R.I., office and can be...

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