Ensuring the Workings

Fire/Life Safety August 2020

The question of whether fire alarm systems will work when needed focuses on operational reliability. Ultimately, the goal for any system is that it will detect the fire early enough and sound the alarm loud enough to ensure the occupants evacuate the building safely. Additionally, if the system is connected to a remote supervising station—and it always should be—it will notify the fire department with a minimum time delay.

Bottom line, to achieve these important goals, you need to make sure that many things are done right in your installation.

Where do you begin? Make sure you have a copy of the NFPA 72 edition that your jurisdiction has adopted. You can find out from your current building code.

Second, have you read the code, or at least reviewed it so you know where to find various installation and application requirements when an issue arises on a project design or installation? If you answered yes, then you are in the top 20% of contractors that install fire alarm systems today.

The code will keep you out of trouble on a project. It will help to ensure the fire alarm system you install will work reliably when called on. The code also guides what to do or not to do in a building.

For example, the code requires a listed mechanical guard to be used to protect any initiating device or notification appliance when it could be subject to physical damage.

When using fixed-temperature or rate-compensated heat detectors, the temperature rating of the detector must be at least 20°F above the maximum expected temperature at the ceiling.

Speaking of heat detectors and ceilings, the spacing of a heat detector is affected by ceiling height. Review the section of the code on high ceilings (17.6.3.5) and specifically look at Table 17.6.3.5.1 for heat-detector spacing reduction based on ceiling height.

Smoke detectors

Smoke detectors are more common in our systems today and can present more problems with false alarms due to misapplications and improper installation, so here are a few highlights from the code to guide you.

NFPA 72 reminds you to select and place smoke detectors by considering both the performance characteristics of the detector and the areas into which the detectors are planned for installation. This will help to avoid nuisance or unintentional alarms after the installation is complete.

Section 17.7.1.8 of the code requires that, “unless specifically designed and listed for the expected conditions, smoke detectors shall not be installed if any of the following ambient conditions exist:

(1) Temperature below 32°F (0°C)

(2) Temperature above 100°F (38°C)

(3) Relative humidity above 93 percent

(4) Air velocity greater than 300 ft/min (1.5 m/sec)”

When you are designing or installing a fire alarm system, you must evaluate the spaces for potential ambient sources of smoke, moisture, dust or fumes and electrical or mechanical influences to minimize nuisance alarms.

There is a different problem with spot-type smoke detectors, which call for reduced spacing on high ceilings. The higher the ceiling, the more likely detection will not occur. This is due primarily to stratification.

As stated in the Annex A 17.7.1.10 of NFPA 72-2016, “Stratification of air in a room can hinder air containing smoke particles or gaseous combustion products from reaching ceiling-mounted smoke detectors or fire-gas detectors.”

This occurs due to the burning products cooling as they rise toward the ceiling until they reach a level where the products are essentially the same temperature as the surrounding air.

There is no table for adjusting the spacing of spot-type smoke detectors for high ceiling placement like there is for spot-type heat detectors. Also, never use Table 17.6.3.5.1 for the placement of spot-type smoke detector on high ceilings. Just don’t use spot-type smoke detector on high ceilings. Don’t.

In high ceiling areas, the use or projected beam-type smoke detection is generally considered a better design. Always keep in mind that a design goal of detecting a smoldering fire in a large space with a high ceiling is next to impossible. Air sampling–type smoke detectors at different levels may be considered, but that is a more expensive option.

Reviewing a few areas of the code, and using it correctly, ensures your alarm installations will work.

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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