Consider the fable about crying wolf. False alarms have the same effect in reducing the credibility of alarms. Individual experiences with home smoke alarms that keep going off from cooking develops a negative culture of response to real emergencies. This negative
response culture is exacerbated when occupants experience a false alarm in a commercial building and are never told the cause.
Based on empirical evidence, most fire alarm system false alarms are caused by three distinct factors: poor design, installation and maintenance.
In some cases, the false alarms caused by poor design can be found and fixed during plans review or the initial acceptance testing. In most cases, the issues revolve around the misapplication of detectors, i.e., using the wrong type of smoke or heat detector. In others, false alarms occur by not observing the environmental conditions where a detector or control component is being placed.
NFPA 72-2007, the National Fire Alarm Code, requires the selection and placement of smoke detectors to account for the performance characteristics of the detector and the areas into which the detectors are to be installed to prevent nuisance alarms or improper operation after installation.
The code also warns the designer that the location of smoke detectors must be based on an evaluation of potential ambient sources of smoke, moisture, dust, fumes, and electrical or mechanical influences to minimize nuisance alarms. Section 126.96.36.199 states, “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 0°C (32°F)
(2) Temperature above 38°C (100°F)
(3) Relative humidity above 93 percent
(4) Air velocity greater than 1.5 m/sec (300 ft/min)”
Most product-testing organizations include tests for temporary excursions beyond the devices’ normal limits as described. However, most designers are not aware of these limits and will push the envelope when applying detection to a space. The designer should consult the manufacturer’s published instructions to ensure the application of a device suits the area in which it will be installed.
Poor installation also contributes to false alarms. Smoke detectors, especially, can be affected by electrical and mechanical influences. For example, evidence shows that electronic ballasts used for fluorescent light fixtures will affect the operation of smoke detectors. These detectors also can be set off by a list of aerosols and particulate matter that may be present in the space where the detector is located. NFPA 72-2007 lists potential environmental issues that can adversely affect the operation of smoke detectors. The list is in the annex of the code, Table A.188.8.131.52 (a), and includes the following:
- Moisture (e.g., humid outside air, humidifiers, showers, etc.)
- Combustion products and fumes (e.g., cleaning fluids, cooking equipment, fireplaces, paint spray, etc.)
- Atmospheric contaminants (e.g., dust or lint, excessive tobacco smoke, linen and bedding handling, sawing, drilling and grinding, etc.)
- Engine exhaust (e.g., diesel trucks and locomotives, engines not vented to the outside, or gasoline forklift trucks)
- Heating elements with abnormal conditions (e.g., dust accumulations, improper exhaust or incomplete combustion).
Table A.184.108.40.206 (b), NFPA 72-2007, contains a partial list of sources of mechanical and electrical influences on smoke detectors, which includes electrical noise and transients from radio frequency, lightning, vibration and power-supply spikes.
It may not be possible to totally isolate the smoke detector location from the environmental, electrical and mechanical effects, but awareness of these issues will help the designer and installer mitigate their effects on detector performance.
Another installation issue that contributes to false alarms is early installation of smoke detectors. The code is clear on this, and it is imperative that the installer ensure the owner or general contractor is aware that section 220.127.116.11 states, “Detectors shall not be installed until after the construction cleanup of all trades is complete and final.”
The requirement’s only exception is where detectors are required by the authority having jurisdiction for protection during construction. If that proves to be the case, the code requires, “Detectors that have been installed during construction and found to have a sensitivity outside the listed and marked sensitivity range shall be cleaned or replaced in accordance with Chapter 10 at completion of construction.”
Finally, the last issue that affects false alarms is that of poor maintenance or even none at all. If smoke detectors are not cleaned at least annually, false alarms can be expected from that system.
Contractors and designers must be aware of the part they play in maintaining the credibility of installed fire alarm systems. Pay attention to the basics.
MOORE, a licensed fire protection engineer, frequent speaker and an expert in the life safety field, is a co-editor of the current National Fire Alarm Code Handbook. Moore is a principal with Hughes Associates Inc. at the Warwick, R.I., office.