A GOOD FIRE ALARM SYSTEM DESIGN becomes a function of how well you understand fire-protection principles as well as the requirements of NFPA 72, National Fire Alarm and Signaling Code, and
NFPA 70, National Electrical Code (NEC). However, I have met too many fire alarm contractors who assume
that, if they know the manufacturer’s installation requirements and have some familiarity with NFPA 72, that’s enough
to design a fire alarm system. These contactors then tell the owner that the system “has been designed to code,”
whatever that means.
As I have stated before, NFPA 72 is not a design manual. Surely a
manufacturer’s installation manual does not qualify either. Why, then,
do contractors and designers feel those two books are all they need to
provide a complete fire alarm system design?
For example, do you know the differences between the various smoke-detection technologies and how to best apply them in a design? Generally
speaking, contractors probably use photoelectric, spot-type smoke detectors. For most applications that need early warning, this type will perform
appropriately. However, if some of the spaces a contractor intends to
protect have high ceilings, a spot-type smoke detector of any technology
will not provide early warning.
Contractors must understand how a fire grows. Smoke cools as it
moves away from fire and the thermals necessary to lift the smoke to
a ceiling-mounted detector diminishes. This reduces the chance that a
smoke detector will provide early warning of a small fire. Since early
warning is a goal of smoke detection, this lapse in understanding defeats
In high-ceiling spaces, it is difficult to satisfy the early-warning goal.
Generally, the fire must become larger before enough thermal lift can
move the smoke to the detector.
Furthermore, because of its relative inaccessibility, a spot-type smoke
detector installed on a very high ceiling is less likely to receive needed
service. As a result, such detectors are prone to locking into alarm, and
resetting the fire alarm system control panel becomes impossible. At that
point, someone will get on a lift to clean the detector or replace it. Unfortunately, the detector is usually replaced with another spot-type detector.
Contractors should consider other types of smoke detection for high-ceiling
spaces. For example, linear projected beam-type smoke detectors may
perform much better than spot-type detectors. Actual ceiling heights and
room configuration will determine if this type of detector better suits
fire-protection goals and design needs. In the case where it makes sense
to use linear projected beam-type detectors, contractors should also under-
stand that the fire size at detection will still have to be larger. Contractors
should consider this fact when “selling” this feature.
What about the environment where the smoke detectors are to be
placed? Annex A of NFPA 72 2016 covers causes of smoke detector false
alarms. Table A. 17. 7. 1. 9(a), Common Sources of Aerosols and Particulate
Matter Moisture, provides guidance on the different ambient conditions
that adversely affect smoke detector operation. These conditions include
moisture, combustion products and fumes, atmospheric contaminants,
and engine exhaust; it also occurs when a heating element experiences
Table A. 17. 7. 1. 9(b) covers sources of electrical and mechanical influ-
ences on smoke detectors, such as electrical noise, transients and airflow.
Smoke detection usually becomes an integral part of any fire alarm
system design, and the design goals always include the requirement for
the system to remain stable—that is, false-alarm-free. Therefore, it is
imperative to stay aware of the above information while developing and
A lack of audibility in nonvoice systems and intelligibility in in-building
fire emergency voice/alarm communications systems (EVACS) presents
even more design issues that regularly rear their ugly heads during acceptance testing. Authorities having jurisdiction are aware of the necessity
for audibility and intelligibility when they witness fire alarm systems acceptance tests.
First, Chapter 18 of NFPA 72 2016, spells out the audibility requirements. Chapter 18 requires contractors to clearly document the audible
notification appliances’ sound-pressure levels. Contractors must make this
information available for use during the system’s acceptance, calculate
the sound pressure losses, and provide additional audible notification
appliances when the calculations show the sound pressure level will fall
below the level required by the code.
FOCUS | FIRE
BY WAYNE D. MOORE