Planning A Fire Detection System - Siemens System 3 Manual

Section V — Detector Application and Spacing

3. PLANNING A FIRE DETECTION SYSTEM

When planning a fire detection system, make
a choice of detector based on the kinds of
fires expected. The type and quantity of fuel,
possible ignition sources, ranges of ambient
conditions, and the value of the property to be
protected should all be considered.
In general, heat detectors have the lowest
cost and false alarm rate, but are the slowest
to respond. Since the heat generated by small
fires tends to dissipate fairly rapidly, heat
detectors are best used to protect confined
spaces, or directly over hazards where flam-
ing fires can be expected. They are usually
installed on a grid pattern at their recom-
mended spacing distances or at reduced
spacing for faster response. The operating
temperature of a heat detector should be at
least 25° F above the maximum expected
ambient temperature in the area protected.
Smoke detectors cost more than heat detec-
tors, but respond faster to fires. They are
better suited to protect large open spaces
than heat detectors because smoke does not
dissipate as rapidly as heat does in the same
size space. Smoke detectors are either
installed according to prevailing air current
conditions or on a grid layout.
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Photoelectric smoke detectors are best
used in places where wire insulation or
other smoldering fires may be expected.
Ionization smoke detectors are useful
where fire would be expected to develop
into a small flaming condition soon after
pyrolysis.
Flame detectors offer extremely fast re-
sponse, but warn of any source of radiation
in their sensitivity range. False alarm rates
can be high if this kind of detector is im-
properly applied. Because flame detectors
are "line of sight" devices, care must be
taken to ensure that they can see the entire
protected area and that they will not be
accidentally blocked by stacked material or
equipment. Their sensitivity is a function of
flame size and distance from the detector.
Although fairly expensive, they are well-
suited to protect areas where explosive or
flammable vapors or dusts are encountered
because they are usually available in
explosionproof housings or have intrinsi-
cally safe ratings.
NFPA 72 Annex A (2002 Edition) has more
specific information on the installation of the
various types of detectors.