NFPA Journal - May/June 2017 - 37

CODES & STANDARDS
& COMPLIANCE

layouts-sometimes the contents being
protected require that a system reflect a
fair bit of artistry in its own right.
Matt Klaus is NFPA technical services lead for fire
protection engineering. NFPA members and AHJs
can use the Technical Questions tab to post queries
on NFPA 13 at nfpa.org/13.

NFPA 72

Fire alarms, signaling, and
emergency communication

Smoke detector spacing
in beamed ceilings
By Wayne Moore

ecently, participants
on the NFPA Xchange
(nfpa.org/xchange)
forum posted questions
regarding the spacing of spot-type smoke detectors for
non-smooth ceiling configurations.
Before addressing question about the
installation requirements for any detection device, however, it's important
to understand the owner's detection
goals-without that knowledge, device
spacing becomes moot.
We will assume that because the
questions centered on spot-type smoke
detector spacing, the owner had set
early detection as an important goal.
The questioner did not mention ceiling
height, but ceiling height significantly
impacts early detection. Essentially, the
higher the ceiling, the larger the fire
must grow before it actuates a detector.
As stated in the Annex A of the 2016
edition of NFPA 72®, National Fire
Alarm and Signaling Code, "in order
for a smoke detector to respond, the
smoke has to travel from the point of
origin [of the fire] to the detector."
Additionally, the code advises that
the designer should understand fire
size and smoke travel concepts to
determine whether or not the design
can meet the early detection goal.
After determining whether or not it's

R

preaction system and single-interlock
preaction system. For a double-interlock system, the piping is filled with
compressed air. For these systems,
the piping will not become filled with
water until both the sprinkler and
detection system are activated. With a
single-interlock system, only the detection system needs to activate to fill the
pipes with water. While either of these
systems can be installed in a museum
or art gallery, the single-interlock
system is seen as the most desirable for
those types of spaces.
The primary use of the double-interlock preaction system is to protect
freezer storage warehouse spaces
where filling the pipe with water can
be extremely costly. In museums, art
galleries, server rooms, and other
spaces sensitive to the discharge of

water, filling the pipes with water is not
nearly as problematic because it can
be easily drained without concern for
freezing that exists in freezer spaces.
One of the primary advantages of the
single-interlock system is that the water
reaches the fire much more quickly
than in a double-interlock system. The
benefit of the faster response is that
the fire will be smaller at activation
and fewer valuables will be exposed
to the fire. This will also cause fewer
sprinklers to activate, which will limit
the impact of water being discharged
within the space.
While NFPA 13 allows both of these
systems to be installed, the designer must
weigh the pros and cons of the system
types when making the ultimate decision on which to choose. Not all system
designs are cookie-cutter, out-of-the-box

N F PA . O R G / J O U R N A L * NFPA JOURNAL

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Table of Contents for the Digital Edition of NFPA Journal - May/June 2017
