NFPA Journal - Summer 2021 - 48

In Compliance
through the pipe. Think about cornering while driving a car, when the force
pushes you toward the outside of the
turn. The same phenomenon occurs
when water makes a turn as it travels
through a pipe, except that the force
acts against the pipe joints.
Designers typically have three
primary options to draw from to address
this problem: thrust blocks, self-restraining fittings, or other special connections.
Thrust blocks are blocks of concrete
that are installed against underground
pipe fittings and fan out in the direction opposite the thrust force until the
surface area is several times larger than
that of the fitting. Resistance is provided
by transferring the thrust force to the
soil through the larger bearing area of
the block. Provided there are stable soil
conditions and adequate space, concrete
thrust blocks can be an effective method
of restraining pipes, and have the added
benefit of using affordable materials
commonly found at a construction site.
On the other hand, thrust blocks can
require more time to install because the
concrete needs to set.
A restrained joint is a special type
of joint designed to provide longitudinal restraint. This method functions
in a similar manner to thrust blocks
because the restrained unit of piping
with the soil balances the thrust
forces. Some examples of these
systems include locking mechanical joints, bolted flange joints, and
pipe clamps with tie rods. NFPA
13, Standard for the Installation of
Sprinkler Systems, contains requirements for the material and sizing for
these methods. Compared to thrust
blocks, restrained joint systems are
typically faster to install.
Finally, designers could also use
connection methods that inherently restrain piping against thrust
forces. These connection types
include threaded, grooved, welded,
or heat-fused connections, as well as
chemical- or solvent-cemented connections. Using these connections does not
require additional restraints as long as
the joints can pass a hydrostatic test
that is part of the acceptance testing
for underground piping.

One of these methods needs to be
used at points where the piping direction changes, such as elbows, bends,
and tee fittings, to prevent unrestrained pipe joints from separating
and to prevent pipe movement in soft
soil. NFPA 13 doesn't mandate which
method should be used-the decision
is left to the designer or the owner.
Determining which piping connections
require restraint is as easy as identifying the connections that change a
pipe's direction. Straight runs of underground pipe do not require restraining
because force on a straight connection
doesn't act in a way that could separate
the piping as easily.
These requirements aren't just for
sprinkler systems-they can be applied to
any water-based fire suppression system.
Requirements for these systems are
found in NFPA 13 and are extracted from
NFPA 24, Standard for the Installation
of Private Fire Service Mains and Their
Appurtenances. Check with the AHJ
as well as local and state regulations
to determine if any additional requirements apply to your specific installation
of underground pipe.
Brian O'Connor is a technical services
engineer at NFPA. NFPA members and AHJs can
use the Technical Questions tab to post queries on
NFPA 13 at nfpa.org/13. To follow the progress
on the upcoming edition of the code, visit nfpa.
org/13next

NFPA 72

Fire alarms, signaling, and
emergency communication

Ensuring the fire
alarm system remains
reliable with a
secondary power supply
By Shawn Mahoney
s we witnessed this
winter in Texas, and
as we've seen in many
other instances, extreme
weather and increased
usage of the power grid can result in a
loss of primary power to a building in

A

the form of power outages or rolling
blackouts. In order to remain operational during the loss of primary power,
fire alarm systems are provided with a
secondary source of power. NFPA 72®,
National Fire Alarm and Signaling
Code®, outlines how the secondary
power must be designed and how it
must be inspected, tested, and maintained to ensure it is operational when
needed the most.
The most common forms of secondary power supplies are batteries or
an emergency generator. Secondary
power supplies are designed to provide
enough capacity to power the entire
system for 24 hours on standby and
then operate the system for at least
5 minutes under emergency conditions (15 minutes for emergency voice/
alarm communication systems). If
a generator is used for secondary
power, batteries are still required, but
only need to provide capacity for four
hours-enough time to get the generator operational if there is an issue.
To ensure that the secondary power
supply is always available, the fire
alarm system itself is able to monitor
for the presence of voltage as well as
the battery charging system, and will
annunciate a trouble signal if there
is an issue with the power supply or
charging system.
Although the fire alarm system can
monitor some aspects of the secondary
power supply, there is some inspection,
testing, and maintenance (ITM) that
needs to be completed to ensure that
the secondary power supply is reliable.
Batteries need to be inspected semiannually to confirm that the connections
are tight and there is no corrosion
on the connections. During inspection, the batteries need to be checked
for damage such as cracks in the case,
bulges, or leaking. The batteries also
need to be marked with the month
and year of manufacture, not the date
of installation-this information is
important for tracking the age of the
batteries. If a battery's age exceeds the
manufacturer's replacement date, the
battery needs to be replaced.
The batteries and charger need to
be tested semiannually. Tests include

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