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Chapter 8 Alarm Systems | Up
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Figure 8-3.—Typical shunt switch circuit. |
NATIONAL ELECTRICAL CODE
REQUIREMENTS FOR SECURITY/
FIRE-ALARM SYSTEMS
Because of the potential fire and explosion hazards
caused by the improper handling and installation of
electrical wiring. certain rules in the selection of
materials and quality of workmanship must be
followed as well as precautions for safety,. The
National Electrical Code® (NEC®) was developed to
standardize and simplify these rules and provide some
reliable guide for electrical construction.
The NEC® is published (and frequently revised)
by the National Fire Protection Association (NFPA),
Batterymarch Park. Quincy, MA 02269. It contains
specific rules and regulations intended to help in the
practical safeguarding of persons and property from
hazards arising from the use of electricity,, including
low voltage. used in the majority of security/fire-alarm
systems.
Article 725 of the NEC® covers remote-control.
signaling, and power-limited circuits that are not an
integral part of a device or appliance. The NEC®
(section 725-1) states:
The circuits described herein (Article 725) are
characterized by usage and electrical power
limitations that differentiate them from light
and power circuits and. therefore. special
consideration is given with regard to minimum
wire sizes, derating factors, overcurrent
protection, and conductor insulation
requirements.
Personnel assigned to install security/fire-alarm
systems should become familiar with Article 725 of
the NEC® as well as Article 760, “Fire Protective
Signaling Systems.”
This article covers the
installation of wiring and equipment of fire-
protective signaling systems operating at 600 volts or
less.
Other NEC® articles of interest to security/fire-
alarm installers include the following:
1. Section 300-21, ‘Spread of Fire or Products of
Combustion.”
2. Articles 500 through 516 and Article 517, Part G
(dealing with installations in hazardous locations).
3. Article 110, “Requirements for Electrical
Installations” and Article 300. “Wiring Methods.”
4. Article 310. -“Conductors for General Wiring.”
5. Fire-protectile signaling circuits and
equipment will be grounded according to Article 250,
except for dc-power limited fire-protective signaling
circuits that have a maximum current of 0.03 amperes.
6. The power supply of nonpower-limited fire-
protective signaling circuits will comply with chapters
1 through 4 and the output voltage will not be more than
600 volts, nominal.
7. Conductors of No. 18 and No. 16 sizes will be
permitted to be used provided they, supply loads that do
not exceed the ampacities given in table 402-5 and are
installed in a raceway or a cable approved for the
purpose. Conductors larger than No. 16 will not supply
loads greater than the ampacities given in tables 310-16
through 310-19.
8. When only nonpower-limited fire-protective
signaling circuits and Class 1 circuits are in a raceway,
the number of conductors will be determined according
to section 300-17. The derating factors given in Note 9
to tables 310-16 through 310-19 will apply if such
conductors carry continuous loads.
9. Where power-supply conductors and fire-
protective signaling circuit conductors are permitted in
a raceway according to section 760-15, the number of
conductors will be determined according to section
300-17. The derating factors given in Note 8 to tables
310-16 through 310-19 will apply as follows:
a. For all conductors when the fire-
protective signaling circuit conductors carry
continuous loads and the total number of conductors
is more than three.
b. For the power-supply conductors only when
the fire-protective signaling circuit conductors do not
carry continuous loads and the number of power-supply
conductors is more than three.
10. When fire-protective signaling circuit
conductors are installed in cable trays. comply with
sections 318-8 through 318-10.
UNDERSTANDING BASIC
INSTALLATION OF SECURITY/
FIRE-ALARM SYSTEMS
The installation of a protective security/fire-alarm
circuit should always start at the protective-circuit
energy source, as if it were an end-of-line battery-a
battery. remote from the control panel-even though it
may actually be a power supply installed in the panel.
A pair of wires is run from this power source to the first
contact location, but just the positive wire is cut and
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