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 TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
a single ground fault in any signaling circuit. A
really have all the tools and test equipment needed
Class B system does not include this emergency
to service these alarm systems. Consequently, the
operating feature. NFPA 72-90 also deals with the
cost will be much less if the routine maintenance is
performed in-house.
styles of supervisory circuits. NFPA 72-90 further
a. Fire detection system. The concept of defense in
requires that the central alarm station be continu-
depth is applied in fire protection when an early
ously manned. Alternative main power supply
warning fire detection system is used to communi-
sources must be provided within the supervisory
cate plant or equipment status to a central location
central station. The signal-initiating device in the
or assigned staff. The first line of defense is the
fire detection system is the fire detector. The three
early warning fire detection system designed to de-
basic types of detectors can detect smoke, heat, and
tect the particles of combustion formed before overt
flame. In addition to these generic types, detectors
signs of fire appear, followed by systems designed to
can be configure as spot type or line type (table
detect fire and release extinguishing agents. The
10-1). In spop-type detectors, such as smoke detec-
system's purpose is to provide the earliest possible
tors, the sensing element is concentrated at a par-
warning of a potential fire hazard, principally by
ticular location. Line-type detectors sense tempera-
the extensive use of ionization smoke detections. One
ture changes along the length of a metal wire. When
of the major advantages of using a remote multi-
heat above a predetermined level reaches the lines
plexing system for fire detection is the ease of add-
strung throughout an area to be protected, an alarm
ing alarm detectors without the requirement of long
or alarm and fire-suppression system is triggered.
conduit and multiple cable runs throughout the
Heat detectors are fixed-temperature, rate-compen-
plant. A sample arrangement of this type of system
sated, or rate-of-rise types. A fixed-temperature de-
is as shown in figure 10-1. The early warning fire
tector is a device that responds when its operating
detection system may be a Class A proprietary pro-
element becomes heated to a predetermined level or
tective signaling system that meets the require-
higher. A rate-compensated detector is a device that
ments of The National Fire Protection Association
responds when the temperature of the air surround-
(NFPA) Standard for the Installation, Maintenance,
ing the device reaches a predetermined level, re-
and Use of Protective Signaling Systems (NFPA 72-
gardless of the rate of temperature rise. A rate-of-
90). Class A and Class B fire detection circuits are
rise detector is a device that responds when the
shown in figure 10-2. Class A means a fire alarm
temperature rises at a rate exceeding a predeter-
can be received and displayed at the central alarm
mined amount.
station in the abnormal presence of a single break of
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