TM 55-1520-240-23-10
12-7
FIRE DETECTION AND EXTINGUISHING SYSTEMS
12-7
FIRE DETECTION SYSTEM DESCRIPTION
The fire detection system consists of a fire detection
loop routed around each engine, a control unit for each
of the two loops, and a test switch and indicator lights in
the cockpit.
Each fire detection loop consists of three sensing
elements joined by connectors. Each element consists
of a thin metallic tube with a wire running through the
center, insulated from the tubing by a salt embedded in
a ceramic matrix. The resistance of the insulating salt
drops and the salt becomes conductive at about 575 F
(300 C). The tubing is connected to ground. The central
wire is connected to the control unit. The three elements
have a combined length of about 19 feet.
The two control units are mounted in the cabin at STA
482.00 left and right. Each unit contains an ac-powered
magnetic amplifier and a dc test circuit. Each unit is
connected to its respective fire detection loop. The units
are sealed, and maintenance is limited to replacement.
A fire indicating light for each engine is located in the
respective fire extinguisher handle on the cockpit center
instrument panel. A test switch between the fire pull
handles allows the light and electrical circuits to be
checked.
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