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 26-15-02
 ALL  ú ú 01 Page 408 ú Jan 28/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A

757 MAINTENANCE MANUAL
 LOWER CARGO COMPARTMENT SMOKE DETECTION -
_________________________________________ DESCRIPTION AND OPERATION
_________________________
 1.  General
_______
 A.  Each of the forward and aft cargo compartments smoke detection systems are dual systems. The smoke detector systems monitor the air in the cargo compartments and provide alerts for smoke conditions.
 B.  The dual smoke detection systems consist of a forward and aft component which are powered by the 28v dc battery bus and 115v ac ground service bus respectively. The system circuit breakers are on the overhead panel P11 (Fig. 2). There are no smoke detection system on/off switches.
 C.  The main components of each smoke detection system are: a pair of smoke detectors, a pair of air-flow blowers, a plenum pressure switch, a fire/overheat logic/test card, and a fire/overheat test panel.
 D.  The operation of both forward and aft systems is the same. Both systems operate in AND logic. Thus, both detectors of a system must sense smoke in order for a fire alarm to be given.
 2.  Component Details (Fig. 1)
_________________
 A.  The forward cargo compartment smoke detector assembly (detectors, blowers, and plenum pressure switch) is located in the forward cargo compartment ceiling just aft of No. 1 cargo door. The aft cargo compartment smoke detector assembly is mounted behind the aft cargo compartment sidewall just forward of No. 2 cargo door.
 B.  Cargo Smoke Detectors
_____________________
 ú (1) The smoke detectors operate on an optical principle. The detector
 ú  consists of two photocells and a Light Emitting Diode (LED) in a
 ú  light-proof chamber plus associated amplifier, output, and test
 ú  circuits on a printed circuit board. The detector has inlet and
 ú  outlet tubes to provide access to air and smoke.
 ú (2) Smoke entering the light-proof chamber reflects the light from the
 ú  LED onto a photo diode. There are two photo diodes in the chamber.
 ú  One controls the intensity of the LED. The other photo diode
 ú  measures the reflected light from the smoke particles. The
 ú  reflected light increases the voltage output across the second photo
 ú  diode. When the smoke concentration gets above 10%, the voltage
 ú  will increase to above the reference voltage monitored by a
 ú  comparator and relay driver. The relay provides an alarm output
 ú  signal to the fire/overheat logic/test card.
 ú  (a) The LEDs are not replaceable parts.
 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú

 26-16-00
 ALL  ú ú 02.1 Page 1 ú Sep 28/02
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL

CARD FILE (P54)CIRCUIT BREAKER PANEL (P11)
SEE A
FIRE/OVHT TEST
SYS FAIL
WHL  ENG/APU 
WELL  ENG  CARGO 
CARGO COMPT 
P-RESET FAIL  CEILING 
A 

FIRE/OVERHEAT TEST PANEL
A
INTAKE ORIFICE INTAKE ORIFICE DUAL DETECTORS
SEE B (RIGHT SIDEWALL) B SEE D
DUAL DETECTORS
(CARGO COMPARTMENT SMOKE CEILING) DETECTOR
SEE C
TUBING
SMOKE DETECTOR TUBING NO.1 CARGONO.2 CARGO DOORDOOR
FWD
AFT CARGO COMPT
INTAKE ORIFICE
SEE B
FWD
FWD CARGO COMPT
Cargo Compartment Smoke Detection System Component Location Figure 1 (Sheet 1)
EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú

 26-16-00
　
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