(2)  
The smoke detectors use an ionization sensor to detect smoke. A very small amount of radioactive material ionizes the air between two electrodes allowing current to flow through the air between the electrodes. Any smoke particles present interfere with this current flow. The change in current flow is sensed by a current amplifier which outputs a signal to the applicable CEU.

B.  Cargo Electronic Unit (CEU)
(1)  
One CEU is installed in the ceiling of each cargo compartment. An access panel is installed over the CEU to keep it from being damaged. The CEU in the aft cargo compartment controls and monitors the smoke detectors in the aft cargo compartment. The CEU in the forward cargo compartment controls and monitors the smoke detectors in the forward cargo compartment.

(2)  
The CEU has two test buttons. The LAMP TEST button will make sure that all the indication lamps on the CEU will come on. The PRESS TO TEST button will initiate a signal to do a test of all the smoke detectors in its cargo compartment.

(3)  
The CEU has lamps for each smoke detector in its cargo compartment. If a smoke detector fails or a smoke alarm is initiated by a smoke detector, then the lamp for that smoke detector will come on.

C.  Cargo Fire Control Panel
(1)  The Cargo Fire Control Panel is installed in P8 panel in the flight compartment. The control panel is used to initiate a test of the cargo smoke detection system and to indicate when a failure or a smoke alarm has been initiated.
(a)  The TEST switch, on the control panel, will initiate a test of the cargo smoke detection system. The test will make sure that the CEU and the smoke detectors are operating correctly.
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Cargo Bay Smoke Detection Test  500 
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500  Cargo Bay Smoke Detection Test 
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(b)  
The DETECTOR FAULT light (Amber) will come on whenever a smoke detector or a CEU has a power failure. The light will also come on if a detector fails to respond to the test.

(c)  
The DET SELECT switches control whether the system will operate in dual loop mode or single loop mode. When the switch is in the NORM position, the system will operate in the dual loop mode. When the switch is in the A position or the B position, the system will operate in the single loop mode.

(d)  
The applicable FWD or AFT ARM warning light (Red) will come on when smoke is sensed in one of the cargo compartments.

3.  Operation
A.  Functional Description (Fig. 2)
(1)  
The Cargo Smoke Detection System usually operates in the dual loop mode. In the dual loop mode, two detectors in a loop must sense smoke before a smoke alarm is initiated. If one of the detectors in a loop fails, then the system will automatically change to the single loop mode. In the single loop mode, the smoke alarm will be initiated if only one smoke detector senses smoke.

(2)  
When smoke is sensed in one of the cargo compartments, a smoke alarm signal will be initiated by the CEU. The CEU will send the signal to the flight compartment and the following conditions will occur.

(a)  
The applicable FWD or AFT ARM light (Red), on the Cargo Fire Control Panel (P8), will come on.

(b)  
The two FIRE WARN lights, on the Glareshield Panel (P7), will come on.

(c)  
The fire warning bell will sound.

(3)  
If you push the BELL CUTOUT switch, on the Engine and APU Fire Control Panel P8-1, the fire warning bell will stop. Also, if you push one of the FIRE WARN lights the bell will stop.

(4)  
If a detector loses power, then the DETECTOR FAULT light (Amber) will come on. The light on the CEU will show which detector or wiring circuit failed.

(5)  
Fault and Fire Indications are shown in Table 1.

500 
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TABLE 1 
LOOP A  LOOP B  INDICATION 
Normal  Normal  Normal 
　
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