*
Backup generator field relay opens because of a fault
*
Engine fire switch is pulled out
*
Engine is shutdown.
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AC GENERATION -BACKUP POWER -BREAKER CONTROL
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AC GENERATION -FUNCTIONAL DESCRIPTION
General
The BPCU and GCUs automatically control the power source selection for each main ac bus. The power sources for a main ac bus have this priority:
*
Related IDG
*
APU generator
*
Opposite IDG.
External power has priority over other sources when you manually choose it with the external power switch.
APB Control
When the APU is running, the APU GCU closes the APB. This permits the APU to supply power to the tie bus. If the left main ac bus has no power, the left GCU closes the left BTB. This connects the tie bus to the left main ac bus.
If the right main ac bus has no power, the right GCU closes the right BTB. This connects the tie bus to the right main ac bus.
GCB Control
When the left IDG operates, the left GCU closes the left GCB. This permits the IDG to supply power to the left main ac bus.
When the right IDG operates, the right GCU closes the right GCB. This permits the IDG to supply power to the right main ac bus.
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BTB Control
One IDG can supply power to both main ac buses at the same time. For example, if the right IDG fails, the right GCU opens the right GCB and closes the right BTB. The left GCU closes the left BTB. This connects the left main ac bus to the right main ac bus.
Ground Service and Ground Handling Buses
When the right main ac bus has power, it supplies power to the ground service bus. When the right main ac bus has no power, the BPCU selects primary external power or the APU generator as the power source.
Primary external power or the APU generator also supply power to the ground handling bus through the ground handling relay.
If both the APU generator and primary external power are available, primary external power has priority.
Backup Generator Power
The backup power system supplies power to one or both transfer buses during these conditions:
*
Loss of all main ac power
*
Single main generator operation
*
Autoland
*
Backup system test after engine start.
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AC GENERATION -FUNCTIONAL DESCRIPTION
The backup generator converter controls the TBBs and CCBs. For example, if the left main ac bus has no power, the converter opens the left TBB and closes the left CCB. This permits the converter to supply power to the left transfer bus.
BITE
The GCUs and backup converter protect and monitor the ac power systems for faults. They perform three kinds of tests:
*
Continuous
*
Power-up/manually-initiated
*
Fault-initiated.
The GCUs and the converter send the test fault data to the AIMS.
Training Information Point
There is a short time delay between when the engine reaches idle and when the IDG starts to supply power. The length of the time delay depends on engine idle speed data from the engine data interface unit (EDIU).
On the ground, if the GCU gets a satisfactory engine idle speed signal from the EDIU, the IDG starts to supply power in 6 seconds. If there is not a satisfactory engine idle speed signal from the EDIU, the GCU gets a different engine speed signal from its own PMG. In this case, the IDG starts to supply power in 16 seconds.
In the air, if the GCU gets a satisfactory engine idle speed signal from the EDIU, the IDG starts to supply power in 10 seconds. If it must get an engine speed signal from the PMG, the IDG starts to supply power in 20 seconds.
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AC GENERATION -FUNCTIONAL DESCRIPTION
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AC GENERATION -IDG AND APU POWER CMCS GROUND TESTS
General
There are three operational tests for the main ac electrical power generation system:
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