24-20-00
 ALL  ú ú 06 Page 13 ú Jun 10/90
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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A
747-400 MAINTENANCE MANUAL
 2) Both External Power Sources Available a) With one APU generator supplying all the airplane buses, pushing the adjacent EXT PWR switch causes the APU to auto-parallel with the external power source and trip the APB same as with one external power source available. b) Pushing the EXT PWR switch opposite the APU generator in use causes the APU generator to synchronize with external power. The XPC automatically closes, momentarily paralleling the APU Generator and external power. The SSB then trips, leaving the external power source supplying power to one side of the sync bus and the APU Generator supplying power to the other side.
Auxiliary Power Operation to External Power Operation - One APU
 Generator Being Used (SSB Open)
 1)  Pushing the EXT PWR switch adjacent to the APU generator in use causes a power transfer similar to that described above with one external power source available. Pushing the opposite EXT PWR switch connects that external power source to its side of the sync bus, leaving the APU generator supplying power to the other side.
Auxiliary Power Operation to External Power Operation - Two APU
 Generators Being Used (SSB Open)
 1)  With both APU generators supplying power to the buses, pushing either EXT PWR switches causes the adjacent APU Generator to auto-parallel with that external power source and then trip the APB. The other half of the system can be brought onto external power in a similar way by pushing the other EXT PWR switch.
External Power Supplied to an Unpowered Airplane
 1)  When one or both external power sources are available on an unpowered airplane, pushing the EXT PWR switch of an available source causes the associated XPC and the SSB to close.
 2)  If the second external power source is available and its EXT PWR switch is pressed, the SSB trips and then the associated XPC closes. Subsequent power transfers operate same as a powered airplane.
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 24-20-00
 ALL  ú ú 09 Page 14 ú Jun 10/90
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
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Auxiliary Power Supplied to an Unpowered Airplane

 1)  The power transfers to supply auxiliary power to a de-energized system are similar to those for external power supplied to an unpowered airplane.

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IDG Power Supplied to an Unpowered Airplane 1) Generator Control Switches ON and Engines Not In Operation

 a)  When an engine is started and reaches idle rpm the GCB for that channel closes. The SSB, if tripped, also closes. Once a single IDG is powering any bus, subsequent power transfers operate same as a powered airplane.
 2) Generator Control Switches ON and Engines In Operation
 a)  In this condition, the IDGs auto-parallel through their GCBs and the SSB. No action is required with the flight deck controls.
 3)  Generator Control Switches OFF and Engines In Operation
 a)  When a GEN CONT switch is placed in the ON position with the associated engine in operation at idle rpm, the GCB closes the system operation is identical to that described above with generator control switches on and engines not in operation.
 4)  Generator Control Switches OFF and Engines Not In Operation
 a)  Starting an engine with the associated GEN CONT switch in the OFF Position causes no change in the system status.
 b)  Operating a GEN CONT switch to ON with the associated engine not operating has no effect other than to cause the associated GCR to close.
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 The 115-volt ac single-phase 400-Hz standby power for the flight critical loads are normally powered from the ac bus No. 3. In the event of a total power loss, the main battery hot bus will supply 24 volts dc to the main static inverter, which converts the 24-volt dc input into 115-volt ac single phase 400 Hz used to power the flight critical loads.

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 With the standby power switch in the auto position and the main battery switch on, the system will automatically switch the ac standby bus to the output of the main static inverter if power is lost on the ac bus No. 3.
　
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