D.  Control of the electrical system is more automatic than for systems which may be operated in parallel. Since the generators in the system cannot be operated in parallel, control of the transfer bus is automatic, so that the operating generator is transferred to pick up bus loads of the tripped generator when that generator stops functioning. The transfer bus contactor has a magnetically held relay with a "center-off" position. The transfer bus control has a preferred source, with the control circuit so arranged that transfer of power under normal operating conditions should not occur. The right and left generator breakers and bus tie breakers, the APU generator breaker and the external power contactor are all electrically interlocked and automatically trip the operating source of the bus when another source is switched to this bus.
Operation
A.  With the airplane on the ground, all engines shut down, external 3-phase power connected to the receptacle, and if the power source is of the correct voltage, (with no negative sequence voltage condition) then the external power contactor (EPC) will close when the ground power (EP) switch is placed on the ON position.
NOTE: Placing the ground power switch to ON sends a trip signal to the APU generator breaker (GB) and to engine generator breakers No. 1 and 2 (GBl-GB2).
With the APU generator powering the airplane, closing the ground power switch (EP) will trip the APU GB and allow the external power contactor (EPC) to close. The bus tie breakers (BTB) will close, connecting external power to the load buses (Fig. 2). The dc power for closing the EPC and BTBs is supplied from a transformer-rectifier (T-R) unit in the bus protection panel (BPP). The transformer-rectifier provides dc power for control and protection when ac power is connected to the external power receptacle. If external power goes into an undervoltage, overvoltage, or negative sequence voltage condition, the EPC will be de-energized.
500 
Nov 01/74  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  24-00 Page 3 

H46871 H45181
Electrical Power System Bus Diagram  510 
24-00  Figure 2 (Sheet 1)  Jun 20/86 
Page 4 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

510  Electrical Power System Bus Diagram 
Jun 20/86  Figure 2 (Sheet 2)  24-00 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

EFFECTIVITY
NU ALL EXCEPT JA8443 THRU JA8492; JA8492 AFTER INCORPORATING SB 24-1040
CA ALL EXCEPT B-2501 THRU B-2505; B-2504 THRU B-2505 AFTER INCORPORATING  SB 24-1040

Electrical Power System Bus DiagramJun 20/86 Figure 2 (Sheet 3) 24-00 Page 4C/4D BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

B.  With the airplane on the ground, all engines shut down, all generator control relays (GCR's) closed, and power being supplied to the main buses through both BTB's from the APU, if engine No. 1 is started (and a few seconds allowed for its generator to come up to speed and voltage), the No. 1 generator breaker (GB) will not close and take over generator bus No. 1 until No. 1 generator control switch is placed in the ON position, and No. 1 BTB is tripped. Then No. 1 GB automatically closes and No. 1 generator supplies the No. 1 main bus. If APU power is removed from the airplane before the No. 2 engine is started, power will be lost to the No. 2 load bus. When the No. 2 engine is started, it will automatically take over the No. 2 load bus when No. 2 generator control switch is placed in the ON position. The same sequence will hold if external power is being used instead of the APU (Fig. 3).
C.  With neither airplane engine running, the APU GCR closed, APU generator voltage and frequency correct, the external power contactor (EPC) tripped, either left or right APU control switch in the ON position, the APU GB will close (Fig. 4). Closing the APU GB connects the APU generator to the TIE BUS (Fig. 2). When the left and/or right APU control switch is placed in the ON position, the associated BTB closes and APU power is connected to the No. 1 and/or 2 main bus. Control and protection of the APU is provided by the APU generator control unit (GCU). The BTB's wil1 close with power from the APU GCU or from the battery. The BTB's wil1 trip when the APU GB is tripped or when either APU switch is momentarily placed in the OFF position. The APU generator will shut down automatically when the APU turbine is shut down. The APU generator field is de-excited by momentarily placing both APU control switches in the OFF position if APU generator is powering both buses.
D.  When the APU generator is supplying power to a main load bus through its associated BTB and an engine is started, the BTB will be tripped when the generator control switch is placed in the ON position and the voltage and frequency are correct. When the BTB is tripped, the engine-driven generator will be automatically connected to the bus and the differential protection switching relays (DPR's) will be de-energized. When the engine is shut down, the engine-driven generator will reach an underfrequency condition, thereby tripping the GB and disconnecting the generator from the load bus. When the generator reaches an undervoltage condition, a time delay is started. After 7 seconds the GCR will automatically trip and de-excite the field. Momentarily placing the generator control switch in the OFF position will also trip the GCR and GB.
　
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