H20908
500  Magnetic Latch Type Circuit Breaker Schematic Diagram 
Aug 15/68  Figure 6  24-21-0 
Page 17 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

10. System Operation
A.  With the airplane on the ground, both engines shut down, APU shut down, external power disconnected, battery switch positioned to ON, generator control switch in the neutral position, all generator breakers (GB), generator control relays (GCR), and bus tie breakers (BTB) will be tripped (Fig. 7). When an engine is started, its associated GCR and GB will not close until the associated generator control switch is positioned to ON. When the switch is positioned to ON, K1 or K2 will close, providing voltage to close the GCR. Then, when generator output voltage and frequency are correct and there is no fault on the system, power ready relay K801 will close and provide voltage to close the GB, thereby energizing the associated load bus (Fig. 8). The same sequence will occur when starting the second engine. Both BTB's will remain tripped. Positioning a generator control switch to OFF will trip the associated GCR and GB.
B.  With both engine generators supplying power to the main load buses, transfer buses No. 1 and 2 will be energized with bus transfer relays No. 1 and 2 both in the normal position (Fig. 8). Bus transfer switch S2 on forward overhead panel must be in the AUTO position. Should generator No. 2 stop functioning, the loads supplied from the No. 2 transfer bus will be automatically transferred to the No. 1 generator. Galley loads will be automatically shed and hydraulic system B pump 2 will not tranfer to Gen Bus No. 1 unless pump 1 is inoperative. The alternate coil of No. 2 bus transfer relay is energized thru normally closed of No. 2 GB and normal coil of No. 2 bus transfer relay is de-energized. A time delay permits the circuit to mechanically clear previous connections before the No. 2 bus transfer relay closes. A similar sequence takes place should generator No. 1 stop functioning instead of No. 2.
C.  With neither engine running, external power contactor (EPC) tripped, APU operating, battery switch positioned to ON, closing either left or right APU control switch will close the APU GCR (Fig. 9). Then, when voltage and frequency are correct and there is no fault on the system, the APU GB will close, connecting the APU generator to the tie bus (Fig. 10). Positioning the left and/or right APU control switch to ON also closes the left and/or right BTB, thereby energizing either or both load buses. Closing a BTB energizes the associated differential protection relay (DPR) which prevents closing the associated engine GB. Positioning either APU control switch to OFF will trip the BTB and second control switch will trip the APU GCR, APU GB and second BTB.
5A8 
24-21-0 Page 18  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  May 20/82 

H20904 H20899
500  Engine-Driven Generator Control Diagram 
Nov 01/74  Figure 7  24-21-0 
Page 19 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

Transfer Bus Control Diagram  500 
24-21-0  Figure 8  Jul 15/70 
Page 20 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

D.  With the APU supplying power to a main load bus through the associated BTB and an engine is started, the BTB will trip when the generator control switch is positioned to ON and engine generator voltage and frequency are correct. Tripping of the BTB will de-energize the DPR and allow the engine GB to close. The load bus will then be supplied by the engine generator.
E.  When an 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.
500 
Jul 15/70  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  24-21-0 Page 21 

H20894 H50567
APU Generator Control Diagram  500 
24-21-0  Figure 9  Feb 01/76 
Page 22 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

5B6  AC Generator System Bus Diagram 
Jun 20/83  Figure 10  24-21-0 
Page 23 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

AC GENERATION - TROUBLE SHOOTING
　
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