(4)  
The source of power switched on automatically takes priority over

 and switches off the power source previously on.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) will be tripped. When an engine is started, its associated GB will not close until the associated generator control switch is positioned to ON. When the switch is positioned to ON, and 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. Positioning a generator control switch to OFF will trip the associated GCR and GB.With both of the main buses energized, transfer buses 1 and 2 will be energized with bus transfer relays 1 and 2 both in the normal position. Bus transfer switch S2 on forward overhead panel must be in the AUTO position.If main bus 1 goes dead, transfer bus 1 automatically transfers to main bus 2. The alternate coil of bus transfer relay 1 is energized through the normally closed contact of transfer control relay 1 R349. The normal coil of bus transfer relay 1 is also de-energized. A time delay prevents transfer during bus switching. A similar sequence occurs if main bus 2 goes dead instead of main bus 1.Load sheds can occur during these conditions:

 (1)  
When APU generator power is on the main buses, the load shed sensor module M1438 monitors the load on the APU generator through the current transformer T9. If the load on the APU generator is more than 162.5 amperes on each phase, the load shed sensor module automatically opens the galley switch S4.

 (2)  
When the gen bus 1 goes dead, the galleys are shed. Since the forward galley 1 is connected to gen bus 1, the forward galley 1 loses power. The aft galley 4B also loses power. This happens as follows:

 (a)  When the gen bus 1 is not energized, the APU generator breaker 1, the external power contactor 1, and the generator breaker 1 are open. Now, there is no path for 28 volts dc to the coil of the transfer control relay 1 R349. The transfer control relay 1 relaxes and disconnects 28 volts dc from the coil of the galley relay R62. The galley relay R62 relaxes and disconnects the aft galley 4B from the gen bus 2.

 (3)
An equivalent sequence occurs if the gen bus 2 goes dead.With neither engine running, external power contactors (EPC) tripped, APU operating, battery switch positioned to ON, closing either left or right APU control switch, when voltage and frequency are correct, will close APU GB 1 or 2, connecting the APU generator to gen bus 1 or 2.

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 24-21-00
 ALL  ú ú 03 Page 20 ú Nov 12/01
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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737-300/400/500MAINTENANCE MANUAL
When the APU is supplying power to a main load bus and an engine is started, the APU GB will trip when the generator control switch is positioned to ON and engine generator voltage and frequency are correct. Tripping of the APU GB will allow the engine GB to close. The load bus will then be supplied by the engine generator.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.Operation During Engine Start (on engines with the VSCF)
 (1)  
At 20 to 30% N2, there is enough voltage from the PMG to operate the GCCU. At 45 to 50% N2, the GCCU commands the inverter to create 15 to 20 volts L-N at 360 Hz. This simulates the residual voltage from the CSD/generator operation.

 (2)  
The GCCU now looks at the input from the voltage regulator in the GCU.

 (a)  
If there is less than 5 volts dc, the GCCU knows that the GCR in the GCU is open. The voltage stays at 15-20 V and 360 Hz until the GCR closes.

 (b)  
If there is more than 5 volts dc, the GCCU knows that the GCR in the GCU is closed. Then, at approximately 50% N2, the GCCU makes the voltage go to 115 V and 400 Hz. Now, you can push the GEN switch to connect the VSCF to the ac buses.

Operation During Engine Shutdown (on engines with the VSCF)
 (1)  
As the engine speed decreases to about 50% N2, the GCCU makes the frequency go back to 360 Hz. The voltage stays at 115 volts. Then, the GCU will see an underfrequency condition and open the generator breaker.
　
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