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Generator busses can be powered from the engine generators by momentarily positioning the related generator switch to ON. This connects the voltage regulator to the generator and connects the generator to its associated generator bus. Selecting a new power source disconnects the existing power source.
When the APU is operating, selecting either APU GEN switch ON connects APU power to its associated generator bus. On the ground, the APU can supply electrical power to both generator busses.
With the airplane on the ground and external power available, selecting the GRD PWR switch ON connects external power to both generator busses. The APU or an engine generator can supply power to one generator bus while external power supplies the other generator bus.
In flight, each engine generator normally powers its own generator bus. If an engine generator is no longer supplying power, the APU generator may be used to power one generator bus. Since the entire electrical system is powered from the two generator busses, all electrical components can be powered with any two operating generators.
Bus Transfer System
The generator busses supply the heavy electrical loads including supplying power to the transfer and main busses. The transfer busses carry the essential electrical loads, and the main busses carry the non-essential loads.
If a generator trips off, its generator bus and main bus will not be powered. Each transfer bus has a transfer relay which automatically selects the opposite generator bus as its power source. The BUS TRANS switch must be in the AUTO position to enable this transfer.
Automatic Galley Load Shedding
In flight, all galley power is automatically removed when operating on one generator. This automatic galley load shedding feature reduces the total electrical load on the remaining generator, protecting it from overload.
Copyright . The Boeing Company. See title page for details.
6.20.4  June 7, 2002

Electrical -System Description
APU Automatic Galley Load Shedding
On some airplanes, galley electrical loads will automatically be shed should the total airplane electrical power requirements exceed design limits with the APU providing electrical power.
AC Power Schematic

 

Airplane Configuration: Battery Switch - ON Standby Power Switch - AUTO Bus Transfer Switch  - AUTO ENGINE GENERATOR CONNECTED  TO RESPECTIVE BUS
Copyright . The Boeing Company. See title page for details.
June 7, 2002  6.20.5
Electrical -System Description 737 Operations Manual

Electrical Power Controls and Monitoring Generator Drive
Each engine driven generator is connected to its engine through a generator drive unit. Each generator drive is a self-contained unit consisting of an oil supply, cooler, instrumentation and disconnect device which provides for complete isolation of the generator in the event of a malfunction.
Operating conditions of the generator drive can be observed on the generator drive oil temperature indicator. Oil temperature is measured as it enters and leaves the generator drive. Temperature of oil entering the generator is indicated on the IN scale. Temperature differential between outlet and inlet is indicated as RISE - (out temperature minus in temperature). During normal operation, the oil temperature rise should be less than 20 deg. C. Readings above 20 deg. C indicate excessive generator load or poor condition of the drive and are used by maintenance in troubleshooting drive problems.
The amber HIGH OIL TEMPERATURE light illuminates when oil temperature in the internal oil tank exceeds limitations. The amber LOW OIL PRESSURE light illuminates when oil pressure is below the operating limit. When the generator has been disconnected, the LOW OIL PRESSURE light will be on, and the HIGH OIL TEMPERATURE light remains on until the oil is cooled.
A generator drive disconnect switch is installed. This switch disconnects the generator from the engine in the event of a generator drive malfunction. Reactivation of the generator may be accomplished only on the ground by maintenance personnel.
AC Voltmeter and Frequency Meter
AC voltage and frequency may be read on the AC voltmeter and frequency meter
for standby power, ground power, generator No. 1, APU generator, generator No.
2 and the static inverter. Frequency is indicated only when the generator is
electrically excited. The voltage regulator automatically controls the generator
output voltage.

Current readings for the two engine generators and the APU generator may be

read on the AC ammeter.
The TEST position is used by maintenance and connects the voltage and
frequency meter to the power systems test module for selection of additional
reading points.

Copyright . The Boeing Company. See title page for details.
6.20.6  June 7, 2002

Electrical -System Description 737 Operations Manual
　
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