APU Generator (GEN) Switch
Three position switch, spring–loaded to center position. OFF - disconnects the APU from the generator bus.
ON – connects the APU generator output to the generator bus if the quality is correct.
Note:  In flight, if one generator bus is powered by the APU and the other APU GEN switch is move to ON, the second generator bus will not connect to the APU generator.
Electrical -Controls and Indicators 737 Operations Manual

Ground Service Switch

GROUND SERVICE Switch
Solenoid held ON, spring-loaded to OFF.
Provides manual control of ground service bus. Enables servicing airplane using external power without activating generator busses.
.  
ON – connects the ground service bus to the external AC bus. Trips off
when the GRD PWR switch is ON

.
OFF – disconnects external AC bus from the ground service bus.

Copyright . The Boeing Company. See title page for details.
6.10.8   December 1, 2000

737 Operations Manual

Introduction
Primary electrical power is provided by two engine driven generators which supply three-phase, 115 volt, 400 cycle alternating current. Each generator supplies its own bus system in normal operation and can also supply essential loads of the opposite side bus system when one generator is inoperative. Transformer rectifier (TR) units and a battery supply DC power. The battery also provides backup power for the AC and DC standby systems. The APU operates a generator and can supply power to both AC generator busses on the ground or one AC generator bus in flight.
There are two basic principles of operation for the 737 electrical system:
.  
There is no paralleling of the AC sources of power.

.  
The source of power being connected to a generator bus automatically
disconnects an existing source.

The electrical power system may be categorized into three main divisions: the AC power system, the DC power system, and the standby power system.
Electrical Power Generation Engine Generators
Primary power is obtained from two engine driven generators. Each generator is part of a generator drive unit which maintains a constant frequency throughout the normal operating range of the engine. The generator is coupled directly to the engine and operates whenever the engine is running.
APU Generator
The APU generator can supply primary power on the ground and can serve as a backup for either generator in flight. The APU generator is identical to the engine generators but has no generator drive unit, since the APU itself is governed and will maintain a constant speed. As the only power source, the APU generator can meet electrical power requirements for all ground conditions and all essential flight requirements.
Electrical -System Description 737 Operations Manual

External Ground Power
An external AC power receptacle located near the nose gear wheel well, on the lower right side of the fuselage, allows the use of an external power source. Status lights on a panel adjacent to the receptacle permit the ground crew to determine if external power is being used. A GRD POWER AVAILABLE light provides flight deck indication that an AC ground power source is attached outside the airplane. A GRD PWR switch allows connection of external power to both generator busses.
The battery switch must be ON for the GRD PWR switch to be operable. Positioning the battery switch to OFF will automatically disconnect the GRD PWR switch.
Ground Service
For ground servicing, a ground service switch is located on the forward attendant’s panel. The switch provides ground power directly to the AC ground service bus for utility outlets, cabin lighting and the battery charger without powering all airplane electrical busses. The ground service switch is magnetically held in the ON position and is overridden when the GRD PWR switch is positioned to ON.
Copyright . The Boeing Company. See title page for details.
6.20.2  December 3, 2004

Electrical -System Description
Electrical Power Schematic

Battery Switch    - ON Standby Power Switch - AUTO Bus Transfer Switch  - AUTO ENGINE GENERATOR CONNECTED TO RESPECTIVE BUS

As installed
Copyright . The Boeing Company. See title page for details.
December 1, 2000  6.20.3
Electrical -System Description 737 Operations Manual

AC Power System
Each AC power system consists of a generator bus, a main bus, and a transfer bus. The left AC power system also includes a ground service bus. Transfer bus 1 supplies power to the AC standby bus. If the source powering either AC power system fails or is disconnected, a transfer relay automatically selects the opposite generator bus as an alternate power source for the transfer bus.
　
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