737 GENERAL -- ELECTRICAL POWER DISTRIBUTION General
These ac buses receive power directly from an ac source:
- AC transfer bus 1 
- AC transfer bus 2 
- Ground service bus 1 
- Ground service bus 2. 

System logic removes loads (load shed) to prevent an overload of an ac source.
These dc buses receive power directly from the transformer rectifier units (TRUs):
-DC bus 1
-DC bus 2
-Battery bus.
These buses receive power directly from the battery or the battery charger:
-Hot battery bus
-Switched hot battery bus.
AC Transfer Buses
These ac sources supply power to the dc transfer buses:
-External power
-APU starter-generator
-Integrated drive generators (IDGs).
737-600/700/800/900 TRAINING MANUAL
The system makes sure that two ac sources can not supply power to the same transfer bus at the same time. One ac source can supply power to the two transfer buses through the bus tie breakers (BTBs).
Each transfer bus supplies power to these components or buses:
-Galleys (as many as 2)
-Main Bus
-Ground service bus
-Transformer rectifier unit (as many as 2).
Ground Service Buses
Each ground service bus receives power in two ways:
-The ac transfer bus on that side has power
-The ground service switch on the forward attendant
panel is in the ON position, and external power is
connected to the airplane.
The two ground service transfer relays control the selection of the power source.
Main Buses and Galley Buses
The main buses and the galley buses receive power from their related ac transfer bus. Load shed relays remove the power to these buses when their loads are more than the normal operation limits. This prevents an overload of the ac source. The bus power control unit (BPCU) controls the load shed function.

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737 GENERAL -- ELECTRICAL POWER DISTRIBUTION DC Buses
DC bus 1 usually receives power from TRU 1. The bus can receive power from TRU 2 or TRU 3 through the bus tie relay. This relay is usually energized.
DC bus 2 usually receives power from TRU 2. TRU 3 supplies power if TRU 2 has a failure. DC bus 2 can also receive power from TRU 1 through the bus tie relay.
Standby Buses
The ac standby bus usually receives power from ac transfer bus 1. The static inverter can also supply power to this bus. A remote control circuit breaker (RCCB) controls power to the static inverter.
The dc standby bus usually receives power from dc bus
1. The hot battery bus can also supply power to the dc standby bus.
Battery Buses
The hot battery bus receives power from the battery or battery charger.
The battery bus normally receives power from TRU 3. The battery bus receives power from the battery if TRU 3 has no output.
The switched hot battery bus receives power from the hot battery bus when the battery switch (P5 panel) is in the ON position.
737-600/700/800/900 TRAINING MANUAL
External Power
External power can supply power to these buses:
-AC transfer buses
-Ground service buses.
External power supplies power to each ac transfer bus through the external power contactor (EPC) and a bus tie breaker (BTB).
APU Power
The APU starter-generator supplies power to an ac transfer bus through the APU breaker (APB) and the necessary BTB. The APU can supply power to the two ac transfer buses on the ground. It can supply power to the two ac transfer buses in flight as a backup to the IDGs. See the ac generation section for more information.
IDG Power
The IDGs are the normal sources of the ac transfer buses. An IDG supplies power through a generator control breaker (GCB).
Battery Charger
The battery charger makes sure there is maximum battery charge. The battery charger also operates as a TRU when not in the charge mode.
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737-600/700/800/900 TRAINING MANUAL
737 GENERAL -- ELECTRICAL POWER DISTRIBUTION
The battery charger inhibit relay removes power to the battery charger during an APU start or when the battery internal temperature is more than limits.
Remote Control Circuit Breaker (RCCB)
The standby power system uses a RCCB to control power input to the static inverter. This RCCB is normally closed. See the standby power system section for more information.
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737-600/700/800/900 TRAINING MANUAL

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737 GENERAL -- ELECTRICAL POWER OPERATION General
You use switches on the forward P5 overhead panel or the forward flight attendant panel to operate the electrical system. See the distribution general description in this section for more information about the operation effect on distribution.
　
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