Electrical -System Description
DC Power System Schematic

Copyright . The Boeing Company. See title page for details.
December 6, 2002  6.20.11
Electrical -System Description 737 Operations Manual

Standby Power System Normal Operation
The standby system provides 115V AC and 24V DC power to minimum essential systems in the event of loss of all engine or APU generator AC power. The standby power system consists of:
.
the battery

.
static inverter

.
AC standby bus

.
DC standby bus

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battery bus

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hot battery bus

.
switched hot battery bus. During normal operation the guarded standby power switch is in AUTO and the battery switch is ON. This configuration provides an alternate power sources in case of partial loss as well as complete transfer to battery power if all normal power is lost. The AC standby bus is normally powered from AC transfer bus 1.

The DC standby bus is powered by DC bus No. 1; the battery bus is powered by TR3; the hot battery bus and switched hot battery bus are powered by the battery.
Alternate Operation
The alternate power source for standby power is the battery. In flight, with the standby power switch in the AUTO position, the loss of all engine and APU power causes the battery to power the standby loads. The AC standby bus is powered from the battery bus, via the static inverter. On some airplanes the AC standby bus is powered by the battery via the static inverter. The DC standby bus, battery bus, and switched hot battery bus are powered by the battery.
In flight, if either transfer bus 1 or DC bus 1 loses power, the AC standby bus automatically switches to the battery bus via the static inverter. On some airplanes the AC standby bus automatically switches to the battery via the static inverter. The DC standby bus switches to the battery bus.
A fully charged battery has sufficient capacity to provide power to the minimum essential flight instruments, communications and navigation equipment for a minimum of 30 minutes.
On the ground, with the loss of all AC power, the AC and DC standby busses are unpowered with the standby power switch in AUTO. The air/ground safety sensor inhibits the transfer to battery power to prevent discharging the battery. If the standby power switch is positioned to BAT, the air/ground safety sensor is bypassed and the AC and DC standby busses are powered.
Copyright . The Boeing Company. See title page for details.
6.20.12  June 7, 2002

Electrical -System Description 737 Operations Manual
Static Inverter
The static inverter converts 24 volt DC power from the battery to 115V AC power to supply the AC standby bus during the loss of normal electrical power. The power supply to the inverter is controlled by the standby power switch and the battery switch on the overhead panel.
Electrical -System Description 737 Operations Manual

ON

OFF
BAT
ON
STANDBY POWER

BAT OFF
AUTO
Airplane configuration - Inflight Battery Switch - ON Standby Power Switch - AUTO GENERATOR BUSSES NOT POWERED

OFF

OFF
BAT ORBAT
ON ON
STANDBY POWER

BAT OFF AUTO
Airplane configuration - Inflight or
 On The Ground Battery Switch - ON Standby Power Switch - BAT GENERATOR BUSSES POWERED
OR NOT POWEREDAs installed

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

Electrical -System Description 737 Operations Manual
Intentionally
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Copyright . The Boeing Company. See title page for details.
June 7, 2002  6.20.15
Electrical -System Description 737 Operations Manual

All Generators Inoperative
The following list identifies the significant equipment that operates when the battery is the only source of electrical power and is powering the standby busses.
Airplane General, Emergency Equipment, Doors, and Windows
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emergency instrument flood lights

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entry lights (dim) (hot battery bus)

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position lights

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standby compass light

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standby forward airstair operation (as installed)

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white dome lights

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flight crew and passenger oxygen

Air Systems
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A/C pack valves

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altitude warning horn

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manual pressurization control

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PACK TRIP OFF lights

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BLEED TRIP OFF lights

Engines, APU
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N1, fuel flow, and EGT indications

.
N2 indications (airplanes with EIS)

.
right igniters

.
starter valves

.
thrust reversers (switched hot battery bus)
　
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