737 Flight Crew Operations Manual
Electrical -
System Description
Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.
D6-27370-866-EGP 6.20.3
AC Power System
Each AC power system consists of a transfer bus, a main bus, two galley busses,
and a ground service bus. Transfer bus 1 also supplies power to the AC standby
bus. If the AC source powering either transfer bus fails or is disconnected, the
transfer bus can be powered by any available source through the tie bus with the
bus tie breakers (BTBs).
With the airplane on the ground and both generator control switches OFF, or with
both engines shut down, selecting the GRD PWR switch ON connects external
power to both transfer busses. Likewise, selecting either APU GEN switch ON
connects APU power to both transfer busses. Whichever source is selected last
powers both busses. It is not possible to power one transfer bus with external
power and one transfer bus with APU power.
The transfer busses can be powered from the engine generators by momentarily
positioning the related generator switch to ON. This closes the related generator
circuit breaker (GCB) and connects the generator to the transfer bus. Whenever
external power or APU is powering both transfer busses, and engine generator
power is applied to its onside transfer bus, external power or APU continues to
supply power to the remaining transfer bus.
In flight, each engine generator normally powers its own transfer bus. If an engine
generator is no longer supplying power, the BTBs automatically close to allow the
other engine generator to supply both transfer busses through the tie bus and
BTBs. The APU can power either or both busses through the BTBs.
The system also incorporates an automatic generator on–line feature in case the
airplane takes off with the APU powering both transfer busses. If the APU is either
shut down or fails, the engine generators are automatically connected to their
related transfer busses. This action occurs only once in flight and only under the
circumstances described above.
Bus Tie System
Either generator or the APU can supply power to both transfer busses. If the BUS
TRANS switch is in the AUTO position and the source powering the transfer bus
is disconnected or fails, the source powering the opposite transfer bus
automatically picks up the unpowered transfer bus through the BTBs.
September 28, 2006
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