737 MAX Flight Crew Operations Manual
Electrical -
System Description
MN-FLT-OH-201
6.20.3
AC Power System
Each AC power system consists of a transfer bus, a main bus, two galley buses,
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 buses. Likewise, selecting either APU GEN switch ON
connects APU power to both transfer buses. Whichever source is selected last
powers both buses. It is not possible to power one transfer bus with external power
and one transfer bus with APU power.
The transfer buses 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 buses, 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 buses through the tie bus and BTBs.
The APU can power either or both buses 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 buses. If the APU is either
shut down or fails, the engine generators are automatically connected to their
related transfer buses. 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 buses. 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.
March 1, 2021
