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.
