AC Power System (Contd.)
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.
Automatic Load Shedding (Engine Generators)
Option - CAB/UTIL Power Switch
For single generator operation, the system is designed to shed electrical load incrementally
based on actual load sensing. The galleys and main bus on transfer bus 2 are shed first; if
an overload is still sensed, the galleys and main bus on transfer bus 1 are shed; if overload
still exists, the IFE buses are shed. When configuration changes to more source capacity
(two generator operation), automatic load restoration of the main busses, galley busses
and IFE buses occurs; manual restoration of galley and main bus power can be attempted
by moving the CAB/UTIL Power Switch to OFF, then back ON.
APU Automatic Load Shedding
In flight, if the APU is the only source of electrical power, all galley busses and main buses
are automatically shed. If electrical load still exceeds design limits, both IFE busses are also
automatically shed. On the ground, the APU attempts to carry a full electrical load. If an
overload condition is sensed, the APU sheds galley busses and main busses until the load
is within limits. Manual restoration of galley and main bus power can be attempted by
moving the CAB/UTIL Power Switch to OFF, then back ON.
