787 Flight Crew Operations Manual
DO NOT USE FOR FLIGHT
Electrical -
System Description
Copyright © The Boeing Company. See title page for details.
D615Z003-TBC 6.20.7
Main 235 Vac Bus Distribution System
There are four main 235 Vac buses, designated L1, L2, R1, and R2. These primary
power system buses are located in the aft E/E bay power panels. They distribute
power directly to a limited number of high voltage AC airplane systems. In
addition, they distribute power to all other airplane bus systems through power
conversion devices. These buses can be energized by several different sources,
depending on source availability and the operation being conducted.
On the ground, with only forward external power selected ON (no engine or APU
generator power), the 235 Vac buses are energized by the 115 Vac distribution
system through power conversion devices.
On the ground, with only the APU power available (no external power or engine
generator power), each APU generator energizes two of the four main 235 Vac
buses.
With one engine running (no APU or external power), that engine’s two related
235 Vac buses are energized directly. The other two 235 Vac buses are energized
by the two powered buses.
With one engine and the APU running, that engine’s two related 235 Vac buses are
energized directly. The other two 235 Vac buses are energized by the APU
generators.
With both engines running, each main 235 Vac bus is energized directly from its
related engine generator. If any engine generator is not available, its related main
bus is energized by another main 235 Vac bus.
Operating indications are provided by the electrical synoptic and EICAS message
systems. The EICAS caution message ELEC AC BUS (L1, L2, R1, R2) shows
when a main 235 Vac bus is inoperative.
115 Vac Bus Distribution System
There are multiple 115 Vac buses. To distribute power, the 115 Vac system uses
both primary power system buses located in the forward E/E bay, and RPDUs
located throughout the airplane. The higher amperage (greater than 10 amps) loads
are distributed directly by the bus system. The lower amperage (10 amps or less)
loads are distributed by the RPDUs.
On the ground, with only forward external power selected ON (no engine or APU
generator power), the 115 Vac system is energized directly. It then energizes the
235 Vac bus system through power conversion devices.
With an engine(s) and/or APU running (external power OFF), the main 235 Vac
buses energize the 115 Vac system through power conversion devices. If forward
external power is then selected ON, it energizes the 115 Vac system directly.
February 15, 2010
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