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switch is in RUN, or
• a drive fail signal is active
The advisory message ELEC GEN DRIVE is displayed when a condition is
detected that requires the flight crew to manually disconnect the generator from
the engine. These include:
• low VFSG oil pressure
• loss of output voltage from both Permanent Magnet Generators (PMG)
(indicating a drive shaft failure)
The VFSG cannot be reconnected by the flight crew.
787 Flight Crew Operations Manual
Electrical -
System Description
6.20.2 Boeing Proprietary
The power sources normally remain isolated from each other throughout the
generation and distribution channels. During power transfers, a brief power
interruption may occur as buses receive power from a new power source (see
Power Transfers in this section).
APU Starter Generator (ASG)
The Auxiliary Power Unit (APU) has two ASGs, which function as both electric
starters and generators. As starters, only one ASG is required for APU start.
The APU start power can be provided from the APU battery, ground power, or a
VFSG powered bus source. For APU starts with the APU battery as the power
source (no VFSG or external power source on-line), the right ASG is always used
for the start. For APU starts with either a VFSG or external power on-line, either
the left or right ASG will be used for the start.
Once the APU is operating, both ASGs function as electrical generators. They
operate in an isolated configuration, with each APU generator powering its
respective 235 Vac ring-tie bus. They provide the necessary electrical power for
ground operations, engine start, and supplemental power during flight.
External Power (EP)
External power consists of two forward and one aft receptacles, with
corresponding control panel switches.
Both forward external power sources are normally used, and are the primary
source for ground power operations. The airplane is capable of powering ground
handling and ground service loads with only a single forward external power
source, though power for additional airplane loads is limited.
Aft external power can provide an alternate means to power the large motor
controllers for engine start, hydraulics, and environmental control systems when
the APU is inoperative or OFF. Connecting external power to the aft receptacle
without first applying forward external power or a generator source has no effect,
and does not supply power for these airplane power loads.
If only 2 external power sources are used to support main engine start, the two
forward external power sources should be used. In this configuration, other
non-essential loads are temporarily shed, and engine start time increases.
A white AVAIL indication illuminates on the control panel switches when power
of acceptable quality is available from their external power sources. Selecting the
switch ON connects power to the airplanes electrical system, and extinguishes the
AVAIL light.
787 Flight Crew Operations Manual
Electrical -
System Description
Boeing Proprietary 6.20.3
Power Source Transfers
There are two types of power transfers, a Break Power Transfer (BPT) which
momentarily interrupts power to a main AC bus, and a No-Break Power Transfer
(NBPT), which maintains power throughout the source transition. The Bus Power
Control Unit (BPCU) is responsible for coordinating all power transfers.
A BPT occurs if power transfer is initiated when:
• airplane is in the air
• a protective trip of the source occurs
• a generator control switch or APU generator control switch is selected
OFF
• the engines are not at idle
All normal AC power transfers on the ground are NBPT unless otherwise
specified in the table below:
Transfer When Transfer Typically Occurs Type of
Transfer
Fwd EP to ASG Prior to flight, when ASG becomes available (APU
running).
No-Break
ASG to VFSG Prior to flight, when VFSG becomes available
(engine running).
No-Break
VFSG to ASG After flight, when VFSG is no longer available
(engine shutdown).
Break
Fwd EP to VFSG Prior to flight, when VFSG becomes available
(engine start using EP).
Break
VFSG to Fwd EP After flight, when VFSG is no longer available
(engine shutdown, no APU).
Break
Fwd EP to Fwd
EP
airplane power up or power down (additional EP
selected ON or OFF).
Break
VFSG to VFSG When VFSG becomes available (with other engine
VFSGs already available) or when VFSG is no longer
available (with other engines VFSGs available).
Break
ASG to ASG When one ASG control switch is selected from OFF
　
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