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overload condition is relieved. Galley busses are shed first, followed by utility
busses. When an additional AC power source is available, loads are restored in
reverse order.
During load shedding, the ELEC UTIL BUS L and R messages and utility off
lights are inhibited. However, the following EICAS alert messages may display in
the order shown depending upon fuel system configuration and the extent of load
shedding:
• FUELPUMP3FWD
• FUELOVRD2FWD
• FUELOVRD3FWD
• FUELOVRDCTRL
• FUELPUMP2FWD
AC Electrical System Power Sources
The main AC electrical power sources are:
• four integrated drive generators (IDGs)
• two APU generators
• two external power sources
December 01, 1999
747 Operations Manual
Copyright © The Boeing Company. See title page for details.
6.20.2 D6-30151-425
Electrical -
System Description
During normal operation, IDG power output to the four AC busses is
synchronized. Each bus is interconnected through a single synchronous bus,
which distributes electrical power and allows individual AC busses to remain
powered when their related generators are inoperative.
Integrated Drive Generators (IDGs)
One IDG is mounted on each engine. Each IDG incorporates an AC generator,
drive unit with self-contained lubrication system, and automatic control and
protection components.
When an engine is started with external power or APU generators powering the
electrical system, Generator Control switches ON, and Bus Tie switches AUTO,
the IDG powers its side of the synchronous bus when voltage and frequency are
acceptable. The previous power source is disconnected. When an engine on the
opposite side of the airplane is started, the IDG powers its side of the synchronous
bus. The previous power source is disconnected, and the SSB closes.
When a single external power source is powering the synchronous bus and an
engine is started, the IDG powers the entire synchronous bus when voltage and
frequency are acceptable. The SSB remains closed and external power is
disconnected.
During power source transfers on the ground, such as switching from IDG to APU
or external power, output from both power sources is momentarily synchronized
before one source is disconnected. This provides smooth, uninterrupted electrical
power switching.
Each IDG can be electrically disconnected from its related bus by pushing the
Generator Control switch OFF. On the ground, an IDG can also be disconnected
from its related bus by selecting an available APU or external power source.
The EICAS message ELEC DRIVE displays and the related generator drive light
illuminates if low oil pressure or high oil temperature is sensed in an IDG.
Pushing the Generator Drive Disconnect switch manually disconnects the IDG
from its engine and opens the related GCB if not already open. The IDG cannot
be reconnected in flight.
Auxiliary Power (External and APU)
Auxiliary power is available from external power sources or generators mounted
on the APU. External power and APU generators can power the main AC busses
through connection to the synchronous bus only when the airplane is on the
ground. A split system breaker (SSB) divides the synchronous bus. This allows
each side of the AC electrical system to be powered by separate auxiliary power
sources. External power and an APU generator cannot be connected to the same
side of the synchronous bus simultaneously.
April 1, 2000
747 Operations Manual
Electrical -
System Description
Copyright © The Boeing Company. See title page for details.
D6-30151-425 6.20.3
On an unpowered airplane, when one auxiliary power source is selected ON with
the related AVAIL light illuminated, the SSB closes and power is provided to the
entire synchronous bus. When another auxiliary power source is applied to the
opposite side of the airplane, the SSB opens and each source powers its side of the
synchronous bus. If one auxiliary power source is disconnected or fails, the SSB
closes maintaining power to the entire synchronous bus.
When IDGs are powering the electrical system with APU generators or external
power available, selecting one auxiliary power source opens the SSB. The selected
auxiliary power source powers its side of the synchronous bus, while the opposite
side remains powered by IDGs. Selecting a second auxiliary power source on the
opposite side completes the power transfer from IDGs to auxiliary power. The
SSB remains open.
AC Electrical Power Distribution
AC power is distributed by the following busses:
• four main busses
• two transfer busses
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