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AC BUS 1 and AC BUS 2
AC BUS 1 and AC BUS 2 are the main AC buses of the aircraft, and receive power
from any of the three generators or from external AC power. AC BUS 1 normally
supplies power to the AC ESS BUS and AC UTIL BUS 1. AC BUS 2 normally
powers AC UTIL BUS 2.
Bus Priority
AC BUS 1 and AC BUS 2 are protected by a bus priority logic to ensure that the buses
remain powered at all times. The generator control units (GCUs) determine the bus
priority logic for AC BUS 1 and AC BUS 2 via the generator line contactors (GLCs)
and generator transfer contactors (GTCs). The bus priority logic for AC BUS 1 and
AC BUS 2 is as follows:
• on-side engine-driven generator (example: AC BUS 1 powered by GEN 1)
• APU generator
• cross-side engine-driven generator (example: AC BUS 1 powered by GEN 2)
• external AC Power
According to the above priority logic, the GLCs and GTCs will automatically tie the
main AC buses to any of the available AC power sources, to ensure the main AC buses
remain powered at all times.
AC BUS 1(2) Automatic Transfer (AUTOXFER) Switch/Lights
The flight crew may manually inhibit the automatic bus transfer priority logic by
pressing the appropriate AUTOXFER switch/light located on the ELECTRICAL
POWER Panel. This prevents the corresponding AC BUS from being powered by any
source except its on-side engine-driven generator. This may be used to isolate a main
AC bus in the event of an electrical emergency (example: electrical smoke or fire).
Selection of the AUTOXFER switch/light is indicated by illumination of the
corresponding white OFF annunciator on the switch/light and the white AUTO XFER
OFF icon on the AC Electrical Synoptic page (see Figure 7-9).
Should a bus fault or generator overcurrent condition occur on a main AC bus, the
automatic bus transfer priority logic will be inhibited, isolating the fault from the
remaining AC electrical system. This condition is indicated by the AC 1(2)
AUTOXFER caution EICAS message, illumination of the corresponding amber FAIL
annunciator on the AUTOXFER switch/light, and the amber AUTO XFER FAIL icon
on the AC Synoptic page (see Figure 7-9).
ELECTRICAL
7-12 For Training Purposes Only
Jan 04
P I LOT T R A I N I N G GU I D E
AC BUS Automatic Transfer
Figure 7-9
ELECTRICAL POWER
BATT MASTER EXT. POWER
APU GEN GEN 2
OFF/
RESET
OFF/
RESET
ON
ON
OFF
ESS. POWER DC POWER
AC ESS XFER MAIN BUS TIE
AC POWER
GEN 1
OFF/
RESET
AVAIL
IN USE
IN
USE
ON
AUTOXFER
FAIL
OFF
FAIL
OFF
ALTN CLOSED
ON
AC DC
P604_07_034
AC ELECTRICAL
UTIL BUS 1
BUS 2
GEN
2
GEN
1
APU
BUS 1
UTIL BUS 2
GEN
ESS BUS
4
115
400
KVA
V
HZ
0
0
0
KVA
V
HZ
0
0
0
KVA
V
HZ
AUTO
XFER
FAIL
SHED
AC BUS 2 AUTOMATIC TRANSFER FAILURE
ELECTRICAL POWER
BATT MASTER EXT. POWER
APU GEN GEN 2
OFF/
RESET
OFF/
RESET
ON
ON
OFF
ESS. POWER DC POWER
AC ESS XFER MAIN BUS TIE
AC POWER
GEN 1
OFF/
RESET
AVAIL
IN USE
IN
USE
ON
AUTOXFER
FAIL
OFF
FAIL
OFF
ALTN CLOSED
ON
AC DC
P604_07_033
AC ELECTRICAL
UTIL BUS 1
BUS 2
GEN
2
GEN
1
APU
BUS 1
UTIL BUS 2
GEN
ESS BUS
4
115
400
KVA
V
HZ
4
115
400
KVA
V
HZ
0
0
0
KVA
V
HZ
AUTO
XFER
OFF
AC BUS 2 AUTOXFER SWITCH/LIGHT SELECTED OFF
ELECTRICAL
For Training Purposes Only
May 03
7-13
P I LOT T R A I N I N G GU I D E
AC ESS (Essential) Bus
The AC ESS BUS supplies power to the equipment essential for flight. It is normally
powered by AC BUS 1, but may also be powered by AC BUS 2 or the ADG BUS. The
AC ESS BUS supplies power to ESS TRU 1, which provides DC power to the DC
ESS BUS and the BATT BUS.
Refer to the ADG section in this chapter for information on the ADG BUS.
AC ESS Bus Transfer
If a loss of power on AC BUS 1 is sensed, the AC ESS BUS is automatically
transferred to AC BUS 2. When an automatic transfer to AC BUS 2 occurs, the AC
ESS XFER switch/light located on the ELECTRICAL POWER Panel illuminates and
the AC ESS ALTN status EICAS message appears (see Figure 7-10).
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