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• TRU 2 normally supplies DC BUS 2 and DC UTIL BUS 2
• ESS TRU 1 normally supplies DC ESS BUS and DC BATT BUS
• ESS TRU 2 normally supplies DC ESS BUS and DC BATT BUS
TRU Power Distribution System
The TRU Power Distribution System supplies the following buses:
DC BUS 1 and DC BUS 2
DC BUS 1 is supplied by TRU 1 and DC BUS 2 is supplied by TRU 2. If either TRU
fails, both buses can be supplied by either TRU by selecting the MAIN BUS TIE
switch/light on the ELECTRICAL POWER Panel (see Figure 7-17).
DC ESS BUS
The DC ESS BUS powers equipment essential for safe flight, and therefore, may be
supplied by numerous DC sources to ensure redundancy of its power supply.
Normally, ESS TRU 1 and ESS TRU 2 power the DC ESS BUS. In the event of a
single ESS TRU failure, either ESS TRU is capable of sustaining the DC ESS BUS
load. Should both ESS TRUs fail with the aircraft in flight, the APU BATT DIR BUS
through the DC ESS solid-state contactor will power the DC ESS BUS.
During ADG operation, the AC ESS BUS powers ESS TRU 1 which in turn powers
the DC ESS BUS. The DC ESS BUS can also be powered by the APU and Main
batteries through the Emergency DC Transfer Contactor. Refer to the ADG section of
this chapter for additional information.
ELECTRICAL
For Training Purposes Only
May 03
7-23
P I LOT T R A I N I N G GU I D E
TRU 1 Failed and MAIN BUS TIE Closed
Figure 7-17
BATT BUS
The BATT BUS powers equipment essential for safe flight, and may be supplied by
several DC sources to ensure redundancy. Normally, ESS TRU 1 and ESS TRU 2
power the BATT BUS when the main AC buses are powered. If the ESS TRUs are not
operating (no AC power or ESS TRU failures), the BATT BUS will be powered by the
APU BATT DIR BUS and MAIN BATT DIR BUS provided the BATT MASTER
Switch is selected ON.
The BATT MASTER Switch located on the ELECTRICAL POWER Panel controls
the APU BATT and MAIN BATT contactors to supply the BATT BUS, as described
above.
The BATT BUS also supplies the DC EMER BUS.
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_039
TRU 1
BUS 1
BUS 2
UTIL BUS 1
UTIL BUS 2
EMER BUS
APU BATT
DIR BUS
MAIN BATT
DIR BUS
AC BUS 1 AC BUS 2 AC ESS BUS
TIE
ESS BUS
BATT BUS
– – V
0 A
TRU 2
28 V
20 A
ESS
TRU 2
28 V
24 A
ESS
TRU 1
28 V
24 A
MAIN
BATT
26 V
0 A
APU
BATT
27 V
0 A
EXT
DC
28 V
19 A
DC ELECTRICAL
ELECTRICAL
7-24 For Training Purposes Only
Jan 04
P I LOT T R A I N I N G GU I D E
DC UTIL BUS 1 and DC UTIL BUS 2
The DC UTIL buses supply power to nonessential cabin equipment. TRU 1 and
TRU 2 power DC UTILITY BUS 1 and 2 respectively.
Whenever the MAIN BUS TIE switch/light is selected following a TRU 1 or 2 failure,
the DC UTIL buses are load shed to reduce the DC electrical load.
Battery Power Distribution System
The Battery Power Distribution System represents a small portion of the aircraft’s total
DC load. The batteries primarily provide the energy needed to start the APU, which
can then be used to supply AC power to the aircraft. The batteries also supply
emergency DC power in-flight should all AC power be lost and the ADG become
inoperative.
Two rechargeable nickel-cadmium (NiCad) batteries are installed in the aft equipment
bay. The main battery is a 24-volt, 17-amp/hour battery. The APU battery is a 24-volt,
43-amp/hour battery.
The batteries supply DC electrical power to the following buses:
• MAIN BATT DIR BUS
• APU BATT DIR BUS
• DC EMER BUS
• BATT BUS
Battery Charging
The main battery charger and APU battery charger are in a charging configuration at
all times when the applicable main AC buses are powered:
• AC BUS 1 for Main Battery Charger
• AC BUS 2 for APU Battery Charger
The battery chargers operate automatically in various modes based on battery
temperatures. Should a battery or battery charger fault be detected, the respective
battery charger will shut down. A MAIN (APU) BATT CHGR FAIL status EICAS
message will appear and the CHGR OFF icon will be displayed on the DC Electrical
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