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MAINTENANCE MANUAL
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MAINTENANCE MANUAL
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MAINTENANCE MANUAL
PMG
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28V DC
BAT BUS
B R GEN
OA
OBOCC
CONT UNIT P6 INTEGRATED
DRIVE GEN
GEN CONTUNIT D
E
F POWERSUPPLY TRIP G AUTO W
HARDWARE
AUTO
LT POWERAUTO LANDSOFTWARE
LBTB
RBTB
RGCB
XFR BUS
FLT INSTR
OI
BTB AUTO
UN
TP
H ISOL
LDTB
U
EMD&T
T T
RGCB
ISLN A
APB
CLOSE I
IN
BTB CONTROL
EPC NTRELAYS
T
R BUS TIE SWITCH LTLGCB
E
E
ON
R
RF
DMB FTO GCU A
AC
C
OFF
E
E BUS ENABLE
BTB
R GEN CONT SWITCH
PROTECTIONLATCH ELEC SYSTEM CONTROL PANEL
DIGITALDATA
JLINK
K R BUS ISOLATED
XCVR
(LEVEL C ALARM)
RIGHT GENERATOR CONTROL UNIT
L/R EICAS CMPTR L
M
Right Bus Tie Breaker Control Schematic
Figure 5 (Sheet 1)
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757 MAINTENANCE MANUAL
When a BUS TIE switch is isolated (ISLN), its BTB cannot close. The amber ISLN light in the switch is on. Also, the EICAS display provides the L (or R) BUS ISOLATED level C EICAS message when the corresponding BUS TIE switch is set to ISLN. The main ac bus will remain isolated from the other power sources even if the bus loses power.The left and right BUS TIE switches control the automatic switching of power to the captain's and first officer's flight instrument transfer buses (Ref 24-51-00). This switching maintains power on the buses if their primary source fails. For automatic control of power to the captain's flight instrument transfer bus, the left BUS TIE switch must be set to AUTO (in). If the captain's instrument transfer bus switches to its alternate source while its main source (left ac bus) still has power, a CAPT INSTR XFER EICAS message appears on the status and maintenance pages. For automatic control of power to the first officer's flight instrument transfer bus, the right BUS TIE switch must be set to AUTO (in). If the first officer's instrument transfer bus switches to its alternate source while its main source (right ac bus) still has power, a F/O INSTR XFER EICAS message appears on the status and maintenance pages.An uncleared differential fault on the ac tie or main ac buses causes the GCUs to lock-out the BTBs (Ref 24-23-00). The BTBs are kept open by the GCUs. If a GCU detects an undervoltage and overload at the same time, the GCU locks-out its BTB. When a BTB is locked-out, its BUS TIE switch ISLN light comes on. The L (or R) BUS ISOLATED message also appears on the EICAS display. A locked-out BTB is reset by first pressing its switch to off (out), then to AUTO (in). If the ISLN light doesn't go off, the BTB is still locked-out. The condition which caused the lock-out still remains.
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