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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
GEN CT
PMG HARDWARESOFTWARE
OPEN PHASE
SEC 3.5
OUTPUT
DETECTION
OVER VOLTAGE
INTEGRATED
VOLT 129V
DRIVE GENERATOR
TRIP UNDER VOLTAGE115V AC
VOLT 107V
SEC 9.0
L/R BUSL/R GCR
SHORTED
CONTROL
SEC2.0
PMG
CLOSE
DETECTION 115V AC
L/R BUS L/R GEN
CONT BRKR
L/R GENLOADS
FREQUENCY SENSE OVER FREQ
UNDER FREQ
 SEC1.0
372.5
HZ
SEC 0.15
350
HZ
427.5
SEC1.0
HZ
440

28 MSECHZ
SHORTED L/R GEN DPCT
L/R BUS TIE DPCT ROTATING
SEC 5.5DIODE
WATCH DOGCIRCUIT GCR
AUX SEC 8.0
UNDER
SPEED
TO
TRIP

AIR/GND
PWR GEN DIFF
TIE BUS
UP I
FAULT
N L/R GEN
CURRENT
P LOADS
PWR T
U
SENSING
GEN CONT SW  UP
L/R GCB CLOSE
CONTROL
15 MSEC
RBTB AUX I
N
L/R BUS RBTB 50 MSEC
TIE BRKR
T FAULT
OVERLAP
OLAPL R
E
10 MSEC
R OVERLAP DPCT L/R GEN DPCT

CURRENT F SENSING A 15 MSEC
L/R BTB E
C GDPR CONTROL
50 MSEC PWRUP
TBDP
DIFF FAULT SENSE
INTERFACE

OLAPR
GDPL APU BUS DIGITAL RBTB SWTIE DPCT
DIGITAL DATA EXT. PWR
MICRO DATA LINK DPCT
PROCESSOR  LINK XCVR
XCVR

L/R BUS
TIE DPCT
LEFT/RIGHT GENERATOR CONTROL UNIT
BUS PWR CONT UNIT

Left/Right Generator Differential Protection Schematic
Figure 2

APU GEN CONTROL UNIT
BUS PWR CONTROL UNIT
APU Differential Protector Schematic
Figure 3

A
757
MAINTENANCE MANUAL

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A

757
MAINTENANCE MANUAL

 (d)  If a breaker has been tripped due to fault isolation. The breaker may be reset by cycling the corresponding control switch. The switch is pressed to OFF (or ISLN for the BUS TIE switch), then the switch is pressed to ON (or AUTO for BUS TIE switch). If the fault condition remains, the breakers remain open (they are locked-out). The amber OFF (or ISLN for BUS TIE switches) light comes on, and the corresponding EICAS message is displayed.
 (3)  
Load Shedding

 (a)  
To reduce generator loads, the utility bus relays (UBRs) and electrical load control units (ELCUs) are de-energized. This removes power from the utility and galley buses. In some cases the shed buses are automatically reset. This is done by relays and circuit interlocks. In other cases reset is accomplished by cycling the UTILITY BUS switches.

 (b)  
The UTILITY BUS switches are on electrical systems control panel M10063. (This panel is on pilot's overhead panel P5.) The switches are push on/push off type which latch in when on. When latched, an ON indicator in the switch is visible. The left UTILITY BUS switch controls the left UBR, and the left galley ELCU. The right UTILITY BUS switch controls the right UBR, and two right galley ELCUs. If a UBR is de-energized, an amber OFF light in the UTILITY BUS switch controlling that UBR comes on. A two step process manually resets the UBR and its associated ELCU(s). The controlling switch is pressed to off (out), then pressed on (in). If the shedding conditions remain, the contactors are again de-energized. (The amber OFF light in the switch again comes on.)

 (c)  
When a UBR is de-energized, the amber OFF light in the associated UTILITY BUS switch comes on. Also, EICAS will display the corresponding level C message, (L or R) UTIL BUS OFF. The EICAS messages are inhibited from being displayed during engine start. (The amber OFF lights in the UTILITY BUS switch can still come on.)

 (d)  
The following paragraphs describe the conditions which cause load shedding. The L and R GEN CONT switches are assumed to be in their normal ON position (latched-in). The L and R BUS TIE switches are assumed to be in their normal AUTO position (latched-in). These switches are on P5.

 (4)  
Pre-Engine Start Load Shedding (Galleys Shed)

 (a)  Pre-engine start load shedding occurs when: 1) The airplane is on the ground, and 2) electrical power is supplied by the APU generator or
　
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