C2 ELEC HYDPUMP SW
OFF
ON
L BUS
C1 ELEC HYD
TIE
CLOSE PUMP SW
L BUS TIE HYD SYS CONT PANELBREAKER
1 WHEN POWER IS APPLIED, RELAY STAYS ENERGIZED FOR 1.5 SECONDS.
Load Shedding Schematic
Figure 4
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A
(3)
(4)
757
MAINTENANCE MANUAL
(b)
Two utility bus relays (UBR's) control utility bus power. The utility bus is powered when its UBR is energized. Two push on/push off UTIL BUS switches on M10063 electrical system control panel provide manual control of the UBR's. Each UBR has its own switch. With the switch in the out position the UBR is de-energized and the utility bus has no power. Pressing the switch in energizes the the UBR's, and a white ON light in the switch comes on.
(c)
Galley buses are also controlled by the UTIL BUS switches. Pressing the switches in energizes the utility bus galley relays, supplying power to the galley buses. The white ON light in the switch is on. The galley buses have no power if the switches are in the out position.
(d) Utility and galley bus shedding occurs during the following conditions: 1) Loss of one or more generators in flight (both utility and galley buses shed). 2) One source overloaded with two sources operating either in flight or on the ground (respective utility and galley buses shed).
3) Overload with one source supplying both buses on the ground (both utility and galley buses shed).
(e)
Galley buses are temporarily shed when the airplane is powered by APU or external power with a hydraulic pump on and load greater than 90 KVA. This protection is for ground mode only.
(f)
Utility and galley buses are automatically reset when returning to two generator operation in flight. Utility and galley buses lost due to overload conditions must be manually reset by cycling the appropriate UTILITY BUS switches.
(g)
Utility and galley bus power is temporarily removed when starting an engine with APU pneumatics and APU electrical power. Only the utility and galley buses being supplied by APU power will be interrupted. The utility and galley buses will be automatically reset when the engine generator comes on line.
(h)
EICAS computers monitor utility bus status.AC Center Bus (Fig. 5)
_____________
(a)
The AC center bus provides a third, independent power source during autoland conditions. The bus is normally powered from the left main AC bus. If an autoland signal is received, the autoland relay energizes, connecting the center bus to the standby power bus. The AC center bus is in P6 panel.
Ground Service Bus (Fig. 6)
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P11 REQUEST RELAY CONT UNIT
Center Bus Schematic
Figure 5
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757
MAINTENANCE MANUAL
BUS POWER CONTROL UNIT GEN CONTUNIT
Ground Service Bus Schematic DiagramFigure 6
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