757
MAINTENANCE MANUAL
(a)
The ground service bus provides power to loads used inflight and on the ground. The bus is normally powered from the right main AC bus. External power or APU generator power can also be used. A red GND SVCE warning light on right generator power panel P32 is on when the bus has power. The ground service bus and circuit breakers are in P32, P6, P37, P34, and P11 overhead circuit breaker panels.
(b)
The BPCU works with a ground service switch to control two main relays: the ground service select relay, and the ground service transfer relay. The ground service select relay is in APU/external power panel P34. The ground service switch is on forward left attendant's panel P21. The switch is a momentary action type. Pushing the switch once turn on the ground service bus, pushing it again turns the bus off. A white light in the switch shows when APU or external power is used.
(c)
The ground service bus is normally powered by the right main AC bus. The GROUND SERVICE switch has no effect. The ground service transfer relay is de-energized.
(d)
If the right main AC bus is unpowered, APU or external power may be used. The BPCU activates the ground service relay to select either APU or external power (external power has priority). Pressing the GROUND SERVICE switch causes the BPCU to energize the ground service transfer relay. The ground service bus is powered. Removing APU or external power causes all relays to de-energize. The GROUND SERVICE light goes off.
(e)
Re-energizing the right main AC bus automatically switches the ground service bus back to right main AC bus power. The GROUND SERVICE light goes off.
(f)
The ground service bus will trip under the following conditions (Ref 24-23-00): 1) over voltage 2) under voltage 3) over frequency 4) under frequency 5) over current 6) open phase
(g)
To reset the ground service bus after a protective trip, cycle the GND SERVICE switch.
(5) Ground Handling Bus (Fig. 7)
(a)
The ground handling bus provides power to loads used on the ground only. The bus is powered from the APU generator or external power (external power has priority). The three position ground handling relay connects power to the bus. The bus and circuit breakers are in P34 panel.
(b)
The ground handling bus is not powered inflight. The BPCU prevents the bus from energizing with APU power inflight.
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A
757
MAINTENANCE MANUAL
BUS POWER CONTROL UNIT CONTROL UNIT
Ground Handling Bus SchematicFigure 7
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757
MAINTENANCE MANUAL
(c)
The ground handling bus will trip under the following
conditions (Ref 24-23-00):
1) over voltage
2) under voltage
3) over frequency
4) under frequency
5) over current
6) open phase
(d)
To reset the ground handling bus after a protective trip, cycle (press and release) the GROUND SERVICE switch.
(6)
Flight Instrument Transfer Buses (Figs. 8 and 9)
(a)
The flight instrument transfer buses allow switching of the captain's or first officer's instruments to the alternate main AC bus if the respective main AC bus loses power. The normal power source for the captain's flight instrument transfer bus is the left main ac bus. The normal power source for the first officer's flight instrument transfer bus is the right main ac bus. The instrument bus voltage sensing units also switch to the alternate bus if main bus power is of poor quality. The alternate main ac bus voltage must be of proper quality for switching to occur. The BUS TIE switches on M10063 electrical system control panel must be in the AUTO position (latched in) for automatic bus switching capability. The flight instrument transfer buses are located in P6 and P11 panels.
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