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General
The BPCU automatically removes power (load shed) to the
galley buses and sometimes the main buses to protect the
power source from overload. There are three types of load
shed:
* Configuration
* Command
* Overcurrent.
Configuration Load Shed
Configuration load shed occurs when the APU is the only
source of AC power in flight. The BPCU opens (de-energizes) all
galley load shed relays. Air/ground inputs come from the
proximity switch electronics unit (PSEU). AC power is provided
by the BPCU to the PSEU for the air or ground modes.
Command Load Shed
This type of load shed occurs for the APU starter-generator
only. The command load shed occurs when the APU electronic
control unit (ECU) senses APU high EGT. The APU ECU
monitors APU performance parameters to determine a possible
overload condition.
The ECU sends an APU load shed signal to the BPCU. The
galley load shed relays open first. The main bus relays will
open if the overload condition continues.
Overcurrent Load Shed
Overcurrent load shed occurs when a generator control unit
(GCU or AGCU) sees an overcurrent condition of a generator or
the BPCU sees an overcurrent conditions of external power.
These are the overcurrent limits for a generator or external
power source to cause load shed:
* 274 amps (nominal) for 300 seconds
* 340 amps (nominal) for 5 seconds.
The number of galleys and main buses which lose power and
the order in which they lose power depends on the
configuration of the electrical power system.
AC ELECTRICAL LOAD DISTRIBUTION - LOAD SHED - FUNCTIONAL DESCRIPTION
EFFECTIVITY
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BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
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AC ELECTRICAL LOAD DISTRIBUTION - LOAD SHED - FUNCTIONAL DESCRIPTION
AC ELECTRICAL LOAD DISTRIBUTION - LOAD SHED - FUNCTIONAL DESCRIPTION
EFFECTIVITY
GUN ALL 24-50-00
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BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
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General
The BPCU removes power (load shed) to the galley buses and
sometimes the main buses to protect the power source from
overload. There are three types of load shed:
* Configuration
* Command
* Overcurrent.
Configuration Load Shed
Configuration load shed occurs when the APU is the only
source of AC power in flight. The BPCU opens (de-energizes) all
galley load shed relays within 100 milliseconds. The galley
relays close (energize) if a second source of AC power is
established.
AIRPLANES WITH GALLEY SWITCH;
After landing, move the galley switch to OFF and back to the ON
position to reset the galley relays.
AIRPLANES WITH CAB/UTIL SWITCH;
After landing, move the cabin utility switch to OFF and back to
the ON position to reset the galley relays.
Command Load Shed
This type of load shed occurs when the APU electronic control
unit (ECU) senses high exhaust gas temperature (EGT). The
APU ECU monitors APU performance parameters to determine
a possible overload condition.
The ECU sends an APU load shed signal to the BPCU. In flight if
the APU is the only source of power, both main bus relays open
(de-energize).
NOTE: Galley relays are already opened due to configuration
load shed.
The main bus relays close when another AC power source is
selected.
If the APU is the only source of power on the ground, all galley
relays will open. The galley relays close when the APU
operates within normal EGT limits, provided the galley switch
stays in the ON position.
If the APU generator supplies power to an AC transfer bus and
an IDG supplies power to the opposite AC transfer bus, the
galley relays that get power from the APU open. The galley
relays close when the APU operates within normal EGT limits if
the galley switch stays in the ON position.
AC ELECTRICAL LOAD DISTRIBUTION - LOAD SHED - OPERATION
EFFECTIVITY
GUN ALL 24-50-00
Page 12
D633A101-GUN Oct 10/2005
BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
24-50-00-110
Overcurrent Load Shed
Overcurrent load shed occurs when a generator control unit
(GCU or AGCU) senses an overcurrent condition of a generator
or the BPCU senses an overcurrent condition of external power.
These are the overcurrent limits for a generator or external
power source to cause load shed:
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