(a)
Remove all power from the BCU to accomplish a power up reset.
(b)
AIRPLANES WITH THE -11 BUS CONTROL UNIT (BCU); Push the external power switch -1 or -2 on the P5 panel.
(6)
Whenever external power with acceptable power quality is properly connected to the airplane, the AC CONNECTED light on the P36 panel will illuminate.
(7)
The PWR NOT IN USE light on the P36 panel illuminates whenever the XPC is open with external power connected.
(8)
The BCU provides control functions to permit closure of the XPC if phase sequence is correct and voltage and frequency are within steady-state, no-strip limits.
(9)
If abnormal voltage, frequency, or phase sequency power is applied, the BCU will inhibit closure of the XPC. In addition, the BCU will cause the XPC to open when the external power plug is withdrawn from the airplane receptacle or when a protection trip occurs.
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(10)
The 115-volt ac external power sent to the BCUs is rectified to 28 volts dc by an internal TRU. The 28 volts dc is used to power the BCU circuitry and the external control switches and indicator lights.
(11)
When the BCUs determine the 115-volt ac external power is good, the BCU will cause the AVAIL light in the EXT PWR switch on the electrical system control module to illuminate.
(12)
Momentary actuation of the EXT PWR switch when the associated XPC is open and external power is available will cause a close signal to the associated XPC via auto-parallel dead bus circuits.
(13)
External power is applied to the sync bus when the BCU applies 28 volts dc to the coil of the XPC. After the XPC is closed, power is removed from one of the coils since it takes less power to hold the XPC closed than it does to initially close it. This is done to reduce the power requirements of the XPC.
(14)
To remove external power, the BCU removes the coil power which allows the spring to open the XPC.
(15)
After the XPC closes, the 115-volt ac external power is applied to the sync bus and the external power ON light in the EXT PWR switch comes on and the POWER NOT IN USE light on the P36 panel goes out.
(16)
If only one external power source is available, the load buses can be supplied through the sync bus with the SSB in the closed position. If two external power sources are available, the load buses can be supplied through each half of the sync bus with the SSB open. An interlock is provided in the system to prevent paralleling of the two external power sources.
(17)
There are four current transformers associated with external power: two ground power current transformers (GPCTs) and two differential protection current transformers (DPCTs). The DPCTs monitor the sync bus current (Ref 24-23-00).
(18)
The GPCTs and the DPCTs provide current information to the BCUs for percent load indications on EICAS, differential fault protection, and load management.
(19)
Each BCU provides the following protective functions to open the associated XPC as a result of external power circuit faults (Ref 24-23-00):
(a)
Ground power overvoltage protection.
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(b)
Ground power undervoltage protection.
(c)
Ground power overfrequency protection.
(d)
Ground power underfrequency protection.
(e)
Ground power phase sequence protection.
(f)
Sync bus negative sequence voltage protection.
(g)
Sync bus differential fault protection.
(h)
Sustained unlike source parallel protection (SUSP).
(i)
Ground power plug fault protection.
(20)
The XPC will also open after a power transfer from external power has occured.
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