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GCU Fail-Safe
The GCU has protection for an internal failure. A fail-safe
condition causes these effects:
* GCR and GCB open
* BTB closes, if open
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* GCU fault indication on the front of the GCU and indication on
the P5 panel.
GCU Automatic Close
The GCU1 sends a closed command to the GCR when all of
these conditions occur:
* Fire switch is in the normal positon
* BTB1 is open
* Airplane is in the air mode
* GCB is open on transition to the air mode and the APU is the
only source of power on the aircraft (GCB2 & BTB2 is also
open)
* Auxiliary power breaker (APB) opens.
This occurs only once in flight. The circuit resets on transition to
the ground mode. The proximity switch electronics unit (PSEU)
supplies the air/ground signals.
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AC GENERATION - GCU - PROTECTION AND EXCITATION
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Built-In Test Equipment
The GCU built-in test equipment (BITE) is circuits and lights to
find and isolate faults in each generation channel. BITE
operates independently of the normal control and protective
circuits. Inputs from other GCUs or the bus power control unit
are not necessary. The GCU has these BITE functions:
* Finds and identifies LRU failures in the generation channel
* Saves fault information in the non-volatile memory (NVM)
* Shows fault information with front panel lights
* Self test of GCU circuits during power-up and whenever you
push the test switch.
Front Panel
The front panel has one momentary TEST switch and seven
lights. You use the TEST switch to turn off lights that come on by
faults and to start a GCU self test.
A front panel light comes on when the GCU finds a fault. Only
one light comes on at a time. There is a priority if the GCU sees
more than one fault at the same time. The highest priority fault
light comes on if there is more than one fault. The lights have
this priority:
* 1 - GCU FAULT
* 2 - IDG FAULT
* 3 - GCB/APB FAULT
* 4 - BTB FAULT
* 5 - FEEDER FAULT
* 6 - DIST/BUS FAULT
* 7 - GCU PASS.
GCU FAULT Light
The GCU FAULT light comes on when the GCU/AGCU fails a
self test or the fail-safe protection starts. An internal failure
causes either one of these conditions.
The fail-safe protection starts when the GCU continuous monitor
BITE functions finds an internal fault.
The GCU fault light also comes on if an undervoltage or
overvoltage trip happens because of a voltage regulator failure.
IDG FAULT Light
For a GCU, the IDG FAULT light comes on when the GCU
protection circuits open the generator control breaker (GCB) for
any of these IDG output problems:
* Underfrequency
* Overfrequency
* Shorted rotating diodes at power source
* Undervoltage (voltage regulator okay).
The IDG FAULT light does not come on when a GCU is installed
in the AGCU position.
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EFFECTIVITY
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GCB/APB FAULT Light
For a GCU, the GCB/APB FAULT light comes on to show that the
GCB did not move to the commanded position. For the AGCU,
the GCB/APB FAULT light comes on to show that the APB did
not move to the commanded position. The logic has a time
delay to let the breakers change position.
BTB FAULT Light
The BTB FAULT light comes and shows that the BTB did not
　
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