Failure of the Neutral-To-Phase System of the APU Generator or the GAPCU

TASK 24-23-00-810-822-A
Failure of the Neutral-To-Phase System of the APU Generator or the GAPCU

1. Possible Causes

GEN-APU [8XS]
wiring
GAPCU [24XG]

2. Job Set-up Information
A. Referenced Information

REFERENCE	DESIGNATION
ASM 242301
AMM 24-23-51-000-001	Removal of the APU Generator
AMM 24-23-51-400-001	Installation of the APU Generator
AMM 24-41-00-861-002	Energize the Aircraft Electrical Circuits from the External Power
AMM 24-41-00-862-002	De-energize the Aircraft Electrical Circuits Supplied from the External Power
AMM 24-41-34-000-001	Removal of the Ground Power Control Unit (GPCU)
AMM 24-41-34-400-001	Installation of the Ground Power Control Unit (GPCU)

3. Fault Confirmation
Subtask 24-23-00-869-061-A ** ON A/C NOT FOR ALL

A. Test
Not applicable.

NOTE: As the APU GEN was set to OFF because of a true failure, the fault confirmation is not necessary.

4. Fault Isolation
Subtask 24-23-00-810-075-A ** ON A/C NOT FOR ALL

A. If the fault symptom is identified by the CFDS message GEN APU (8XS)/ GAPCU (24XG)/ WRG: GEN APU FEEDER and the upper ECAM-DU warning APU GEN FAULT:

do a check of the resistance of the neutral-to-phase connections ASM 242301 of the APU generator between:
. terminal T1 and N (9.35 milliohms plus or minus 1.25 milliohms)
. terminal T2 and N (9.35 milliohms plus or minus 1.25 milliohms)
. terminal T3 and N (9.35 milliohms plus or minus 1.25 milliohms).

(1) If the resistance values are out of the specified limits:

replace the GEN-APU [8XS] AMM 24-23-51-000-001 and AMM 24-23-51-400-001.

(2) If the resistance values are in the specified limits:

do a check of the wiring of the APU generator neutral-feeder for an open circuit or a high resistance between the terminal N of the APU generator and the airframe ground ASM 242301.

(a) If the wiring is not correct:

repair or replace the neutral feeder as necessary.

(b) If the wiring is correct:

replace the GAPCU [24XG] AMM 24-41-34-000-001 and AMM 24-41-34-400-001.

1 If the fault continues:

replace the GEN-APU [8XS] AMM 24-23-51-000-001 and AMM 24-23-51-400-001.

Subtask 24-23-00-860-063-A ** ON A/C NOT FOR ALL

B. Make sure that the aircraft electrical circuits operate correctly in APU configuration.
(1) Energize the aircraft electrical circuits from the APU AMM 24-41-00-861-002.
(2) Let the APU generator operate for five minutes.
(3) De-energize the aircraft electrical circuits from the APU AMM 24-41-00-862-002.

[Rev.8 from Aug 2018] 2026.04.04 04:26:41 UTC