AC ESS Feed and Transfer

The ESS (essential) family is the network's survival floor. The overview made the design point in one FCTM sentence — the distribution is "designed to fly, navigate, communicate and ensure passenger comfort" — and concluded that the fly/navigate/communicate minimum set must have several "step-fathers" so it cannot be lost in a single stroke. This article covers how that family changes hands in the normal and half-failed world: the AC ESS FEED transfer circuit — the automatic 3-second move to AC BUS 2, the timing of the manual ALTN selection, and the two reset rules. The two emergency-world fathers (the emergency generator and the static inverter) are established in Emergency Generator and Static Inverter; this article completes the half that lives in normal and single-failure operation.

A point worth fixing at the outset: the AC ESS FEED transfer is wholesale, not retail. When the switching contactor moves, it does not carry the AC ESS BUS alone — the AC ESS SHED BUS and the ESS TR (and with it the whole DC ESS family) go with it. AC BUS 2 takes over the entire essential household, not one bus.

1. Scope and boundaries

This article owns the 3XC switching circuit, the automatic and manual transfers, the two reset rules, and the LAND RECOVERY logic seen from the switching circuit. It leaves to neighbouring articles: the complete AC ESS BUS FAULT procedure (see AC ESS Bus Fault and Shed); how the ESS family is fed in the emergency configuration (see Emergency Generator and Emergency Electrical Configuration); and the precise busbar-numbering family — 9XP / 901XP / 4XP / 401XP / 931XP segmentation (see AC Distribution and Busbars).

By the end you should be able to answer five questions:

1. In the normal configuration, who supplies the AC ESS family — and for the automatic transfer, what is the trigger, the executing contactor, and the time delay?
2. After AC BUS 1 recovers, how do the reset rules differ in flight versus on the ground?
3. When must the crew press AC ESS FEED manually, and what do they watch to confirm success?
4. What does the FAULT light precisely mean (it is not "AC BUS 1 is lost")?
5. What does LAND RECOVERY add when read from the 24-25 switching-circuit view (the APPR PROC prompt and the timing)?

2. Architecture — the switching circuit

The whole transfer hangs on a single contactor, 3XC, sitting between the two AC BUSes and the essential household. Read off the AC Essential Generation Switching schematic (AMM Fig. 24-25-00-15050) and the ASM switching schematic (ASM 24-25-01):

        AC BUS 1 (1XP)                         AC BUS 2 (2XP)
             | normal supply (via 1XC)              | altn supply (via 2XC)
             v                                       v
        +-- 3XC-A --+                          +-- 3XC-B --+
        |           |   3XC = AC ESS BUS       |           |
        +-----+-----+   SWITCHING contactor    +-----+-----+
              |         (740VU, zone 121, two mechanically
              |          interlocked half-sections, mutually exclusive)
              v
   AC ESS BUS (9XP / 901XP) --+--> AC ESS SHED BUS (4XP / 401XP)
              |               +--> 115/26 V auto-transformer --> 931XP (26 V AC)
              +---------------+--> ESS TR --> DC ESS BUS (4PP) / DC ESS SHED (8PP)

  Panel controls:  AC ESS FEED pb-sw   4XC  (235VU, guarded)
                   LAND RECOVERY pb-sw 8XC  (EMER ELEC overhead panel)
  Emergency-world feeders (articles 05 / 13):
                   EMER GEN --> EMER GLC 2XE        STAT INV --> 2XB / 3XB

The wholesale nature of the transfer is visible at a glance: 3XC does not move the AC ESS BUS only — the AC ESS SHED BUS and the ESS TR (and with it the whole DC ESS family) travel with it. AC BUS 2 inherits the entire essential household.

Three findings from reading the schematic (eyes-on ASM 24-25-01 + AMM Fig. 24-25-00-15050):

1. 3XC is one physical contactor carrying two sets of contacts — 3XC-A (to AC BUS 1, through 1XC) and 3XC-B (to AC BUS 2, through 2XC) — that are mechanically interlocked and mutually exclusive (the dashed mechanical link on the figure). Only one set can close; the two paths are never connected at once.
2. The relay 22XH (BUS 901XP CTL, 740VU) is the physical element that senses whether the AC ESS BUS has power and drives the FAULT legend. The FCOM definition "FAULT = AC ESS BUS not supplied" lands on this relay.
3. The relay 9XN (BUS 1XP CTL) is the detector that triggers the automatic transfer — it senses AC BUS 1 under-voltage — working with 13XN (BUS 2XP CTL).

The switching relay chain, read off the same schematic and the AMM operational-check circuit-breaker naming:

3. Automatic transfer — the 3-second move

Start from the normal state. The AC ESS BUS and the AC ESS SHED BUS are fed in parallel from AC BUS 1 (3XC-A closed, 3XC-B open):

"are normally supplied in parallel from AC BUS 1."

Per AMM 24-25-00. When AC BUS 1 is lost, the transfer runs automatically:

"In case of AC BUS 1 loss, the AC ESS BUS, AC ESS SHED BUS and the ESS TR supply is automatically transferred from AC BUS 1 busbar to AC BUS 2 busbar via the contactor 3XC-B. On the ECAM system: the ELEC AC page is displayed showing the AC BUS 1 indication in amber, and after 3 seconds (time delay of the relay 11XC), the connection of the essential network to the AC BUS 2."

Per AMM 24-25-00. So the trigger is AC BUS 1 loss (and only that), the executor is 3XC-B, and the delay is the 3 seconds of relay 11XC.

Those 3 seconds are the ESS family's brief blackout window — equipment on the AC ESS family (including some flight-deck displays) flickers off for about 3 seconds and returns. Why not switch instantly? The reasoning is the same as the CSM/G's own 3-second delay in Emergency Generator: filter the transient. A "loss" of AC BUS 1 may be nothing more than a momentary dropout during a source change; switching immediately would cause needless back-and-forth, so the circuit waits 3 seconds to confirm a real loss, paying the price of one controlled short interruption. The overview's FCOM principle underwrites this in advance — "Electrical transients are acceptable for equipment" (per FCOM DSC-24-10-10).

[!warning]- A failure of the transfer circuit itself loses both the E/WD and the SD

A subtle boundary: if the automatic transfer circuit itself fails (not AC BUS 1 lost, but 3XC-B / 11XC failing), the consequence is worse than "one ESS feed short". Because the E/WD and SD displays draw from the AC ESS family, they go dark together. The AC BUS 1 FAULT procedure states it plainly:

"If the automatic transfer of AC ESS BUS is inoperative, the ECAM's E/WD and SD DUs are simultaneously lost. The ECAM/ND SEL must be used to recover the E/WD on the NDU, in order to apply the ECAM procedure."

Per FCOM PRO-ABN-ELEC (ELEC AC BUS 1 FAULT). The companion manual-recovery lines appear in the same procedure: AC ESS FEED ... ALTN (shown when the automatic transfer has failed) and, if the manual transfer also fails, EMER ELEC PWR ... MAN ON.

4. The two reset rules — manual in flight, automatic on ground

"In flight, in case of the AC BUS 1 recovering, this transfer can only be reset using the AC ESS FEED pushbutton switch (push, then push in). On ground, in case of the AC BUS 1 recovering, this transfer is automatically reseted."

Per AMM 24-25-00. In flight the transfer can only be reset by hand (AC ESS FEED out, then in); the system will not switch back on its own. On the ground it resets automatically.

The philosophy is the same as the emergency generator's "once coupled, do not hand back" rule (see Emergency Generator): in the air, do not trust a source that has only just recovered. AC BUS 1 is back, but why it failed has not yet been established, so the ESS family stays on the stable AC BUS 2 and the decision to revert is left to the crew. On the ground the risk is low, so the system reverts itself.

This is a common troubleshooting puzzle — "all the buses are green again, why is AC ESS still hanging on bus 2?" — and the answer is "because you are airborne". The AMM operational check gives the hard evidence: after a manual ALTN, even restoring the normal-supply breaker leaves it unchanged —

"close the circuit breaker 1XC. There is no change."

Per AMM 24-25-00. There is no automatic revert; AC ESS FEED must be pressed again.

5. Manual transfer and the true meaning of FAULT

The pushbutton's FAULT legend defines the whole manual case, and its subject matters:

"FAULT lt: Comes on amber associated with an ECAM caution when the AC ESS BUS is not electrically supplied."

Per FCOM DSC-24-20. The subject is the AC ESS BUS itself has no power — not "AC BUS 1 has no power". Two cases split from this:

• Caused by AC BUS 1 loss: the automatic transfer completes within 3 seconds, so FAULT only flickers (it extinguishes the moment the move is done). The primary caution you read is AC BUS 1 FAULT.
• AC BUS 1 healthy, AC ESS lost on its own (a feeder fault or a 3XC-A contactor fault): the automatic logic does not act — it only recognises the "AC BUS 1 under-voltage" trigger — so FAULT stays on, and this is your cue to act:

"This P/BSW enables to transfer the AC essential busses and the ESS TR supply from AC BUS 1 to AC BUS 2. This P/BSW will be used when the loss of the AC essential busses normal supply does not result from the AC BUS 1 loss. The white ALTN legend comes on on the AC ESS FEED pushbutton switch (the FAULT legend remains on if the AC ESS BUS is still not supplied)."

Per AMM 24-25-00. The trigger sentence, with the 901XP busbar number, opens the same passage:

"In case of loss of the AC ESS BUS (901XP), the FAULT legend on AC ESS FEED pushbutton switch comes on."

Per AMM 24-25-00. The moment the AC ESS BUS (FIN 901XP) loses power, FAULT comes on.

The confirmation chain. Press the button → white ALTN legend on + FAULT out + a green bar appears on the SD between the supplying bus and the AC ESS bus:

"a green bar comes into view between the AC 1 and the AC ESS busbar. This shows that the AC 1 busbar energizes the AC ESS busbar."

Per AMM 24-25-00. Once switched to AC 2, that green bar moves to lie between AC 2 and AC ESS — this is the mechanism behind "the feed comes out of AC 1 or AC 2" in Network Priority and Normal Supply: the green bar shows who is supplying. ALTN on but FAULT not out means changing fathers did not revive it — the fault is in the AC ESS bus itself or downstream (a short circuit), and you are now deep inside the AC ESS BUS FAULT procedure (see AC ESS Bus Fault and Shed).

[!warning]- "All buses green but AC ESS on bus 2" is a normal airborne state, and FAULT is not "AC BUS 1 lost"

Two related misreadings, predicted then corrected:

1. "AC BUS 1 has recovered, so why is AC ESS still on AC 2?" — By design. In flight there is no automatic revert (§4); the crew reverts deliberately with AC ESS FEED, or it happens by itself only on the ground.
2. "FAULT is on, so AC BUS 1 must be lost." — Not necessarily. FAULT means the AC ESS BUS itself has no power. If AC BUS 1 is healthy and AC ESS is lost (a feeder / 3XC-A fault), the automatic logic stays put and FAULT is precisely the signal to select ALTN by hand.

The ECAM message names differ between the two scenarios (confirmed verbatim from the AMM operational check):

• Automatic transfer caused by AC BUS 1 loss: besides ELEC AC BUS 1 FAULT, the AC ESS path raises ELEC AC ESS BUS ALTN — the FCOM defines this caution as triggering "when the AC ESS BUS is abnormally supplied by AC 2 BUS" (per FCOM PRO-ABN-ELEC). Both cautions appear together.
• AC ESS lost on its own (feeder / 3XC-A fault): the EWD shows ELEC AC ESS BUS FAULT + ELEC AC ESS BUS SHED; pressing AC ESS FEED out clears them, FAULT extinguishes and ALTN comes on.
• The AMM confirmation sentence for the automatic case: "The MASTER CAUT, single chime and the amber message (AC BUS 1 FAULT) on the EWD confirm this failure" (per AMM 24-25-00).

6. LAND RECOVERY from the switching-circuit view

The 24-25 section adds two details that the emergency-generator article does not carry:

Who prompts you to press it.

"An APPR PROC status warns the crew to push the EMER ELEC/LAND RECOVERY pushbutton switch."

Per AMM 24-25-00. The APPR PROC area of the ECAM STATUS page lists the item — you do not rely on memory.

When to press it.

"before the extension of the slats, the crew must push the EMER ELEC/LAND RECOVERY pushbutton switch."

Per AMM 24-25-00 — before the slats are extended, with the emergency generator on line. The reasoning chains back to the emergency-generator article: the object of the recovery list is "mainly slat extension" (the slat extension itself needs this recovered supply); and in the RAT scenario the CSM/G drops off the moment the slats extend, so 407PP / 805PP must be reconnected before that point. The AC ESS GND bus 805PP is in any case not supplied in the all-engines-stopped (RAT) scenario — "not supplied in case of total engine flame out" (per AMM 24-25-00), which cross-confirms the sub-bus table in the emergency-generator article.

The complete recovery list, from the FCOM panel description:

"(4) LAND RECOVERY pb-sw — ON: When pressed, with the emergency generator running, the AC LAND RECOVERY and the DC LAND RECOVERY buses are recovered and the following equipment are restored: ‐ LGCIU 1 ‐ SFCC 1 (flap channel is not recovered, if the emergency generator is powered by the RAT). ‐ BSCU channel 1 (not recovered, if the emergency generator is powered by the RAT) ‐ LH windshield anti-ice (not recovered, if the RAT) ‐ LH landing light (not recovered, if the RAT). The remaining fuel pump (if any) is lost. Note: The remaining fuel pump will be shed at 260 kt, if the emergency generator is powered by the RAT, or at LAND RECOVERY selection, whichever occurs first. When the LAND RECOVERY pb-sw is pressed in EMER ELEC configuration, ADR 3 is lost, and consequently, AP 1 is lost."

Per FCOM DSC-24-20. One precise point: what is "recovered for the slats" is the slat channel of SFCC 1; the flap channel cannot be recovered in the RAT scenario (the wording "SFCC 1 (flap channel is not recovered if RAT)" agrees with the AMM "mainly slat extension" — both point to the slats). The remaining fuel pump is lost (shed at 260 kt or at LAND RECOVERY selection, whichever comes first, in the RAT scenario), and pressing it in the EMER ELEC configuration loses ADR 3 and, with it, AP 1. This is the same cost sheet as the emergency-generator article — that article reads it from the EMER GEN view, this one from the switching-circuit view, and the two align.

7. The ESS family's full lineage of supplies

The whole chapter's "essential" idea is this one table — five layers of supply, four changes of father:

That AC BUS 2 takes the load automatically when AC BUS 1 fails, and the emergency generator and static inverter feed AC ESS directly when both AC BUSes are lost, is exactly what "the ESS family must have several step-fathers" means — the engineering definition of the word "essential" is this column of fall-backs.

A mental model. The AC ESS family is a child whose registered guardianship follows the guardian. The legal guardian is AC BUS 1 (normal); if the guardian is incapacitated, the court imposes a 3-day cooling-off period (the 3 seconds of 11XC — to confirm it is not a false alarm) and then transfers guardianship to the uncle, AC BUS 2. Once transferred away from home (airborne), it is not transferred back even if the original guardian recovers — reverting needs the person to appear and sign in (the AC ESS FEED cycle); at home (on the ground) it reverts automatically. And if the child gets into trouble while the guardian is fine (AC ESS lost while AC BUS 1 is healthy), the court does not act on its own — you must file the application yourself (the manual ALTN).

8. Flight-deck scenarios

1. Cruise, AC BUS 1 FAULT. ESS equipment flickers for about 3 seconds and returns automatically (11XC's delay). ELEC AC page: AC BUS 1 amber, AC ESS feed now drawn from AC 2. You do not need to touch AC ESS FEED — the automatic transfer is already done.
2. AC BUS 1 recovered (in flight). AC ESS still hangs on AC 2 — by design (no automatic in-flight revert). If the procedure calls for restoring NORM: AC ESS FEED out, then in. On the ground it reverts to NORM automatically.
3. AC ESS BUS FAULT while AC BUS 1 and 2 are both green. The textbook manual case — press AC ESS FEED (guard up), watch the three-in-a-row: white ALTN on + FAULT out + SD turns green. If FAULT does not extinguish, the bus itself is faulted; follow the ECAM into the AC ESS BUS FAULT procedure (see AC ESS Bus Fault and Shed).
4. EMER CONFIG approach preparation. The APPR PROC area of the STATUS page lists LAND RECOVERY — press it before extending the slats (to recover SFCC 1 and the other landing equipment), keeping the cost sheet in mind (ADR 3 / AP 1).
5. ESS equipment flickers for 3 seconds during a ground source change. First think of 11XC's delay as normal (a source change briefly dropped AC BUS 1 and triggered it) — a single occurrence is explainable; only repeated occurrences warrant checking 3XC / the feeder.

9. Dispatch view (MEL)

The operator MEL cuts a clean line between "the bus itself has no power" and "the switching path is broken" — exactly the distinction §5 draws around the FAULT light:

Per the operator MEL (MI-24-25). Read it this way: bus with no power (ECAM on) = no dispatch; but if only the path used to change fathers has latently failed, the current bus is still being supplied normally, and there is no ECAM, then MI-24-25-01/02 allow a conditional Category C dispatch — provided the redundant DC TIE contactor chain is healthy, because it must step up if a real transfer is ever needed. The LAND RECOVERY function, by contrast, is a hard no-go. (Note: the AC ESS FEED → ALTN action also appears in the SMOKE/FUMES isolation procedure, for a different purpose — isolating a burning electrical network — which belongs to the smoke article.)

Self-test

[!note]- Q1. What are the trigger, mechanism, time delay, and "removal list" of the automatic transfer?

Trigger = AC BUS 1 loss (the only trigger); mechanism = the 3XC-B contactor; delay = the 3 seconds of relay 11XC (to filter transients); removal list = AC ESS BUS + AC ESS SHED BUS + ESS TR (and with it the whole DC ESS family) — the entire household moves to AC BUS 2. The ELEC AC page shows AC BUS 1 amber, then connects the essential network to AC BUS 2 after the 3-second delay.

[!note]- Q2. What are the two reset rules?

In flight, when AC BUS 1 recovers — manual reset only (AC ESS FEED out, then in); the system does not revert automatically (it does not trust a just-recovered source). On the ground, when AC BUS 1 recovers — automatic reset to NORM. The AMM operational check confirms the no-auto-revert in flight: after a manual ALTN, restoring the 1XC breaker produces "no change".

[!note]- Q3. What does the FAULT light precisely mean, and what are the two cases it splits into?

FAULT = the AC ESS BUS itself is not electrically supplied (not "AC BUS 1 is not supplied"). If caused by an AC BUS 1 loss, the automatic transfer takes over and FAULT only flickers. If AC BUS 1 is healthy and AC ESS is lost (a feeder / 3XC-A fault), the automatic logic does not act, FAULT stays on, and a manual ALTN is required.

[!note]- Q4. What is the success-confirmation chain for the manual ALTN, and what does failure mean?

White ALTN legend on + FAULT out + the SD shows AC ESS turning green with the feed now drawn from AC 2 (the green bar showing who energises AC ESS). ALTN on but FAULT not out = AC BUS 2 cannot revive it either — the fault is in the AC ESS bus itself or downstream, and you proceed into the AC ESS BUS FAULT procedure.

[!note]- Q5. When is LAND RECOVERY pressed, and where does the prompt come from?

With the emergency generator on line, before the slats extend (the object of the recovery is "mainly slat extension" equipment = SFCC 1; in the RAT scenario the CSM/G drops off as the slats extend, so it must be done first). The prompt comes from the APPR PROC area of the ECAM STATUS page, not from memory. Note that 805PP is in any case not supplied in a total engine flame-out.

Key takeaways

References

Per AMM 24-25-00 D/O (normal parallel supply, automatic transfer via 3XC-B with the 3-second 11XC delay, the in-flight-manual / on-ground-automatic reset rules, the manual-transfer use case with the white ALTN legend and the persisting FAULT, the 901XP loss trigger, the SD green-bar evidence and the "no change" operational check, the APPR PROC / before-slats LAND RECOVERY prompt and "mainly slat extension", 805PP not supplied in a total flame-out, the EWD confirmation sentence); FCOM DSC-24-20 (the AC ESS FEED pb-sw Normal/ALTN/FAULT states and the total-loss note, the LAND RECOVERY recovery list with SFCC 1 slat/flap, BSCU, windshield, landing light, the remaining fuel pump 260 kt shed, and ADR 3 → AP 1); FCOM PRO-ABN-ELEC (AC BUS 1 FAULT — automatic-transfer-inoperative loses the E/WD and SD DUs, recovered via ECAM/ND SEL; the AC ESS BUS ALTN "abnormally supplied by AC 2 BUS" trigger); FCOM DSC-24-10-10 ("electrical transients are acceptable" general principle); the operator MEL (MI-24-25 — ECAM-triggered AC ESS family no-dispatch; the automatic- and manual-transfer latent-failure Category C conditions with the pb-normal and three-DC-TIE requirements, ALTN adding no-ETOPS; LAND RECOVERY must be operative); component designations read off the AC essential switching schematic (ASM 24-25-01 / AMM Fig. 24-25-00-15050 — 3XC-A/3XC-B mechanical interlock, 22XH = the FAULT-light sensing relay, 9XN/13XN = the AC BUS loss detectors, and the 1XC/2XC/5XC/6XC/11XC/13XC relay chain). The "3 seconds filter the transient" note, the "do not trust a just-recovered source in flight" philosophy, the lineage table and the guardianship mental model are integrative syntheses of the above and contain no facts from outside the library.

Independent study material, not an Airbus publication. Refer to current operator FCOM, FCTM, and QRH for operational use.