AC ESS BUS FAULT and SHED

The essential-load family — AC ESS BUS (9XP) and its sheddable sub-segment AC SHED ESS BUS (4XP / 401XP) — is the bus group the overview singled out as the network's protected core: the fly, navigate, communicate minimum, deliberately housed on side 1 and given several "step-fathers" so it cannot be lost in a single stroke. The AC ESS Feed Transfer article owns the switching machinery that gives it those step-fathers. This article picks up where that machinery meets the cockpit — the two ECAM cautions the family raises when it loses power for itself, ELEC AC ESS BUS FAULT (the parent 9XP is not supplied) and ELEC AC ESS BUS SHED (only the sheddable segment 401XP is not supplied), plus the status-level reminder ELEC AC ESS BUS ALTN (the family is now living on its alternate source).

By the end you should be able to answer five questions: (1) in the FAULT procedure, what does the crew do after the ALTN selection fails, and why? (2) which two procedure lines appear only when "AC ESS BUS SHED is still available"? (3) whose equipment does each INOP list take away — and why the FAULT list is the more frightening of the two? (4) what does the AP2 note on the SHED page actually mean? (5) where does PAX OXY MASKS appear, and what does its presence imply for a real cabin event?

1. The AC ESS family and its three annunciations

Read the family as a short vertical chain hanging off AC BUS 1, with one switch — the AC ESS FEED pushbutton — deciding where the top of the chain is fed from:

            AC ESS FEED pb
        NORM = AC BUS 1 (1XP)  ◄── normal source
        ALTN = AC BUS 2 (2XP)  ◄── manual fall-back source
                  │
                  ▼
      ┌───────────────────────────┐
      │     AC ESS BUS  9XP        │ ◄── ELEC AC ESS BUS FAULT
      │   (the protected parent)   │     (parent busbar not supplied)
      └─────────────┬─────────────┘
                    │
      ┌─────────────┴─────────────┐
      │  AC SHED ESS BUS  4XP      │ ◄── ELEC AC ESS BUS SHED
      │   (401XP sub-segment)      │     (sheddable segment not supplied)
      └───────────────────────────┘
            16XH sheds this segment in
            ELEC EMER CONFIG (see art. 05)

The pushbutton's FAULT legend is the entry point to the whole topic. The AMM defines exactly what that legend means — and the wording is deliberately not "AC BUS 1 has failed":

"In case of loss of the AC essential busses, FAULT legend on the AC ESS FEED pushbutton switch comes on : this P/BSW enables to transfer the AC essential busses supply from AC BUS 1 to AC BUS 2, in particular when the loss of the AC essential busses normal supply does not result from AC BUS 1 loss."

Per AMM 24-00-00. So the legend says "AC ESS itself has no power". If AC BUS 1 is healthy but the essential bus is lost through a feeder or contactor fault, the automatic transfer does not act and the crew must select ALTN by hand — the case the AC ESS Feed Transfer article works through. That single fact is the seed for the two cautions below, and for why "select ALTN" can come back unsuccessful.

The family raises three distinct annunciations, and keeping them apart is the whole point of this article:

Per FCOM PRO-ABN-ELEC. The ALTN line is the normal end-state of a successful FAULT recovery — once ALTN has been selected and the essential bus is alive again off AC BUS 2, the crew carry the AC ESS BUS ALTN status to the end of the flight (§5, scenario 1). It is a reminder, not a fault.

2. ELEC AC ESS BUS FAULT — the parent bus is dead

The caution triggers when the AC ESS BUS (9XP) itself is not supplied. The procedure has a two-stage shape: first try to re-feed the bus from its alternate source; if that does not bring it back, stop trying to revive the bus and reroute the captain's displays and air data to "number 3" resources instead.

The ECAM procedure, as displayed:

ELEC AC ESS BUS FAULT
   AC ESS FEED .......................... ALTN
    IF UNSUCCESSFUL:
   CAPT EFIS DMC ........................... 3      ← shown only if AC ESS BUS SHED available
   AIR DATA SWTG .................... CAPT ON 3      ← shown only if AC ESS BUS SHED available
    If AC BUS 1 or AC BUS 2 is lost:
   AVOID ADVERSE WEATHER

The conditional lines carry an explicit FCOM caveat, verbatim on both:

"This line is only displayed if AC ESS BUS SHED is available."

Per FCOM PRO-ABN-ELEC. The logic of the whole sheet reads cleanly once the topology is in mind:

ELEC AC ESS BUS FAULT
        │
        ▼
 AC ESS FEED ── ALTN     (transfer the source AC BUS 1 ► AC BUS 2)
        │
   ┌────┴───────────────┐
   │  bus restored?     │
   └──┬──────────────┬──┘
   yes│              │no  ── IF UNSUCCESSFUL
      ▼              ▼
 AC ESS BUS ALTN   reroute the captain side to "No.3" feeds:
 status carried     CAPT EFIS DMC ........ 3   (DMC 1 is INOP)
 to landing         AIR DATA SWTG .. CAPT ON 3 (ADR 1 is INOP)
                    └─ both only if the SHED segment is still alive

Why reroute, rather than keep trying? A successful ALTN means AC BUS 2 is now feeding the essential bus through the feed transfer, and everything comes back. Unsuccessful means the essential bus body cannot be revived at all — exactly the feeder/contactor fault the AMM legend warns about, where moving the source upstream changes nothing downstream. At that point the procedure abandons the bus and borrows the captain's instruments onto resources that do not depend on the dead essential bus: the captain's EFIS is switched to DMC 3, and captain air data is switched to ADR 3 (AIR DATA SWTG ... CAPT ON 3). These two reroute lines are themselves conditional — they are only shown while AC ESS BUS SHED is available, because the equipment they hand the captain onto sits downstream of the sheddable segment; if that segment is gone too, there is nothing to reroute onto and the lines are suppressed.

The last conditional line, "If AC BUS 1 or AC BUS 2 is lost: AVOID ADVERSE WEATHER", ties the procedure to the loss of terrain awareness in the INOP list below (GPWS + GPWS TERR are both lost), and only appears when a main AC bus has also gone.

What the FAULT takes away

The STATUS/INOP page for AC ESS BUS FAULT is the larger of the two lists — it strips the captain's whole side plus several safety systems. Verbatim from the FCOM INOP block:

Per FCOM PRO-ABN-ELEC. Three entries deserve to be read with weight, not skimmed:

• PAX OXY MASKS (auto + manual) hang on AC ESS. With AC ESS lost, both the automatic and the manual cockpit release paths for the cabin masks are gone — the backup release means belong to the oxygen system (ATA 35), not to ATA 24.
• GPWS + GPWS TERR — the whole terrain-awareness function is down on this side, which is exactly why the procedure adds AVOID ADVERSE WEATHER when a main bus is also lost.
• SDAC 1 + FWC 1 — half of the alerting/monitoring chain is offline, so the surviving ECAM alerting is itself degraded.

[!warning]- Counter-intuitive: with AC ESS lost, the passenger oxygen masks lose their normal release

It is tempting to assume the cabin masks "have their own circuit" and will drop regardless. They do not — PAX OXY MASKS (auto + manual) are listed INOP on the AC ESS BUS FAULT page, so during an AC ESS loss the two normal release paths are both unavailable; the backup release is an oxygen-system (ATA 35) function. Treat a depressurisation overlaid on an AC ESS loss as a case where mask deployment cannot be taken for granted.

The list does not mean every item is necessarily gone. Both AC ESS cautions carry the same caveat, verbatim:

"The warning may be caused by a sub BUS failure. Consequently, only a part of the above-listed systems may be lost."

Per FCOM PRO-ABN-ELEC. This is the same "discounted reading" met on the AC bus faults page — handle the INOP list against what is actually lost on the SD, not against the full printed list.

3. ELEC AC ESS BUS SHED — judge the nature first, then act

This caution triggers when the sheddable segment (AC SHED ESS / 401XP) is not supplied. Before doing anything, decide why it is unpowered, because the answer changes whether it is even a fault at all:

• In an ELEC EMER CONFIG scenario — running on the RAT, or on batteries only — the segment is legitimately shed by contactor 16XH as a designed part of the degraded configuration (see Emergency Generator). It is not handled in isolation; it is one symptom of the bigger picture.
• A standalone SHED caution, with the main network otherwise healthy, is a genuine sub-segment fault — and that is what this procedure is for.

The procedure is two switch selections:

ELEC AC ESS BUS SHED
   AIR DATA SWTG .................... CAPT ON 3      (CAPT AOA is on the SHED segment)
   ATC/XPDR ............................. SYS 2      (ATC 1 is on the SHED segment)

Both selections follow directly from the INOP list: with CAPT AOA lost the captain air data switches to source 3, and with ATC 1 lost the transponder switches to system 2.

What the SHED takes away

Verbatim from the FCOM INOP block:

Per FCOM PRO-ABN-ELEC. Note that ESS TR + AFT XFR appear INOP on both the FAULT and the SHED pages — the ESS TR / aft-transfer path is taken down either way (its detail belongs to Transformer-Rectifiers and DC Network Transfer). Note too that CVR is on this segment, so a standalone SHED has already stopped the cockpit voice recording.

The page carries two notes. The first is the same sub-BUS caveat as the FAULT page (only part of the list may actually be lost). The second is the gem of the topic:

"AP2 pb light is lost. AP2 engagement can be checked on the FMA."

Per FCOM PRO-ABN-ELEC. The AP2 pushbutton light circuit hangs on the sheddable segment — so the light goes dark, but the autopilot itself is unaffected. A dark AP2 light does not mean AP2 is unusable; its true engagement status is read on the FMA. This is the cleanest example in the chapter of "the indication chain is broken while the control chain is intact" — do not infer loss of function from loss of a legend.

[!warning]- Counter-intuitive: a dark AP2 light is not a dead AP2

On AC ESS BUS SHED the AP2 pushbutton light is INOP because its lamp circuit sits on the sheddable segment. The autopilot function is not on that circuit. Per FCOM, "AP2 pb light is lost. AP2 engagement can be checked on the FMA." Read the FMA for the real engagement state — never conclude "AP2 has failed" from a dark button. Likewise, the CVR being INOP on this page means the recording has stopped even though nothing in the cockpit "looks" different.

4. FAULT vs SHED at a glance

5. Flight-deck scenarios

1. AC ESS BUS FAULT, ALTN restores it. Select AC ESS FEED ALTN; the FAULT legend extinguishes, the ALTN legend (white) comes on, and the SD confirms the essential bus alive off AC BUS 2 (the confirmation chain is in AC ESS Feed Transfer). The flight continues normally carrying the ELEC AC ESS BUS ALTN status to the end — a reminder, not a fault.
2. AC ESS BUS FAULT, ALTN unsuccessful. The captain's screens stay dark — execute CAPT EFIS DMC 3 and AIR DATA SWTG CAPT ON 3 to borrow the captain side onto the number-3 resources, and hold in mind the safety losses on the INOP list: GPWS / GPWS TERR (terrain awareness — weight the AVOID ADVERSE WEATHER caveat heavily) and PAX OXY MASKS. The dead essential bus is not coming back in flight.
3. A standalone AC ESS BUS SHED in cruise. With the main network healthy this is not EMER CONFIG — it is a sub-segment fault. Do the two switch selections (CAPT ON 3, XPDR SYS 2), and note two quiet consequences: the CVR has stopped recording, and the AP2 light is dark — check AP2 engagement on the FMA, not on the button.

[!warning]- Common misconceptions — predict, then check

Read each statement, decide true or false, then check the truth in brackets.

1. "The AP2 light went out on AC ESS BUS SHED, so AP2 cannot be used." — False. The AP2 pushbutton lamp circuit is on the sheddable segment, so the light is lost, but the autopilot is not — "AP2 engagement can be checked on the FMA." Indication chain broken, control chain intact.
2. "A standalone AC ESS BUS SHED in cruise means we are in the RAT/battery scenario and have entered EMER CONFIG." — False. With the main network healthy, a SHED on its own is a sub-segment fault, not the designed shed of EMER CONFIG; only in the RAT / battery-only scenario is the segment legitimately shed by 16XH.
3. "AC ESS being lost is no big deal — the passenger oxygen masks will still drop on their own." — False. PAX OXY MASKS (auto + manual) are on AC ESS; during an AC ESS loss both normal release paths are gone, and the backup release is an ATA 35 function.
4. "If the ALTN selection in the FAULT procedure is unsuccessful, try again to revive the essential bus." — False. Unsuccessful means the bus body cannot be revived; the procedure deliberately stops trying and reroutes the captain's displays to DMC 3 and air data to ADR 3.
5. "Once an AC ESS BUS FAULT or SHED appears, every system on the INOP list is gone." — Conditional. Both pages carry the sub-BUS caveat: "only a part of the above-listed systems may be lost." Handle against the actual SD INOP, as on the AC bus faults page.

Self-test

[!note]- Q1. In the FAULT procedure, what is done after "IF UNSUCCESSFUL", and why?

The essential bus body cannot be revived (a feeder/contactor fault, not an AC BUS 1 loss), so the procedure stops trying to bring the bus back and instead reroutes the captain side onto independent resources: CAPT EFIS DMC ... 3 (captain EFIS onto DMC 3, since DMC 1/EFIS DMC 1 is INOP) and AIR DATA SWTG ... CAPT ON 3 (captain air data onto ADR 3, since ADR 1 / CAPT PITOT is INOP). Both lines are conditional — displayed only while AC ESS BUS SHED is still available, because the equipment they reroute onto sits downstream of the sheddable segment.

[!note]- Q2. Which two FAULT procedure lines are shown only under a condition, and what is it?

CAPT EFIS DMC ... 3 and AIR DATA SWTG ... CAPT ON 3. Per FCOM, each carries "This line is only displayed if AC ESS BUS SHED is available." If the sheddable segment is also gone there is nothing to reroute the captain side onto, so the lines are suppressed.

[!note]- Q3. What does the AP2 note on the SHED page mean, and what principle does it teach?

"AP2 pb light is lost. AP2 engagement can be checked on the FMA." The AP2 pushbutton lamp circuit is on the sheddable segment, so the light goes dark while the autopilot function is unaffected — engagement is confirmed on the FMA. The principle: a lost indication is not a lost function; the indication chain can break while the control chain stays intact.

[!note]- Q4. How do you judge a standalone AC ESS BUS SHED, and what are its quiet consequences?

With the main network healthy, a SHED on its own is a sub-segment fault — not the EMER CONFIG shed performed by 16XH on the RAT / battery-only scenario. Do the two procedure selections: AIR DATA SWTG CAPT ON 3 (CAPT AOA INOP) and ATC/XPDR SYS 2 (ATC 1 INOP). Quiet consequences from the INOP list: the CVR has stopped recording and the AP2 light is dark (check the FMA).

[!note]- Q5. Name the three FAULT INOP entries that carry real operational weight, and the caveat that limits all of them.

PAX OXY MASKS (auto + manual) — both normal mask-release paths gone (backup is ATA 35); GPWS + GPWS TERR — terrain awareness lost, hence AVOID ADVERSE WEATHER when a main bus is also lost; SDAC 1 + FWC 1 — half the alerting/monitoring chain offline. The limiting caveat on both AC ESS pages: "The warning may be caused by a sub BUS failure. Consequently, only a part of the above-listed systems may be lost."

Key takeaways

References

Per FCOM PRO-ABN-ELEC (ELEC AC ESS BUS FAULT triggering condition, procedure with the IF UNSUCCESSFUL reroute and the two SHED-conditional lines, AVOID ADVERSE WEATHER, full INOP list and the sub-BUS note; ELEC AC ESS BUS SHED procedure, INOP list, the sub-BUS note and the AP2-FMA note; ELEC AC ESS BUS ALTN annunciation — "abnormally supplied by AC 2 BUS"); AMM 24-00-00 D/O (the AC ESS FEED pushbutton FAULT-legend meaning and AC BUS 1 → AC BUS 2 transfer); cross-references to Emergency Generator (16XH shedding the SHED segment in EMER CONFIG), AC ESS Feed Transfer (the transfer machinery and confirmation chain), AC Bus Faults (the sub-BUS discounted reading), DC ESS Bus Fault and SHED (the DC-side mirror), and Emergency Electrical Configuration (the ESS family in the full emergency picture). The FAULT-vs-SHED comparison table and the "judge the nature first" framing 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.