AC Distribution and Busbars

The earlier articles in this chapter kept introducing busbar numbers in passing — 9XP, 901XP, 905XPB, 107XP, 401XP — without ever laying the family tree on the table. This article does exactly that. It sorts the AC distribution side into three layers (generation → main busbars → sub-busbars → loads), names the family relationships between parent buses and their sub-segments, and explains the single device that sits in the middle of it all: the RCCB (Remote Control Circuit Breaker).

It is also where several loose ends from earlier articles are tied off: the relationship between 9XP and 901XP (raised under AC ESS Feed and Transfer), the meaning of the "-B" suffix on 905XP (raised under Static Inverter), and the exact shed list of the COMMERCIAL pushbutton (flagged but not unpacked in the Electrical System Overview). By the end you should be able to answer five things: what the three-layer distribution model is and what device separates a main bus from a sub-bus; the parent/child structure of the AC ESS family (9XP / 901XP / 931XPA / 905XPB / 4XP / 401XP); what an RCCB is and how it differs from an ordinary breaker and from a contactor; exactly which segments the COMMERCIAL pushbutton sheds; and which three transformers produce the 26 VAC sub-rail.

A scope note first. The bus-by-bus, load-by-load wiring (the seventy-plus schematics in the AMM/ASM wiring set) is maintenance reference, not crew material — this article works at the family-structure level, not the wiring-diagram level. Configuration differences between the main passenger fleet and the freighters are not opened up here.

1. The three-layer distribution model

Read the AC distribution side as three stacked layers. Power produced and converted at the top is routed down through devices that get progressively more local:

┌──────────────────────────────────────────────────────────────────┐
│ LAYER 1  —  Generation / conversion                                │
│   GEN 1 · GEN 2 · APU GEN · EXT A · EXT B · EMER GEN · STAT INV     │
│   (the whole story of articles 01–13)                              │
└─────────────────────────────────┬──────────────────────────────────┘
                                   │  feeds
                                   ▼
┌──────────────────────────────────────────────────────────────────┐
│ LAYER 2  —  Main busbars      1XP · 2XP · 9XP · 4XP                 │
└─────────────────────────────────┬──────────────────────────────────┘
                                   │  via RCCB (remote-control C/B),
                                   │  or contactor / relay
                                   │  ── opened/closed by the ECMUs ──
                                   ▼
┌──────────────────────────────────────────────────────────────────┐
│ LAYER 3  —  Sub-busbars                                            │
│   107XP · 901XP · 905XPB · 931XPA · 115XP · 208XP · …               │
└─────────────────────────────────┬──────────────────────────────────┘
                                   │  via ordinary circuit breakers (C/B)
                                   │  ── status reported by the CBMU ──
                                   ▼
┌──────────────────────────────────────────────────────────────────┐
│ LOADS                                                              │
└──────────────────────────────────────────────────────────────────┘

The single most useful idea in this article is the boundary between Layer 2 and Layer 3. Everything that "manages power in whole segments" — load shedding, commercial isolation, the AC ESS feed transfer — acts on that boundary, through RCCBs, contactors or relays commanded by the ECMUs (see ECMU and Contactor Management). Everything below it, at the load level, is protected by ordinary circuit breakers whose status is reported by the CBMU (see Circuit Breakers and the CBMU).

Physically, the main busbars live in the main electrical power centre (panels 710VU/715VU), while the sub-busbars are distributed across the distribution boards (721VU/722VU/742VU and the like) and the cabin 5001VE-series panels — the other face of the circuit-breaker-location detail quoted in the overview article.

2. The AC busbar family

2.1 The numbering convention

Before the table, fix the numbering scheme — once you hold it, every bus number in the chapter decodes itself:

• XP = AC, PP = DC. (You have already seen the DC side — 1PP, 3PP, 4PP — in DC Network and Transfer.)
• A short number is the parent bus body; a three-digit number with a hundreds digit is a sub-segment. So 9XP is the parent, 901XP is one of its sub-segments.
• The leading digit groups the family: 9-series = ESS, 4-series = SHED ESS, and 1-/2-leading three-digit numbers are the side-1 / side-2 service or sheddable segments.
• An -A / -B suffix marks a further sub-segmentation of a sub-bus (901XP, 905XPB, 931XPA).

This is why the AMM appears to call the same thing by different names in different sections: under one task it writes "AC ESS buses (4XP, 9XP)", under another "AC ESS BUS (901XP)". These are not different buses — they are different levels of the same family, named by whichever level the local context needs.

2.2 The family table

The RCCB and contactor FIN numbers above are read off the ASM schematic set; the parent/child structure and the verbatim that anchors it come from the AMM.

The decisive verbatim for the 9XP family is AMM 24-52-00:

"The AC essential busbar (9XP) supplies three-phase 115 VAC/400 Hz power directly to sub-busbar 901XP. The AC essential busbar (9XP) supplies single-phase 115 VAC/400 Hz power: - To sub-busbar 931XPA (26 VAC/400 Hz) through the 115/26 VAC transformer (3XH) - To sub-busbar 905XPB through relay 1XH. Sub-busbar 905XPB is shed in emergency configuration in flight."

That single paragraph closes three cross-article questions at once. As a tree:

                  ┌────────────────────────┐
                  │   AC ESS BUS  (9XP)     │
                  └──┬──────────┬────────┬──┘
                     │          │        │
                     ▼          ▼        ▼
               ┌──────────┐ ┌────────┐ ┌──────────┐
               │  901XP   │ │ 931XPA │ │  905XPB  │
               │ AC ESS   │ │ 26 VAC │ │ AC ESS   │
               │  core    │ │  ESS   │ │  GND     │
               └──────────┘ └────────┘ └──────────┘
               3-phase,     via 3XH     via relay 1XH;
               direct       (115/26     SHED in EMER
                            VAC xfmr)   CONFIG in flight

[!warning]- "AC ESS GND has 'ESS' in its name, so it must be powered in flight."

Incorrect. 905XPB (AC ESS GND) is shed in emergency configuration in flight — it is fed through relay 1XH and survives only on the ground on batteries. The "ESS" in its name is a family label, not a guarantee of in-flight power. It is the physical bus behind the "ground-only supply" point made under Static Inverter.

3. The RCCB — the distribution layer's "electric circuit breaker"

The device sitting on the Layer-2/Layer-3 boundary for most segment switching is the RCCB. It is the hybrid of a circuit breaker and a contactor. AMM 24-50-00 gives its two functions:

"The Remote control Circuit Breaker (RCCB) is used to fulfill the following functions: - electrical line protection (tripping function), - electrical load switching (switching function)."

So it protects like a circuit breaker (a temperature-controlled bimetal strip trips it on overload) and it switches like a contactor (it accepts a remote electrical command). The full loading particulars are worth reading in full rather than summarising — they are the article's skeleton:

"The loading particulars of the RCCB are: - three pole, trip-free circuit breaker, - monostable type remote control by electrical signal, - 35 A and 50 A rating on 115 VAC, 400 Hz, - temperature-controlled bimetal strip, - control locking in case of tripping, - RESET pushbutton switch on the front panel resetting the RCCB into operation after tripping, - indicating: visual indication on the front face of the RCCB: status of the main contacts: open or closed message, in case of tripping, RESET pushbutton switch; electrical indication: status of the main contacts by two auxiliary reversing switches, in case of tripping, TRIP signal."

Note the specifics: it is three-pole, trip-free; monostable, remote-controlled by an electrical signal; rated at 35 A and 50 A on 115 VAC, 400 Hz; tripped by a temperature-controlled bimetal strip; its control is locked when it trips; a front-panel RESET pushbutton restores it to operation after a trip; and it reports two ways — a visual indication on its front face and an electrical indication, the latter sending a TRIP signal on a trip.

The RESET pushbutton has two stable positions, and the operation paragraph spells them out:

"(a) RESET pushbutton switch pushed (IN position) When the RESET pushbutton switch is pushed, the electrical control is authorized. On the front face of the RCCB, the visual indication shows the CLOSE or OPEN message in function of main contact status. (b) RESET pushbutton switch released (OUT position) The RESET pushbutton switch is released if there is an overload on the main circuit. In this case the RCCB cannot be electrically controlled. The main contacts are locked in the OPEN status. The TRIP signal is generated to the CBMU for monitoring. On the front face of the RCCB, the visual indication shows the OPEN message. Push the RESET pushbutton switch to reactivate the RCCB control."

Three consequences a pilot should carry:

• The TRIP signal goes to the CBMU — so an RCCB trip is visible on the ECAM C/B page, one of the CBMU's data sources (see Circuit Breakers and the CBMU).
• After a trip, the electrical control is dead and the main contacts are locked open. You cannot command it shut again from anywhere — not from the flight deck, not via the ECMU.
• The reset is a physical action at the device. The RESET pushbutton is on the RCCB body in the avionics compartment / distribution boards. In flight it is unreachable.

[!warning]- "An RCCB tripped, so the crew can just reset it from the flight deck."

Incorrect. The overload latch is on the device body, the RESET pushbutton is in the avionics compartment / distribution boards, and a tripped RCCB cannot be electrically commanded — the ECMU cannot drive it shut either. This is the same rule as the TR latch-open (see Transformer-Rectifiers) and the tripped C/B (article 17): on the A330, the "reset" action almost never lives in the cockpit.

4. The COMMERCIAL pushbutton — one-touch commercial shedding

The COMMERCIAL pushbutton is the crew's master switch for the commercial (passenger-service) load family — the embodiment of the design rule "commercial supply has secondary priority" met in the overview.

4.1 Two views of the same action

Bus view. With the pushbutton ON (pressed in), a relay group drives a set of RCCBs (6XN1 / 7XN1 / 6XN2 / 7XN2 / 5XN / 51XN / 39XN) and contactor 35XN closed, so the commercial sub-buses and the galleys are all supplied. With the pushbutton OFF (released, OFF light on), the same devices open and seven segments are shed — 107XP / 113XP / 115XP / 212XP / 214XP / 208XP / 220XP — plus all galleys, and the SD ELEC AC page shows COMMERCIAL OFF.

Equipment view. FCOM lists the same action by the equipment it removes, and the two lists are the two faces of one switch action:

"OFF : The following equipment is shed: Galleys; Cargo loading system; Electrical service; Escape slide lock mechanism ice protection; Water/waste (drain mast) ice protection; Lavatory and cabin lights; Water heater; In-seat power supply; Passenger entertainment system."

The display priority is also defined verbatim — the galley legend appears by order of priority (1 = highest):

"The following legend appears in white, when applicable, depending on the order of priority (1: highest priority): 1. COMMERCIAL OFF 2. GALLEY SHED 3. GALLEY PARTIALLY SHED."

4.2 The boundary with the GALLEY pushbutton

The GALLEY pushbutton is not simply "the galley-only version" of COMMERCIAL — it sheds more than the galleys:

"OFF : All galleys are shed. Water/Waste (drain mast) ice protection is lost. The electrical supply of the heating floor panels is shed."

So GALLEY OFF = galleys + drain-mast (water/waste) ice protection + heating floor panels (three items), whereas COMMERCIAL OFF reaches wider (the nine equipment classes / seven bus segments above). The two switches overlap on the galleys but differ in span. Crucially, neither one touches the ESS family or any flight equipment — both act only on the commercial side.

4.3 The pilot-usage boundary (QRH)

A precise point worth holding: COMMERCIAL OFF is not the response to any electrical smoke. In the QRH SMOKE/FUMES/AVNCS SMOKE procedure it appears only in the "If CABIN EQPT smoke/fumes suspected" branch (QRH 24.01B), as a cabin-equipment isolation step alongside PAX SYS OFF / EMER EXIT LT and similar. In other words it is a directed isolation for "smoke suspected to come from cabin commercial equipment", not a general smoke switch. Press it and you cut the seven commercial segments / nine equipment classes — the cabin goes dark in places (lavatories, entertainment, water heater) — but the flight deck and the ESS family feel nothing.

5. The 26 VAC sub-rail — three transformers

A small 26 VAC / 400 Hz rail is produced by three 115/26 V transformers, one hanging off each of the three relevant parents:

The architecture is "follow the parent": if a main bus is lost, its 26 V segment dies with it; nothing is gained by suspecting the transformer first. The ESS 26 V segment (931XPA) inherits the full ESS fall-back set — which is why 931XP shows power throughout the emergency configurations in the overview's distribution table.

[!warning]- Three sourcing boundaries on the 26 VAC rail (kept honest against the primary library)

1. The 26 VAC load population is not stated. The primary D/O only says the 115/26 VAC transformer feeds a 26 VAC/400 Hz sub-busbar; it does not name the specific users. "Instrument lighting" is a reasonable engineering inference (and is how the overview phrases it) but is not written as fact here.
2. Naming layer. The AMM names these units "115/26 VAC transformer (131XPC and 232XPA)", while the ASM schematics label the transformers 21XN1 / 21XN2 and treat 131XP / 232XP as their output bus names — one object, two coordinate systems. This article uses the AMM names; the ASM FINs stay in the wiring-diagram layer.
3. Fleet configuration. The main passenger fleet uses 131XPC / 232XPA; a few older passenger aircraft and the freighters use 131XPA / 232XPA. This article takes the main passenger group and does not open up the configuration spread. (A maintenance-layer ESS 26 VAC segment, 931XPB via a second autotransformer 9XH, exists in the parts index but is not part of the family proper and is not drawn here.)

6. Main-bus alerts and the AC ESS FEED supply chain

6.1 Main-bus alerts

A lost main bus raises its own caution. Loss of 1XP → AC BUS 1 FAULT; loss of 2XP → AC BUS 2 FAULT. Both are Level 2 (MASTER CAUT + single chime + automatic ELEC AC page); the handling procedure is treated under AC Bus Faults. The authoritative source for the trigger and level is FCOM PRO-ABN-ELEC:

"L2 This alert triggers when the AC 1 busbar is not supplied." "L2 This alert triggers when the AC 2 busbar is not supplied."

Sub-busbars have no independent alert of their own. A lost sub-bus surfaces either as an upstream main-bus caution or as an equipment-level effect on the various ECAM system pages. That asymmetry — main bodies are alarmed, sub-segments are not — is the "parent vs sub-segment" distinction expressed in the alert design.

6.2 The AC ESS FEED supply chain

How the 9XP family is normally fed, and where it transfers when the feed is lost, is given by the AC ESS FEED pushbutton description (FCOM DSC-24-20):

"Normal : The AC ESS BUS is supplied from AC BUS 1. It is automatically supplied by the AC BUS 2 when the AC BUS 1 is lost. ALTN : The AC ESS BUS is supplied from AC BUS 2. … Note: In case of total loss of main generators the AC ESS BUS is automatically supplied by the emergency generator or by the static inverter if the emergency generator is not available."

This strings the 9XP family's defences into one chain:

AC BUS 1  ──(lost)──►  AC BUS 2  ──(all main GEN lost)──►  EMER GEN
                                                              │
                                                  (EMER GEN unavailable)
                                                              ▼
                                                        static inverter

The feed transfer itself is covered under AC ESS Feed and Transfer and the static-inverter end under Static Inverter; the verbatim above is the authoritative overview text that ties them together.

7. The FCOM distribution table

The overview article quoted the distribution table as a skeleton; with articles 01–13 behind you, every row should now be derivable.

[!warning]- Reading the FCOM distribution table — two presentation traps

1. The FCOM table uses functional bus names only (AC BUS 1 / AC ESS BUS / AC ESS GND / AC ESS SHED / AC LAND REC) — there are no numeric bus numbers anywhere in it. The numbers 9XP / 4XP / 901XP / 905XP / 903XP are AMM/ASM-layer designations; they appear in the table below only as bracketed aids for cross-article matching, not because FCOM prints them.
2. The FCOM table only covers EMER CONFIG (3 rows) + ON GROUND (2 rows) — there is no "normal" row. The "normal" row below is a teaching supplement, derived from the AC ESS FEED logic of §6.2, and is italicised to mark it as such.

Two footnote classes share the same number "(1)" on the same FCOM page and are very easy to confuse. The page-top footnotes govern TR / generator interlocks:

"(1) Lost in case of overcurrent on the faulty TR." "(2) In case of differential protection failure : • The affected generator is not replaced ; • The associated TR is switched off."

Footnote (1) here is the 4PC interlock of Transformer-Rectifiers; footnote (2) is the double cost of the differential protection of GCU and AC Generation Control — the affected generator is not replaced and the TR it feeds is switched off. The AC sub-table carries its own local footnote, also numbered (1):

"(1) Supplied when LAND RECOVERY pushbutton is at ON."

That local footnote is the basis for the (1) on the two EMER GEN rows in the table above: in the EMER GEN configurations, AC LAND REC is supplied only with the LAND RECOVERY pushbutton at ON, whereas in the battery-only row the STAT INV supplies it regardless of the pushbutton position.

[!warning]- The opposite pushbutton dependence in the AC LAND REC column

The two EMER GEN rows depend on the LAND RECOVERY pushbutton being ON to feed AC LAND REC; the battery-only row feeds it through the static inverter regardless of pushbutton position. The dependence is reversed between the two scenarios — the key counter-intuitive point of that column.

8. Flight-deck application

8.1 Four quick scenarios

1. An ECAM system page shows a cluster of equipment lost, but the ELEC page main buses are all green. Think three-layer model: what is lost may be a sub-busbar (an RCCB has tripped), not a main bus. Cross-check on the C/B page for a TRIP record — the RCCB's TRIP signal goes to the CBMU.
2. The QRH smoke procedure's "cabin equipment suspected" branch sends you to COMMERCIAL OFF. Recognise it as the directed isolation step of SMOKE/FUMES 24.01B, not a general smoke switch. Have the list in mind — seven commercial segments and all galleys cut instantly; the cabin goes dark in places (lavatories / entertainment / water heater) but the flight deck and ESS feel nothing. Recovery is simply pressing the pushbutton back in (re-closing that RCCB group).
3. An RCCB trips in flight — what can the crew do? Nothing, in flight: the RESET is on the device body in the avionics compartment / distribution boards. This is the same law as the TR latch-open (article 10) and the tripped C/B (article 17): the A330's reset actions almost never live in the cockpit.
4. Checking an abnormal 26 V load. Look at the parent first — 131XP follows 1XP, 232XP follows 2XP, 931XPA follows 9XP. Only with the parent alive does the transformer (131XPC / 232XPA / 3XH) itself become a suspect.

8.2 Misconceptions to clear

[!warning]- "If an ECAM system page shows equipment lost, the ELEC page must also flag a fault."

Incorrect. Sub-busbars have no independent alert. A tripped RCCB can drop a sub-bus while the main buses stay all green. Cross-check the C/B page for the TRIP record; only the main-bus level produces AC BUS 1 / 2 FAULT.

[!warning]- "COMMERCIAL OFF and GALLEY OFF are much the same — both cut the galleys."

Half right. COMMERCIAL OFF sheds seven commercial segments + all galleys (107XP / 113XP / 115XP / 212XP / 214XP / 208XP / 220XP, covering cargo loading, electrical service, lavatory/cabin lights, in-seat power, entertainment and more); GALLEY OFF is also more than the galleys — by FCOM it is galleys + drain-mast ice protection + heating floor panels — but it spans less than COMMERCIAL. Both leave the ESS family and flight equipment untouched.

[!warning]- "The 26 V instrument lighting is acting up — check the 26 V transformer first."

Wrong order. The 26 V segments follow their parent: 131XP follows 1XP, 232XP follows 2XP, 931XPA follows 9XP. Suspect the transformer (131XPC / 232XPA / 3XH) only once the parent bus is confirmed alive; a lost main bus takes its 26 V segment with it, and that is not the transformer's doing.

Self-test

[!note]- Q1. What is the three-layer distribution model, and what device sits between a main bus and a sub-bus?

Layer 1 = generation/conversion → main busbars (1XP / 2XP / 9XP / 4XP). Layer 2 → Layer 3: main bus to sub-bus, via an RCCB (ECMU-commanded) or a contactor / relay. Layer 3 → loads, via ordinary circuit breakers (CBMU-monitored). All "whole-segment" management — load shedding, commercial isolation, the AC ESS feed transfer — acts on the Layer-2/Layer-3 boundary.

[!note]- Q2. Describe the parent/child structure of the AC ESS family.

9XP is the AC ESS parent: it feeds 901XP directly (the three-phase core), feeds 931XPA (26 VAC) through transformer 3XH, and feeds 905XPB (AC ESS GND) through relay 1XH — and 905XPB is shed in emergency configuration in flight, surviving only on the ground on batteries. 4XP is the AC ESS SHED parent (sub-segment 401XP), shed wholesale by 16XH when on RAT / batteries only. The AMM names the same family by parent or by sub-segment depending on context.

[!note]- Q3. How does an RCCB differ from an ordinary circuit breaker and from a contactor?

It is the hybrid of both. Its protection function works like a circuit breaker (temperature-controlled bimetal strip, 35 A / 50 A ratings, trip-free, three-pole); its switching function works like a contactor (monostable, remote-controlled by an ECMU electrical signal). After a trip the RESET pushbutton releases (OUT), the main contacts are locked open, the electrical control is dead, and a TRIP signal is generated to the CBMU. Reset is a physical push at the device body — unreachable in flight.

[!note]- Q4. Exactly what does COMMERCIAL pb OFF shed, and what stays untouched?

Bus view: seven segments (107XP / 113XP / 115XP / 212XP / 214XP / 208XP / 220XP) plus all galleys, by opening the RCCB group + contactor 35XN. Equipment view (FCOM): galleys, cargo loading system, electrical service, escape-slide-lock and water/waste ice protection, lavatory/cabin lights, water heater, in-seat power, passenger entertainment. The SD shows COMMERCIAL OFF (the highest-priority galley legend). The ESS family and flight equipment are untouched.

[!note]- Q5. Which three transformers produce the 26 VAC rail, and what is the diagnostic rule?

131XPC (1XP → 131XP, left), 232XPA (2XP → 232XP, right), 3XH (9XP → 931XPA, ESS). Each 26 V segment follows its parent bus — a lost main bus takes its 26 V segment with it. 931XPA inherits the full ESS fall-back set (powered throughout the emergency configurations). Suspect the transformer only once the parent is confirmed alive.

Key takeaways

References

Per AMM 24-50-00 (three-layer distribution principle; full RCCB description — two functions, loading particulars, RESET IN/OUT operation, TRIP signal to the CBMU), AMM 24-51-00 (main-bus dual supply; 26 V transformers 131XPC / 232XPA; sub-bus-to-RCCB mapping; COMMERCIAL pb shed group), AMM 24-52-00 (the AC ESS family verbatim — 901XP direct, 3XH → 931XPA, 1XH → 905XPB shed in emergency configuration in flight); FCOM DSC-24-20 (COMMERCIAL pb equipment list, GALLEY pb OFF, galley-indication priority, AC ESS FEED supply chain), FCOM DSC-24-10-30-40 (distribution table, page-top footnotes (1)/(2), AC sub-table LAND RECOVERY footnote), FCOM DSC-24-10-30-30 (AC ESS GND / AC LAND REC ground vs battery-only logic), FCOM PRO-ABN-ELEC (AC BUS 1 / 2 FAULT triggers and L2 level); QRH 24.01B (COMMERCIAL OFF as the cabin-equipment isolation step in SMOKE/FUMES). The numbering convention of §2.1, the bus-view/equipment-view comparison of §4, the italicised "normal" row of §7, and the city-water analogy are integrative synthesis built on the above, not verbatim source. The RCCB and contactor FIN numbers are read from the ASM schematic set and stay in the wiring-diagram layer.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.