Network Priority and Normal Supply

The hardware is now fully named: the four AC sources and the static inverter (overview, IDG through emergency generator), and the box that switches the contactors (ECMU). This article strings them into operation: the precise order in which five sources queue up for two AC buses, how the network flows through a normal day on the ground and in flight, and who supplies whom when several sources are present at once.

This is the article that reads the FCOM operations chapter (DSC-24-10-30) as a set of rules you can run forward. By the end you should be able to answer five questions:

1. Given any combination of sources (say "APU + EXT A + EXT B all connected"), who feeds AC BUS 1 and AC BUS 2?
2. How do you replace the FCOM priority "tongue-twister" with one table and one principle?
3. What is the single exception to the no-parallel rule, and what hardware enforces it?
4. What two conditions close the BUS TIE pushbutton in AUTO, and what is OFF for?
5. What does the ELEC AC/DC page look like in normal flight, and which "white standby" symbols are present all the time?

What stays with neighbouring articles: the mechanism of the transfer instant (No-Break Power Transfer); failure reconfiguration (Automatic Reconfiguration); the maintenance bus (Ground Service / Maintenance Bus); the full field-by-field ECAM semantics (ECAM ELEC Page).

1. The per-busbar priority ladder

Each AC bus has a "queue" of sources ranked by priority. The AMM states the two queues as a single compact line:

"1XP = IDG 1/APU GEN/EXT PWR B/EXT PWR A/IDG 2, 2XP = IDG 2/EXT PWR A/APU GEN/EXT PWR B/IDG 1."

Per AMM 24-22-00. So 1XP (AC BUS 1) ranks IDG 1 > APU GEN > EXT B > EXT A > IDG 2; 2XP (AC BUS 2) ranks IDG 2 > EXT A > APU GEN > EXT B > IDG 1. This single table is the main source; the same ranking is reprinted "for reference only" in the transfer-circuit and external-power sections and in the NBPT description, with identical wording and no busbar mismatch.

        AC BUS 1 (1XP) queue            AC BUS 2 (2XP) queue
        +------------------+            +------------------+
   1    |  IDG 1 (own gen) |       1    |  IDG 2 (own gen) |
   2    |  APU GEN         |       2    |  EXT PWR A       |
   3    |  EXT PWR B       |       3    |  APU GEN         |
   4    |  EXT PWR A       |       4    |  EXT PWR B       |
   5    |  IDG 2 (relief)  |       5    |  IDG 1 (relief)  |
        +------------------+            +------------------+
 Principle: own-side generator > "middle sources" by physical
            proximity > other-side generator (relief)
 Physical proximity (overview read-out): APU/EXT B tap the LEFT
            corridor segment, EXT A taps the RIGHT segment.

FCOM gives the same logic in two forms — both are just this table expanded sentence by sentence (memorise the table, not the tongue-twister).

Form 1 — the EXT B pushbutton Note (five priority lines):

"The APU generator has priority over external power (A and B) for AC BUS 1. The external power A has priority over the APU generator for AC BUS 2. The APU generator has priority over external power B for AC BUS 2. The engine generators have priority over the external power or APU. The external power B has priority over external power A for AC BUS 1."

Per FCOM DSC-24-20.

Form 2 — the operations narrative:

"Each AC BUS is supplied in priority order by: the corresponding engine generator. the APU generator or the external power A (if both are connected, the APU generator has priority for the left side bars, and the external power has priority for the right side bars). the external power B (if both external powers are connected, B has priority for the left side bars and A has priority for the right side bars). the other side engine generator."

Per FCOM DSC-24-10-30-10. The physical root of the table comes from the overview read-out of the system diagram: the EXT B contactor is physically wired into the left corridor segment (next to the APU), EXT A into the right segment — whoever is nearer needs to cross fewer contactors, so it ranks higher. The "middle source" priority is just proximity made into a rule.

A teaching image that holds the whole table: think of a hospital triage rule. Each consulting room (bus) calls its own resident specialist first (own-side IDG); if the specialist is out, it calls the nearest general doctor (the middle source by proximity — the left room calls the APU first, the right room calls EXT A first); only when nobody is left does it borrow the specialist from the opposite room (the other-side IDG, the rank-5 relief). The hall announcer only ever lets one doctor into one room at a time (no parallel), and the few seconds of overlap during a shift change (NBPT) must be timed by the head nurse (the ECMU).

2. The three operating rules

FCOM compresses the entire operating philosophy into three sentences:

"The APU generator or an external power may supply all the network. One generators can supply all the network (with galley shedding in case of overload detection). The generators cannot be connected in parallel (except on ground during No Break Power Transfers)."

Per FCOM DSC-24-10-30-10. Three sentences, three rules:

1. Any single source can carry the whole network. This is the premise that makes the priority table meaningful — a low queue rank does not mean "cannot carry the load", only "yields to a higher-ranked source if one is present".
2. What it cannot carry is solved by shedding galleys — the "commercial supply has secondary priority" mechanism, applied on overload detection (see the three galley-shed scripts in ECMU).
3. Never in parallel. The only exception is the controlled, sub-second on-ground NBPT instant. The enforcer is the ECMU's Inadvertent Paralleling Trip, backed by the GCU/GAPCU layer (see ECMU and No-Break Power Transfer).

3. Normal supply in flight — the split-bus network

The FCOM IN FLIGHT paragraph is the standard flow, end to end:

"Each engine driven generator supplies its associated AC BUS (1 and 2) via its Generator Line Contactor (GLC 1 and GLC 2). AC BUS 1 normally supplies the AC ESS BUS via a contactor. AC BUS 1 supplies TR 1 which normally supplies DC BUS 1, DC BAT BUS. AC BUS 2 supplies TR 2 which normally supplies DC BUS 2. AC BUS 1 supplies ESS TR which normally supplies DC ESS BUS. The two batteries are connected to the DC BAT BUS if they need charging. When they are fully charged the Battery Charge Limiter disconnects them."

Per FCOM DSC-24-10-30-20. Notice that the whole paragraph never mentions the BTC or SIC — in normal flight the corridor is open, and the two half-networks live separate lives (split-bus operation). The AMM nails the split-bus character down harder than the FCOM omission can:

"In normal flight configuration each IDG supplies its own distribution network via its line contactor (GLC). The two IDGs are never electrically coupled."

Per AMM 24-00-00.

The supply tree (AMM 24-00-00 skeleton): AC BUS 1 raises three DC-network children — AC ESS BUS (through a contactor), TR 1 (→ DC BUS 1 + DC BAT BUS), and the ESS TR (→ DC ESS BUS + DC SHED ESS BUS); AC BUS 2's main network feeds only TR 2 (→ DC BUS 2). So the "left-heavy" asymmetry is 3 : 1 in operation — more lopsided than it first appears, and the reason the captain-side essential family survives longest in degradation.

[!warning]- The APU TR draws on AC BUS 2 but is NOT a "DC-network child" of it

On the ELEC DC page the APU TR is labelled "AC2", and it does electrically draw on AC BUS 2:

"In normal configuration, the transformer rectifier APU TR (1PU3) is supplied from the busbar 2XP."

Per AMM 24-32-00. But the APU TR is not in the same class as TR 2 — the AMM 24-00-00 main supply tree deliberately leaves it out of networks 1 and 2. It is a side branch: it draws on 2XP but serves the APU sub-network (charging the APU battery, starting the APU, feeding the APU-dedicated buses). So state it precisely: "AC BUS 2's main network raises TR 2; a separate side branch, the APU TR, draws on 2XP and serves the APU" (detailed in Transformer-Rectifiers and Batteries and the BCL). The trap to avoid: a missing field in the FCOM IN FLIGHT paragraph does not mean the field does not exist — the diagram and a different AMM chapter may carry the evidence.

4. The multi-source ground scripts

On the ground the same priority table produces four standard pictures.

Script A — one source carries the whole network (APU GEN or one external source):

"Either the APU generator or external power may supply the complete system (with some galley shedding in case of overload)."

Per FCOM DSC-24-10-30-20. The corridor is fully closed (BTC1 + SIC + BTC2 as required), one source feeding both half-networks.

Script B — APU + EXT A both connected. Split by the table: the left network takes the APU (rank 2), the right network takes EXT A (rank 2), and the SIC opens the corridor in the middle. The AMM power note:

"up to a maximum power of 115 KVA for the APU generator and 90 KVA for the EXT PWR A (the technical loads is approximately 50 KVA)."

Per AMM 24-41-00. Note this is a 115 kVA APU ceiling and a 90 kVA EXT A ceiling each carried separately — not 205 kVA summed — against a technical load of only about 50 kVA. The two-source split exists to power full commercial load (galleys, cargo work, ground air conditioning): one 90 kVA source is enough for a "bare" aeroplane, but with all commercial loads on you need two sources, each owning a half-network.

Script C — all three sources connected (APU + EXT A + EXT B). FCOM writes this case explicitly:

"If external power A, external power B and the APU generator supply the entire system, the APU generator has priority over external power B. The display is then the same as the one for APU generator plus external power."

Per FCOM DSC-24-10-30-20. Running the table: the left-network queue holds the APU (rank 2) and EXT B (rank 3) — APU wins; the right network is won by EXT A (rank 2). EXT B sits on the bench the whole time (its AUTO light still on — see External Power). So three sources connected gives exactly the Script B picture, with EXT B as a pure backup.

Script D — ground service only. The maintenance-bus switch hangs the AC/DC GND/FLT buses directly on external power A without waking the whole network (see Ground Service / Maintenance Bus).

5. The BUS TIE pushbutton — the manual corridor master switch

The BUS TIE pushbutton is the crew's manual control over the corridor:

"AUTO: The three BUS TIE contactors open or close automatically according to the priority logic in order to maintain power supply to all AC buses. The three contactors close when: only one engine generator supplies the aircraft, or only the APU generator or single ground power unit supplies the aircraft. OFF: The three BUS TIE contactors open."

Per FCOM DSC-24-20. "The three" = BTC1 + BTC2 + SIC (decoded in GCU and AC Generation Control). The two AUTO-close conditions share one essence: only one source is carrying the whole network, so the corridor must be fully closed. OFF is for fault isolation — for instance, when a bus short is suspected of propagating across the corridor, OFF cuts the two halves fully apart (procedural scenarios in AC Bus Faults and ECMU Fault). Note the bet OFF places: after it, if either side loses its source, you have personally closed the relief path from the opposite side.

Do not confuse the two BUS TIE MEL items:

1. The ELEC BUS TIE OFF ECAM caution is not an MEL matter. It is the status display of a crew-selected OFF, not an equipment failure, so it carries no dispatch restriction (per the operator MEL list, item 24-01-01).
2. A failed OFF indicator light on the BUS TIE pushbutton is Category C, dispatchable with a placard (per the operator MEL, item 24-01-01-14). The first is "the state you selected"; the second is "a lamp burned out" — two different things.

6. The normal ECAM picture

Burn this "normal baseline" into your eyes so the abnormal jumps out. The picture below is a direct read-out of the merged ELEC AC + DC SD page in the normal configuration (the threshold definitions behind each field are in the FCOM text and in ECAM ELEC Page):

+- ELEC DC ----------------------------------------------+
|  DC BAT v        DC APU v                               |
|  BAT1 25V/5A   BAT2 26V/0A   APU BAT 25V/5A             |
|  DC1    DC ESS (STAT INV v)    DC2                      |
|  TR1      ESS TR      TR2       APU TR                  |
|  28V/50A  28V/50A   28V/50A    25V/100A                 |
|  ^AC1     ^AC1       ^AC2       ^AC2   <- feed-source    |
+- ELEC AC ----------------------------------------------+
|        ESS TR^   +-AC ESS-+                             |
|  EMER GEN >          < STAT INV    (two white triangles |
|  TR1^           TR2^   APU TR^      = standby in place)  |
|  AC 1                 AC 2                              |
|  GEN1 32%/116V/400HZ  APU GEN^  GEN2 32%/116V/400HZ     |
|  IDG1 123 C/22 RISE    (white)   IDG2 122 C/22 RISE     |
+--------------------------------------------------------+

Four features worth fixing in the eye:

• The AC ESS feeder branches off AC 1 (the NORM-position visual cue): "AC ESS FEED — 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." Per FCOM DSC-24-20. Select ALTN and the same line redraws from AC 2 (see AC ESS Feed and Transfer).
• APU GEN as plain white text, no box (stopped, standby); EMER GEN ▷ and ◁ STAT INV as two white triangles. The APU GEN symbol is green when the APU generator supplies one or more AC busbars, white otherwise; the EMER GEN / STAT INV triangles are white when their contactors are open (per FCOM DSC-24-20). The two white triangles are not a fault — they are the "fall-back layer present and standing by" marker.
• The IDG field: oil outlet temperature + a RISE value (123 C / 22 RISE on the diagram). RISE is a real ELEC-page field; its precise semantics are pinned down in ECAM ELEC Page.
• The ELEC DC page: four TRs (TR1 / ESS TR draw on AC1, TR2 / APU TR draw on AC2, the feed-source label sits under each TR box). The diagram shows the APU TR at 25V/100A — this snapshot includes an APU-start transient (with the APU off in cruise the APU TR should read 0 A). Read the battery rows by their arrows: the ↓ at the top of DC BAT / DC APU means charging in progress; BAT 2 at 26V/0A is charge current fallen to zero (fully charged, the arrow about to vanish), not "disconnected" — while the battery box is still displayed the BCL is still connected. A true disconnect removes the box entirely ("Battery Line Contactor open: nothing displayed"), confirming the Batteries and the BCL point that a charged battery is opened by its BCL.

7. A day in the life of the network

Stringing the rules into one normal day (each step's evidence traces to the article noted):

Cold cabin   HOT BUS always live -> BAT ON: batteries + static
             inverter light the ESS core
Plug EXT A   GAPCU wakes the ECMU -> EPC A closes -> whole network
             energised, batteries switch to charge
Start APU    APU GEN on line: left net switches to APU (rank 2 > EXT),
             right net stays EXT A (rank 2 > APU rank 3)
Unplug EXT   APU carries the whole network alone (corridor closed, Script A)
Start ENG 1  GEN 1 on line (seamless NBPT) -> left net reverts to IDG 1;
             right net still APU
Start ENG 2  GEN 2 on line -> right net reverts to IDG 2; APU GEN drops
             to hot standby -> shut down APU
Cruise       split-bus: GLC 1/2 closed, corridor open, batteries idle
             (the section 3 baseline)
Land/secure  reverse hand-back: GEN off -> APU/EXT take over by rank ->
             back to the cold cabin

The precise crew-action SOP for each step lives in the normal procedures (plug-in external power and battery check, APU start, pre-pushback external-power removal, post-start GEN take-over and APU shutdown, cruise ELEC scan, landing/securing) — per FCOM PRO-NOR-SOP. One operating hard rule: start both engines before unplugging external power (otherwise, single-engine, NBPT is not available and disconnecting external power gives a transient — see No-Break Power Transfer and External Power).

8. Worked deductions and flight-deck scenarios

8.1 Deduction drill (the answer is the priority table applied)

The last row is the common trap: for EXT B to actually supply anything, the APU GEN must be off line — exactly why the EXT B pushbutton is built as AUTO (see External Power).

8.2 Flight-deck scenarios

1. Ground asks "do you still want external power?" after the APU is running. Judge by the load demand (the ~50 kVA technical-load rule, External Power). High-power work → keep EXT A for the two-source split; an EXT B-only connection is basically dead weight (benched), so it is better to move the ground unit to receptacle A.
2. Confirm the source configuration on the ELEC AC page before start. With both external sources + the APU all connected, the picture only shows "APU + EXT A supplying" — do not assume EXT B is doing work just because its AUTO light is on.
3. Cruise cross-check of the normal picture. A 30-second sweep of the section 6 checklist (load %, voltage/frequency green, AC ESS line off AC1, two white triangles in place, no battery arrows) — this "normal baseline" is the reference frame for every failure read in articles 21 through 31.
4. Single-engine taxi-in (GEN 2 already off). Left network IDG 1, right network fed across the corridor from IDG 1 (the rank-5 relief) or from the APU — the galleys may already be shed to a single feeder per the ECMU script, so a cabin report of "the ovens are dead" is normal.
5. Maintenance asks for BUS TIE OFF for an isolation test. Understand that you are pressing the master switch for all three contactors; during the test, any further source loss on either side is a true loss (no relief). Do not make any source switch before restoring AUTO.

Self-test

[!note]- Q1. State the per-busbar priority table and the principle behind it.

1XP (left) = IDG 1 > APU GEN > EXT B > EXT A > IDG 2; 2XP (right) = IDG 2 > EXT A > APU GEN > EXT B > IDG 1. Principle: own-side generator > middle sources by physical proximity (APU / EXT B tap the left corridor segment, EXT A taps the right) > other-side generator as relief. Memorise the table, not the five-line tongue-twister.

[!note]- Q2. All three ground sources connected (APU + EXT A + EXT B) — who supplies whom?

Left network APU (rank 2 beats EXT B's rank 3), right network EXT A (rank 2 beats APU's rank 3); the SIC opens to split; EXT B supplies nothing (benched with its AUTO light on). The picture is identical to the "APU + EXT A" two-source case — FCOM states this explicitly.

[!note]- Q3. What is the exception to the no-parallel rule, and what enforces it?

The only exception is the controlled, brief on-ground parallel during a No-Break Power Transfer (inside the synchronisation window). Enforcement: the ECMU's Inadvertent Paralleling Trip (opens the corridor contactor, then the source contactor, and latches), backed by the GCU/GAPCU layer. Detail in articles 06 and 08.

[!note]- Q4. What two conditions close the BUS TIE pushbutton in AUTO, and what does OFF cost?

Both conditions reduce to "one source carrying the whole network": only one engine generator supplies the aircraft, or only the APU generator / a single ground power unit supplies it. OFF opens all three contactors (BTC1 / BTC2 / SIC), fully isolating left from right; the cost is giving up the cross-corridor relief — after OFF, any source loss on a side means losing that side's bus.

[!note]- Q5. What is the normal-flight ELEC AC/DC baseline picture?

AC page: GEN 1/2 load % + 116 V / 400 Hz green, AC ESS feeder off AC 1, APU GEN white standby text, EMER GEN ▷ / ◁ STAT INV white triangles always present, IDG oil temp + RISE. DC page: TR1/TR2/ESS TR around 28 V each feeding their own buses, APU TR per APU state, batteries with no arrows (BCL disconnected) or a brief ↓ for charging. Corridor open, no amber anywhere.

Key takeaways

A line to hold the table: own-side generator takes the front row; middle sources board by the nearest platform (B left, A right); the other-side generator queues last; one source feeds the whole network and the galleys give way; parallel is allowed only for the ground instant.

References

Per FCOM DSC-24-10-30-10 (priority-order narrative, the three operating rules: single source / galley shed / no parallel), DSC-24-10-30-20 (IN FLIGHT standard supply tree, ON GROUND single-source and three-source cases, ground-service line), DSC-24-20 (AC ESS FEED NORM from AC 1, BUS TIE pushbutton AUTO/OFF, the EXT B pushbutton five-line priority Note and AUTO-light-remains note, ECAM APU GEN / EMER GEN / STAT INV symbol colour logic), DSC-24-10-30 read as the operations chapter; AMM 24-22-00 D/O (the per-busbar 1XP/2XP five-rank line, the main priority source reprinted for reference in 24-23 / 24-41 and in the 24-29 NBPT section); AMM 24-00-00 D/O (the split-bus statement "the two IDGs are never electrically coupled" and the full AC-DC supply tree — AC BUS 1 raising AC ESS + TR1 + ESS TR, AC BUS 2 raising TR 2); AMM 24-32-00 D/O (the APU TR / 1PU3 drawing on 2XP as an APU-sub-network side branch); AMM 24-41-00 D/O (the 115 kVA APU / 90 kVA EXT A separate ceilings and ~50 kVA technical load); the operator MEL 24-01-01 / 24-01-01-14 (the selected BUS TIE OFF caution is not an MEL matter, the failed OFF lamp is Category C); FCOM PRO-NOR-SOP (the day-in-the-life crew actions, both engines started before unplugging external power); the ELEC AC + DC normal SD page (the baseline picture, GEN 32 % / 116 V / 400 HZ, IDG 123 °C / 22 RISE, the white standby triangles, the AC ESS feeder off AC 1, the four-TR feed-source labels, BAT 2 at 0 A as fully charged not disconnected, the APU TR start transient at 100 A). The "physical proximity sets the middle-source rank" principle, the day-in-the-life narrative, the deduction table, and the triage analogy 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.