Electrical System — Overview

The electrical system sits upstream of almost everything else on the aeroplane: hydraulic pumps need electrical control, the flight-control computers need power, and ECAM itself needs power. Studying ATA 24 is, at heart, studying one network — who supplies whom, and who takes over when a source is lost. This article is the map for the whole chapter. It builds the skeleton first — where the power comes from (generation), how it is routed (distribution), who manages it (control), and what is left at each step down the degradation ladder — and leaves each component's detailed treatment to its own article.

One feature is specific to an overview: it must both name everything once (every source, bus, and control box appears) and state the design philosophy (the FCTM line that defines the whole network's intent in a single sentence). So this article carries two layers a component article does not: the FCTM design philosophy (§8) and the dispatch view (§9) — the two keys a pilot needs to understand why the system is shaped this way and whether it can be signed out with a fault.

A useful point to fix at the outset: the A330 electrical system has no dedicated chapter in FCOM Limitations. There is no "Electrical" entry in the LIM table of contents. This is not an oversight — it is a design fact: the system's "limits" all live inside the abnormal/degraded QRH procedures (the ELEC EMER CONFIG cruise limits, §8), not in a limitation table a pilot must memorise. In the normal configuration the electrical system imposes no operating limit at all; every "do not exceed" belongs to some failure configuration.

1. Two rails, three layers of defence

FCOM defines the whole system in three sentences. The first sets the two rails:

"The electrical power system consists of a three phase 115/200 V 400 Hz constant-frequency AC system and a 28 V DC system. Electrical transients are acceptable for equipment. Commercial supply has secondary priority."

The second defines the normal source set:

"In normal configuration, the electrical power system provides AC power. The electrical power system is constituted of 2 engine generators and 1 APU generator. Each generator can provide AC power to all electrical bus bars. A part of this AC power is converted into DC power for certain applications."

The third defines the fall-back:

"In the event that normal AC power is not available, an emergency generator can provide AC power. In the event that all AC power is not available, the electrical power system can invert DC power from the batteries into AC power."

Per FCOM DSC-24-10-10, three things fall out of these sentences:

AC is the source; DC is derived. The A330 is an "AC aeroplane": the generators produce only 115/200 V 400 Hz three-phase AC, and 28 V DC is a by-product converted from AC by the transformer-rectifiers (TRs). So if the AC network fails, the DC network is necessarily affected — but not the reverse: the batteries can sustain DC independently.
These three sentences are the three layers of defence. Normal layer (three 115 kVA generators) → emergency layer (a hydraulically driven emergency generator) → last-resort layer (batteries + static inverter). FCOM writes the chapter's entire failure logic in its opening paragraph.
Two easily skipped phrases carry weight. "Electrical transients are acceptable" — equipment is designed to ride through transients, so source switching is allowed to drop power briefly (this is why No-Break Power Transfer is done only on the ground; in flight transfers are hard switches — see No-Break Power Transfer). "Commercial supply has secondary priority" — passenger-service equipment is a second-class citizen by design, shed first on overload (see Galley and Commercial Loads).

2. The AC single-line picture

Read the network from the bottom up: generation at the bottom, through contactors into the AC bus layer, through the TRs into the DC bus layer, with the batteries and hot buses on top. Power flows up; in degradation, the fall-back sources take over from the top down.

══════════════════ Battery layer (last-resort) ══════════════════
 701PP HOT BUS 1 ←─ BAT 1            BAT 2 ─→ HOT BUS 2  702PP
   (permanently connected, no contactor)   (permanently connected)
                       │                │
                  BAT contactors ── close on: charging / BAT-only /
                       │                │      ground or in-flight 7 s
        ┌──────────────┴────────────────┴──────────┐   APU BAT
        │            DC BAT BUS  3PP                │   (APU start)
        └──┬──────────────────────────────────┬────┘   708PP APU HOT
           │ DC TIE contactor (L)   DC TIE (R)│
═══════════╪══════════════ DC layer ══════════╪══════════════════
   ┌───────┴──┐  ┌──────────┐      ┌───────────┴┐  ┌──────────┐
   │ DC BUS 1 │  │ DC ESS   │      │ DC BUS 2   │  │ DC SVCE  │
   │   1PP    │  │ BUS 4PP  │      │   2PP      │  │ BUS 6PP  │
   └────▲─────┘  └──▲───────┘      └─────▲──────┘  └────▲─────┘
        │          │ ├─ DC ESS SHED 8PP   │              │
        │          │ ├─ DC LAND RCVRY 407PP              │
      TR1        ESS TR    STAT INV     TR2          (from TR2)
        │          │  │  (28→115 V 1φ)    │              │
════════╪══════════╪══╪══════════════════╪══════════════╪══════
   ┌────┴───────┐  │  │   ┌──────────────┐  ┌────────────┐
   │  AC BUS 1  │──┘  └───│ AC ESS BUS   │  │ AC BUS 2   │
   │    1XP     │ AC ESS  │    9XP       │  │    2XP     │
   └────▲───────┘ FEED    └──┬───────────┘  └────▲───────┘
        │      NORM=AC1 ┌────┼─ AC SHED ESS 4XP  │
        │      ALTN=AC2 │    ├─ AC ESS GND 905XP │
        │   EMER GEN ───┤    └─ AC LAND RCVRY 903XP
        │   LINE CNTOR ┌┴────────┐
        │              │EMER GEN │← Green hydraulic drives CSM/G
        │              │  8 kVA  │
        │              └─────────┘
   ─────┼─ AC BUS TIE (L) ┄┄ BUS TIE pb ┄┄ AC BUS TIE (R) ─────
        │      │         │          │          │       │
   ┌────┴─┐ ┌──┴───┐ ┌───┴────┐ ┌───┴──┐ ┌─────┴──┐
   │GEN 1 │ │EXT B │ │APU GEN │ │EXT A │ │ GEN 2  │
   │IDG 1 │ │90 kVA│ │115 kVA │ │90 kVA│ │ IDG 2  │
   │115kVA│ └──────┘ └────────┘ └──────┘ │ 115 kVA│
   └──────┘  EXT B feeds the LEFT half;  └────────┘
             EXT A feeds the RIGHT half.

Two reading notes that come from the actual FCOM system diagram (p.1678), not from drawing convention:

EXT B is wired into the left half-network and EXT A into the right. This is physical wiring, and it is the root of the priority rule you will meet later ("EXT B has priority for the left bus, EXT A for the right") — see Network Priority and Normal Supply.
Busbar numbering. The AC sheddable essential bus is named 4XP (AC SHED ESS BUS) in the AMM overview; 401XP is its sub-segment number, appearing in feeder/sub-bus contexts. They are two levels of the same bus, not two buses.

3. Generation — four AC sources and one "pseudo-generator"

Engine generator (IDG). The engine HP rotor drives the generator through the accessory gearbox at a speed that varies with thrust, but the generator must produce a constant 400 Hz — so a hydromechanical constant-speed unit sits in between. The generator and that unit together are the IDG (Integrated Drive Generator):

"The engine HP compressor drives each main generator via the accessory gearbox and via an integrated hydromechanical speed regulator which transforms the engine variable speed into constant speed for the generator."

Per AMM 24-00-00. A mechanical fault can be cleared by disconnecting the IDG from the flight deck (guarded IDG pushbutton), but disconnection is a one-way ticket — only maintenance can reconnect it, on the ground (see Integrated Drive Generator).

APU generator. Same rating as an IDG (115 kVA), but it needs no constant-speed unit — the APU itself runs at constant speed, so direct drive is already constant-frequency. ATA 49 confirms the drive arrangement ("a single shaft gas turbine which delivers mechanical shaft power for driving the accessory gearbox (electrical generator)") — the APU generator is also accessory-gearbox driven, the only difference from the IDG being the APU's constant single-shaft speed. That is what lets it replace any main generator. ATA 49 also gives its electrical role:

"On ground: It supplies electrical power to the electrical system. In Flight: It backs up the Electrical system … The APU may obtain power for starting from the batteries specifically assigned to the APU, or in combination with the external power, or normal aircraft supply. APU start is permitted throughout the normal flight envelope, except when APU battery only is supplying."

Per FCOM DSC-49-10-10. So APU start power has three possible sources (dedicated APU battery / in combination with external power / normal aircraft supply), and the one boundary a pilot should hold is no in-flight start when APU battery only is supplying — battery capacity cannot take the start surge in that state.

External power A / B. Two receptacles near the nosewheel, 90 kVA each:

"Two ground power connectors near the nosewheel allow ground power to be supplied to all busbars (with some galleys shed in case of overload). Two ground power units (90 KVA MAX each) can supply the aircraft."

Per FCOM DSC-24-10-20-10. The twin receptacles allow parallel supply for high-power ground work, but each external source has its own "home" half (A → right, B → left), arbitrated by the GAPCU/ECMUs (see External Power). Note "some galleys shed in case of overload" again — load-shedding the galleys is the system-wide "secondary priority first" mechanism.

Emergency generator (CSM/G). Strictly, this is not "another generator" but a hydraulic-to-electrical converter. The AMM gives its speed-control mechanism and its Green-pressure sources:

"When emergency conditions prevail, a hydraulic motor (CSM) controlled by a servo-valve speed regulator drives a generator … In electrical emergency condition following the loss of the two generators, two main hydraulic pumps driven by engines 1 and 2 supply the Green hydraulic system. In emergency condition, the loss of two engines, the Ram Air Turbine (RAT) pump supplies the Green hydraulic system. The loss of the AC BUS 1, and AC BUS 2 generates the automatic control of the CSM/G."

Per AMM 24-00-00. Two pressure-source paths must be kept distinct (this is the overview-level root of the "EDP scenario vs RAT scenario" in Emergency Generator and Emergency Electrical Configuration): engines still turning (a pure electrical disaster) → the engine-driven pumps supply Green pressure, and the CSM/G runs at its full 8 kVA; both engines stopped → the RAT pump supplies Green pressure, and output falls to 3.5 kVA. Note that the trigger is loss of both AC BUSes, not loss of both engines.

Static inverter ("pseudo-generator"). It does not generate; it disguises the batteries' 28 V DC as 115 V single-phase AC, 2.5 kVA. FCOM gives its source and its flight/ground activation difference:

"A static inverter transforms DC power from the DC ESS bus into 2.5 KVA of single-phase 115 V 400 Hz AC power, which is then supplied to part of the AC essential bus. In flight, the inverter is automatically activated, if nothing but the batteries is supplying electrical power to the aircraft, regardless of the position of the BAT 1 and BAT 2 pushbuttons. On ground, the inverter is activated, if only the batteries are supplying electrical power to the aircraft, and both BAT 1 and BAT 2 pushbuttons are on."

Per FCOM DSC-24-10-20-10. The flight/ground difference is a test point: in flight it ignores the BAT pushbuttons (survival first, automatic activation); on the ground both BAT pushbuttons must be on (a manual gate, to save the batteries). Its input is the DC ESS bus — so on batteries only the chain is "battery → DC ESS → static inverter → AC ESS" (see Static Inverter).

4. Distribution — the left-heavy pair of half-networks

The AMM divides the AC distribution into two independent networks, but the two are not symmetric:

"There are two distribution networks: network 1 associated to left side (side 1) generator (IDG 1), network 2 associated to right side (side 2) generator (IDG 2). 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.

Network 1 (left): AC BUS 1 (1XP) + AC ESS BUS (9XP) + AC SHED ESS BUS (4XP) — the essential-load family all hangs on the left network.
Network 2 (right): AC BUS 2 (2XP) — a single clean bus.

The DC side is asymmetric the same way: TR1 supplies DC BUS 1 (1PP) + DC BAT BUS (3PP); TR2 supplies DC BUS 2 (2PP) + DC SERVICE BUS (6PP); the ESS TR (which also draws from AC BUS 1) supplies DC ESS BUS (4PP) + DC SHED ESS BUS (8PP).

Why left-heavy? The official design intent in one sentence (FCTM):

"The electrical distribution has been designed to fly, navigate, communicate and ensure passenger comfort."

Per FCTM PR-AEP-ELEC. The ESS (essential) family is the minimum set needed to fly, navigate, communicate — it must have several "step-fathers" so it cannot be lost in one stroke: if AC BUS 1 fails it can transfer wholesale to AC BUS 2 (the AC ESS FEED transfer, see AC ESS Feed and Transfer); if both AC BUSes are lost, the emergency generator and static inverter feed AC ESS directly. "Passenger comfort" (galleys etc.) is secondary, shed first on overload. Putting the ESS family on side 1 is just an engineering choice — having chosen side 1, side 1 becomes "heavy". The captain-side key instruments, ADIR 1, VHF 1 and so on follow ESS / side 1, which is why in many failure procedures "the left-side equipment survives longer".

The AC ESS FEED pushbutton has a use beyond the "AC BUS 1 lost" case:

"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 FAULT legend means "AC ESS itself has no power", not necessarily "AC BUS 1 has no power": if AC BUS 1 is healthy but AC ESS is lost (a feeder/contactor fault), the automatic transfer does not act and the crew must select ALTN manually.

A 26 V AC sub-rail also exists: the AC ESS BUS and AC BUS 2 each feed a 115/26 V auto-transformer producing 26 V AC 400 Hz for small loads such as instrument lighting (per AMM 24-00-00) — the origin of the small-fry busbars like 931XP in the bus list (see AC Distribution and Busbars).

5. DC layer — four TRs, three batteries

TR (transformer-rectifier) family — four identical units, different roles:

"Two main Transformer Rectifiers TR1 and TR2 and one essential TR supply the aircraft's electrical system with DC current. A fourth TR is dedicated to APU start or APU battery charging. Each TR controls its contactor by internal logic."

Per FCOM DSC-24-10-20-20. Note the weight of the last sentence: the TR contactors are the only power contactors not managed by the ECMUs (which manage every other AC/DC main contactor). Each TR judges overheat / overcurrent / minimum current / open-or-short and opens its own contactor (see Transformer-Rectifiers). The reset path is in the AMM:

"In case of failure, the TR generates a FAULT signal to the CMS and SDAC for maintenance purposes. It is possible to reset the TR from the CMS or through a pushbutton switch located on the TR front face."

Per AMM 24-00-00. The reset button is on the TR front face in the avionics compartment — a pilot cannot reach it, so an in-flight TR latch-open stays open until landing. The fourth TR has two uses ("charge the APU battery, start the APU alone or coupled with the APU battery").

The TRs recover symmetrically:

"(a) TR1 If there is loss of the TR1, the TR2 automatically restores supply to the DC BUS 1 and the DC BAT BUS. (b) TR2 TR2 loss leads to the symmetrical recovery of DC BUS 2 from the TR1. (c) In the event of ESS TR loss and if the TR 2 is available, the DC ESS BUS and DC SHED ESS BUS are automatically recovered by the TR1 through the DC BAT BUS and the DC BUS 1. If the TR 2 is not available, the DC ESS BUS and the DC SHED ESS BUS are no longer supplied and the bus 2PP is supplied by the TR 1. (d) TR1 and TR2 In the event of TR1 and TR2 loss, the DC ESS BUS and DC SHED ESS BUS remain supplied by the ESS TR."

Per AMM 24-00-00. Two counter-intuitive points: (c) ESS TR cover has a precondition — before TR1 takes the ESS TR's job it must first hand its own job to TR2, so with only one main TR left, DC ESS is dropped (and 2PP then draws from TR1); (d) with both main TRs lost but the ESS TR alive, DC ESS is preserved (the ESS TR is an independent third unit, outside the main-TR fault chain). This "both mains lost but DC ESS alive" is the key background for the DC BUS 1+2 failure (see DC Bus Faults).

Batteries — three identical, 2 main + 1 APU:

"Two main batteries, each with a normal capacity of 40 Ah, are permanently connected to the two hot busses. An identical third battery (40 Ah) is dedicated to APU start. Each battery has an associated Battery Charge Limiter (BCL). The BCL monitors battery charging and controls its battery contactor."

Per FCOM DSC-24-10-20-20. (A manual-source note: the AMM overview cites "37 Ah" in one place; the dedicated AMM 24-38 chapter states a nominal capacity of 40 Ah, in agreement with FCOM. 40 Ah is taken as the value.)

"Permanently connected to the hot buses" means HOT BUS 1/2 (701PP/702PP) always have power (as long as the battery is physically present and charged) — the reason some equipment is still alive after an overnight stop, and the reason the refuel busbars hang on the hot buses (refuelling without energising the whole aircraft). But the batteries connect to the DC BAT BUS only conditionally, gated by the BCL (charging demand / battery-only supply / in-flight DC-all-lost limited to 7 s). In the normal configuration the batteries are disconnected most of the time (see Batteries and the BCL) — one of the chapter's biggest counter-intuitive points.

6. Control layer — who manages the network for you

The A330 network reconfigures fully automatically; crew switch actions are minimal. The automation is split across three levels (per AMM 24-00-00):

Source level — GCU / GAPCU. Each generator has a "housekeeper". The GCU regulates frequency (IDG only), regulates voltage, and protects (trips its own GLC and de-excites on a fault). The GAPCU does three jobs in one: it manages the APU generator, manages both external sources, and is the system-wide BITE "letter box" forwarding the GCU fault codes to the central maintenance system. (FCOM keeps a simpler "GPCU" variant in the overview — a unit that controls only external power, not the APU; this aircraft fits the GAPCU — see APU Generator and GAPCU.)
Network level — ECMU 1 / 2. The network dispatcher:

"Two identical Electrical Contactor Management Units (ECMUs) provide: ‐ AC and DC contactors control (excepted TR contactors, which are controlled by the TR itself) ‐ Galley shedding control ‐ No Break Power Transfer control (NBPT) ‐ Monitoring and indicating. … ECMU 1 controls: GLC 1, BTC 1, the APU Generator Contactor, both DC Tie Contactors, the BUS TIE Contactor, the External Power Contactor B. ECMU 2 controls: GLC 2, BTC 2, the External Power Contactor A, both DC tie Contactors."

Per FCOM DSC-24-10-20-30. The split follows the half-networks, but note the crossover: the APU contactor belongs to side 1, EXT A to side 2, EXT B to side 1 — matching the "EXT B on the left, EXT A on the right" wiring of §2. ECMU 1 also manages the BUS TIE contactor (ECMU 2 does not). The two DC tie contactors are controllable by either ECMU (redundancy). The hardware that enforces the "generators never parallel" rule is the AMM-level IPT (Inadvertent Paralleling Trip) — see No-Break Power Transfer.

End level — BCL / TR internal logic / CBMU. The battery contactors belong to the BCLs, the TR contactors to the TRs themselves, and circuit-breaker status reporting to the CBMU. The ends are autonomous; the network level does not interfere.

The transfer circuit's half/full-network logic:

"The Bus Transfer Contactors (BTC)s and the System Isolation Contactor (SIC) are automatically controlled by the ECMUs. They enable supply of all the aircraft electrical network or half of it. The supply only depends on the availability of one of the power sources: GENx, APU GEN, EXT PWR x."

Per AMM 24-00-00. This is the overview-level reading of "three BUS TIE contactors = BTC1 + BTC2 + SIC": with one source feeding the whole network the corridor is fully closed (BTC + SIC); with two sources split between the halves, the SIC opens the corridor in the middle.

The priority rule and the no-parallel exception (the decisive verbatim, worked through in Network Priority):

"Each AC BUS is supplied in priority order by: ‐ the corresponding engine generator. ‐ the APU generator or the external power A … ‐ the external power B … ‐ the other side engine generator. … The generators cannot be connected in parallel (except on ground during No Break Power Transfers)."

Per FCOM DSC-24-10-30-10.

No circuit breakers in the cockpit:

"The circuit breakers are located on the front face of panels 710VU, 740VU, 743VU and 5058VE in the avionics compartment, underneath the cockpit. The other circuit breakers are located on the left of the APU CONTROL BOX … in the bulk cargo compartment. All commercial circuit breakers are located on the panels 5001VE, 5002VE, 5005VE, 5006VE and 5060VE in the cabin and cargo compartments."

Per AMM 24-00-00. The A330 has no circuit-breaker panel in the cockpit. If a breaker trips in flight you cannot reach for it — you only see it on the ECAM C/B page (reported by the CBMU), and handle it by procedure. The commercial breakers in the cabin/cargo panels are the ones outside the CBMU monitoring circle (a cabin report of "the coffee maker is dead" with a NORMAL C/B page is no contradiction — see Circuit Breakers and the CBMU).

7. The power ladder

Source	Power	Type	Role
GEN 1 / GEN 2 (IDG)	115 kVA each	3-phase 115/200 V 400 Hz	Primary, one per half-network
APU GEN	115 kVA	as above	Versatile substitute: whole network on ground; in flight replaces one or more main generators
EXT A / B	90 kVA each (max)	as above	Ground power, twin receptacles
EMER GEN	8 kVA	as above	Feeds the ESS family after total main-generator loss
EMER GEN (RAT-driven, < 260 kt)	3.5 kVA	as above	Further reduced; above 260 kt the output rises from engine windmill effect (§8)
Static inverter	2.5 kVA	single-phase 115 V 400 Hz	The batteries' AC avatar, the last layer
Batteries × 3	40 Ah	28 V DC	DC last-resort + APU start

Memorise the ladder as 115 → 90 → 8 → 3.5 → 2.5 — each step down shrinks the population served by one ring, from "whole aircraft" to "half-network" to "ESS family" to "ESS core". That is defence-in-depth expressed as numbers. Note the FCOM wording: the APU GEN can replace "one or more" main engine generators — not just one — which matters most when both IDGs have failed.

8. Emergency philosophy, the degradation ladder and the QRH numbers

§§1–7 describe what the system is; the FCTM adds why it degrades this way and what it feels like at the bottom — the layer that distinguishes an overview from a component article.

Two classes of cause (which decide whether you should try the APU):

"The electrical emergency configuration is due essentially to the loss of all main AC BUS … In most cases, this is due to an anomaly on the electrical network, e.g. a short circuit. All engine generators trip and the emergency generator is driven by the EDP. In this scenario, the probability to restore electrical power using the APU generator is low. Another cause … could be a combination of electrical failures and engine failures. In this scenario, the flight crew may restore electrical power using the APU generator."

Per FCTM PR-AEP-ELEC. So "try the APU to restore" is not a cure-all: in the pure-electrical short-circuit type, the generators have all tripped and the APU will most likely be refused by the same faulty network; only the combination type is likely to be recoverable by APU. Trying costs nothing, but do not count on it.

The 260 kt windmill effect (the why behind that 3.5 kVA number):

"If the speed is higher than 260 kt IAS, the emergency generator electrical power is increased as it takes credit from the engine windmill effect."

Per FCTM PR-AEP-ELEC. With both engines stopped and the RAT supplying pressure, the windmilling engines still drive an engine pump a little; the faster the airflow, the stronger the windmill, and 260 kt is the gain threshold.

The degradation ladder (the master map for the whole failure chapter):

Normal: GEN1 + GEN2 ────────────────── whole network 100%
  ↓ lose one generator (21/22)
single-source reconfig: APU GEN substitutes, or other GEN
   carries whole network ──── network held (galleys shed on overload)
   ⚠ red line 1: an overcurrent-tripped generator is NOT reconfigured;
                  its AC BUS is lost
   ⚠ red line 2: differential-protection failure → generator not
                  replaced AND its TR switched off (AC + DC both shrink)
  ↓ lose one AC BUS (23)
half-network: AC ESS family transfers auto/manual to AC BUS 2 ── ESS held
  ↓ AC BUS 1+2 both lost = ELEC EMER CONFIG (29)
emergency layer: EMER GEN 8 kVA (engine pumps supply Green) ── ESS family only
  ↓ both engines also stopped (RAT supplies Green, 3.5 kVA < 260 kt)
emergency reduced: AC ESS SHED is shed ──── ESS core
  ↓ EMER GEN also unavailable (30)
last-resort: BAT + static inverter ── DC ESS + AC ESS (no SHED),
                                       DC BAT BUS lost after 7 s

The two red lines, verbatim (FCOM distribution-table notes):

"Note: If a generator is lost due to overcurrent detection, reconfiguration does not occur and the related AC BUS is lost." "(2) In case of differential protection failure: • The affected generator is not replaced; • The associated TR is switched off."

Per FCOM DSC-24-10-30-30/-40. An overcurrent-tripped generator or TR is not reconfigured and its bus is accepted as lost; on a differential-protection failure the affected generator is not replaced and the TR it feeds is also switched off — one differential trip shrinks both the AC and DC layers.

EMER GEN two sub-states (FCOM Operations version, finer than the diagram): with the EDP driving, the emergency generator feeds AC ESS BUS + AC ESS SHED, and via the ESS TR feeds DC ESS BUS + DC ESS SHED; with the RAT driving, AC ESS SHED + DC ESS SHED are lost (only AC ESS BUS + DC ESS BUS remain); in both sub-states AC ESS GND is lost and the LAND RECOVERY buses are shed until the pushbutton restores them (per FCOM DSC-24-10-30-30).

Batteries only, the 7 s and the 50 kt split (FCOM distribution table): in flight on batteries only, the DC BAT BUS is "Lost after 7 s" (the battery bridge drops 3PP after 7 s to save charge); on the ground the split is V > 50 kt vs V ≤ 50 kt (at ≤ 50 kt the DC BAT BUS is fed directly by BAT 1/2 and AC ESS GND is restored through the static inverter) — per FCOM DSC-24-10-30-40.

The hard QRH numbers (degradation is not just qualitative — the QRH fixes the cruise limits):

"ELEC EMER CONFIG SUMMARY — CRUISE: SPD BRK: DO NOT USE / MAX SPD: 330/.82 / ALTN LAW: PROT LOST / AUTOFLIGHT: AP1+2 LOST / CAT 2 INOP … For slats extension: RAT MAN ON, MINIMUM RAT SPEED 140 KT, LAND RECOVERY ON / L/G gravity extension: MAX SPD 200 KT / LANDING: Only 3 spoilers per wing, Direct law, NO REVERSER."

Per QRH 16.01A.

EDP scenario vs RAT scenario, what is left to fly with:

"the aircraft is in a much better configuration when the EDP powers the emergency generator, rather than the RAT. … FLY [EDP] PFD 1, pitch trim, rudder trim, alternate law … [RAT] PFD 1, alternate law. The AP, pitch trim and rudder trim are not available. … The aircraft will be out of trim in roll due to right outboard aileron upfloat … At slats extension, the emergency generator disconnects (in order to dedicate the RAT for flight controls) and landing is performed on batteries."

Per FCTM PR-AEP-ELEC.

The LAND RECOVERY logic (why, when, and why again after restoration):

"The LAND RECOVERY pb should be pressed prior to commencing the approach. This action will not be delayed … When ELEC EMER CONFIG occurs, the LAND RECOVERY AC and DC BUS bars are initially shed and will remain shed until the LAND RECOVERY pb is set to ON. This remains true if normal electrical configuration is restored. This is the reason why the flight crew will also set the LAND RECOVERY pb to ON for approach following a restoration from an ELEC EMER CONFIG."

Per FCTM PR-AEP-ELEC. Human factors: with only PFD 1 available the left seat becomes PF; workload rises sharply; with only the E/WD available the ECP must be used with discipline.

9. Dispatch view (MEL)

The MEL cuts the electrical system into roughly thirteen dispatch sub-systems (24-22 main generation / 24-23 APU generation / 24-24 emergency generation / 24-25 AC ESS FEED / 24-26 galley & commercial / 24-28 static inverter / 24-29 ECMS / 24-32 DC main generation / 24-34 DC emergency / 24-35 DC conversion / 24-38 DC batteries / 24-41 external-power control / 24-53 C/B monitoring).

One overview-level dispatch philosophy worth the mental arithmetic: one main AC generation system (IDG/GCU/line contactor) inoperative may be dispatched (Category C, 2 installed / 1 required), with hard conditions — no ETOPS beyond 180 min, the APU and AC auxiliary generation system operating and used throughout the flight, and all busbars energised (per the operator MEL, MI-24-22). The logic is clean: dispatching with one main generator gone leaves the aircraft on a single main generator; losing one more drops straight into EMER CONFIG, so the MEL requires the APU GEN on line for the whole flight to pre-restore the redundancy, and bars ETOPS beyond 180 min. (The AC ESS FEED automatic-transfer dispatch requires the three DC TIE contactors to work — the official "three DC TIE" confirms the pair of DC TIE contactors drawn in §2.)

The full dispatch philosophy across all sub-systems is in MEL Dispatch View. The three dispatch "states" worth carrying: no-go (static inverter / ESS TR — the survival floor), Category C go (galley, CBMU, one generator), and not in the MEL coordinate system at all (the ground service network, a non-flight function).

Self-test

[!note]- Q1. What are the two electrical rails on the A330, and how are they related?

A three-phase 115/200 V 400 Hz constant-frequency AC rail is the primary supply (produced by two IDGs and one APU generator); 28 V DC is the secondary rail, derived from AC by four transformer-rectifiers (TR1/TR2 for the normal DC network, the ESS TR for the DC ESS network, the APU TR for APU start / APU battery charging). AC is the source, DC is the derivative. When all AC is lost, DC can be sustained independently by the batteries, and the static inverter (fed from the DC ESS bus) inverts 2.5 kVA single-phase AC back out.

[!note]- Q2. Rank the AC sources by power and state what each layer serves.

IDG × 2 at 115 kVA each (one per half-network, either can carry the whole network) ≈ APU GEN 115 kVA (whole network on ground, replaces one or more main generators in flight) > EXT A/B 90 kVA each (ground, galleys shed on overload) > EMER GEN 8 kVA (EDP-supplied Green, ESS family) > EMER GEN on RAT < 260 kt 3.5 kVA (ESS core, SHED shed; above 260 kt the windmill raises it) > static inverter 2.5 kVA (AC ESS BUS only, single-phase). Each step down shrinks the population served by one ring.

[!note]- Q3. Which contactors does each ECMU control, and which power contactor is managed by neither?

ECMU 1 controls GLC 1, BTC 1, the APU generator contactor, the BUS TIE contactor, External Power contactor B, and both DC tie contactors; ECMU 2 controls GLC 2, BTC 2, External Power contactor A, and both DC tie contactors (the DC ties are controllable by either). The four TR contactors are controlled by each TR's own internal logic, not the ECMUs. Battery contactors belong to each BCL.

[!note]- Q4. Why is the network called "left-heavy", and what is the official design objective?

Network 1 (left) = AC BUS 1 + AC ESS BUS + AC SHED ESS BUS (parent 4XP); on the DC side TR1 also carries the DC BAT BUS, and the ESS TR draws downstream of AC BUS 1. Network 2 (right) is only AC BUS 2 + DC BUS 2 + DC SERVICE BUS. The official objective (FCTM): the distribution is "designed to fly, navigate, communicate and ensure passenger comfort" — the ESS family is the fly/navigate/communicate minimum set, needing multiple fall-backs (transfer to AC BUS 2 / EMER GEN / static inverter), so it forms a separate family and is housed by default on side 1.

[!note]- Q5. List what is left at each rung of the degradation ladder down to batteries only, with the QRH cruise numbers.

(1) Lose one generator → automatic reconfiguration holds the whole network (galleys shed; two red lines: overcurrent is not reconfigured / differential failure also switches off the TR). (2) Lose AC BUS 1 → the AC ESS family transfers to AC BUS 2, half-network running. (3) AC BUS 1+2 both lost → EMER CONFIG, EMER GEN 8 kVA (EDP) feeds the ESS family; QRH limits MAX SPD 330/.82 + AP1+2 lost + CAT 2 INOP. (4) RAT only → 3.5 kVA, AC/DC ESS SHED shed, right outboard aileron upfloat puts the aircraft out of roll trim, slats extension drops the EMER GEN and the landing is on batteries. (5) EMER GEN unavailable → batteries + static inverter: DC ESS + AC ESS (no SHED), DC BAT BUS held only 7 s.

Key takeaways

#	Point
1	Three-phase 115/200 V 400 Hz + 28 V DC; AC is the source, DC is what the TRs make from it.
2	Power ladder 115-90-8-3.5-2.5; each step shrinks the served ring; above 260 kt the RAT windmill raises the EMER GEN output.
3	Generators never parallel (except the on-ground NBPT instant, three permitted pairs); the hardware enforcer is the ECMU IPT.
4	Two red lines: overcurrent trip is not reconfigured / differential-protection failure also switches off the TR — the network would rather drop a bus than feed a faulty source into a healthy one.
5	Left-heavy = FCTM "fly, navigate, communicate"; the ESS family has multiple fall-backs and is housed on side 1.
6	No circuit-breaker panel in the cockpit, and no reachable TR reset — C/Bs and TRs are in the avionics bay; in flight you only watch ECAM.

References

Per FCOM DSC-24-10-10 (general / two-rail / three-layer), DSC-24-10-20-10 (sources, GPCU/GAPCU variants, static inverter, external power), DSC-24-10-20-20 (TRs, batteries), DSC-24-10-20-30 (ECMU functions and split, NBPT), DSC-24-10-30-10 (priority order, no-parallel), DSC-24-10-30-30/-40 (degradation, red-line notes, EMER GEN sub-states, 7 s / 50 kt, discharge horn); AMM 24-00-00 D/O (network split, FIN/zone, SIC/BTC half-network, TR symmetric recovery, CSM/G servo-valve, CBMU principle); FCTM PR-AEP-ELEC (design objective, EMER CONFIG causes, 260 kt windmill, EDP vs RAT, LAND RECOVERY); QRH 16.01A (ELEC EMER CONFIG cruise summary); FCOM DSC-49-10-10/-20 (APU electrical role); the operator MEL 24-22/24-25 (dispatch). The "designed to fly, navigate, communicate" sentence is the FCTM design statement; the degradation-ladder narrative is an integrative synthesis of the above.

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