Static Inverter

The static inverter is the last rung of the power ladder. Where every other AC source on the aeroplane generates AC, the inverter does not generate anything — it disguises the batteries' 28 V DC as single-phase 115 V 400 Hz AC, 2.5 kVA, and feeds it to the core of the AC essential network. The word "static" is literal: there are no rotating parts, no bearings, no excitation — it is pure power electronics. It is the final entry in the AC ESS "step-father" lineage (see AC ESS Feed and Transfer) and, in effect, the AC avatar of the batteries (see Batteries and the BCL).

This is a deliberately short article. The static inverter's source coverage in the manuals is one of the smallest in the chapter — a single AMM generation sub-chapter and a handful of FCOM lines. The depth is honest to that: the whole weight falls on two sets of activation conditions and one counter-intuitive protection rule. By the end you should be able to answer:

1. Which contactors does the supply chain pass through, and which busbars does the output feed?
2. How do the in-flight and on-ground automatic activation conditions differ, and where does 50 kt appear in that logic?
3. What are its five protections, and which one — and only one — actually opens a contactor, and why?
4. When STATIC INV FAULT is displayed, is it still supplying power?
5. On the ELEC page, when does its standby white triangle become a real "supplying" indication?

1. Architecture — the supply chain and what it feeds

The static inverter sits at the bottom of a chain of three contactors, each with a distinct job. Read it top-down from the batteries:

BAT 1 + BAT 2   (ata-24-12)
      |  2PC / 3PC -- the battery gate (ata-24-11)
      v
   2XB  -- static inverter contactor (740VU)
      |
      v
+--------------------------------------------------+
|  Static inverter   1XB   (avionics bay, zone 121) |
|        28 V DC  --invert-->  single-phase         |
|        115 V AC / 400 Hz,   2 500 VA max          |
+--------------------------------------------------+
      |
      v
   3XB  -- EMER BUS SPLY contactor
      |--> 901XP    AC ESS BUS (core segment)
      |--> 931XP-A  26 V AC ESS segment (via 3XH 115/26 V xfmr)
      +--> 905XP-B  AC ESS GND  -- GROUND ONLY
  control C/B: 742VU "STAT INV CTL" (6XB)

Each contactor on the chain owns one decision:

• 2PC / 3PC decide whether the batteries supply the essential side at all — the battery gate, covered in DC Network Transfer. The AMM is explicit that these contactors feed both the DC ESS bus and the inverter from the two main batteries: "These contactors serve to supply the DC ESS BUS (4PP) and the static inverter from the two main batteries." Per AMM 24-35-00. So the inverter's input is the DC ESS bus, itself battery-fed through 2PC/3PC.
• 2XB decides whether the inverter is energised.
• 3XB (the EMER BUS SPLY contactor) decides where the inverted AC is routed.

The rating and the input are fixed by FCOM:

"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."

Per FCOM DSC-24-10-20-10. Note "part of the AC essential bus" — the inverter does not feed the whole essential family. In the functional-bus view, it supplies the AC ESS BUS and the AC LAND RECOVERY bus (whatever the LAND RECOVERY pushbutton position), and on the ground additionally the AC ESS GND; the AC ESS SHED is never fed by it (it is automatically shed in the emergency configuration). The AMM sub-bus numbers cross-confirm the FCOM functional names: 901XP is the AC ESS BUS core segment, 931XP-A is a 26 V AC essential segment derived through the 3XH 115/26 V auto-transformer (per AMM 24-52-00), and 905XP-B is the AC ESS GND.

The flight/ground asymmetry of the output is the one thing to fix here. FCOM gives two distribution pictures for "batteries only", and the only line that moves between them is AC ESS GND. In flight on batteries only:

"When emergency generator is not available, the batteries supply: the DC ESS BUS, the DC LAND RECOVERY (whatever the LAND RECOVERY pushbutton position is) and through the STAT INV: the AC ESS BUS, the AC LAND RECOVERY (whatever the LAND RECOVERY pushbutton position is). The AC ESS SHED, the DC ESS SHED and the SHED LAND RECOVERY are not supplied. The AC ESS GND is lost."

Per FCOM DSC-24-10-30-30. On the ground on batteries only:

"Provided they are both selected AUTO, batteries supply the: DC ESS BUS, DC BAT BUS, DC LAND RECOVERY (whatever the position of the LAND RECOVERY pushbutton). And, through the static inverter, the: AC ESS BUS, and AC ESS GND, AC LAND RECOVERY (whatever the LAND RECOVERY pushbutton position is). The AC ESS SHED, the DC ESS SHED and the SHED LAND RECOVERY are not supplied."

Per FCOM DSC-24-10-30-30. So 905XP / AC ESS GND is supplied only on the ground — and, per the distribution table, only after the aircraft has slowed to ≤ 50 kt (the V > 50 kt row still shows AC ESS GND as not supplied; the ≤ 50 kt row restores it through the static inverter, per FCOM DSC-24-10-30-40). In flight on batteries only, this segment is simply given up.

2. Automatic activation — three cases, and the two roles of 50 kt

FCOM states the activation rule in two sentences (in flight it ignores the BAT pushbuttons; on the ground both must be on):

"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 AMM resolves those two sentences into three precise cases — all premised on the emergency configuration (busbars 1XP and 2XP lost):

"The static inverter starts operating automatically in the following cases: In flight, in emergency configuration (busbars 1XP and 2XP lost): — as longer as the Constant Speed Motor/Generator (CSM/G) is not connected to the essential network (CSM/G starting phase), — if the CSM/G is not available (at slat extension with Ram Air Turbine (RAT) extended) and the aircraft speed above 50 knts, — at landing, with aircraft speed lower than 50 Knts and if the two BAT 1 and BAT 2 pushbutton switches are pushed."

Per AMM 24-28-00. The three cases map onto three points on the emergency timeline:

1. CSM/G starting phase — for the first seconds of the emergency configuration, the emergency generator has not yet come on line. The inverter bridges the gap (see Emergency Generator). When the CSM/G connects to the essential network, the inverter drops back to standby.
2. CSM/G unavailable — in the RAT scenario, the emergency generator is deliberately dropped at slat extension to dedicate the RAT to flight controls. From that point, while speed is above 50 kt, the inverter carries the AC essential core unconditionally.
3. At landing, below 50 kt — once the aircraft has slowed through 50 kt on rollout, the activation condition changes: it now requires both BAT 1 and BAT 2 pushbuttons pushed.

[!warning]- 50 kt is the air/ground stitch — and the only point where the BAT pushbuttons matter

50 kt does two jobs at once. First, it is the air/ground switching speed: above it the inverter activates unconditionally (regardless of BAT pushbutton position); below it the activation gate changes to "both BAT pushbuttons pushed". This is exactly the seam between FCOM's two sentences — "in flight, regardless of the BAT pushbuttons" versus "on ground, both BAT pushbuttons on". Second, 50 kt is the dividing line between the two ground battery-only distribution rows (V > 50 kt versus V ≤ 50 kt), and it is at ≤ 50 kt that AC ESS GND is restored through the inverter. The operational consequence: on the rollout, do not touch the BAT pushbuttons — below 50 kt, switching one off is now enough to drop the inverter, and with it the AC ESS core.

3. Protection — five alarms, one trip

The AMM lists five protection functions and one warning output. Every one of them raises a fault signal; only one of them opens a contactor:

"In case of failure, the static inverter generates a permanent ground signal to the System Data Analog Concentrator(s) (SDAC). This signal is generated in case of tripping of any of the following protection functions: — fan failure, — overheat: temperature higher than 110 °C, + or - 5 °C, — output over voltage: output voltage higher than 130 V rms, — output undervoltage: output voltage lower than 103 V rms, — output over/under frequency: 320 Hz/480 Hz. The warning message: STATIC INV FAULT is displayed on the Engine/Warning Display (EWD)."

Per AMM 24-28-00. The contactor behaviour is then nailed by a single NOTE:

"NOTE: Contactors 2XB and 3XB only open (trip) in case of output overvoltage (ie, remain closed in case of output undervoltage, overheat, fan failure or output over/under frequency)."

Per AMM 24-28-00. Tabulated:

[!warning]- Predict before you read on: "the inverter overheats — does the system cut it off?"

Following the protection philosophy of every component up to this point — the GCU trips its generator on a fault, each TR latches its contactor open on a fault — the instinctive answer is yes, it trips. It does not. This is the chapter's cleanest reversal. The static inverter is the last meal of the AC ESS core: the only time it is ever needed is when the aircraft is on batteries only, and opening its contactor means the AC ESS bus goes dark — the standby instruments behind it go with it. So the design trade is deliberately the opposite of "trip on fault": poor power quality (undervoltage, frequency drift), an overheating unit, even a failed cooling fan — it keeps supplying through the fault and merely raises STATIC INV FAULT to tell the crew. The one exception is overvoltage: above 130 V rms the output would damage the downstream equipment it is meant to protect, so supplying is worse than not supplying, and only then does it trip. Every earlier protection in the chapter says "rather drop the bus than feed a faulty source"; this one says the reverse, for exactly one reason — there is nothing behind it.

4. Fault indication and display

On any of the five protection trips, the inverter sends a permanent ground signal to the SDAC, which drives STATIC INV FAULT on the EWD. The signal is "crew awareness" in character — there is no associated flight-crew procedure; it tells you the last rung has a fault, nothing to action.

[!warning]- The alert level is reported two ways — present both, do not pick one

The two source coordinate systems disagree, and they should not be merged. The AMM 24-28-00 figure caption labels it STAT INV FAULT (Class 1 - Level 1) - EWD/SD — this is the AMM/CMS maintenance classification (Class 1 = a recordable failure). FCOM PRO-ABN-ELEC carries the same alert as L2 — this is the FWC cockpit alert level (Master Caution + single chime). The two scales are not interchangeable: reading the AMM's "Level 1" as a cockpit level (and concluding it is "one rung below GEN FAULT") is an over-reading. What is certain: it is a crew-awareness item with no procedure; the cockpit level for a pilot is FCOM's L2, while the AMM "Level 1" belongs to the maintenance layer. The detailed reconciliation against PRO-ABN belongs to ECAM Warnings Summary.

On the ELEC pages the inverter has two indications. On the ELEC AC page it follows the emergency generator's logic:

"Static Inverter: Same logic as the emergency generator."

Per FCOM DSC-24-20. In the standby state it is the white ◁STAT INV triangle (facing the EMER GEN▷ triangle across the bus); when it is actually supplying, the triangle is replaced by live voltage/frequency parameters. On failure it goes amber. The ELEC DC page carries its own indication:

"Static Inverter: normally white; amber when the static inverter is faulty."

Per FCOM DSC-24-20. The detailed read of both pages is in ECAM ELEC Page.

5. Pilot scenarios and dispatch

Four scenarios put the whole article to work:

1. First seconds of EMER CONFIG. Half of "why the screens did not all go black" is the inverter (the other half is the DC side of the batteries) — case 1, the CSM/G starting-phase bridge. Once the CSM/G is on line, the inverter falls back to its white triangle.
2. RAT scenario, slats out for approach. The CSM/G is dropped to dedicate the RAT to flight controls; the inverter takes over seamlessly (case 2, unconditional above 50 kt), so the AC ESS core does not flicker. On rollout through 50 kt, activation passes to the BAT pushbuttons (case 3) — this is precisely when not to touch them; switching one off now drops the AC ESS bus. (See Emergency Electrical Configuration and Battery-Only Flight.)
3. STATIC INV FAULT displayed (non-overvoltage). It is still supplying — fault-but-alive. The message is a health flag on the last rung, weighting the case to land soon; there is no "isolate" action, and none is wanted.
4. Ground battery-only check. Both BAT pushbuttons must be on for the inverter to work (the 2PC/3PC closing condition). Working with a single battery means there is no AC on the AC ESS bus — that is normal, not a fault.

Dispatch (MEL): another no-go. The static inverter must work — it cannot be dispatched inoperative, and ELEC STATIC INV FAULT is a no-go item under the operator MEL. It sits in the same tier as the ESS TR (see Transformer-Rectifiers) and the main batteries: every part of the "emergency/survival floor" is required serviceable. This mirrors the article's design reading — the last rung, with nothing behind it, is naturally not something dispatch will let you leave behind. The full dispatch picture is in MEL Dispatch View.

Self-test

[!note]- Q1. Trace the supply chain and the output. Which busbar is supplied only on the ground?

BAT 1 + BAT 2 → 2PC / 3PC (battery gate) → DC ESS bus → 2XB → static inverter 1XB (28 V DC inverted to single-phase 115 V AC / 400 Hz, 2 500 VA max) → 3XB (EMER BUS SPLY) → 901XP (AC ESS BUS core) + 931XP-A (26 V AC ESS, via the 3XH transformer), with 905XP-B (AC ESS GND) added only on the ground. In flight on batteries only, AC ESS GND is lost; on the ground (≤ 50 kt) it is restored through the inverter. AC ESS SHED is never fed by the inverter.

[!note]- Q2. State the three automatic activation cases and the two roles of 50 kt.

In the emergency configuration (1XP and 2XP lost): (1) during the CSM/G starting phase (bridging before the emergency generator is on line); (2) when the CSM/G is unavailable (slat extension with the RAT extended) and speed is above 50 kt — unconditional activation; (3) at landing below 50 kt, requiring both BAT 1 and BAT 2 pushbuttons pushed. 50 kt is, first, the air/ground switching speed (activation changes from unconditional to "BAT pushbuttons on"); and second, the dividing line between the two ground battery-only distribution rows, below which AC ESS GND is restored.

[!note]- Q3. List the five protections and identify the only one that opens a contactor. Why only that one?

Fan failure / overheat (> 110 °C ± 5 °C) / output undervoltage (< 103 V rms) / output over- or under-frequency (320 Hz / 480 Hz) — all four raise STATIC INV FAULT only, contactors stay closed** (supply through the fault). The only trip of 2XB / 3XB is **output overvoltage (> 130 V rms), because above 130 V the output would burn the downstream equipment, making supply worse than no supply. Philosophy: the last rung has nothing behind it, so it would rather supply faulted than drop the AC ESS core.

[!note]- Q4. STATIC INV FAULT is displayed. Is the inverter still supplying power?

Unless the fault is overvoltage (in which case the contactors have already tripped), yes — under the other four fault types it keeps supplying. The message is a health flag, not an isolation notice; it is crew-awareness with no procedure. Note that the alert level is reported two ways (AMM figure caption Class 1 - Level 1, a maintenance classification, versus FCOM PRO-ABN-ELEC L2, the cockpit level) — present both, do not pick one.

[!note]- Q5. How do you read the inverter on the ELEC pages?

Standby = the white ◁STAT INV triangle on the ELEC AC page (permanent, not a fault), facing the EMER GEN▷ triangle. Supplying = the triangle is replaced by live voltage/frequency parameters. Faulty = amber (the same logic as the emergency generator). The ELEC DC page also carries a STAT INV indication: normally white, amber when faulty.

Key takeaways

References

Per FCOM DSC-24-10-20-10 (rating 2.5 kVA single-phase, DC ESS bus input, flight/ground activation difference), DSC-24-10-30-30 (battery-only distribution, flight versus ground, AC ESS GND), DSC-24-10-30-40 (ground battery distribution table, V > 50 kt versus V ≤ 50 kt), DSC-24-20 (ELEC AC / ELEC DC page display logic); AMM 24-28-00 (architecture, the three automatic activation cases, the five protection functions and the overvoltage-only-trip NOTE, the SDAC permanent ground signal and STATIC INV FAULT); AMM 24-35-00 (contactors 2PC/3PC supplying the DC ESS bus and the static inverter from the two main batteries); AMM 24-52-00 (the 26 V AC essential segment via the 3XH 115/26 V transformer); FCTM PR-AEP-ELEC and the QRH ELEC EMER CONFIG summary (emergency-configuration context); the operator MEL 24-28 (no-go dispatch). The "last meal of the AC ESS core" reading of the protection trade-off is an integrative synthesis of the verbatim protection behaviour; the alert-level note is presented in both coordinate systems per the manuals' own wording, without choosing between them.

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