Ground Service Network and MAINT BUS

Overnight cleaning, refuelling and loading need only a handful of electricity. Waking the whole network for that work wastes both power and ground-power-unit capacity. The ground service network is the answer: a small, self-contained "sub-grid" in which external power A feeds one set of service sub-busbars while the rest of the aeroplane stays asleep. The switch that arms it lives on the forward cabin attendant panel — FCOM calls it the MAINT BUS switch, the AMM calls it the GND SELECT CTL switch (FIN 16XX). Two names, one device; this article keeps both straight.

What makes the ground service network worth a dedicated study is that it is the one electrical configuration that energises real loads without closing the main external-power contactor. Understanding it means tracing two separate supply paths (AC and DC) out of the same EXT A receptacle, seeing why the arming switch is magnetically held rather than mechanically detented, and recognising that this whole configuration sits outside the MEL coordinate system because it serves no flight function.

By the end you should be able to answer five questions:

1. Which busbar segments make up the ground service network (AC and DC sides), and what is the supply path to each?
2. What is the precondition for the MAINT BUS switch to stay ON, and what are the three ways it returns to OFF?
3. What jobs can this small network do — the FCOM service list — and what is not on it?
4. On the DC side, why is 601PP treated differently from 602PP / 603PP?
5. With the switch OFF, who owns these busbar segments?

This article reuses three earlier threads: EXT A as the "primary receptacle" (External Power), TR2's "second identity" (Transformer-Rectifiers and DC Network and Transfer), and the service-segment numbering (AC Distribution and Busbars).

1. Architecture — two supply paths out of one receptacle

The ground service network is fed from a single source — external power A — but it splits immediately into an AC path and a DC path, and neither path goes through the main external-power contactor:

GPU ──► EXT PWR A receptacle  (A only — see article 04)
          │
          ├── AC path
          │     via RCCBs 1XX–5XX + contactor 19XX
          │     (19XX = the 115XP BUS GRND SPLY contactor)
          │     ──► AC ground service sub-busbars
          │         (segment count varies with FSN — see table)
          │     note: ECMU 2 commands the RCCBs and 6XX; 19XX is
          │           not in that set — its position is fed back
          │           to ECMU 1
          │
          └── DC path
                via 6XX  (TR2 ground-supply contactor)
                ──► TR2  (115 VAC → 28 VDC — TR2's second identity, art. 10)
                ──► 1PX  (DC-service-bus ground-supply contactor)
                     ├──► 601PP  via C/B 3PX, upstream of the shed
                     │           contactor — never shed
                     └──► 6PX (shedding contactor) ──► 602PP / 603PP
                                                       (some configs)

Switch: GND SELECT CTL / MAINT BUS  (FIN 16XX "SW-MAINT/BUS",
        on the forward cabin attendant panel 5001VE)
Key: the path never closes EPC A — the main network stays dark and asleep

Two points fix the topology. First, the AC sub-busbars are taken straight from the ground power unit through the remote-control circuit breakers and a pair of contactors, with the RCCBs and 6XX commanded by ECMU 2 — the contactor 19XX (the 115XP bus ground-supply contactor) sits in the same path but feeds its position back to ECMU 1, so it is not part of the ECMU 2 command set. Second, the DC path is the ground-supply face of TR2: the GPU drives 6XX, which excites TR2, which then feeds the DC service buses through 1PX. The whole thing is a parallel-universe version of the normal DC service supply you met in article 11 — same TR2, same 6PP-family buses, but a different set of entry and exit contactors.

1.1 The AC service segments are configuration-dependent

The list of AC service sub-busbars is not "the same on every aeroplane" — the AMM gives three tiers by manufacturer serial number (FSN):

"* ON A/C FSN 105-150 The AC ground service network comprises the following sub-busbars: - 111XP, - 113XP, - 115XP, - 212XP, - 214XP, - 216XP."*

Per AMM 24-42-00 §3. The middle tier covers most passenger configurations; the freighter tier (FSN 401-450) carries the most segments. The contrast with the normal configuration closes the loop opened in article 11: in flight / normal operation this DC service family is the 6PP family, supplied by TR2 through 1PN (the flight-supply contactor); in ground service mode the same family is supplied by the GPU through 6XX → TR2 → 1PX (the ground-supply contactor). One TR2, one set of buses, two sets of doors.

2. The MAINT BUS / GND SELECT CTL switch

2.1 A magnetic latch, and three ways back to OFF

The AMM describes the switch in two halves — what holds it ON, and what drops it:

"ON: this position is electromagnetically latched, this position controls the ground service network supply from the ground power unit through EXT PWR A (if available and via the ground power receptacle). … OFF: The switch returns to the OFF position if: it is manually put in this position, the Ground and Auxiliary Power Control Unit (GAPCU) triggers a protection further to a failure of the ground power unit, EXT PWR A is disconnected or powered down."

Per AMM 24-42-00 §4.A. The word to dwell on is electromagnetically latched: the switch is not held by a mechanical detent but by a "parameters-normal" signal from EXT A. FCOM states the precondition for that latch directly:

"The selector latches magnetically, provided the external power A parameters are normal (AVAIL light is on). … OFF: The AC and DC GRND/FLT busbars are connected to AC BUS 2 and DC BUS 2."

Per FCOM DSC-24-20 FORWARD CABIN. So the latter two of the three OFF cases are really one mechanism: lose the holding signal and the switch springs back. If the GPU fails (a GAPCU protection — one of the nine causes in External Power) or is unplugged, the small network closes itself down at once; there is no "left it on overnight" hazard. With the switch OFF, these segments revert to the right-hand network (connected to AC BUS 2 and DC BUS 2) — which is exactly why they carry the "GND/FLT" (ground / flight) dual-state names.

[!warning]- The three-case OFF enumeration is AMM, not FCOM

FCOM's only statement on returning to OFF is one line — "The switch trips, when the external source is removed." (DSC-24-20 FORWARD CABIN). The explicit three-case enumeration (manual / GAPCU protection / EXT A disconnected or powered down) appears only in AMM 24-42-00 §4.A. Do not quote the three cases as FCOM text; they are two layers of description from two manuals.

2.2 Latching only grants authority — ECMU 2 does the connecting

Latching the switch is not the same as energising the buses. The AMM adds the release step:

"When the GND SELECT CTL switch is in the ON position, the ECMU2 controls the RCCBs: the ground service network can then be supplied."

Per AMM 24-42-00 §4.A. The latch grants permission; the actual power-up is ECMU 2 closing the RCCBs. This is the same ECMU 2 that owns EXT A elsewhere in the chapter (see ECMU and Contactor Management).

2.3 Two names, one switch

FCOM's name for the device — MAINT BUS switch — comes from the external-power description, not the forward-cabin page:

"This configuration is selected via the MAINT BUS switch, located in the forward entrance area."

Per FCOM DSC-24-10-30-20. The AMM's FIN table, in turn, lists the switch (position 16XX) with the designation "SW-MAINT/BUS" — so both manuals really do call it a MAINT BUS switch; "GND SELECT CTL" is simply the panel legend the AMM uses in its operating description.

3. What the small network powers

The FCOM service list is the clearest statement of intent — nineteen items, every one of them a transit or overnight job:

"the following services can be energized: Passenger compartment lighting / Galley lighting / Entrance area lights / Lavatory lighting and service / Vacuum cleaner outlets / Flight compartment service outlets / Flight compartment flood lighting / Fuel quantity indications / Refueling / Lower deck cargo compartment lighting and power outlets / Main and nose landing gear compartment lighting / Hydraulic compartment lighting / Landing gear compartment service outlets / Ground call / Equipment compartment lights and service outlets / Navigation lights / Escape slide locking mechanism ice protection / Parking brake / Cargo door hydraulic pump."

Per FCOM DSC-24-20 FORWARD CABIN. Read the list by purpose and it sorts itself: cleaning (lighting + vacuum-cleaner outlets), refuelling (fuel-quantity indications + the refuel system), loading (cargo-compartment power + cargo-door hydraulic pump), and safety (navigation lights + parking brake). There is no flight equipment on the list at all. FCOM states the design intent in one sentence:

"This switch allows maintenance and ground service personnel to energize electrical circuits for ground servicing, without energizing the aircraft's entire electrical system."

Per FCOM DSC-24-20 FORWARD CABIN. That is the whole point of the small network: do ground servicing without waking the aeroplane.

A useful mental model: the MAINT BUS is the night-maintenance breaker behind a shopping mall. The cleaners and stock crew do not need the escalators and air-conditioning of the whole building running — there is a small breaker on the service corridor that lights the hallways, runs the goods lift and powers the cleaning outlets (the service segments). And it is magnetically held: cut or disturb the mains feed (EXT A) and the breaker drops out on its own, so it is never "left on all night". To open the building for business (prepare for flight) you still close the main breaker in the distribution room (the flight-deck EXT A pushbutton).

4. The DC side — 601PP always supplied, 602PP / 603PP on COMMERCIAL

The DC service buses are not equal citizens. The AMM pins down 601PP's "always supplied" status:

"In flight, in normal condition and with the ELEC/COMMERCIAL pushbutton switch (22XN) pushed or released, the electrical power source always supplies the 28DC service bus (601PP) through the TR2 (1PU2) and the DC-service-bus ground-supply contactor (1PX)."

Per AMM 24-43-00 §3. (The source prints "28DC" — read it as 28 VDC. Note also that the COMMERCIAL pushbutton is FIN 22XN, while the commercial relay below is FIN 32XN; the two are easy to confuse.) Reading the schematic, 601PP is taken upstream of 1PX through its own C/B 3PX, ahead of the shed contactor 6PX (per ASM 24-43-01) — which is precisely why it is "always supplied". By contrast, 602PP and 603PP hang downstream of 6PX, and 6PX's own 28 VDC coil supply is not a single element. The AMM lists three in series:

"… through: - The 602PP-603PP circuit-breaker (7PX) - The TR2 ground-supply contactor (6XX) - The electrical commercial relay (32XN)."

Per AMM 24-43-00 §3. So pushing the COMMERCIAL pushbutton (22XN) energises 32XN, which lets 6PX close. The DC service trio therefore divides into two grades: 601PP is "always-on" grade (it lives or dies only with TR2 / 1PX), while 602PP / 603PP are "commercial" grade — they fall with COMMERCIAL OFF, the DC-side extension of the seven-segment shed list from article 14.

[!warning]- The 6PX / 32XN commercial tiering is configuration-dependent

The 6PX / 32XN / 7PX commercial-shedding mechanism appears only in the §3 text of certain FSN ranges (004-050 / 105-150 / 201-250 / 301-350 / 352-400 and similar). Another FSN range (003-003 / 101 / 103-104 / 401-450) does not mention 6PX / 32XN / 7PX in its §3. Do not generalise the commercial tiering as universal aircraft behaviour.

The loss-of-TR2 case ties back to articles 10 and 11:

"A TR2 fault results in the automatic opening of the contactor 1PX, the busbar 6PP is no longer supplied."

Per AMM 24-43-00 §3 / 24-42-00 §4.A NOTE. DC BUS 2 itself can still be recovered by TR1, but the service segments are not on that rescue list — so a TR2 fault takes the entire DC half of the ground service network with it.

5. MAINT BUS ON is not "aircraft powered"

Two things are easy to conflate, and pilots and ground crew should keep them apart: MAINT BUS ON is not the same as energising the aeroplane.

• MAINT BUS / GND SELECT CTL runs the bypass small network: EPC A never closes, the main busbars stay dark, no ECAM page comes alive — only the service segments are powered.
• The flight-deck EXT A pushbutton energises the whole network: EPC A closes, the GAPCU wakes the ECMUs, and every busbar comes up.

Both share the same precondition — EXT A AVAIL (the receptacle light) — but they go to entirely different places. Overnight cleaning uses the former (it spares GPU capacity and wakes only the few loads needed); pre-flight preparation uses the latter.

6. Flight-deck scenarios

1. An overnight turn that is only cleaning and refuelling. Tell the ground crew to use the MAINT BUS (forward cabin attendant panel) rather than asking you to select flight-deck external power — the small network is enough, and it drops out by itself if the GPU fails.
2. The refueller reports "no power on the fuel gauges". Walk the chain: is EXT A AVAIL lit → has the MAINT BUS latched ON (failure to latch = an EXT A parameter problem, the IMR / EPRR territory of article 04)? Fuel-quantity indications and refuelling are both on the FCOM service list, so if the chain is good they should be live.
3. COMMERCIAL OFF in flight, and what the ground service segments do. On the DC side, 602PP / 603PP follow (6PX loses its coil supply) while 601PP holds; on the AC side the commercial-related segments shed too — how many segments depends on the FSN configuration (see the table in §1.1), consistent with the article 14 shed list.
4. TR2 FAULT on the day, and ground service. 1PX opens automatically and the 6PP family loses supply — the DC half of the ground service network is dead (the AC half can still run on its direct RCCB path). When maintenance asks "why are the service outlets dead?", this is the answer.

7. Dispatch and procedure view

The ground service network has no MEL dispatch entry, because it is not a flight-critical item. In the operator MEL the 24-series numbering steps from the external-power-control item straight to the AC-circuit-breaker-monitoring item, with no entry for the ground service network in between. The network is used only for ground servicing (refuelling / cleaning / lighting / parking brake) and plays no part in flight — so it neither affects dispatch nor appears as a MEL item. This is different again from the no-go static inverter (article 13) or the Category C galley (article 15): the ground service network is simply not in the MEL coordinate system at all (see MEL Dispatch View).

[!warning]- Mind the event layer — TR2's two channels are not one dispatch case

In the MEL, TR2 is the flight-supply channel (TR2 through 1PN feeding 6PP in the flight configuration). The ground service supply through 1PX is a separate channel. Do not read "the DC ground service is dead" as a TR2 dispatch problem — they are different contactors feeding the same buses by different routes.

The switch does appear in a normal procedure, though. The QRH "Securing the Aircraft" flow lists GND SELECT CTL sw … AS RQRD (per QRH Normal Procedures, "Securing the Aircraft") — selecting GND SELECT CTL as required before leaving the aircraft or overnighting is exactly where this small network is operated.

Self-test

[!note]- Q1. What busbar segments make up the ground service network, and what is the supply path to each?

AC side: the service segments are FSN-dependent (middle-tier passenger config = 113XP / 115XP / 212XP / 214XP / 216XP / 220XP; FSN 105-150 = the 111XP tier; freighter FSN 401-450 = 8 segments — see §1.1). The GPU feeds them through the EXT A receptacle → RCCBs 1XX–5XX + contactor 19XX (= the 115XP BUS GRND SPLY contactor); ECMU 2 commands the RCCBs + 6XX. DC side: 601PP / 602PP / 603PP, fed GPU → 6XX → TR2 (rectification) → 1PX → 601PP (via C/B 3PX, upstream of 6PX, never shed) / 6PX → 602PP / 603PP (some configs). The path never closes EPC A, so the main network stays asleep.

[!note]- Q2. What is the precondition for the MAINT BUS switch to stay ON, and what are the three OFF cases?

ON = electromagnetically latched, and the latch holds only while EXT A parameters are normal (AVAIL light on). It returns to OFF in three cases: manually put to OFF / the GAPCU triggers a protection following a GPU failure / EXT A is disconnected or powered down. The latter two are really one mechanism — lose the holding signal and the switch springs back — which prevents "leaving it on".

[!note]- Q3. What is on the service list, and what is conspicuously absent?

The full set of transit jobs: passenger and compartment lighting, service outlets, vacuum-cleaner outlets, fuel-quantity indications and refuelling, cargo-compartment power and the cargo-door hydraulic pump, ground call, navigation lights, escape-slide ice protection, and the parking brake. There is no flight equipment of any kind.

[!note]- Q4. How is 601PP treated differently from 602PP / 603PP?

601PP is "always-on" grade — in flight, regardless of COMMERCIAL position, TR2 supplies it through 1PX continuously (taken upstream via C/B 3PX, ahead of the shed contactor 6PX). 602PP / 603PP are "commercial" grade — downstream of 6PX, whose coil is fed through 7PX → 6XX → the commercial relay 32XN, so they drop when COMMERCIAL is released. A TR2 fault opens 1PX automatically and all three are lost.

[!note]- Q5. What is the difference between MAINT BUS ON and "energising the aircraft"?

MAINT BUS ON = a bypass small network (EPC A not closed, main buses dark, no ECAM), powering only the service segments. The flight-deck EXT A pushbutton ON = the whole network energised (EPC A closed, ECMUs woken, all busbars up). Both need EXT A AVAIL, but their uses differ: overnight servicing uses the former, pre-flight preparation the latter. With the switch OFF, the GND/FLT segments revert to AC BUS 2 / DC BUS 2 (the origin of the "GND/FLT" dual-state names).

Key takeaways

References

Per AMM 24-42-00 §2/§3/§4.A (the AC service-segment FSN tiers verbatim; 16XX = "SW-MAINT/BUS"; 19XX = the 115XP BUS GRND SPLY contactor with ECMU 2 commanding the RCCBs and 6XX; the ON magnetic-latch description and the three OFF cases; the ON → ECMU 2 RCCB release; the TR2-fault NOTE opening 1PX); AMM 24-43-00 §3 (the DC trio 601PP / 602PP / 603PP; the 6XX → TR2 → 1PX → 6PX chain; the 601PP "always supplied" verbatim; the 6PX coil through 7PX → 6XX → 32XN, configuration-dependent; the 22XN pushbutton versus the 32XN commercial relay); FCOM DSC-24-20 FORWARD CABIN (the magnetic-latch precondition and AVAIL light; the nineteen-item service list verbatim; OFF reverts to AC BUS 2 / DC BUS 2; the "trips when the external source is removed" line; the "without energizing the aircraft's entire electrical system" design statement); FCOM DSC-24-10-30-20 (the "MAINT BUS switch" naming); ASM 24-43-01 / 24-42-01 (601PP taken via C/B 3PX upstream of 6PX, read off the schematic); the operator MEL and QRH Normal Procedures (no dispatch entry for the ground service network; "Securing the Aircraft" GND SELECT CTL "AS RQRD"). The "lose the holding signal and it springs back" reading, the bypass-versus-full-network contrast, the "not in the MEL coordinate system" framing, and the night-maintenance-breaker 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.