DC Network and Transfer

Transformer-Rectifiers dealt with the sources of DC power — the four TRs that make 28 V DC from the AC network. This article deals with the network those sources feed: how the DC busbars are spread across three physical power centres, what each of the five transfer contactors actually does, and how the DC ESS bus draws from a three-source supply spectrum that degrades step by step.

One idea unlocks the whole chapter. On the AC side the "corridor" through which a healthy source rescues a starved bus is the BTC1–SIC–BTC2 chain (see Network Priority and Normal Supply). On the DC side the corridor is the DC BAT BUS (3PP). Every mutual-rescue current — TR1 covering TR2, TR1 covering the ESS TR, a battery feeding the essential network — physically passes through 3PP. Hold that picture and the five contactors stop being a list to memorise and become a set of gates around one junction.

1. The three power centres

The DC network does not live in one box. The AMM distributes it across three physically separate power centres (per AMM 24-60-00):

The geography is itself a piece of design language (integrative reading): normal supply lives with normal supply (710VU); the survival network lives by itself in a centre whose very name is emergency power centre (740VU); and the APU forms its own small republic near the tail (5000VU), so the APU battery sits close to the starter and feeds the heavy start current over short, light cable instead of a long run forward.

2. The DC network single-line picture

Read the network from the sources at the top down into the busbars, then down again into the essential layer. Power normally flows down from each TR into its own bus; in degradation, the rescue current flows sideways through 3PP into whichever bus has been starved.

   TR1                 TR2                     APU TR
    │ 5PU1              │ 5PU2                  │
    ▼                   ▼                       ▼
┌──────────┐     ┌──────────┐  1PN  ┌──────────┐   ┌────────────────┐
│ DC BUS 1 │     │ DC BUS 2 │──────►│ DC SVCE  │   │ 309PP / 909PP  │
│   1PP    │     │   2PP    │       │ BUS 6PP  │   │ (APU TR; 309PP │
└────┬─────┘     └────┬─────┘       └──────────┘   │  ← APU BAT on  │
     │ 1PC1           │ 1PC2                        │  APU TR loss)  │
     │ (bidirectional)│ (failure-only)             │ 709PP APU HOT  │
     ▼                ▼                             └────────────────┘
┌──────────────────────────────┐     BAT 1 ─ 6PB1 ─┐
│       DC BAT BUS  3PP         │◄──────────────────┤
│   (the DC-side "corridor")    │     BAT 2 ─ 6PB2 ─┘
└──────────────┬───────────────┘
               │ 4PC  (recovery gate; latched open by 2PE on ESS TR overcurrent)
               ▼
┌──────────────────────────────┐
│       DC ESS BUS  4PP         │◄── 3PE ───── ESS TR (normal / CSM/G in EMER)
│                               │◄── 2PC/3PC ─ batteries direct ──► STAT INV
└───┬───────────┬──────────────┘
    │ 1PH       │ 9PH
    ▼           ▼
 DC SHED     407PP / 805PP         470PP ◄─ 14PH ─ CIDS emergency functions
 ESS 8PP     LAND RECOVERY

Hardware verification — the ASM block diagram

The topology above was cross-checked against the ASM 24-60-01 DC Load Distribution Block Diagram (read directly from the figure). The block diagram resolves one further level — the sub-busbars hanging off each main bus through their own contactors. A pilot need not memorise these finer designations, but they confirm the topology is drawn correctly and give maintenance a reference:

Three points the figure confirms: (1) 3PP is simultaneously the crossroads and is labelled "DC BUS 3" — the same busbar under a second name, which is exactly the "3PP is the DC junction" mental model; (2) the emergency-configuration note at the foot of the diagram reads "28VDC FROM ESS TR OR FROM BATTERIES", confirming the three-source spectrum of §4; (3) the batteries reach the hot buses and the refuel busbars through shunts — the basis for refuelling without energising the whole aircraft (covered in Batteries and the BCL).

3. The five transfer contactors

Five contactors gate the DC junction. Their switching semantics are the heart of this article.

1PC1 (DC BUS 1 / BAT BUS supply) — the bidirectional everyday contactor. It carries normal current down into 3PP, and reverses to rescue 1PP when TR1 is lost:

"This contactor is controlled by the Electrical Contactor Management Unit 1 (ECMU1) and ECMU2 and serves to supply: ‐ in normal configuration, the DC BAT BUS (3PP) from the DC BUS 1 (1PP), ‐ in case of Transformer Rectifier 1 (TR1) fault or TR1 contactor open, the DC BUS 1 (1PP) from the DC BAT BUS (3PP)."

Per AMM 24-35-00. So 1PC1 is closed and conducting in normal operation, and merely changes direction when TR1 drops — it is not a "failure-only" device.

1PC2 (DC BUS 2 / BAT BUS supply) — the failure-only contactor. It stays open in the normal configuration and closes only in three fault cases:

"This contactor controlled by the ECMU1 and ECMU2 is only used in failure conditions: ‐ in case of TR1 fault or TR1 contactor open, it is used to restore the DC BAT BUS (3PP) from the DC BUS 2 (2PP), ‐ in case of TR2 fault or TR2 contactor open, it is used to restore the DC BUS 2 (2PP) from the DC BAT BUS (3PP), ‐ in case of 1PC1 fault, it is used to restore the DC BAT BUS (3PP) from the DC BUS 2 (2PP)."

Per AMM 24-35-00. Note the third case: 1PC2 also covers a fault of 1PC1 itself, taking 2PP→3PP to bypass the failed everyday contactor. When Transformer-Rectifiers said "TRs substitute for one another through the 1PC chain", the precise statement is this: the rescue current always enters 3PP first and then flows out of 3PP toward the starved bus — the DC BAT BUS is the DC-side crossroads.

[!note]- A useful mental model — the central roundabout

Treat the DC BAT BUS (3PP) as the roundabout at the centre of a small estate. Three exits lead to three districts (1PP, 2PP, 4PP); in normal times everyone uses their own main road (each TR feeds its own bus directly) and the roundabout carries only routine traffic (battery charging, supplying 3PP's own residents). When a main road collapses (a TR is lost), every relief convoy detours through the roundabout (the 1PC / 4PC chain). The roundabout has its own rules: if the district is on fire (ESS TR overcurrent) the barrier stays shut (2PE latches 4PC open); and in a city-wide blackout (both main TRs lost) the roundabout runs on the emergency battery for 7 seconds, then goes dark to save the battery.

4PC (BAT BUS / ESS BUS supply) — the recovery gate. When the ESS TR is lost and both main TRs are available, 4PC restores DC ESS from 3PP:

"In case of ESS TR loss, and only if the TR1 and TR2 are available, this contactor is used to restore the DC ESS BUS (4PP) from the DC BAT BUS (3PP)."

Per AMM 24-35-00. The "only if TR1 and TR2 are available" precondition is the reason DC ESS is dropped when only one main TR survives — TR1 must first hand its own job to TR2 before it can carry the ESS TR's load (the asymmetric-recovery logic established in Transformer-Rectifiers). And on an ESS TR overcurrent specifically, 4PC is latched open through relay 2PE — the executing element of the latch-out logic — so a faulty ESS TR cannot be fed from a healthy 3PP (per AMM 24-35-00 §4).

2PC + 3PC (ESS BUS / static-inverter supply) — the battery gate. This pair connects the two main batteries directly to DC ESS and the static inverter:

"These contactors serve to supply the DC ESS BUS (4PP) and the static inverter from the two main batteries in the following cases: (a) In emergency configuration: ‐ as long as the Constant Speed Motor/Generator (CSM/G) is not connected to the essential network (CSM/G starting phase), ‐ when the CSM/G is unavailable, i.e.: from slat extension with the RAT extended. (b) On the ground: When the batteries are the only power source available and the BAT1 and BAT2 line contactors 6PB1 and 6PB2 are closed."

Per AMM 24-35-00. Two reading notes. First, the AMM gives the event ("CSM/G starting phase"), not a duration — the often-quoted "about 10 s" is the length of the CSM/G start sequence, sourced from the emergency-generator article, not from this section. Second, the control inputs include aircraft speed: FCOM gives the battery-to-DC-ESS connection as a three-state rule:

"The batteries are connected to the DC ESS BUS when batteries only are supplying: ‐ In flight, ‐ On ground, speed above 50 kt, ‐ On ground, speed below 50 kt, provided they are both selected auto."

Per FCOM DSC-24-20. The DC BAT BUS itself is treated differently on the ground: on batteries only, "Provided they are both selected AUTO, batteries supply the DC ESS BUS, DC BAT BUS and DC LAND RECOVERY" applies at ≤ 50 kt — i.e. the DC BAT BUS is brought back into supply only below 50 kt, not above it (per FCOM DSC-24-20).

A ground protection red line is buried in a NOTE:

[!warning]- Counter-intuitive: on the ground, one battery below 23 V drops the whole essential network

"On ground, if one battery voltage < 23 V, the essential network is automatically disconnected." (Per AMM 24-35-00.) On the ground there is no reason to drain a battery keeping the essential network alive — the charge is better kept to start the APU. Do not confuse the two 23 V thresholds. This NOTE disconnects the essential network (no stated delay). The threshold treated in Batteries and the BCL — battery voltage < 23 V for 16 s, on the ground, with main power lost — opens the battery contactor (battery itself disconnected). Same number, different object and different delay: one drops the ESS network, the other drops the battery.

4. The DC ESS three-source spectrum

DC ESS is the most heavily defended DC bus, drawn from three sources in a degradation order (per AMM 24-62-00):

The point worth holding is that the ESS TR does not leave the stage in the emergency configuration — this is directly supported in the AMM (not an inference): AMM 24-62-00 lists the ESS TR, via 3PE, still supplying DC ESS "in emergency configuration during the CSM/G operation." The CSM/G produces AC, so DC ESS must still rely on the ESS TR to rectify it to DC — which is why the emergency supply list in Emergency Generator reads "via the ESS TR". So "DC ESS fed directly from the batteries" happens only in the darkest case, when there is not even a CSM/G.

The fall-back boundary closes the spectrum (an AMM NOTE in agreement with the TR article): with the ESS TR lost and TR1 or TR2 also lost, the DC ESS network is no longer supplied — unless the aircraft has entered the emergency configuration, in which case the batteries take over through 2PC/3PC.

5. Main distribution and the APU buses

The main DC network is an iron triangle (per AMM 24-61-00): 1PP + 3PP fed by TR1; 2PP + 6PP fed by TR2; a single loss is covered by substitution through the 3PP crossroads, while a double loss takes everything (3PP surviving only 7 seconds on the battery — "In the event of TR1 and TR2 loss, DC BUS 1 and DC BUS 2 are immediately lost and DC BAT BUS after 7 seconds", per AMM 24-30-00).

The DC SERVICE BUS (6PP) has a dual identity: in the normal configuration it is fed from TR2 through 1PN (the SERVICE BUS FLT SPLY contactor); in the ground-service configuration it is energised by the GPU driving TR2 directly, without waking the whole network (covered in Ground Service and Maintenance Bus).

The APU forms its own republic:

"Busbars 309PP and 909PP are supplied by APU TR; in case of loss of the APU TR, the busbar 309PP is supplied from the APU BAT."

Per AMM 24-61-00. The APU TR is never reconfigured by another TR (its "orphan" property from the TR article), but 309PP has the APU battery as a back-stop — so APU start capability does not vanish with the APU TR.

6. 470PP — the CIDS emergency override supply

The most counter-intuitive supply path in the DC network keeps current flowing to a sub-busbar after the crew has switched the batteries off:

"After an emergency configuration, when the EMER LT switch is in the ARM or ON position, the 6PP busbar is lost and the BAT1 and BAT2 pushbutton switches are in the OFF position, the two main batteries supply the 470PP sub-busbar."

Per AMM 24-62-00. What does 470PP actually feed? The CIDS emergency functions — not the emergency lighting. AMM 23-73-00 (Cabin Intercommunication Data System) names 470PP explicitly as the CIDS essential busbar:

"To supply all CIDS components with electrical power this essential busbar has to be energized: ‐ 470PP."

and lists the minimum functions kept on essential power as "Passenger Address, Cabin Interphone, Smoke Indication, EVAC." (Per AMM 23-73-00.) So the EMER LT switch being in ARM/ON is merely one of the conditions for 470PP supply — an indication that an evacuation is imminent — not the load itself. The emergency lighting proper is an independent ATA-33 system with its own power source and has nothing to do with 470PP.

[!warning]- Counter-intuitive: the batteries are OFF, yet the PA and EVAC signal stay alive

With both BAT pushbuttons OFF, the batteries still feed 470PP (the CIDS emergency segment, via contactor 14PH). This is the hard guarantee of the evacuation case: the crew switches the batteries off by procedure, but the cabin address and evacuation signal (CIDS) must not go silent with them. The FCOM line for the BAT pb OFF state — "OFF: the DC ESS BUS is not connected to the battery (except in flight in emergency configuration)... Hot buses remain supplied" (per FCOM DSC-24-20) — is the other face of the same design philosophy: OFF cuts the BCL control chain, not the last supply the evacuation needs. (Hot buses and 470PP are different busbars; what they share is the principle "switching the batteries off still preserves what evacuation requires.") Do not suspect "the power-down was not done cleanly" when the cabin PA still works.

7. The 1PH shed and the 9PH recovery

Two further contactors configure the DC essential network in degradation (per AMM 24-62-00).

1PH sheds the DC SHED ESS bus (8PP) on two precise triggers:

"A part of this network (busbar 8PP and the associated sub-busbars) is shed through the contactor 1PH: ‐ in emergency configuration when the network is supplied by the CSM/G powered by the RAT PUMP ‐ or by the batteries only."

Per AMM 24-62-00. That is: 8PP is shed by design (1) when the CSM/G is pressurised by the RAT pump, and (2) on batteries only. This is the mechanism behind the "DC ESS BUS SHED can be legitimate" entry in the caution table of §8 — under the EDP scenario, 8PP is not shed.

9PH recovers the LAND RECOVERY busbars (407PP / 805PP):

"In emergency configuration (CSM/G), a part of the DC essential network (sub-busbars 407PP and 805PP) is recovered through the contactor 9PH as soon as the LAND RECOVERY pushbutton switch (8XC) located on the EMER ELEC control panel 211VU (overhead panel) is pushed."

Per AMM 24-62-00. The LAND RECOVERY pushbutton is 8XC on the overhead EMER ELEC panel 211VU; pressing it recovers 407PP / 805PP through 9PH — the DC-side execution of the LAND RECOVERY action described in Emergency Generator and AC ESS Feed and Transfer.

[!warning]- Counter-intuitive: on batteries only, LAND RECOVERY needs no pushbutton

"On batteries, the LAND/RECOVERY busbars are always supplied." (Per AMM 24-62-00.) In the battery-only configuration the LAND RECOVERY busbars are energised regardless of the pushbutton — matching the FCOM distribution table, where the batteries-only row carries DC LAND RECOVERY "whatever the position of the LAND RECOVERY pushbutton". The pushbutton gates recovery only under the CSM/G scenario.

8. The six DC bus cautions

Six DC busbars each raise their own EWD caution (per AMM 24-61-00 and 24-62-00):

All six are MASTER CAUT + single chime + automatic call-up of the ELEC DC page. Two reading notes. The DC BUS 1+2 caution involves a double-EIVMU interaction unpacked in the fault article (integrative; traced to DC Bus Faults). And the "SHED" wording on 8PP matters: it can be shed legitimately (the RAT scenario / batteries only, where 1PH opens by design — see §7), so when the caution appears the first question is "is this a designed shed or a genuine fault?" before acting (unpacked in DC ESS Bus Fault and Shed).

9. Pilot scenarios

1. TR1 FAULT, reading the ELEC DC page. Trace the current path in your head: TR2 → 2PP → 1PC2 → 3PP → 1PC1 → 1PP. Both tie contactors closed, all three buses green = reconfiguration succeeded.
2. DC BAT BUS FAULT on its own. 3PP has been lost by itself (e.g. a dual 1PC failure). The battery charging path is broken and the main APU start path is affected, but 1PP / 2PP / 4PP are still each fed by their own TR.
3. The first seconds of EMER CONFIG. Where does DC ESS get its power? 2PC/3PC are open and the batteries feed it directly — the DC-side channel of the "transition-phase battery bridge" in the emergency-generator timeline. Once the CSM/G comes on line, the ESS TR resumes duty and 2PC/3PC drop out.
4. Evacuation procedure, switching BAT 1+2 off. The cabin PA and evacuation signal (CIDS) are still alive — the 470PP override is working (EMER LT in ARM/ON). Do not doubt the power-down because "the batteries are off, yet the cabin still has PA". The emergency lighting is a separate ATA-33 system with its own supply, independent of 470PP.
5. A long ground job on batteries only. Watch the 23 V red line — when either battery falls below 23 V the essential network disconnects automatically (the system saving charge for you). That is not a fault; it is the cue to connect external power or start the APU.

Self-test

[!note]- Q1. Name the three power centres and what each holds.

710VU main electrical power centre (avionics compartment) — network 1 (1PP / 3PP) and network 2 (2PP / 6PP). 740VU emergency electrical power centre — the ESS network (4PP / 8PP), the DC ESS switching contactors, and the main-battery contactors. 5000VU APU power centre (bulk cargo compartment) — 309PP / 709PP / 909PP and the APU battery / starter contactors.

[!note]- Q2. How do 1PC1 and 1PC2 divide the work?

1PC1 is the bidirectional everyday contactor — it feeds 1PP→3PP normally and reverses to 3PP→1PP to rescue DC BUS 1 on a TR1 fault. 1PC2 is failure-only — open in normal operation, closing only in three cases: TR1 fault (2PP→3PP), TR2 fault (3PP→2PP), and a 1PC1 fault (2PP→3PP, bypassing the failed contactor). All rescue current passes through the 3PP crossroads, and both contactors are controlled by ECMU1 and ECMU2.

[!note]- Q3. Give the DC ESS three-source spectrum.

(1) ESS TR via 3PE — normal (fed from AC1, on AC1 loss from AC2; in the emergency configuration it still rectifies the CSM/G output and does not leave the stage). (2) TR1 via 1PP→3PP→4PC — when the ESS TR is lost (not by overcurrent) and both main TRs are available; an ESS TR overcurrent latches 4PC open through relay 2PE. (3) Batteries via 2PC/3PC — CSM/G start-phase bridging, after slat extension on the RAT, and on the ground on batteries only (V > 23 V, with the speed gate).

[!note]- Q4. Explain the 470PP override logic.

After an emergency configuration, with the EMER LT switch in ARM or ON, 6PP lost, and both BAT pushbuttons OFF, the two main batteries still supply 470PP — the CIDS emergency segment (Passenger Address / EVAC / cabin interphone / smoke indication, ATA-23-73, via 14PH). This keeps the cabin PA and evacuation signal alive after the crew powers the batteries down. The EMER LT switch is only a condition; the emergency lighting proper is an independent ATA-33 system. The design parallels "hot buses remain supplied with BAT OFF".

[!note]- Q5. List the six DC bus cautions and the catch on the SHED one.

DC BUS 1 FAULT (1PP) / DC BUS 2 FAULT (2PP) / DC BUS 1+2 FAULT (both) / DC BAT BUS FAULT (3PP) / DC ESS BUS FAULT (4PP) / DC ESS BUS SHED (8PP). All are MASTER CAUT + single chime + automatic ELEC DC page. 8PP's SHED has designed cases (the RAT scenario / batteries only, where 1PH opens by design), so judge "designed shed vs genuine fault" before acting.

Key takeaways

References

Per AMM 24-35-00 (1PC1 bidirectional supply, 1PC2 three failure cases, 4PC recovery gate, 2PC/3PC battery-supply conditions, the ground 23 V essential-network NOTE), AMM 24-30-00 (TR1/TR2 symmetric recovery, DC BAT BUS lost after 7 s on dual TR loss, APU TR fourth-unit role), AMM 24-60-00 (the three power centres and the three-level distribution), AMM 24-61-00 (main-network triad, 6PP dual identity, APU 309PP/909PP supply and APU BAT back-stop, DC main-network cautions), AMM 24-62-00 (DC ESS three-source spectrum with the ESS TR retained in EMER CONFIG, the 470PP override supply, the 1PH shed and 9PH LAND RECOVERY logic via pushbutton 8XC, the batteries-only LAND RECOVERY note), AMM 23-73-00 (470PP as the CIDS essential busbar and its minimum functions), FCOM DSC-24-20 (battery-to-DC-ESS three-state rule, the ground ≤ 50 kt DC BAT BUS supply, the BAT pb OFF / hot-buses-remain-supplied statement); the topology and sub-busbar designations were cross-checked against the ASM 24-60-01 DC Load Distribution Block Diagram read directly from the figure. The geographic reading of the three power centres, the roundabout mental model, the "hot buses ≈ 470PP same-family design" parallel, and the DC BUS 1+2 / EIVMU linkage are integrative synthesis, drawing no fact from outside the library.

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