DC ESS BUS FAULT and SHED

DC Bus Faults called the DC ESS bus the "live wire that brings you home" in a DC BUS 1+2 failure. This article is about the world where that live wire itself breaks. ELEC DC ESS BUS FAULT is the heaviest single-bus procedure on the DC side: it drags down three AC-side segments with it, takes the captain-side "number 1" resources offline wholesale, and even changes the method for shutting the engines down on the ground. It is the exact DC mirror of AC ESS BUS FAULT and SHED — and like that article, it raises two distinct ECAM cautions, ELEC DC ESS BUS FAULT (the parent 4PP is not supplied) and ELEC DC ESS BUS SHED (only the sheddable segment 8PP is not supplied).

By the end you should be able to answer five questions: (1) why does losing DC ESS take down AC ESS SHED, DC ESS SHED and AC LAND RCVRY with it? (2) what is the mechanism behind "to shut down the engines on the ground, use the fire pb"? (3) why can the trim-tank fuel become trapped? (4) which two fuel pumps on the INOP list close the loop opened in DC Bus Faults? (5) how does the SHED handling differ from the FAULT?

1. ELEC DC ESS BUS FAULT — the live wire breaks

The caution triggers when DC ESS (4PP) loses power. The first thing the page tells you is not about the DC side at all — it is the collateral on the AC side:

"AC ESS SHED, DC ESS SHED and AC LAND RCVRY buses are also lost."

Per FCOM PRO-ABN-ELEC. Read the topology and the collateral becomes obvious — three segments hang their control on DC ESS, so when DC ESS dies they go dark with it:

        ┌───────────────────────────┐
        │      DC ESS  4PP           │ ◄── ELEC DC ESS BUS FAULT
        │  (the captain-side core)   │
        └─────────────┬─────────────┘
        control power │ (relay chains 1PH / 16XH / 903XP)
        ┌─────────────┼─────────────┐
        ▼             ▼             ▼
   AC ESS SHED    DC ESS SHED   AC LAND RCVRY
   (also lost)    (also lost)   (also lost)

The mechanism (an integrative reading back to DC Network and Transfer and AC Distribution): the supply/control relay chains of these three segments — the 1PH / 16XH / 903XP chains — draw their control power from DC ESS. Lose the control power and the controlled segments all go dark with it. The opening collateral note is itself followed by a procedural pointer, "For AC ESS SHED and DC ESS SHED BUS FAULT, refer to the corresponding cautions" — the three segments each carry their own caution beneath this parent one.

The ECAM procedure, as displayed:

ECAM SWTG DMC ........ 3          ← recover the STATUS page (the DMC chain is hit)
AUDIO SWTG ........... SELECT     ← audio onto the backup
VHF 2 OR 3/ATC 2 ..... USE        ← the number-1 comms row is down
BARO REF ............. CHECK      (with DC 2 also lost: PFD BARO REF STD ONLY)
GPWS SYS ............. OFF        ← GPWS supply abnormal — off, to prevent false alerts
T TK UNUSBL PROC ..... APPLY      ← trim-tank fuel per the trapped-fuel procedure

The three beats of the procedure are recover sight (the STATUS page back via DMC 3), recover speech (audio onto backup, the surviving number-2 comms), and recompute fuel (the trim-tank trapped-fuel procedure). Two L12 notes are the gold of this article:

"Note: 1. To shut down the engines on ground, use the fire pb. 2. Trim tank fuel may be trapped and become unusable, depending on the position of the valves (associated with the forward transfer) at the moment the failure occurs. In this case, Refer to PRO-ABN-FUEL [QRH] TRIM TANK FUEL …"

Per FCOM PRO-ABN-ELEC.

[!warning]- Counter-intuitive: pulling the ENG MASTER does not shut the engine down on the ground

It is tempting to assume the ENG MASTER is the universal ground shut-down. It is not here. The ENG MASTER shut-down chain (the HP fuel SOV breaker) hangs on the 28 VDC ESS bus — with DC ESS lost, the MASTER cannot drive the fuel valve, and the procedure explicitly switches to the fire pushbutton: "To shut down the engines on ground, use the fire pb." The fire pb runs an independent shut-off chain (LP valve close + de-excitation in one stroke). This is not an escalation — it is the designed backup means. Cross-referenced with ATA 26 / 70-16.

[!warning]- Counter-intuitive: the fuel gauge shows fuel, but the trim-tank fuel may be unreachable

The forward-transfer valve's electrical control is on the DC ESS chain — at the instant of failure the valve position freezes, and if it freezes closed the trim-tank fuel cannot be drawn (T TK UNUSBL PROC). FMS fuel predictions are no longer reliable, so recompute the usable fuel per the trapped-fuel procedure (PRO-ABN-FUEL TRIM TANK FUEL). The fuel is "on the gauge" but trapped behind a frozen valve.

What the FAULT takes away

The STATUS highlights: AVOID ICING CONDITIONS (WING A. ICE is INOP — wing anti-ice is lost); in severe icing MIN SPD VLS+10 / G DOT / MANEUVER WITH CARE; GPWS FLAP MODE OFF; SLATS/FLAPS SLOW; BOTH PFD ON SAME FMGEC; CAT 3 SINGLE ONLY; and FUEL CONSUMPT INCRSD + FMS PRED UNRELIABLE (footnote action "Disregard FMS fuel predictions"). Two conditional lines also appear: a conditional speed limit "If ELEC DC BUS 2 FAULT or HYD B SYS LO PR is triggered: MAX SPEED REFER TO PRO-ABN…" and a conditional spoiler note "If above FL 200 and ELEC DC BUS 2 FAULT is triggered: SPD BRK DO NOT USE".

The INOP list is the number-1 resource clean-out — the captain's whole side comes off:

Per FCOM PRO-ABN-ELEC. (The CAPT ND / number-1 display overlap with the AC ESS SHED INOP block is flagged for SME review — the two INOP blocks border each other.) One entry deserves weight: L + R FUEL PUMP 2 — both number-2 pumps draw from DC ESS, so this fault loses them; but conversely they survive a DC BUS 1+2 loss (the L PUMP 2 "see-saw" feeding both engines), which is exactly the loop the DC Bus Faults note opened. The number-1 pumps belong to DC 1 / DC 2 separately.

2. ELEC DC ESS BUS SHED — judge the nature first

This caution triggers when DC SHED ESS (8PP) loses power. Before acting, decide why (see DC Network and Transfer): in a RAT scenario / batteries-only, 1PH opens legitimately as a designed part of EMER CONFIG; only a standalone occurrence is a genuine sub-segment fault. Collateral: AC ESS SHED is lost too (refer to its caution).

The procedure is leaner:

FM SOURCE ............ BOTH ON 2      ← FM 1 is on the ESS side, FM 2 on side 2
FOR LDG .............. FLAP LVR 3      (in severe icing — land in CONF 3)

Note the mirror of DC BUS 2 FAULT's BOTH ON 1 — FM 1 sits on the ESS side and FM 2 on the side-2 side, so you use the survivor's counterpart. The anti-ice-avoidance and severe-icing set is the same as the FAULT version (AVOID ICING / VLS+10 / in severe icing land in CONF 3), and a conditional speed limit appears — note the hydraulic side differs from the FAULT version: "If ELEC DC BUS 2 FAULT or HYD G SYS LO PR is triggered: MAX SPEED…".

The SHED STATUS carries FUEL CONSUMPT INCRSD, FMS PRED UNRELIABLE, MANEUVER WITH CARE, GPWS FLAP MODE OFF (a leaner version of the FAULT set); the SHED INOP includes WING A. ICE, AP 1, FM 1, CAT 3 DUAL, BRAKES SYS 1, L WSHLD HEAT / L WNDOW HEAT. SHED is the "discounted version" of FAULT — it loses only the subset hung on the ESS SHED segment; handle by the actual INOP on the SD.

3. FAULT vs SHED at a glance

4. Flight-deck scenarios

1. Cruise DC ESS BUS FAULT. Three beats: (1) recover sight — ECAM SWTG DMC 3; (2) recover speech — AUDIO SWTG + VHF 2/3; (3) recompute fuel — T TK UNUSBL PROC, the FMS prediction is no longer trustworthy. Avoid icing conditions throughout (wing anti-ice is gone), and carry the heavy INOP picture (the whole captain side, half the fire detection) into the approach plan.
2. Shutdown after landing. Pull the MASTER — no response. Use the fire pb to shut down per the procedure — it is not an escalation but the designed backup shut-off chain (LP valve + de-excitation in one stroke).
3. A standalone DC ESS BUS SHED in cruise. The main network is intact, so this is a sub-segment fault, not EMER CONFIG — FM BOTH ON 2, avoid icing; the severe-icing plan is CONF 3 + VLS+10.

[!warning]- Common misconceptions — predict, then check

Read each statement, decide true or false, then check the truth in brackets.

1. "DC ESS is just one DC bus — losing it affects only the DC side." — False. It drags down three AC-side segments (AC ESS SHED / DC ESS SHED / AC LAND RCVRY) and takes the captain's number-1 resources offline wholesale ("AC ESS SHED, DC ESS SHED and AC LAND RCVRY buses are also lost.") — the heaviest single-bus procedure on the DC side.
2. "Pulling the ENG MASTER shuts the engine down on the ground — it is the basic action." — False. With DC ESS lost the MASTER cannot drive the HP fuel SOV (the chain is on the 28 VDC ESS bus); the procedure switches to the fire pb: "To shut down the engines on ground, use the fire pb."
3. "The gauge shows fuel, so the trim-tank fuel is always available." — Conditional. The forward-transfer valve's electrical control is on the DC ESS chain; at failure the valve freezes, and if frozen closed the trim-tank fuel is trapped (T TK UNUSBL PROC; FMS prediction unreliable).
4. "Fuel pumps are paired, so the number-2 pumps survive faults as well as the number-1 pumps." — Half true. L + R FUEL PUMP 2 both draw from DC ESS: this fault loses both at once; but conversely they survive a DC BUS 1+2 loss (the L PUMP 2 see-saw feeding both engines), while the number-1 pumps belong to DC 1 / DC 2 separately.
5. "DC ESS SHED opening always means a fault." — False. Judge the nature first: on the RAT / batteries-only scenario 1PH opens legitimately as part of EMER CONFIG (a designed shed); only a standalone occurrence is a sub-segment fault.

Self-test

[!note]- Q1. Why does losing DC ESS take down three other segments?

The supply/control relay chains of AC ESS SHED / DC ESS SHED / AC LAND RCVRY (the 1PH / 16XH / 903XP chains) draw their control power from DC ESS — lose the control power and the controlled segments go dark too. The page itself points on: "For AC ESS SHED and DC ESS SHED BUS FAULT, refer to the corresponding cautions."

[!note]- Q2. Why the fire pb for shutdown?

The ENG MASTER shutdown chain (the HP fuel SOV breaker) hangs on the 28 VDC ESS bus — with DC ESS lost, the MASTER is dead, so the fire pb's independent shut-off chain (LP valve + de-excitation in one stroke) becomes the ground-shutdown means: "To shut down the engines on ground, use the fire pb."

[!note]- Q3. Why can trim-tank fuel be trapped?

The forward-transfer valve's electrical control is on the DC ESS chain — at failure the valve position freezes, and if frozen closed the trim-tank fuel cannot be drawn out (T TK UNUSBL PROC; FMS fuel prediction is unreliable, so recompute per PRO-ABN-FUEL TRIM TANK FUEL).

[!note]- Q4. What do the two number-2 fuel pumps reveal?

L + R FUEL PUMP 2 both draw from DC ESS — this article loses them; but conversely they survive a DC BUS 1+2 loss (the L PUMP 2 "see-saw" feeding both engines), while the number-1 pumps belong to DC 1 / DC 2 separately. This is the source of the loop opened in DC Bus Faults.

[!note]- Q5. How does SHED handling differ from FAULT?

FAULT is the heavy procedure (recover sight + speech + trapped fuel + fire-pb shutdown + the number-1 clean-out + CAT 3 SINGLE); SHED is judged first for design-shed (legitimate EMER CONFIG via 1PH) vs a sub-segment fault, then a leaner procedure (FM BOTH ON 2 + the anti-ice set + CONF 3 in severe icing; CAT 3 DUAL). SHED is the discounted version of FAULT.

Key takeaways

References

Per FCOM PRO-ABN-ELEC (DC ESS BUS FAULT full procedure: the three-segment collateral and its "refer to the corresponding cautions" pointer, the fire-pb shutdown note, the trapped-fuel note in full, the conditional speed/spoiler lines, the STATUS set, the number-1 INOP list with both number-2 pumps; DC ESS BUS SHED: FM BOTH ON 2, CONF 3 in severe icing, the conditional HYD G speed line, STATUS/INOP), cross-referenced with ATA 70-16 (the MASTER shutdown chain / HP SOV on the ESS bus) and ATA 28-12/23 (the forward-transfer valve / trapped-fuel procedure). The "three segments fall because the control power is lost" is an integrative reading; the relay-chain attribution is from the DC topology of DC Network and Transfer. This article is the DC mirror of AC ESS BUS FAULT and SHED. DC ESS FAULT is a downstream fault, so this article carries no separate MEL section.

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