Total Power Loss: Supply Fault, Main Switch, RCD or Consumer Unit Diagnosis

Quick Answer: Total power loss in a property is most likely a supply fault (check the DNO cut-out fuse and meter), a tripped main switch or RCDs in the consumer unit, or a failed MCB affecting the whole board. Before investigating the consumer unit, always confirm with neighbours whether the outage is street-wide, and check the UK Power Networks (or relevant DNO) outage map. All investigation inside the consumer unit beyond resetting switches must be carried out by a competent person under BS 7671:2018+A2:2022.

Summary

A total power loss call requires systematic diagnosis starting at the supply and working inward. Many apparent "total loss" calls turn out to be a supply outage affecting multiple properties — something any homeowner can check — so a quick neighbour check or DNO outage check at the start of the call can save a wasted journey. When the supply is confirmed present, the fault is almost always in the consumer unit itself.

Modern consumer units (post-2016, metal enclosures as required by Amendment 3 to BS 7671) typically have a main switch at the top, followed by RCDs or RCBOs. A nuisance-tripped RCD can take out half or all of the board depending on how the board is configured. Split-load boards have two RCDs, each protecting half the circuits — a tripped RCD on one side will lose exactly half the circuits, which can be mistaken for a total loss if the customer doesn't understand what they're looking at.

The DNO cut-out fuse (the large sealed fuse in the meter cupboard, before the meter) is the responsibility of the Distribution Network Operator, not the property owner or electrician. Do not open or interfere with the cut-out — this is the energy supplier's equipment. If you suspect this fuse has blown, call the DNO emergency line.

Key Facts

Quick Reference Table

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Check Method Indicates
Neighbours have power Ask neighbour or check DNO app DNO supply fault if all affected
Street lights/other properties Look outside Wide-area supply fault
Meter display lit Check meter display If dark, supply or cut-out fault
Consumer unit main switch position Look at switch — up=on, down=off If off/tripped, reset as first step
RCD positions in board Check all RCDs — up=on, down=off Any down = tripped circuit group
RCBO positions Check each RCBO Single tripped RCBO = single circuit
Main switch thermal trip Try resetting main switch If won't stay on, downstream fault
DNO cut-out fuse Look at fuse carrier (do not open) Visible burn/discolouration = DNO call

Detailed Guidance

Systematic Diagnosis Flow

TOTAL POWER LOSS

Step 1: Is this a supply issue?
├── Do neighbours have power?
│   ├── NO  → DNO fault; call 105. Not your issue.
│   └── YES → Proceed to Step 2
│
Step 2: Check the meter
├── Is the meter display lit/active?
│   ├── NO  → Supply to property is dead
│   │         ├── Check DNO cut-out visually (do not open)
│   │         └── Call DNO emergency 105
│   └── YES → Supply present; fault in consumer unit
│
Step 3: Consumer unit inspection
├── Is main switch in OFF position?
│   ├── YES → Reset main switch (push up)
│   │         ├── Stays on → Test circuits
│   │         └── Trips again → Fault on meter tails or within CU
│   └── NO  → Main switch is on; check RCDs
│
Step 4: Check RCDs / split-load
├── Are any RCDs tripped (down)?
│   ├── YES → Reset RCD
│   │         ├── Stays on → Test. Check for fault on those circuits.
│   │         └── Trips again → Fault on a circuit; use half-split method
│   └── NO  → All up; check individual MCBs/RCBOs
│
Step 5: Check individual MCBs/RCBOs
├── Any in OFF or tripped position?
│   ├── YES → Reset; if trips again, fault on that circuit
│   └── NO  → All up; possible internal CU fault or neutral failure
│             → Call DNO if in doubt about supply

Working Safely at the Consumer Unit

Before opening the consumer unit cover:

  1. Confirm you are a competent person under BS 7671 (or a qualified electrician)
  2. Inform the customer what you are about to do
  3. The consumer unit main switch isolates the circuits, but the meter tails REMAIN LIVE — treat the incoming cables as live at all times
  4. Do not reach above the main switch — the meter tails are unprotected

If resetting doesn't restore power and there is no obvious fault, check for any loose connections or signs of burning inside the consumer unit (cover removed, main switch off). A burning smell, scorched terminals, or discolouration of the busbar indicates a fault requiring full investigation and likely consumer unit replacement.

Split-Load Board — Identifying Which Half Is Lost

A split-load board has two banks of circuits, each protected by an RCD. When one RCD trips, exactly half the circuits lose power:

The customer will typically describe losing some lights but not others, or some sockets but not the cooker. Ask specifically which circuits are dead to confirm it's a split-load situation rather than a total loss.

To identify the faulty circuit: reset the tripped RCD. If it trips again immediately, disconnect all circuits on that RCD (turn each MCB off), then reset the RCD. Turn MCBs back on one at a time until the RCD trips — this identifies the faulty circuit. This is the half-split method described in more detail in the RCD tripping article.

When It Is the DNO Cut-Out Fuse

The cut-out fuse (also called the service fuse or head fuse) is a cartridge fuse rated 60A, 80A, or 100A. It is located in the sealed section of the meter cupboard, usually in a grey or black carrier. Signs that this has blown:

The cut-out is sealed with a tamper-evident seal and is the property of the DNO. Never break this seal — call 105. The DNO will attend (usually within 3 hours for a total supply loss) and replace the fuse for free.

Neutral Failures and Partial Power Loss

A neutral failure at the consumer unit can cause unusual symptoms: some circuits losing power, appliances behaving erratically, lights flickering. This is because neutral is shared across phases; a broken neutral bar connection or a broken neutral in the meter tails can create unbalanced voltages. This is a potentially dangerous situation — if you suspect a neutral failure, advise the customer not to use high-power appliances and call the DNO if the fault appears to be in the supply side.

Frequently Asked Questions

All the switches in the consumer unit are up — why is there still no power?

If all switches are in the on position and there is no power, the fault is on the supply side (cut-out fuse, meter, or meter tails) or there is an internal consumer unit fault (failed busbar connection, failed neutral, corroded terminals). This requires a DNO call for the supply side, or an EICR investigation for the CU side.

Can I reset the consumer unit main switch myself?

Yes, if you are a competent person — or a homeowner resetting their own board. Simply push the main switch back up. If it trips again immediately with no circuits connected, there is a fault within the consumer unit itself that requires professional investigation.

The power came back on by itself — should I be worried?

Intermittent faults can indicate a loose connection, a faulty RCD with poor contact, or supply voltage fluctuations. These should be investigated — an intermittent fault that causes a trip once will usually get worse. Book an EICR or at minimum have the consumer unit terminals checked.

How do I know if the problem is the DNO's or mine?

If your neighbours have power, the problem is yours (inside your property). If your neighbours have no power, it's the DNO's. If you're unsure, call 105 — the DNO will confirm whether there's a known outage at your address.

Regulations & Standards