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

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

• DNO cut-out fuse — sealed fuse before the meter; only the DNO can replace this; never open the cut-out housing
• Main switch — the first isolator after the meter tails; if this has tripped (some have thermal protection), resetting it restores the whole board
• Split-load boards — have two RCDs protecting two groups of circuits; a tripped RCD loses half the circuits, not all
• RCBO boards — each circuit has its own RCBO; a single circuit fault cannot take out adjacent circuits
• RCD test button — pressing the test button intentionally trips the RCD; if the customer pressed this accidentally, resetting it restores power
• Amendment 3 (2015) — requires all new consumer units in domestic properties to have metal enclosures; plastic boards still legal to maintain but must not be replaced like-for-like
• DNO outage line — 105 (free UK-wide number) connects to the local Distribution Network Operator emergency line
• Meter tails — the cables between the DNO meter and the consumer unit main switch; typically 25mm² or 35mm²; do not touch without appropriate isolation
• BS 7671:2018+A2:2022 — current edition of the IET Wiring Regulations; all fixed electrical installation work must comply
• Main switch rating — typically 100A in modern boards; 80A or 60A in older boards; check rating label inside consumer unit door
• Neutral bar — if a neutral connection has failed at the consumer unit, specific circuits will lose power but the supply side will still read voltage; dangerous to handle
• Inspection and testing — if the cause of the trip cannot be identified by resetting and isolating, a full EICR may be needed

Quick Reference Table

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:

• Left bank (RCD 1): typically lighting upstairs, some sockets
• Right bank (RCD 2): typically ring final sockets, kitchen circuits

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:

• Supply is completely dead (meter display off, no power anywhere)
• No other properties in the area are affected
• Meter test confirms no voltage on the meter output terminals (competent person only)

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

• BS 7671:2018+A2:2022 — IET Wiring Regulations: requirements for all fixed electrical installations, consumer units, and protective devices

• Electricity Safety, Quality and Continuity Regulations 2002 — DNO obligations to maintain supply quality; basis for 105 emergency line

• Building Regulations Part P — Notifiable electrical work in dwellings; consumer unit replacement is notifiable

• HSE Electricity at Work Regulations 1989 — Safe working practices around live conductors and consumer units

• IET Guidance Note 1: Selection & Erection — Consumer unit requirements under Amendment 3

• UK Power Networks — Outage Checker — Check supply faults by postcode

• NICEIC Technical Guidance — Consumer unit diagnosis and safe isolation

• 105.co.uk — UK power cut helpline and DNO contacts

• rcd tripping — Detailed RCD fault-finding with half-split method

• consumer units — Consumer unit types, RCBO vs split-load, metal enclosure requirements

• part p notifications — When electrical work is notifiable