Consumer Unit Replacement

Purpose

This document covers the procedure for replacing a domestic consumer unit (fuseboard). Consumer unit replacement is a notifiable job under Part P — it must be self-certified via our competent person scheme.

When Is a Consumer Unit Replacement Needed?

Old fusebox with rewirable fuses — no RCD protection, does not meet modern standards
Nuisance tripping on a split-load board — full RCBO board gives each circuit independent protection
Insufficient circuits for new additions (EV charger, heat pump, etc.)
Consumer unit damaged or failed
Landlord requiring compliance before letting

Before Starting — Survey and Quote

Establish:

Supply size: What is the main fuse / cut-out rating? (Typically 60A, 80A, or 100A for domestic)
Number of circuits: How many circuits are present? The new board must accommodate all of them plus spare ways.
Earthing arrangement: TN-S, TN-C-S/PME, or TT? This affects the type of SPD required.
Meter tails condition: Are the tails in good condition? If they need replacing, quote accordingly.
Existing installation condition: A consumer unit replacement on deteriorating wiring requires an EICR to confirm the existing installation is satisfactory.

Materials

Consumer unit: Correct number of ways + 2–4 spare. Metal or metal-lined enclosure (BS EN 61439-3). Hager, Schneider, Wylex, Crabtree.
RCBOs: One per circuit — rated to match the circuit (6A lighting, 32A ring, 40–45A cooker/shower)
Meter tails: 25mm² — buy more than needed
Earth cable: 16mm² green/yellow
SPD (Surge Protection Device): Type 2 SPD required for TN-C-S (PME) earthing under 18th Edition Amendment 2
Cable clips, trunking for meter tail protection

Procedure

Step 1 — Notify via Competent Person Scheme

Step 2 — Isolate and Test Dead

Switch off the consumer unit main switch. Test that all circuits are dead using a voltage indicator before working — do not assume.

Wilsons cannot isolate the supply side of the meter. If the meter tails must be disconnected, contact the DNO for a temporary disconnection.

Step 3 — Label All Circuits

Before disconnecting anything, label every circuit cable. Take a photograph of the existing board wiring for reference.

Step 4 — Disconnect the Old Board

Switch off the main switch
Disconnect each circuit cable — label as you go
Disconnect the earthing and bonding conductors
Disconnect the meter tails at the main switch
Remove the old consumer unit

Step 5 — Prepare and Fix New Consumer Unit

Fix the back box to the wall — confirm level and that meter tails will reach
Install consumer unit body
Install RCBOs in correct positions (larger loads near the main switch for shorter internal runs)
Fit the SPD as specified in the consumer unit instructions

Step 6 — Connect the Incoming Supply

Meter tails → main switch: Brown/red (L) and Blue/black (N)
Earthing conductor → earth bar
Main bonding conductors (gas and water) → earth bar

Meter tails must be protected where exposed — use trunking or conduit.

Step 7 — Connect the Circuits

For each circuit:

Live (brown/red) → RCBO input terminal
Neutral (blue/black) → neutral bar
Earth (green/yellow) → earth bar

Tighten all terminals to the manufacturer's torque specification — loose terminals are a fire risk.

Step 8 — Pre-Energisation Tests

Before switching on:

Continuity of earth conductors: Test continuity from each circuit earth back to the earth bar.

Polarity: Confirm live and neutral are not reversed on each circuit.

Insulation resistance: With all RCBOs off and main switch off, test L-E, N-E on each circuit. Minimum: 1 MΩ. Disconnect LED drivers and sensitive electronics before testing.

Record all readings.

Step 9 — Energise and Final Tests

Switch on main switch
Switch on each RCBO — confirm each circuit comes on without tripping
Confirm each circuit supplies the correct area
Test each RCBO with the test button — confirm trip and reset
RCD trip time test using MFT — record results
Confirm SPD indicator shows healthy

Step 10 — Documentation

Electrical Installation Certificate (EIC): Required for a new consumer unit. Complete all sections including test results.

Notify via NICEIC to self-certify and generate the Building Regulations compliance certificate.

Leave with customer: Copy of EIC, consumer unit operating manual, instructions for resetting RCBOs.

Common Issues

Problem	Fix
RCBO trips immediately on energising circuit	Fault on that circuit — do not leave on; investigate
Insulation resistance below 1MΩ	Fault in circuit wiring — locate and repair before energising
Neutral bar overcrowded	Use double-entry terminal or additional neutral bar
Meter tails won't reach new board position	Extend with same cable size (25mm²) and correct connector, or reposition board
SPD alarm light on	SPD has operated — check if replacement needed