How to replace a consumer unit

Helpful video reference. We use Chris from CJR Electrical's video "Consumer Unit Replacement - Start to Finish" as the video reference here. Chris is a self-employed electrician working in and around Oxfordshire and gets the UK-specific steps right throughout: isolating at the DNO fuse, working to BS 7671, correct RCBO selection and the full test schedule before power is restored.

1. Plan the job and contact the DNO

Before work begins, your electrician should assess the existing installation: number of circuits, cable sizes, the type of earthing (TN-C-S, TN-S or TT) and whether any bonding needs attention. A new circuit schedule is drawn up so the right number of ways and the correct RCBO ratings are ordered.

The DNO (UK Power Networks, Western Power Distribution, etc.) must be contacted to arrange removal of the sealed service fuse. Most DNOs do this free of charge for consumer unit changes. Without this step, the meter tails remain live and the job cannot proceed safely.

2. Confirm dead and disconnect the old board

Once the DNO has removed the service fuse and replaced the service head seal, your electrician proves the meter tails dead with an approved voltage indicator (AVI) before touching them. Never rely on the main switch alone.

With supply confirmed dead, each circuit is disconnected from the old board one at a time. Bare CPC conductors are sleeved in green-and-yellow as they come out. Every cable is photographed in situ first - a phone camera takes seconds and prevents confusion when 12 similar-looking cables are lying loose.

3. Fix and prepare the new consumer unit

The new metal-enclosure consumer unit (required in all domestic properties since the 18th Edition) is fixed to the wall. Meter tails are fed through a cable clamp into the main switch. The DIN rail is populated with RCBOs (or MCBs and an RCD where a dual-RCD arrangement is being used), following the new circuit schedule.

An RCBO-per-circuit arrangement is the current best practice: each circuit has its own overcurrent protection and 30 mA RCD protection, so a fault on the washing machine does not knock out the fridge or the lights.

4. Connect and label every circuit

Each circuit cable is connected to its designated RCBO or MCB. Conductors are stripped to the correct length, CPCs sleeved, and terminals torqued to the manufacturer's specification - a torque screwdriver is needed, not just a firm hand. Every way is labelled clearly on the circuit schedule inside the board door.

Earthing and bonding are checked at this stage: the main earthing conductor goes to the earth bar, and main equipotential bonding conductors to gas and water services must be the correct size for the supply type.

5. Carry out dead tests before restoring supply

With supply still off, the full test schedule starts. For each circuit this includes:

• Continuity of the circuit protective conductor (CPC).
• Ring final circuit continuity (for socket ring circuits).
• Insulation resistance between live conductors and earth (min 1 MΩ at 500 V d.c.).
• Polarity checks.

Results are recorded on the Electrical Installation Certificate as they go. Any circuit failing insulation resistance is investigated before power is restored.

6. Restore supply and carry out live tests

Once the dead tests are satisfactory, the electrician calls the DNO back to refit the service fuse (or the DNO re-attends on the same day). With supply restored, the remaining tests are carried out:

• Earth fault loop impedance (Ze at the origin and Zs at each socket and outlet).
• RCD operation: each RCBO or RCD is tested at 1× and 5× its rated tripping current. The 40 ms requirement at 1×I∆n (30 mA) is the key pass criterion.
• Prospective fault current at the origin.

7. Issue the certificate and notify Part P

An Electrical Installation Certificate is issued, covering the new consumer unit. The work is notified to the local authority within 30 days via the electrician's Part P self-certification scheme. You receive a copy of the certificate and a Building Regulations compliance certificate from the scheme.

Keep both documents: they are required when selling the property and may be asked for by your insurer.

What this typically costs

A standard replacement in a 3-bedroom home runs a full day. The price covers the new 12-way RCBO unit, all labour, the test schedule, the Electrical Installation Certificate and the Part P notification. In east Kent, Richard charges from £600 for a straightforward consumer unit change. Remedial work identified during testing is always quoted separately before proceeding.