Consumer unit replacement: the pre-work checklist
A board change looks like a two-hour job until you open the old one. The work that determines whether it goes well happens before you quote, not on the day.
Updated 2026-09-30
Survey before you price
The single most common cause of a loss-making board change is quoting without lifting the cover. You are taking responsibility for the whole installation the moment you energise your new board, so the survey is where you find what that responsibility includes.
- · Earthing arrangement: TN-S, TN-C-S or TT, confirmed not assumed
- · Measured Ze and prospective fault current at the origin
- · Main protective bonding to water, gas and any other services — present, sized and labelled
- · Meter tail size and length, and whether the supplier's cut-out fuse rating is known
- · Condition of existing circuit conductors, especially rubber or lead-sheathed cable
- · Insulation resistance of each circuit before you commit to a date
- · Space: is there room for a larger enclosure, and is it accessible per Regulation 132.12
Insulation resistance is the deal-breaker
Test every circuit at 500 V before quoting. A circuit reading below 1 MΩ needs investigation, and old lighting circuits with deteriorated insulation frequently do. Discovering this with the new board fitted and the customer waiting is a bad afternoon.
Where readings are poor, quote the board change and the remedial work separately so the customer understands what they are buying.
Earthing and bonding must be right first
You cannot fit a new board onto inadequate earthing and call the job finished. Main protective bonding on a TN-C-S supply is normally 10 mm², and 6 mm² on TN-S depending on the supply conductor size.
Where there is no earth at all, or a TT installation with no electrode, that work comes first and needs to be in the quote. On a TT installation the whole board must be RCD protected and the electrode resistance measured.
The enclosure itself
Regulation 421.1.201 requires consumer units in domestic premises to have an enclosure of non-combustible material, or be enclosed in one. In practice that means a steel board. Plastic boards are no longer acceptable for new installations in dwellings.
Site it so the main switch is between 1350 mm and 1450 mm from finished floor level where reasonably practicable, and keep it out of cupboards under stairs where it will become inaccessible behind stored belongings.
Protective devices: what to specify
The default in 2026 is RCBOs on every circuit. Split-load boards with two RCDs are cheaper but mean one earth fault takes out half the house, and they make fault-finding harder for whoever comes next.
- · RCBOs per circuit, 30 mA Type A as a minimum — Type AC is no longer suitable for most loads
- · Surge protection: required unless a documented risk assessment says otherwise, and hard to justify omitting
- · AFDDs: required for certain higher-risk premises, worth discussing for older installations
- · Correct breaker curves — Type B for general circuits, Type C where inrush demands it
- · Spare ways for an EV charger or heat pump, which the customer will want within five years
On the day
Isolate properly, including arranging with the supplier if the cut-out needs pulling — cutting the seal yourself is not your call to make. Photograph the existing board and the circuit identification before you strip it.
Label every conductor as you remove it. Fit the new board, terminate to torque, and identify each circuit on the schedule with its actual function, not 'sockets 1'.
Testing and certification afterwards
A board change requires an Electrical Installation Certificate, not a Minor Works certificate. You are certifying the new consumer unit and the circuits connected to it.
- · Continuity of protective conductors and R1 + R2 for every circuit
- · Ring final circuit tests where applicable
- · Insulation resistance, circuit by circuit
- · Polarity, then Ze and Zs, and prospective fault current
- · RCD operation at rated current and five times rated current, plus the test button
- · SPD indicator checked and functional test of AFDDs where fitted
- · Building control notification through your competent person scheme
Limitations belong on the certificate
If circuits are buried and untestable, or the customer declines remedial work you recommended, record it as a departure or limitation on the certificate and give them a written copy. That record is your protection.