The short answer
A consumer unit replacement follows a defined sequence: the incoming supply is isolated, the old board is removed, the new consumer unit is fitted and wired, the circuits are tested, and the Electrical Installation Certificate (EIC) is issued. The job normally takes around 4–8 hours with the power off for most of that time. The step that most often extends the day is testing: if a circuit fails, the fault must be traced and corrected before that circuit can be connected to the new board and certified. A registered electrician handles the entire sequence, including the Part P notification.
Understanding what actually happens during a consumer unit replacement helps you plan for the day — particularly the period without power and the possibility that testing might uncover something in the existing wiring.
The day in brief
- Total time (straightforward)~4–8 hours
- Power offFor most of the day
- Key document producedElectrical Installation Certificate (EIC)
- Part P notificationMade by the registered electrician
- What extends the jobFaults found on testing
Step 1 — Preparation and isolation
Before any work on the board, the registered electrician isolates the supply. The main switch on the existing board is switched off, and the supply into the board is made safe. It is important to understand that the incoming cables from the electricity meter to the board remain live throughout — only the distribution network operator (DNO) can de-energise these. A competent electrician works safely around this live section using appropriate insulation and protection.
At this stage the electrician will also carry out an initial assessment of the existing installation: checking the cable arrangement, identifying each circuit, noting the existing protective devices, and looking for any immediately obvious defects before work begins. Any questions about the scope of work and how unexpected faults would be handled should be asked at this point if not agreed in advance.
Step 2 — Removing the old board
With the supply safely isolated, the old consumer unit is removed. This involves:
- Disconnecting each circuit's conductors from the old devices, labelling them carefully to identify which circuit each belongs to.
- Removing the old enclosure from the wall, noting the cable positions and any surface or buried conduit arrangements.
- Preparing the mounting surface for the new board, which may involve adjustments to the backboard or trunking if the new board is a different size.
The old unit is retained until the end of the job in case any wiring details need to be confirmed from the original arrangement.
Step 3 — Fitting and wiring the new consumer unit
The new consumer unit — a metal-enclosured unit meeting the 18th Edition requirements of BS 7671 — is fixed to the wall. The main switch (isolator) is fitted first, then the busbars and the individual protective devices (MCBs, RCDs or RCBOs as specified). Each circuit's conductors are then connected:
- Live conductors to the relevant MCB or RCBO.
- Neutral conductors to the neutral bar.
- Earth conductors to the earth bar.
Connections are made to the correct torque settings, as specified by the device manufacturer and required by BS 7671. A loose connection is a potential source of heat and fire; correct torquing is a non-negotiable step. If a surge protection device (SPD) is being fitted, it is installed and connected to the busbar at this stage.
Once wired, the board is visually inspected before testing begins.
Step 4 — Testing
Testing is the step that takes the most time and is the most consequential for whether the job stays within its original scope. With the new board wired, the electrician carries out a series of tests on each circuit before it is energised:
- Continuity of circuit protective conductors: confirms the earth path is intact throughout each circuit.
- Insulation resistance: checks that cable insulation is sound and there are no shorts between conductors or to earth.
- Polarity: verifies live, neutral and earth are connected correctly at every outlet and fitting.
- Earth fault loop impedance: assesses the fault path impedance to confirm protective devices will operate fast enough in a fault condition.
- Prospective fault current: measures the maximum expected fault current at the board.
- RCD / RCBO trip times: tests that residual-current devices disconnect within the required time at their rated operating current.
If any circuit fails, the electrician traces and corrects the fault before retesting. Only circuits that pass can be recorded on the EIC. This is the stage where older wiring sometimes produces surprises — degraded insulation, a poor connection made years ago, or an incorrect polarity that was never caught by the old board.
| Test | What it checks | Why it matters |
|---|---|---|
| Continuity of protective conductors | Earth path is intact | Ensures fault current can flow safely to earth |
| Insulation resistance | Cable insulation is sound | Detects degraded or damaged cables |
| Polarity | Live, neutral, earth are correct throughout | Prevents live connections at switches or sockets |
| Earth fault loop impedance | Fault path resistance | Confirms protective devices operate in time |
| RCD trip times | RCD disconnects within required time | Verifies shock protection is effective |
Standard tests carried out during a consumer unit replacement to BS 7671 18th Edition.
Step 5 — Certification and Part P notification
Once all circuits have passed their tests, the electrician completes the documentation:
- Electrical Installation Certificate (EIC): records the installation details — board type, device ratings, cable types — and the test results for every circuit. The EIC is signed by the electrician and issued to the homeowner. Keep it with your property documents.
- Part P notification: as a registered electrician under a competent-person scheme, they notify the work to the scheme, which in turn notifies the local authority's building control department. The homeowner receives a Building Regulations compliance certificate confirming this notification — usually posted or emailed within a few weeks of the job.
The job is complete when you have both documents. If you receive an EIC but no Building Regulations certificate within a reasonable period, follow up with the electrician or the scheme directly.
Frequently asked questions
How long will I be without power during a consumer unit replacement?
For most of the day — typically 4–8 hours for a straightforward job. The supply is isolated while the old board is removed, the new one fitted and wired, and the circuits tested. It is restored once testing is complete.
What happens if the electrician finds a fault in my wiring?
Any circuit that fails its tests cannot be connected to the new board and certified until the fault is corrected. Minor faults are often fixed on the day; significant wiring defects may require a second visit and a separate repair quote. It is worth discussing upfront how your electrician handles this.
What documents should I receive after a consumer unit replacement?
Two: an Electrical Installation Certificate (EIC) recording the technical details and test results to BS 7671, and a Building Regulations compliance certificate confirming the Part P notification. Both are required for this type of notifiable work. Keep them with your property documents for when you sell.
Sources & further reading
- Electrical Safety First — replacing a consumer unit (best practice guide)
- Total Skills — consumer unit replacement cost and process
- IET — BS 7671 18th Edition wiring regulations overview
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property and installation. They are guidance, not a quotation.