A consumer unit replacement always needs an Electrical Installation Certificate (EIC), never a Minor Works certificate, because it affects every circuit and the main earthing and bonding. In a dwelling in England or Wales it is notifiable under Part P of the Building Regulations. Before you quote, test what you are inheriting: earthing arrangement, Ze, prospective fault current, and the insulation resistance, continuity and polarity of every existing circuit. The new board needs a non-combustible enclosure in domestic premises (Regulation 421.1.201), Type A RCDs or RCBOs, a surge protection device unless the owner declines (Section 443), correct labelling (Section 514) and verified main bonding before it is energised.
- Certificate: EIC with a full schedule of test results; Part P notification in England and Wales.
- Test before you quote: Ze, PFC, insulation resistance, R1+R2, ring continuity and polarity on every circuit.
- A circuit that fails insulation resistance is repaired or left disconnected and recorded, never reconnected "on an RCD".
- Domestic consumer units must be non-combustible under 421.1.201; fit Type A RCDs/RCBOs and a Type 2 SPD unless declined.
- Check main bonding and earthing first (132.16); record any limitation or defect you could not put right.
Which certificate does a consumer unit change need?
An Electrical Installation Certificate is the BS 7671 document for a new installation, a new circuit, or an alteration affecting more than one circuit or the origin of the installation. A Minor Electrical Installation Works Certificate covers only additions and alterations to one existing circuit, which a consumer unit change is not. See EICR vs EIC and Minor Works: when to use it.
The EIC for a board change must include:
| Part of the EIC | What it must record for a consumer unit change |
|---|---|
| Description and extent | "Replacement of consumer unit and reconnection of existing circuits"; existing wiring not renewed |
| Supply, earthing and bonding | Earthing arrangement, Ze, prospective fault current, main fuse; earthing conductor and main bonding conductor sizes, with confirmation each is connected |
| Comments on existing installation | Any defect found that you were not asked to fix; BS 7671 Chapter 64 requires this to be recorded |
| Schedules | Schedule of inspections for the new board, and a schedule of test results for every circuit: device, cable sizes, R1+R2, ring continuity, insulation resistance, polarity, Zs, RCD operating time |
Is a consumer unit replacement notifiable under Part P?
Yes. In England and Wales, replacing a consumer unit in a dwelling is notifiable under Part P of the Building Regulations. Registered competent persons (NICEIC, NAPIT, STROMA, BSI) notify through their scheme, which issues the compliance certificate and informs the local authority; anyone else must notify building control before starting and pay a fee. See Part P notification explained. Scotland uses the Building (Scotland) Regulations; a board change there does not normally need a warrant but must still be certified to BS 7671.
What must I test before changing a consumer unit?
Discovering faults after the old board is in the skip is the most common cause of a bad board change. Test first, on the survey visit if possible.
Supply and earthing
- Identify the earthing arrangement: TT needs RCD protection and an electrode test; TN-C-S (PME) needs 10 mm² main bonding.
- Measure Ze and prospective fault current (PFC); the new devices' breaking capacity must exceed the measured PFC.
- Check the cut-out and tails; damage or undersizing means involving the DNO or supplier first.
Every existing circuit
| Test | Why before the change |
|---|---|
| Insulation resistance | Finds circuits that will trip a new RCD; the minimum for a 230 V circuit is 1 MΩ, and a healthy circuit is normally well above that |
| Continuity of protective conductors (R1+R2) | Confirms every circuit has a cpc; old lighting circuits without one need a decision before you fit a metal board |
| Ring final continuity (r1, rn, r2) | Finds broken rings and interconnections |
| Polarity | Reversed polarity is common in older installations and must be corrected |
| Borrowed neutrals | Shared neutrals trip RCBOs instantly; landing lights are the usual culprit |
Record the results: they are your evidence of what you inherited, and they let you tell the client before the job what extra work is needed.
What do I do when an existing circuit fails insulation resistance?
Do not reconnect it. Low insulation resistance means current leaking to earth or between conductors, and a new RCD or RCBO will either trip immediately or trip intermittently and get blamed on the new board.
- Separate line-to-earth from neutral-to-earth to see which conductor is faulty.
- Disconnect loads: immersion heaters, cooker elements, outside lights and old fluorescent fittings are frequent culprits, and a reading that recovers when a load is removed is an appliance fault, not a cable fault.
- Split the circuit at junction points to isolate the section.
- Check for a borrowed neutral.
If the fault can be fixed on the day, fix it, retest and record the new reading. If not, leave the circuit disconnected at the new board, label it at the board and the accessories, record it on the EIC as not reconnected with the reason, and tell the client in writing. An RCD is a protective device, not a repair.
What does the new consumer unit need to comply with?
Non-combustible enclosure
Regulation 421.1.201 requires consumer units and similar switchgear assemblies in domestic (household) premises to have an enclosure of non-combustible material, or to be housed in a cabinet of non-combustible material. Steel consumer units satisfy this; keep the enclosure intact with proper glands and close unused knockouts.
RCD and RCBO selection
BS 7671 requires RCDs for socket-outlets up to 32 A (Regulation 411.3.3), domestic lighting circuits (411.3.4), cables concealed in walls at less than 50 mm without earthed mechanical protection (522.6.203) and bathrooms (Section 701). On a board change the practical answer is RCD protection for every final circuit, either a split-load board or individual RCBOs; RCBOs cost more per way but avoid one fault taking out half the house.
The RCD type matters. Type AC RCDs detect only sinusoidal AC residual currents and are unsuitable for loads with electronic power supplies, inverter drives and EV chargers; Regulation 531.3.3 requires the RCD type to suit the residual current the load can produce. Type A is now the practical minimum for domestic circuits, with Type B or F for specific loads.
Surge protection
Section 443, as revised in Amendment 2 in 2022, requires protection against transient overvoltages in most situations and, for a single dwelling, allows it to be omitted only where the owner declares it is not required. Fit a Type 2 SPD with short leads as the default, and if the client declines record that on the certificate. See SPD requirements.
Arc fault detection devices
AFDDs are recommended by BS 7671 for socket-outlet circuits up to 32 A in domestic premises and required in higher-risk residential buildings (HMOs, student accommodation, care homes and residential buildings above 18 m). For an ordinary house, discuss the decision with the client and note it.
Main switch and labelling
The main switch must be a double-pole isolator rated for the supply, typically 100 A domestically; leave spare ways and follow the manufacturer's torque settings. Section 514 sets the labelling requirements: a durable circuit chart, the RCD test notice, the periodic inspection notice with the next inspection date, an SPD label where fitted, and a label where a circuit has been left disconnected.
Do I have to check the bonding and existing installation before adding to it?
Yes. Regulation 132.16 of BS 7671 requires that no addition or alteration is made to an existing installation unless the rating and condition of the existing equipment, including the distributor's, are adequate for the altered circumstances, and the earthing and bonding are adequate. A consumer unit change is squarely within this. In practice:
- Confirm main protective bonding to incoming water, gas and other extraneous-conductive-parts, correctly sized (10 mm² for PME) with measured continuity to the main earthing terminal.
- Confirm the earthing conductor is adequately sized and, on TT, the electrode is sound.
- Check the supply equipment is in acceptable condition and rated for the load, and assess the existing wiring for rubber insulation, missing cpcs and overheating.
Where you find a defect the client has not asked you to fix, BS 7671 Chapter 64 requires you to record it on the EIC in the comments on the existing installation. Where you could not test something, record it as a limitation. A clear record of what you inherited, what you were asked to do and what you left is the difference between a clean assessment and an argument.
Scheme assessors often pick board changes as the sample job because they exercise most of BS 7671 at once. BS 7671:2018+A4:2026 was published on 15 April 2026 and A2 is withdrawn on 15 October 2026; state the edition and amendment on the certificate.
How Certio helps
Certio produces the EICR, EIC, Minor Works certificate and PAT records on the BS 7671 model forms and turns them into branded PDFs. Photograph the consumer unit and the circuit schedule drafts itself; point the phone camera at a Megger MFT-X1 and the reading goes straight into the schedule of test results, or dictate it while your hands stay on the probes. Suggested observation wording with the regulation reference is there for you to check and confirm. The judgement stays yours; the typing goes. Starter is free for 7 days, then £5 a month.