Electrical Terms Explained

Consumer unit

Also called: Fuse box, Fuse board, Distribution board

A consumer unit is the modern name for your fuse box — the grey or white box that takes the incoming electricity supply and splits it into the separate circuits that run your lights, sockets, cooker and shower.

The word matters because it marks a generation change. An old board with rewireable fuses (porcelain or plastic carriers with fuse wire in them) protects against fire but not against electric shock. A consumer unit uses MCBs and RCDs instead, and since 2016 the enclosure has had to be metal.

See also: MCB , RCD , RCBO

MCB

Also called: Miniature Circuit Breaker, Trip switch

An MCB is the switch in your fuse box that trips when a circuit draws too much current. It does the job an old fuse wire did, but it resets with a flick instead of being rewired.

It protects the cable, not you. An MCB stops a circuit overheating and starting a fire — it will not react fast enough to a person touching a live wire. That is the RCD's job.

See also: RCD , RCBO , Consumer unit

RCD

Also called: Residual Current Device

An RCD is the device in your fuse box that protects people. It compares the current flowing out with the current coming back, and cuts the power within about 30 milliseconds if any is leaking to earth — for example through someone touching a live part.

This is why an RCD trips for reasons an MCB never would: damp in an outdoor socket, a failing oven element, a nail through a cable. Nuisance tripping is a real fault being detected, not the RCD misbehaving.

See also: MCB , RCBO , Earthing and bonding

RCBO

An RCBO is an RCD and an MCB combined into one device protecting a single circuit.

The practical difference is what happens when something trips. On a board with two shared RCDs, one fault kills half the house. On a board of RCBOs, only the faulty circuit drops — the rest of the house stays on. That is usually worth the extra cost on a rewire or a board upgrade.

See also: RCD , MCB , Consumer unit

Earthing and bonding

Earthing gives fault current a deliberate low-resistance path back to the supply so protective devices trip. Bonding connects metal pipework — gas, water, heating — to that same earth so two metal things you might touch at once can never sit at different voltages.

It is the least visible part of an installation and one of the most common things an EICR flags. Older houses often have undersized or missing main bonding conductors to the incoming gas and water pipes.

See also: EICR , RCD

EICR

Also called: Electrical Installation Condition Report, Periodic inspection, Landlord safety certificate

An EICR is a formal inspection and test of a property's fixed wiring, producing a report that grades every problem found and states whether the installation is safe to keep using.

Landlords in England must have one for each rented property at least every five years and give tenants a copy. Homeowners are not legally required to have one, but buyers, insurers and mortgage lenders increasingly ask. The report ends in one word — SATISFACTORY or UNSATISFACTORY — decided entirely by whether any C1 or C2 items were found.

See also: C1 — Danger present , C2 — Potentially dangerous , C3 — Improvement recommended , FI — Further investigation , EIC

C1 — Danger present

C1 means there is a danger right now and someone is at risk of injury. Exposed live parts, for instance.

A C1 makes the report UNSATISFACTORY and cannot wait. Any competent electrician finding one should make it safe before leaving the property, with your permission, and tell you what they did.

See also: EICR , C2 — Potentially dangerous

C2 — Potentially dangerous

C2 means it is not dangerous at this moment, but it would become dangerous if something else failed or someone made contact. No earth on a lighting circuit is the classic example.

C2 is the code that catches people out. It makes the report UNSATISFACTORY exactly as a C1 does, so a landlord's certificate does not pass until every C2 is put right and the work confirmed.

See also: EICR , C1 — Danger present , C3 — Improvement recommended

C3 — Improvement recommended

C3 means the installation is safe as it stands, but it does not meet the current standard and could be improved.

C3 items do NOT fail a report. An installation can carry several C3s and still be SATISFACTORY. Be wary of anyone quoting to 'fix all the C3s' as though the certificate depends on it — it does not.

See also: EICR , C2 — Potentially dangerous

FI — Further investigation

FI means something could not be determined during the inspection and needs investigating before it can be graded.

Like C1 and C2, an FI makes the report UNSATISFACTORY — because an unknown cannot be called safe. It usually points at something that could not be accessed or a reading that did not make sense.

See also: EICR , C2 — Potentially dangerous

EIC

Also called: Electrical Installation Certificate

An EIC is the certificate issued for NEW work — a new consumer unit, a new circuit, a rewire. It records the design, the installation and the test results, and is signed by the person responsible.

An EIC is not the same as an EICR. The EIC certifies work that has just been done; the EICR reports on the condition of an installation that already exists. Keep the EIC: you will be asked for it when you sell or remortgage.

See also: EICR , Part P

Part P

Part P is the section of the Building Regulations covering electrical safety in homes in England and Wales. Certain work — a new consumer unit, a new circuit, work in a bathroom — is 'notifiable', meaning it legally has to be signed off and recorded with Building Control.

An electrician registered with a Competent Person Scheme can self-certify and notify on your behalf. If unnotified notifiable work is discovered later, regularising it means a council application and an inspection you pay for — which is why it tends to surface, expensively, during a house sale.

See also: Competent Person Scheme , BS 7671 , EIC

BS 7671

Also called: The Wiring Regulations, 18th Edition

BS 7671 is the British Standard that sets out how electrical installations must be designed, installed and tested. It is the technical rulebook the trade actually works to.

It is not retrospective. Work that met the standard when it was installed does not become illegal when a new edition lands — which is exactly why an EICR grades older installations C3 (improvement recommended) rather than failing them for age alone.

See also: Part P , EICR , C3 — Improvement recommended

Competent Person Scheme

Also called: NAPIT, NICEIC

A Competent Person Scheme is a government-authorised body that assesses electricians and allows registered members to self-certify their own notifiable work under Part P.

Membership is assessed and ongoing, not a one-off purchase: work is inspected on a rolling basis and registration is withdrawn if standards slip. Checking an electrician is registered is the single most useful thing you can do before letting anyone near your consumer unit.

See also: Part P

First fix and second fix

First fix is the cabling run through walls, floors and ceilings before they are closed up. Second fix is fitting the sockets, switches and light fittings once plastering and decorating are done.

The split matters for scheduling a rewire: the disruptive, dusty half is first fix, and there is usually a gap before second fix while other trades work.

See also: Rewire

Rewire

A rewire replaces the fixed wiring throughout a property — new cable, new consumer unit, new accessories — rather than repairing sections of the old installation.

The strongest signs a property needs one are rewireable fuses, rubber or fabric-insulated cable, sockets set into skirting boards, and no earth at the light fittings. Age alone is a rough guide; an EICR gives a definitive answer and its findings feed straight into the quote.

See also: EICR , Consumer unit , First fix and second fix

Notifiable work

Notifiable work is electrical work that must be reported to Building Control under Part P. In practice: a new or replacement consumer unit, any new circuit, and work in a room containing a bath or shower.

Adding a socket to an existing circuit in a bedroom is generally not notifiable. Replacing your fuse box always is. If in doubt, ask before the work starts — retro-fixing an unnotified job costs far more than doing it properly.

See also: Part P , EIC