RCDs explained: why your EICR mentions them

An RCD (residual current device) is a safety switch in your consumer unit that turns the power off in a fraction of a second if electricity leaks where it shouldn’t — through a damaged cable, a faulty appliance or a person touching something live. Your EICR checks whether the circuits that need RCD protection have it, and tests that each RCD switches off quickly enough. A missing or faulty RCD is usually coded C2 (potentially dangerous) or C3(improvement recommended), depending on the risk.

What an RCD does

In a healthy circuit, all the current that flows out along the line (live) conductor comes back along the neutral. An RCD constantly compares the two. If some current is missing, it must be leaking somewhere — to earth through a fault, or through someone’s body — and the RCD switches the circuit off.

The Health and Safety Executive (HSE) puts it like this: an RCD “detects some, but not all, faults in the electrical system and rapidly switches off the supply, reducing the potential for injury caused by a common type of electric shock.”

That “some, but not all” matters. An RCD can’t prevent every kind of shock, and it doesn’t protect against overloads or short circuits — that’s the job of the circuit-breakers or fuses. An RCBO combines both jobs in one device for a single circuit.

Why 30 mA?

RCDs are rated by how much leakage makes them trip. For protecting people, HSE advises a tripping current of not more than 30 milliamps (mA) — a tiny current compared with what a kettle draws. In HSE’s words, an RCD rated at no more than 30 mA “limits the energy in a particular type of electric shock and can save your life.”

That’s why the wiring regulations, BS 7671, ask for 30 mA RCDs as additional protection wherever people are most at risk.

Where RCD protection is expected

The inspection checklist on every EICR looks for 30 mA RCD protection on:

• socket-outlets rated up to 32 amps (the few exceptions allowed by risk assessment apply outside homes);
• circuits supplying equipment used outdoors;
• cables buried less than 50 mm deep in walls or partitions without other protection — and cables in walls with metal parts, whatever the depth;
• lighting circuits in homes;
• every circuit in a room with a bath or shower.

Many homes were wired before some of these rules existed. The standard report form itself says an installation built to an earlier edition that doesn’t fully comply with the current one “is not necessarily unsafe for continued use, or require upgrading”. So the inspector has to judge the risk — which is where the codes come in.

Why a missing RCD can be a C2 or a C3

Electrical Safety First publishes the industry’s reference guide to coding EICR observations. Its examples aren’t exhaustive, and the inspector still uses their judgement, but these are the usual outcomes:

A C2 means the lack of protection could lead to danger — a mower cable nicked in the garden, say, or a socket near water. A C3 means the installation falls short of today’s standard but isn’t potentially dangerous as it stands.

A single RCD for the whole home isn’t coded as a defect, but it means one fault switches everything off, lights included. Spreading the circuits across more RCDs or RCBOs avoids that.

A C2 makes the report unsatisfactory. For a landlord in England, the remedial work must then be done within 28 days, or sooner if the report says so. A C3 doesn’t affect the result and has no legal deadline — but adding RCD protection is well worth considering in an older home. See EICR codes explained and unsatisfactory EICR: what happens next.

How RCDs are tested during an EICR

The electrician tests each RCD with an instrument that simulates a leak at the RCD’s rated current — 30 mA for a 30 mA device — and records how long it takes to switch off, in milliseconds. A standard (non-delay) RCD must disconnect within 300 milliseconds. Since the 2022 amendment to BS 7671, a second test at five times the rated current is no longer required.

They also:

• press each RCD’s test button to check the mechanism works;
• switch the RCDs and circuit-breakers off and on by hand to prove they disconnect;
• check there’s a notice at the consumer unit telling you to test your RCDs regularly.

More on the other tests in what an EICR checks.

Test your own RCDs every six months

You don’t need an electrician for this. The guidance printed with the standard EICR form says an RCD “should be tested six-monthly by pressing the button marked ‘T’ or ‘Test’”. The RCD should switch off; switch it back on to restore the supply. If it doesn’t switch off, seek expert advice.

Pick a time when losing power briefly won’t matter, and reset clocks afterwards.

If your RCD keeps tripping

An RCD that trips is doing its job: it has detected leakage. Common causes include a faulty appliance, damp getting into an outdoor socket or light, or a wiring fault. Unplug everything on the affected circuits, reset the RCD and plug things back in one at a time. If it trips with nothing plugged in, call an electrician. Never tape, wedge or bypass an RCD.

Adding RCD protection

How it’s done depends on your consumer unit. Sometimes RCDs or RCBOs can be added to the existing board. Where the board is old, full or unsuitable, a new consumer unit with RCD protection built in is usually the answer — and replacing a consumer unit is notifiable building work that must be signed off under the Building Regulations.

Before fitting RCD protection, the electrician tests the circuits, because RCDs can reveal hidden faults an older board never noticed. See when an EICR says the consumer unit must goand our consumer unit replacement service.

Key points

• An RCD switches the power off fast when current leaks to earth.
• 30 mA RCDs are the standard for protecting people.
• A missing RCD is usually a C2 where it could lead to danger — outdoors, bathrooms — and a C3 otherwise.
• RCDs are tested for trip time (within 300 ms for a standard device) and test-button operation.
• Press your RCD test buttons every six months.

Book your EICR

We test every circuit and every RCD to BS 7671, explain your report in plain English, and quote clearly for any RCD upgrades, with no obligation to use us. Prices start at £90 inc VAT (£75 + VAT) for a studio to two-bedroom flat with one consumer unit. Get your price, see our homeowner EICR service, or call 07301 010244.