What is RCD Protection and Do You Actually Need It? (2026 UK Guide)

What does an RCD actually do?

An RCD — Residual Current Device — is the single most important life-safety device in a modern electrical installation. It compares the current flowing out of your home on the line conductor with the current returning on the neutral. If the two differ by more than 30 mA for more than 40 ms, it trips.

That imbalance means current is leaking somewhere it should not be — through a person, through wet plaster, into a metal appliance case. Trip in 40 ms and you get a shock. No trip, and you can die.

Every socket circuit, every cable in a wall zone, and every high-risk circuit (bathroom, garden, cooker) in the UK must have RCD protection at 30 mA.

RCD vs RCBO vs Type A/F/B

You will see all these terms on a modern consumer unit. Quick guide:

• RCD — residual current only. Trips on earth leakage. Needs a separate MCB for overload/short-circuit.
• MCB — miniature circuit breaker. Trips on overload/short. No shock protection.
• RCBO — combined RCD + MCB in a single module. One per circuit — this is the modern standard.
• Main switch RCD — a large single RCD covering the whole board. Discouraged today because one fault cuts everything.

Which "Type" do you need?

• Type AC — the old default. Only trips on pure sine-wave AC faults. Not used in new installs since Amendment 2.
• Type A — the modern minimum. Trips on AC + pulsating DC faults. Suitable for LED, dimmers, most inverters, EV chargers with internal DC-fault detection.
• Type F — Type A + high-frequency faults. Good for VFD kit, some heat pumps, motor drives.
• Type B — highest sensitivity, trips on smooth DC. Required for certain solar inverters and EV chargers without internal DC-fault protection.

Get the type wrong and the RCD may fail to trip when it should, or nuisance-trip constantly. This is one of the top C2 observations we log on EICRs.

How to test your own RCD

Every RCD has a TEST button on the front. Manufacturers recommend pressing it every 3 months.

The correct sequence:

1. Save any open work on computers.
2. Press the TEST button firmly.
3. The RCD should trip immediately — its lever flips to OFF.
4. Reset by lifting the lever back to ON.

If the RCD does not trip, or the lever will not stay on, book an electrician the same day.

What the test button does *not* prove

The internal test button confirms the mechanism works — it does not measure the trip time or trip current. That is why a full installation test every 5 – 10 years is essential. We measure:

• I∆n — actual trip current (should be ≤ 30 mA on a 30 mA device)
• t (trip time) — must trip within 40 ms at 5× rated residual (150 mA)
• Zs — earth loop impedance is low enough to guarantee disconnection

Common RCD problems in London homes

• Old Wylex Standard boards — no RCD anywhere, common in 60s and 70s houses
• Split-load boards with the main switch labelled "RCD" but wired as an isolator
• Failed test buttons in boards over 15 years old
• Nuisance tripping caused by cumulative leakage from LEDs, laptops and kettles on one RCD
• Cascade tripping because Type AC devices cannot handle modern DC-content loads

Why RCBOs are worth the upgrade

If a shared RCD trips, half your house goes off, including the freezer. On a full-RCBO board only the faulty circuit drops. It is the single biggest reliability upgrade you can make short of a full rewire.

The average full-RCBO consumer unit for a 3-bed London home is £700 – £950 fitted and certified. See our consumer unit upgrade guide for full details.

Get your RCDs tested

If you have not tested your RCDs in the last 3 months — do it now. If you have never had a full test, book an electrician or call 0203 503 1240. We are NICEIC-registered and cover the whole of Greater London.