What is an RCD?
By Azzeddine Kebbi, NAPIT-registered electrician and director of Kebbi Electrical Services · Updated 10 November 2026
An RCD (residual current device) is a safety switch that turns off the electricity in a fraction of a second if it detects current leaking to earth, for example through a person touching a live part. It is the main protection against a fatal electric shock.
How it works
An RCD constantly compares the current going out on the live wire with the current coming back on the neutral. If they differ, some current is escaping, so it trips. Most RCDs in homes trip at 30 milliamps, which is low enough to protect people.
Types you might have
In the consumer unit
Protects a group of circuits. Has a T or test button.
RCBO
An RCD and circuit breaker in one, protecting a single circuit.
RCD socket
A socket with built-in RCD protection, often used for outdoor equipment.
Plug-in RCD
A portable adaptor for lawnmowers and power tools.
How to test your RCD
- Press the T or test button on the RCD.
- It should trip straight away and switch off the circuits it protects.
- Switch it back on.
- If it does not trip, call an electrician.
The notice on your consumer unit says how often to test, usually every six months.
Common questions
What does an RCD do?
It switches off the power very quickly if current leaks to earth, which protects people from fatal shocks and reduces the risk of fire.
What is the difference between an RCD and a circuit breaker?
A circuit breaker (MCB) protects the cable from overload and short circuit. An RCD protects people from electric shock. An RCBO does both.
Why does my RCD keep tripping?
Usually a faulty appliance, moisture or a wiring fault. See why an RCD keeps tripping.
Do I need RCD protection?
Modern wiring rules require 30 mA RCD protection for socket circuits and most other circuits in homes. Older boards may have none, which an EICR will flag.
No RCD protection?
We upgrade consumer units so every circuit has RCD protection. See our consumer unit upgrades.