An RCD (residual current device) is the safety switch in your fuse board that cuts the power in a fraction of a second if electricity starts leaking somewhere it shouldn’t, such as through a person. It’s one of the most important safety devices in a home, and one of the most common things we find missing when we test older properties.
What does an RCD actually do?
In a healthy circuit, the current flowing out along the live conductor is the same as the current coming back along the neutral. An RCD constantly compares the two.
If some of that current goes missing, it has found another route to earth. That route could be a damaged cable, water in a fitting, or a person touching something live. When the difference reaches the RCD’s trip level, it switches the circuit off.
For protecting people, that trip level is 30 milliamps (30 mA). That’s a tiny current, and the RCD disconnects quickly enough to greatly reduce the risk of a fatal electric shock. A normal fuse or circuit breaker can’t do this. It only reacts to large overloads and short circuits, so it won’t protect you from the small current that can stop a heart.
RCDs also help reduce the risk of fire caused by earth leakage from damaged cables.
Which circuits need RCD protection?
The wiring regulations (BS 7671) have steadily widened RCD protection. For a new installation or a new circuit in a home, 30 mA RCD protection is now required for:
- Socket outlets rated up to 32 A
- Lighting circuits in homes
- Bathroom circuits
- Outdoor equipment such as garden sockets and outside lights
- Cables buried in walls at shallow depth without other mechanical protection
In practice, almost every circuit in a modern domestic fuse board is RCD protected. Older boards often protect only some circuits, or none at all.
You don’t have to bring an older installation up to the latest standard just because the rules changed. But a missing RCD is one of the first things an electrician will flag during an inspection, and it’s usually the strongest case for replacing an old consumer unit.
RCD, RCBO and split-load boards
You’ll hear a few different terms:
RCD (main switch or split-load board). One RCD protects a group of circuits. In a split-load board, the circuits are shared across two RCDs. It works, but a single fault on one circuit trips everything on that side of the board. You might lose the kitchen sockets and the upstairs lights together, for example.
RCBO. An RCBO combines an RCD and a circuit breaker in one device, and each circuit gets its own. If one circuit develops a fault, only that circuit switches off and the rest of the house stays on. It also makes faults much quicker to trace. Most of the boards we fit now are full RCBO boards.
RCD sockets and plug-in RCDs. These protect a single socket or appliance, which is useful for garden tools. They are not a substitute for proper protection at the board.
There are also different RCD types (AC, A, F and B), depending on what kind of current they can detect. Modern appliances like induction hobs, washing machines, heat pumps and EV chargers can produce currents that an older Type AC device may not respond to properly. That’s why Type A devices are now the usual choice for domestic boards, and EV chargers need their own specific protection. It’s one reason we check the type of RCD, not just whether one is fitted.
How to test your RCD
Every RCD has a test button, usually marked “T” or “Test”. Pressing it creates a small artificial leak and should make the RCD trip straight away.
- Warn everyone first, because the power to those circuits will go off.
- Press the test button firmly.
- The RCD should switch off immediately.
- Switch it back on, then reset any clocks or appliances that lost power.
The label next to the board tells you how often to do this. Newer labels say every six months, and older ones say every three months. If the RCD doesn’t trip when you press the button, or won’t reset, stop and call an electrician.
The test button only proves the mechanism works. During an EICR, an electrician also uses a meter to check that the RCD trips at the right current and fast enough.
My RCD keeps tripping. What should I do?
An RCD that trips is usually doing its job and has detected a real leak. Common causes are a faulty appliance (kettles, washing machines and outdoor equipment are frequent culprits), water getting into an outside light or socket, or a damaged cable.
Unplug everything on the affected circuits, reset the RCD, then plug things back in one at a time to see what trips it. If it trips with nothing plugged in, the fault is in the fixed wiring and needs an electrician. Our guide on why your electric keeps tripping walks through this step by step.
Never tape over, bypass or replace an RCD to stop it tripping. That removes the protection without fixing the fault.
What a missing RCD means on an EICR
During an Electrical Installation Condition Report, the electrician records where RCD protection is missing and codes it based on the risk. Missing RCD protection for socket outlets is commonly coded C2 (potentially dangerous) where sockets could be used for equipment outdoors, and C3 (improvement recommended) in other situations. The exact code depends on the property and how the circuits are used.
For landlords this matters. In England, a C1 or C2 code means the report is unsatisfactory, and the remedial work must be done within 28 days, or sooner if the report says so. Adding RCD protection, usually by upgrading the consumer unit, is one of the most common remedials we carry out. See our guide to EICR codes and our EICR and compliance service.
Quick checklist: is your home protected?
- Look at your fuse board. Do you see a device labelled RCD or RCBO, usually with a test button?
- Are all circuits covered, or only some?
- Does each RCD trip when you press the test button?
- Is the board an old wooden or rewireable-fuse type? If so, it almost certainly needs updating.
If you’re not sure what you’re looking at, send us a photo of your fuse board and we’ll tell you what you have.
Get your RCD protection checked
Positive Contractors is an NICEIC Approved Contractor based in Chigwell, covering London and Essex. We carry out EICRs, add RCD protection and fit new consumer units for homeowners, landlords and letting agents. Call 020 8227 0035 or get a quote.
We cover London and Essex, with our own engineers working every day across Chigwell, Loughton, Buckhurst Hill, Woodford, Ilford, Romford, Brentwood and Epping.