How to test your RCD

Your fuse box has one switch whose only job is to save your life — and it comes with its own test button. Pressing it is completely safe, takes about thirty seconds, and costs nothing. Here's how to check an RCD is working, how often to do it, and what it means if the test fails.

What an RCD actually is

An RCD (residual current device) is the switch in your fuse box — properly called a consumer unit — that cuts the power in a fraction of a second if electricity leaks to earth. "Leaks to earth" includes leaking through a person: a nicked lawnmower lead, a damaged cable, a faulty appliance. Without an RCD, that current can deliver a fatal shock. With one, the power is gone before you've registered anything happened.

Finding it in your consumer unit

Open the flap on your consumer unit and look for the wider switch with a small button marked T or Test. In many Reading homes there's a single RCD covering half the board. Newer boards use RCBOs instead — slimmer switches, one per circuit, each with its own tiny test button. The test works exactly the same way; you just have more buttons to press.

How to test an RCD, step by step

• Pick your moment. Nothing important should be running — computer work saved, oven off, nobody mid-shower.
• Press the T button firmly. The switch should snap off instantly and that section of the house goes dead. That bang of the switch is the pass.
• Reset it. Push the switch fully down and then back up — some types need pushing down first to re-latch before they'll switch on again.
• Check the power's back. That's it. Thirty seconds, done.

How often should an RCD be tested?

Every three months. The easiest way to remember is to tie it to something you already do quarterly — the same weekend you do a seasonal garden tidy, say. There's a practical reason for the routine: the tripping mechanism can stick if it's never exercised, and a stuck RCD is no protection at all. Keeping it moving is the whole point of the button.

If it does not trip when you press the button

Treat that section of the board as unprotected. Stop using those circuits for anything with a plug outdoors, in the bathroom or in the kitchen, and call an electrician. An RCD that won't trip on its own test button is not protecting you.

If it trips but won't reset — or keeps tripping by itself

That's almost never a worn-out RCD. It's a fault on one of the circuits it protects, and the RCD is doing its job by refusing to stay on. Don't keep forcing it back up — it needs proper fault-finding to trace which circuit, and why.

What the button can't tell you

An honest limit: the test button only proves the mechanism moves. It doesn't prove the RCD trips fast enough — that takes a calibrated RCD tester measuring trip times in milliseconds, which is exactly what we do during an EICR or landlord safety report. And no, you can't test an RCD properly with a multimeter, whatever some videos claim.

When to call an electrician

• No RCD in the fuse box at all — common in older Berkshire homes with rewireable fuses or plain breakers only.
• The test button does nothing.
• The RCD won't reset after tripping.
• Nuisance tripping — it keeps going off for no obvious reason.

Beckley Electrical is a family-run firm based in Tilehurst, covering Reading and the surrounding villages. If your RCD failed its test — or your fuse box doesn't have one — get in touch and we'll take a look. And if it passed: well done, see you again in three months.