What is an RCD and how do I test it?

Most people have an RCD, few know which switch it is, and almost nobody presses the button. It is the single device most likely to prevent a fatal electric shock in a home, and it is the one that fails silently.

Safety

Step by step

1. Understand what it does that a fuse does not.A fuse or breaker protects the wiring from overload. An RCD protects people: it compares current flowing out with current returning, and if some is going missing — through a person to earth, for instance — it disconnects in milliseconds.
2. Find it in the consumer unit.Usually a larger switch with a small test button marked T or TEST, often covering a group of circuits. Some units have RCBOs instead, which combine both functions in each individual breaker.
3. Work out what it actually protects.Not everything necessarily. Older installations often protect only sockets and outdoor circuits, leaving lighting unprotected. Newer ones cover much more. Knowing which circuits are covered is worth establishing.
4. Prepare before testing.Shut down computers, note anything that must not lose power, and have a torch to hand if lighting is on the affected circuits. The test genuinely cuts the supply.
5. Press the test button.Firmly. The RCD should trip immediately — a definite click and the switch drops. Then reset it by pushing the lever fully back up. That is the whole test.
6. Do it every three months.The standard recommendation. RCDs can seize internally over years of never operating, and a seized one gives no indication that anything is wrong.
7. Treat a failed test as urgent.If it does not trip, it is providing no protection at all while appearing to. That needs a registered electrician promptly rather than at some point.
8. Understand nuisance tripping.Frequent tripping with no obvious cause is usually cumulative small leakage from many appliances, or one faulty item. Both are worth investigating rather than living with — it is telling you something.
9. Use a plug-in RCD where there is no fixed one.For outdoor tools, garden equipment and older installations, a plug-in RCD adaptor provides the same protection at the socket. Inexpensive, and essential for a lawnmower or hedge trimmer.
10. Check what protection an older house has.A consumer unit with rewirable fuses and no RCD at all is common in older properties and offers none of this. An electrician can advise on upgrading, which is one of the more worthwhile electrical improvements.
11. Have the installation inspected periodically.A periodic inspection report is recommended roughly every ten years for an owner-occupied home, and more often for rentals, where it is a legal requirement.
12. Do not rely on it as a substitute for care.An RCD is a last line of defence, not permission to work on live circuits or use damaged equipment. It reduces the consequence of a fault rather than preventing one.

Tips

• Set a recurring quarterly reminder. It takes thirty seconds and it is the only way anyone actually does it.
• Label the consumer unit while you are there. Knowing which breaker serves which room saves considerable time in every future problem.
• If the RCD trips when you press the test button but not otherwise, that is correct behaviour and the device is working.

Common mistakes

• Never pressing the test button — RCDs can seize internally over years of inactivity, and give no indication. An untested RCD may be protecting nothing.
• Repeatedly resetting an RCD that keeps tripping — It is detecting a genuine earth fault — the kind of fault whose path can be through a person. Find the cause instead.

If it doesn't work

Questions people ask