What is this?
An RCD (Residual Current Device) is a life-saving device that disconnects the power in milliseconds if it detects an earth fault — the type of fault that causes electric shock. Every RCD has a test button that should be pressed regularly (at least every 3 months). If pressing the test button does not trip the RCD, the device has failed and is no longer providing protection against electric shock.
Common causes
- RCD has failed internally due to age or a power surge
- Test button mechanism is jammed or broken
- RCD has been in service too long without being tested (mechanisms can seize)
- Wiring fault preventing the test circuit from operating correctly
- RCD is not the correct type for the installation
Is it dangerous?
A non-functioning RCD means you have no protection against earth faults on the circuits it covers. Earth faults are the primary cause of electric shock injuries and fatalities in the home. If your RCD does not trip when you press the test button, those circuits are unprotected. This is a serious safety issue that needs urgent attention.
Can I fix it myself?
No. If the RCD test button does not trip the device, do not attempt to disassemble or repair it. You should avoid using high-risk appliances on the affected circuits until the RCD is replaced. As a temporary measure, you can use a plug-in RCD adapter (available from DIY stores) on individual appliances, but this is not a substitute for a proper repair.
When to call an electrician
Call an electrician as soon as possible if your RCD fails its test button check. This is a safety-critical failure. The electrician will need to replace the RCD, which requires working inside the consumer unit.
What will an electrician do?
Test the RCD with professional equipment to confirm the failure
Isolate the supply and remove the faulty RCD
Fit a new RCD of the correct type and rating
Test the new RCD for correct operation (trip time and test button)
Test all circuits on the RCD for earth fault loop impedance
Typical cost
£100 – £250
RCD replacement is a relatively quick job. Costs include the new device and testing. If the consumer unit is very old, the electrician may recommend a full board upgrade.
