RCD Test Button Not Working

What's happening?

An RCD (residual current device) watches for current leaking to earth, for example through a person touching something live, and cuts the supply fast enough to prevent most fatal shocks. Its test button creates a small deliberate leak inside the device. Press it with the power on and the RCD switch should drop to off straight away, turning off the circuits it protects. BS 7671 asks for a notice at the consumer unit telling occupants to test it six-monthly. If nothing happens, one of three things is true: the RCD mechanism has seized or failed, the button itself is broken, or that section of the board has no supply at all. The test button only proves the trip mechanism moves. It does not measure how fast the RCD trips, which is why an electrician uses an RCD tester to time it. An RCD that will not stay on when you reset it is a different fault, covered on the RCD keeps tripping page.

What usually causes it?

• The RCD's internal trip mechanism has seized, often in a device that has never been tested
• A broken or jammed test button
• Internal electronic failure after a power surge or lightning strike
• Damp or corrosion inside the consumer unit, often in garages and outbuildings
• No supply to that side of the board, so there is nothing for the test to trip
• Old age: RCDs are mechanical and wear like any switch

Is it dangerous?

Yes. An RCD that does not trip on test may not trip in a real earth fault either, which is the situation it exists for: someone touching a live part, a damaged cable, a wet appliance. Circuit breakers (MCBs) protect cables from overload and short circuits; they do not protect people from shock. Until the RCD is proven, treat the circuits it covers, especially sockets used outdoors and in kitchens and bathrooms, as unprotected.

Can I fix it myself?

1. Check the RCD is actually on: the switch should be up and the circuits it covers should have power. 2. Press the T or Test button firmly once. It should trip immediately. 3. If it does not, do not keep pressing or try to force it, and do not open the consumer unit. 4. Avoid using lawnmowers, pressure washers, outdoor extension leads and appliances near water on those circuits. A plug-in RCD adaptor adds protection for a single appliance in the meantime, but it does not protect the fixed wiring. 5. Note the RCD's rating and any labels so the electrician can bring a replacement.

When should you call an electrician?

Book an electrician promptly if the test button does not trip the RCD. Ask for the RCD to be tested with an RCD tester and replaced if it fails. Make it urgent if anyone has had a shock or tingle, if the board is warm or smells of burning, or if the consumer unit is old or plastic and has other faults. Electrical Safety First's advice is the same: if the test button does not work, get a registered electrician to investigate.

What will the electrician actually do?

• Confirm the RCD has a supply and check the test button operates the mechanism
• Test trip time and trip current with an RCD tester
• Check the circuits it covers for earth leakage and insulation faults before re-energising
• Replace the failed RCD with a compatible device, or advise on RCBO protection per circuit
• Fit or renew the six-monthly RCD test notice at the consumer unit
• Record the results so you have evidence the protection now works

Who fixes this

• Electrical fault finding

What it usually costs

Electrical Fault Finding Cost

Priced on survey

Questions people ask

Sources & References

1. Electrical Safety First: Your questions answered (Can an RCD fail?)
2. IET: BS 7671 FAQs, Consumer units and protective devices
3. Electrical Safety First: Advice for older and vulnerable people (finding the RCD Test button)
4. BS 7671: IET Wiring Regulations (18th Edition), Regulation 514.12.2