The residual current device (RCD), sometimes called a GFCI or earth-leakage breaker, is what protects people from electric shock. It is one of the most important parts of your consumer unit and deserves a few minutes of attention.
How it works
An RCD constantly compares the current flowing into a circuit with the current flowing back. If some of it “leaks” to earth, for example through a person’s body or a faulty appliance, it detects the imbalance and cuts the power in a fraction of a second.
For personal protection, RCDs with a sensitivity of 30 mA are normally used: fast enough to greatly reduce the risk of serious injury.
RCDs and circuit breakers are not the same
- RCD: protects people from current leakage.
- Circuit breaker (MCB): protects cables from overloads and short circuits.
The two are often combined in a single device (an RCBO), but they do different jobs and both are needed.
How to test it in 3 steps
- Find the test button on the RCD, usually marked with the letter T.
- Press it: the device should trip immediately and cut the power.
- Push the switch back up to restore power.
It’s a good idea to repeat the test once a month, or as often as the manufacturer recommends. Appliances on that circuit will switch off during the test.
If the RCD doesn’t trip when you press T, it can no longer be trusted: have a qualified electrician check it and replace it if necessary.