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What is an RCD?

The life-saving device in your consumer unit, and why older boards often lack one.

Published · Updated · By 2K Electrical Services

Hand with a screwdriver at a consumer unit with RCDs, circuit breakers and neatly dressed cables
How an RCD works Line and neutral pass through a sensing coil. On a healthy circuit the current going out equals the current coming back, so nothing happens. If a fault inside an appliance makes its metal case live and someone touches it, part of the current flows through that person to earth instead of coming back. The coil sees the difference and the RCD switches the circuit off. A 30 milliamp RCD must trip within 300 milliseconds at 30 milliamps. HOW AN RCD PROTECTS YOU Residual current device: it compares what goes out with what comes back. LINE · current out NEUTRAL · current back Δ = 0 ? sensing coil same current out and back, nothing to detect TRIP T T = test button fault: live case APPLIANCE metal case earth current through the person to earth Healthy circuit: out = back, the coil sees no difference. Fault: current leaks to earth, here through a person touching the case. Difference of 30 mA or more: the RCD switches off within 300 ms, in a real fault usually under 40 ms, before the heart rhythm is disturbed. Press the T button every six months. If it does not trip, call an electrician.
How an RCD compares the current going out with the current coming back.Swipe sideways to see the whole diagram.

An RCD, or residual current device, is a safety switch that cuts the power in a fraction of a second when electricity starts flowing somewhere it should not, such as through a person who has touched a live wire. Fuses and ordinary circuit breakers protect cables from overload; they will not save you from a shock. An RCD will.

How it works

The RCD compares the current going out on the live conductor with the current coming back on the neutral. In a healthy circuit they are equal. If some current is leaking to earth, through a damaged cable, a wet appliance or a person, the two no longer match. When the difference reaches the device’s rated residual current, 30 mA for personal protection, the RCD must trip within 300 ms; a single test at that rated current is how BS 7671 verifies it. In a real fault the current is usually much higher and a good RCD trips far faster: the RCD product standard requires it to trip within 40 ms at five times its rating. That is fast enough to prevent the heart rhythm being disturbed, which is what makes electric shocks fatal.

Types you will find in a home

  • Fixed RCDs in the consumer unit. These protect whole circuits and everything plugged into them. Modern boards use RCBOs, which combine an RCD and a circuit breaker for each circuit, so a fault in the kitchen does not black out the lights.
  • RCD socket outlets. A socket with its own RCD built in. It protects only what is plugged into that socket.
  • Portable RCD adaptors. Plugged in between the socket and an appliance, typically for lawnmowers and hedge trimmers. Useful where there is no fixed protection, but easy to forget.

Since the 17th Edition in 2008, virtually every circuit in a new or rewired home has needed RCD protection. The 18th Edition went further: sockets, cables buried in walls, bathroom circuits and most lighting circuits all require it, and the type of RCD now has to suit the equipment on the circuit (Type A devices for appliances with electronic controls, for example).

Test it every six months

Every RCD has a button marked T or Test. Press it every six months; the IET suggests doing it when the clocks change in spring and autumn. The RCD should trip immediately. Reset it and carry on. If pressing the button does nothing, or the device will not reset, stop using the circuit and call an electrician. There should be a label on or near the consumer unit reminding you to test.

Do I already have one?

Open the consumer unit cover and look for devices with a test button. If there are none, or if your board has rewirable fuses, you have no RCD protection and an upgrade is the single most effective safety improvement you can make. An EICR will record the absence of RCD protection as a C2 or C3 depending on the circuit.

Remember

An RCD reduces the risk of a fatal shock; it does not remove it, and it does not protect against every kind of fault. Have your installation inspected at least every ten years if you own your home, every five if you let it, and get any fault looked at by a registered electrician.

More guides

Earth fault loop impedance explainedWhy electricians measure Zs and Ze, and what the limits in BS 7671 mean for your installation.Bathroom electrics: common questionsCan you have a socket, a switch, a fan or a light over the bath? Plain answers from Section 701.Bathroom zones under BS 7671How zones 0, 1 and 2 are measured, and the RCD and bonding rules that come with them.EICR codes explainedWhat C1, C2, C3 and FI mean on your report, and which ones you must fix.IP ratings: what the numbers meanReading the two digits on a fitting before you put it in a bathroom or garden.

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