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What a residual current device actually does

Every modern consumer unit in a UK home contains at least one residual current device, usually shortened to RCD. Unlike a fuse or a miniature circuit breaker, which exist to protect the wiring from overload and short circuits, an RCD is there for you. Its job is to keep a constant watch on the current flowing out along the live conductor and back along the neutral, and to cut the supply the instant those two currents stop matching.

In a healthy circuit, everything that leaves down the live conductor returns along the neutral. The two currents are equal and opposite, so the RCD sees nothing to worry about. If some of that current finds a different route to earth — through a damaged cable, a wet appliance, a faulty power tool, or a person — the balance is broken. That missing current is called residual current, and it is exactly what the device is designed to detect.

How quickly does an RCD respond?

Household RCDs are rated at 30 milliamps of residual current. That is a deliberately small figure: 30 mA is generally below the level that causes dangerous disruption to the heart rhythm in an adult during a brief shock. Speed matters just as much as sensitivity. A standard 30 mA RCD must trip within 300 milliseconds at its rated current, and considerably faster — typically within 40 milliseconds — when the residual current is higher, which is the situation you get with a serious fault.

So an RCD does not stop you receiving a shock. What it does is limit how long the shock lasts, and that is what saves lives. A few tens of milliseconds is usually enough to escape with a fright rather than a funeral.

RCDs, RCBOs and the different types explained

Not every protective device in a consumer unit works the same way, and it helps to know what you are looking at.

• RCD (fixed or residual current device): a single device protecting a group of circuits, often half the board. A fault on one circuit knocks out everything on that side.
• RCBO (residual current breaker with overcurrent): an RCD and an MCB combined in one module. Each circuit gets its own protection, so a trip affects only that circuit. RCBOs are increasingly common in new consumer units for precisely this reason.
• Type AC: responds only to alternating residual currents. Historically the default, but unable to see the smooth direct currents produced by some modern electronics.
• Type A: responds to alternating and pulsating direct residual currents. Now the sensible minimum for most domestic circuits, and essential for equipment such as electric vehicle chargers, heat pumps and many appliances with electronic speed control.
• Type F and Type B: specialist devices for equipment that can produce particular waveforms or smooth DC fault currents. These are usually specified for specific installations rather than whole boards.

If your board carries a mixture of Type AC and Type A devices, that is not necessarily a fault, but it is worth asking an electrician whether any circuits have outgrown their protection.

Why the test button matters

Every RCD has a small test button marked with a "T". Pressing it creates a tiny, safe residual current inside the device to prove that the trip mechanism actually operates. This is not a substitute for proper instrument testing, which measures trip times and current thresholds, but it does catch the most common failure: an RCD that has become mechanically stuck and will no longer trip at all.

Press the button on each RCD roughly every three months. The supply will go off for that protected group of circuits, so pick a quiet moment, warn the household, and remember to reset clocks and check freezers afterwards. If a device refuses to trip, or trips but will not reset, stop relying on it and arrange for an electrician to look at it.

Beyond the button, RCD trip times should be verified during an Electrical Installation Condition Report. Owner-occupied homes are commonly advised to have one every ten years, and rented properties at least every five, or on change of tenancy.

When your RCD trips: nuisance tripping and fault finding

RCDs do occasionally trip when there is no danger. Small leakage currents add up, and a handful of appliances each contributing a milliamp or two can push a healthy circuit over the threshold. Damp outdoor sockets, garden lighting, kettle elements, washing machines and ageing fridge compressors are frequent culprits.

If the device will not reset, try this:

• Switch off and unplug everything on the affected circuits, including anything plugged into outdoor sockets.
• Reset the RCD. If it holds, plug items back in one at a time, testing as you go.
• If it trips again with everything disconnected, the fault is in the fixed wiring and needs an electrician.
• If it trips again immediately after you plug in a particular item, that appliance is the likely cause. Have it checked rather than simply avoiding it.

Water ingress is a common seasonal cause, so a trip after heavy rain often points to outside wiring rather than anything inside the house.

What RCDs cannot protect you from

It is important not to over-trust the device. An RCD will not detect a shock between live and neutral, because no current escapes to earth and the balance stays intact. It will not help if you touch two live conductors, and it will not prevent an overload or a short circuit — that is the MCB's job. Nor will it protect a circuit that is not wired through it, such as a lighting circuit on the non-RCD side of an older split-load board.

RCDs are a vital layer of protection, not the whole story. Correct earthing, sound wiring, sensible use of sockets and a periodic check by a competent electrician all work together. Treat the test button as part of your household routine, and if anything about your consumer unit feels unfamiliar or outdated, a qualified electrician can tell you plainly where you stand.