What Is an RCD and How Do You Test One?
Updated: Oct 9
An RCD (residual current device) is a safety switch that compares the current flowing out along the live conductor with the current returning along the neutral. If the two do not match, some current is escaping to earth, possibly through a person, and the RCD cuts the supply. Electrical Safety First says an RCD can disconnect in as little as 40 milliseconds, fast enough to reduce the severity of a shock that a fuse or circuit breaker would not even notice.
How does an RCD work?
In a healthy circuit, every amp that goes out on the live comes back on the neutral. An RCD watches for any difference, called the residual current. Home RCDs are rated at 30 mA (0.03 A), which the IET describes as thousands of times more sensitive than the fuses and circuit breakers that protect the cables.
A 30 mA RCD is designed not to trip below half its rating, 15 mA, and in practice the IET says a typical one trips at somewhere between 18 and 23 mA. When an electrician tests it with an instrument at its full rated current, BS 7671 expects it to disconnect within 300 ms for a standard, non-delayed type.
What an RCD does not do
An RCD is not a complete protection system on its own.
It does not protect against overload. The IET's FAQ is clear that an RCD without built-in overcurrent protection (strictly an RCCB) will not trip if a circuit is overloaded. That is the circuit breaker's job, or an RCBO, which combines both.
It cannot see a fault between live and neutral. If someone touches both conductors with no path to earth, the currents stay balanced and the RCD has nothing to detect.
It is not designed to stop arcing at a loose connection. That is what an arc fault detection device is for, which we explain in our guide to AFDDs.
Where does BS 7671 require 30 mA RCD protection?
The IET Wiring Regulations require additional protection by a 30 mA RCD in most places a householder is likely to meet electricity:
socket outlets rated up to 32 A (regulation 411.3.3)
mobile equipment up to 32 A used outdoors
all lighting circuits in homes (regulation 411.3.4, added in the 18th Edition)
cables buried less than 50 mm deep in a wall or partition that are not otherwise protected against nails and screws
In practice the IET's guidance says that, generally, all circuits in a home will need RCD protection. That applies to new work. An older installation built to earlier rules does not have to be upgraded just because the rules changed, but it will be noted on an inspection. Electrical Safety First's coding guide for EICRs marks missing RCD protection on sockets likely to power equipment outdoors as C2 (potentially dangerous), and on most other sockets and on lighting as C3 (improvement recommended).
If your board has no RCD at all, a consumer unit replacement is the usual fix, because it brings every circuit under RCD protection at once. In the meantime Electrical Safety First suggests plug-in RCDs on your sockets, and an RCD for all outdoor electrical equipment.
How to test an RCD with the test button
Every RCD and RCBO has a test button marked T or Test. Older notices asked householders to press it every three months; under the current BS 7671 notice it is every six months. A simple habit is to do it when the clocks change, so it happens twice a year without needing a reminder.
Warn the household and save any work on a computer, because everything the RCD protects will switch off.
Open the consumer unit's front cover (not the fixed panel behind it) and find the RCDs or RCBOs. Each has a small button marked T.
Press the button firmly once. The switch on the device should flick down and the power to its circuits should go off.
Push the switch back up to restore power. On a board with several RCDs or RCBOs, test each one in turn.
Reset clocks and timers on the cooker, boiler controls and so on.
If pressing the button does nothing, the RCD has failed its most basic check. The IET's guidance is to seek advice from a registered electrical contractor.
The test button only proves the mechanism works. It does not measure how fast the RCD trips or at what current. That needs an instrument test, which is part of an EICR or of the testing after any new work.
What if the RCD trips and will not reset?
Most RCD trips are caused by an appliance, not the wiring. The IET's consumer guidance sets out a simple check you can do safely:
Unplug everything on the affected circuits. Take the plugs out of the sockets rather than just switching them off.
Reset the RCD.
If it stays on, plug appliances back in one at a time. When it trips again, you have found the likely culprit. Stop using that item until it is repaired.
If the RCD will not reset with everything unplugged, the fault is probably in the fixed wiring or a hard-wired appliance, and it needs an electrician. The IET warns that resetting a device onto a fault again and again can damage it, so keep attempts to a minimum. Faults in the wiring are more likely after DIY, such as a nail or screw through a cable when hanging a picture or shelf.
Types of RCD you might have
Electrical Safety First describes three main forms. Fixed RCDs sit in the consumer unit and protect whole circuits. Socket-outlet RCDs are built into a particular socket and protect whatever plugs into it. Portable RCDs plug into an ordinary socket and protect one appliance or lead, which is handy for garden tools.
Inside the board there is a second distinction, the device type, marked AC, A, F or B. Type AC detects alternating leakage only, Type A also handles pulsating DC, and Type B also detects smooth DC, the kind an EV charger can produce. The IET's FAQ notes that Type AC RCDs are only suitable for sinusoidal leakage and can be blinded by the DC content from modern electronics, and Electrical Safety First's guidance for a replacement board recommends Type A devices. If you are unsure what your board contains, our guide to what a consumer unit is walks through each part.
We test and upgrade RCD protection across Shifnal, Telford, Wolverhampton and the surrounding towns, including anyone looking for an electrician in Shrewsbury. If yours has failed its test button check, or trips with nothing plugged in, SM Electrical can find the cause and put it right.
RCD questions, answered
Is an RCD the same as a trip switch?
People often call any device that switches off a "trip switch", but the circuit breakers and the RCD do different jobs. If a whole group of circuits went off at once, it was probably the RCD. If just one circuit went off, it was probably its breaker or RCBO.
Does an RCD stop you getting a shock?
It does not stop the shock starting, but by cutting the supply in a fraction of a second it greatly reduces how long current flows through the body, which is what makes a shock more survivable.
Why does my RCD trip when it rains?
Water getting into an outside socket, light fitting or junction box creates a leakage path to earth, which is exactly what an RCD detects. The trip is the RCD doing its job, and the fitting that let the water in needs attention.
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