Guide
What is an RCD? RCD vs RCBO and which you need
An RCD cuts the power in a fraction of a second when current leaks to earth, and in a home it is set to trip at 30 milliamps. An RCBO does the same for a single circuit and adds the overload protection of a circuit breaker, so one fault switches off one circuit instead of half the house.
What an RCD actually measures
A residual current device compares the current going out along the line conductor with the current coming back along the neutral. In a healthy circuit the two are equal. If they differ, the missing current is going somewhere it should not, through damaged insulation, through water, or through a person who has touched something live, and the device switches the circuit off.
The 30 milliamp rating is what makes it a life saving device. The IET's consumer guidance puts it plainly: RCDs are thousands of times more sensitive than circuit breakers or fuses. When an electrician tests one, it has to disconnect within 200 milliseconds at its rated current and within 40 milliseconds at five times that.
What an RCD does not do is protect a circuit from overload or from a short between line and neutral. That is the job of the circuit breaker or fuse, which is why the two are always used together.
MCB, RCD and RCBO: three devices, three jobs
| Device | Protects against | Usually covers |
|---|---|---|
| MCB (miniature circuit breaker) | Overload and short circuit | One circuit |
| Fuse, rewireable or cartridge | Overload and short circuit | One circuit |
| RCD | Earth leakage and electric shock | A group of circuits |
| RCBO | All of the above | One circuit |
Two newer devices sometimes appear on quotes. A surge protective device guards the installation against voltage spikes, and current wiring regulations call for one in most new installations. An arc fault detection device watches for the electrical signature of a loose or damaged connection, and the regulations recommend rather than require it in most homes.
Split load board or a board full of RCBOs
Most boards fitted in the last twenty years are split load, sometimes called dual RCD: two RCDs, each guarding half the circuits. The IET describes this as the usual arrangement, with lighting split across the two so that a trip never leaves the whole house in the dark. The weakness is that one faulty kettle on the downstairs sockets switches off everything else on that RCD as well, which can include the freezer.
A board with an RCBO on every circuit isolates the fault to the circuit it is on. It also spreads the small leakage that every appliance produces. The IET advises keeping standing leakage below 30% of the RCD's rating, 9 milliamps on a 30 milliamp device, and a single RCD shared by a dozen appliances gets closer to that line than an RCBO feeding three of them. On a board of up to ten circuits, a split load board is typically £450 to £650 fitted and an RCBO board £550 to £800.
Where the wiring regulations ask for RCD protection
BS 7671, the IET Wiring Regulations, requires 30 milliamp RCD protection on socket outlets rated up to 32 amps, and since the 2018 edition on every lighting circuit in a home. Cables buried in walls at a shallow depth without mechanical protection need it too, and so does almost everything in a bathroom.
The regulations apply to new work. A house wired before RCDs were routine is not illegal, and nobody has to rip out a working board because a new edition was published. What happens instead is that an inspection records the missing protection and codes it according to the risk, and a landlord or buyer then decides what to do about it.
Type AC, A, F and B
RCDs come in types according to the kinds of fault current they can detect. Type AC handles only the smooth alternating current of older appliances. Many modern appliances, from induction hobs to washing machines with inverter motors, can produce currents with a direct current component that can stop a Type AC device working properly, and the IET has said Type AC is being restricted for that reason. Type A is the usual domestic choice now, Type F covers equipment with variable speed drives, and Type B handles smooth direct current. If a quote for a new board names Type AC devices, ask why.
Testing yours
Every RCD and RCBO has a button marked T or Test. Pressing it should switch the device off straight away, and you then switch it back on. The IET's advice is to do this every six months, and the label on the board usually says the same. Expect clocks on cookers and microwaves to reset.
If the device does not switch off when the button is pressed, it is not protecting anybody, and it needs an electrician rather than another try.
Common questions
Is an RCBO better than an RCD?
It gives the same shock protection with more selectivity, because a fault only switches off its own circuit. It costs more per circuit and needs a board designed to take it.
Can I add RCD protection without a new consumer unit?
Sometimes. A separate RCD enclosure can be fitted ahead of an existing board, and a plug in RCD protects whatever is plugged into it. On an old fuse box the usual advice is a new board, because the other faults found during testing need fixing anyway.
Why did pressing test switch off several circuits?
Because on a split load board one RCD protects a group of circuits. Everything on that side goes off with it, which is normal.
Does an RCD stop all electric shocks?
No. It responds to current leaking to earth. Someone touching line and neutral together, with no path to earth, can still receive a shock, which is why the rest of the installation still has to be safe.