The three jobs of an RCD, and additional protection for socketsDoes Mrs Smith need a new board?

The three jobs of an RCD

An RCD is used for three jobs. There's no such thing as buying a "fault protection RCD" or a "fire protection RCD". The same kind of device does all three. What changes is the rating (mA) the job needs.

Additional protection: sockets and lighting

Strictly, the regulation says a rated residual operating current not exceeding 30 mA. In practice that means a 30 mA device.

1. Socket-outlets up to and including 32 A

30 mA RCD protection must be installed for all new socket-outlets rated up to and including 32 A.

You may see the exception given as: unless a risk assessment shows they will only be used by a BA4 (instructed) or BA5 (skilled) person. BA5 is an electrically skilled person, such as an electrician. BA4 is someone working under a skilled person's instruction, such as their apprentice.

A typical example is a locked switch room with a distribution board, where someone wants a socket to plug in lights and battery chargers. Only skilled people, or people under their instruction, should be in there. So that socket might not need 30 mA RCD protection, but a risk assessment has to be provided.

Any socket that anyone can use, for example in a house, an office or a school corridor, must have 30 mA additional protection.

BS 7671 · Reg 411.3.3 · additional protection for socket-outlets ≤ 32 A

"Outlets", not "circuits"

Note the word OUTLETS. The regulation says socket-outlets, not socket circuits. That matters when you add a socket to an existing circuit.

Mrs Smith has an old rewireable-fuse consumer unit with no RCDs.
She wants one more socket in the hallway for her hoover.

Many electricians: "You need 30 mA RCD protection, so you need a new board."
  → around £800 to £1,000 before she even gets her socket

But the regulation says OUTLETS.
  → fit an RCD socket-outlet. The new outlet has 30 mA protection
    and the board doesn't need changing.

RCD sockets might not be Mrs Smith's cup of tea. They're bulkier than a normal socket and have test and reset buttons on the front. But they are an option.

There's another regulation to keep in mind with this. It's the one about cables hidden in walls, covered in section 5.

2. Domestic lighting circuits

30 mA RCD protection must be installed for all domestic lighting circuits. This means domestic only, not commercial or industrial. It's the next regulation after 411.3.3.

This applies to new circuits. If you're altering an existing domestic lighting circuit, it's good practice to consider adding 30 mA protection too. That's hard to get around, because there's no lighting equivalent of an RCD socket.

One option people use (and it isn't wrong) is to fit a 30 mA RCD fused connection unit next to the old board. The existing lighting circuit leaves the fuse carrier, goes into the RCD FCU, and carries on to the luminaires.

BS 7671 · Reg 411.3.4 · domestic lighting circuits · the regulation after 411.3.3