RCDs for hidden cables and special locationsDoes a cable buried in a wall need an RCD?

Additional protection: cables hidden in walls

30 mA additional protection also has to be installed when the circuit is installed to 522.6.202 or 522.6.203, unless one of the alternatives in 522.6.204 has been used instead.

522.6.202: cables in walls and partitions

This is for cables concealed in a wall or partition: a masonry wall (brick or block) or a stud wall. If the cable is less than 50 mm deep from the surface, and it's run in the safe zones as it should be, it must have 30 mA RCD protection unless you use 522.6.204 instead.

This is very common. When you chase a wall, you're unlikely to go deeper than 50 mm. So once you've chased it in the safe zones, the question is: do you put it on a 30 mA RCD? That's the easiest answer. If you don't want to, 522.6.204 gives you alternatives.

522.6.203: cables in partition walls

A partition wall is a stud wall (metal or wood) with plasterboard on both sides. Sometimes it's thermally insulated and sometimes it isn't. It's a quick way to put walls up, and you'll see it inside most modern buildings.

This regulation only applies if the wall or partition has metallic parts, such as metal studs, and it applies whatever the depth of the cable. Wooden studs don't bring in 522.6.203, but a cable less than 50 mm deep in a wooden-stud wall still falls under 522.6.202. Most studs now are metal, so you'll usually be following 522.6.203. Again, 522.6.204 can be used instead of the RCD.

522.6.204: the alternatives

• A cable with an earthed metallic covering, such as SWA. The regulation lists the British Standards the cable must meet.
• Earthed metallic conduit, trunking or ducting slid over the cable. This must be rigid. People often ask whether flexible metal conduit counts. It doesn't. There's a regulation in Chapter 54 that says flexible conduit can't be used for this.
• Suitable mechanical protection that stops nails and screws getting through, for example a suitable metal plate with the right impact rating.
• The circuit is part of a SELV or PELV circuit.

BS 7671 · Reg 522.6.202 · cables in walls or partitions, less than 50 mm deep · p.152 BS 7671 · Reg 522.6.203 · cables in walls or partitions with metallic parts, any depth BS 7671 · Reg 522.6.204 · alternatives to the RCD · p.152

BS 7671 · Chapter 54 · flexible metal conduit not accepted Chapter 54 lists flexible metal conduit among the things that can't be used as a protective conductor.

Additional protection: Part 7 locations

The last time you need additional protection is whenever Part 7 asks for it. Part 7 covers the special installations and locations that BS 7671 recognises. If Part 7 says you need 30 mA RCD protection, you fit it. No question.

BS 7671 · Part 7 · special installations or locations (Sections 701 to 753)