Safe zones, depth and metal stud wallsWhere can a cable run in a wall without extra protection?

Cables concealed in a wall less than 50 mm deep must be protected in one of these ways:

1. An earthed metallic covering, or enclosed in earthed metal conduit, trunking or ducting.
2. Mechanical protection against penetration by nails and screws.
3. Run in the permitted zones. This also needs 30 mA RCD additional protection.
4. Part of a SELV/PELV circuit (extra-low voltage, covered later).

• Why the zones? Nobody is likely to hang a picture 150 mm from the ceiling or right in a corner. Cables in line with an accessory are where people expect them.
• Running across into a different room through a partition isn't "in line with an accessory", so it needs extra protection.
• Metal or plastic capping over cables in a chase is good practice, because it stops nails and screws. Capping doesn't count as conduit or trunking.
• Under floors and in ceilings: unarmoured cables through joists must be at least 50 mm from the top or bottom of the joist, or be protected (earthed steel conduit, etc.). This is Reg 522.6.201. Where an old, thin joist can't give 50 mm top and bottom, the cable must meet 522.6.204 instead.

Electrician's Guide to the Building Regs · Permitted cable routes / safe zones figure and <50 mm depth rules · p.43

BS 7671 · Reg 522.6.201: cables through joists · p.151 BS 7671 · Regs 522.6.202, 522.6.203, 522.6.204: cables in walls and metal stud partitions · p.152 In the 18th Edition exam you find regs by number, not page. Practise.