Topic 3 of 5

Installing cables safely

How a cable is run matters as much as which cable you choose. Hidden cables need to be where people expect them, and supported so they stay put.

Cables hidden in walls

A cable buried less than 50 mm from the surface of a wall or partition must:

• run in a safe zone: within 150 mm of the top of the wall or of a corner, or in a straight horizontal or vertical line to an accessory, and
• have 30 mA RCD additional protection.

The alternative is a cable with an earthed metal covering, earthed metal conduit or trunking, or enough mechanical protection to stop nails and screws getting through. Cables in partitions with metal parts inside need the same protection, whatever their depth.

Support

• Fix cables at regular intervals so they don't sag or strain their terminations.
• BS 7671 requires wiring systems to be supported so they won't collapse early in a fire and block escape routes or trap firefighters. Plastic clips or trunking on their own aren't enough. Use metal clips, ties or fixings as well.

Fire stopping

Where a cable or containment passes through a fire-rated wall or floor, seal the hole afterwards to restore its fire resistance.

Other good practice

• Segregation: keep mains (low voltage) cables apart from data, telephone and fire alarm cables, or use partitions in trunking.
• Heat: keep cables away from hot pipes and flues, and avoid burying them in thermal insulation where possible, because it reduces how much current they can carry.
• Bends: don't bend cables tighter than the maker's minimum radius.
• Joists: drill holes or cut notches only where the Building Regulations guidance allows, so the timber isn't weakened.

Exam tip: safe zone cables less than 50 mm deep need both conditions: in the zone and RCD protected.