Wiring Systems, Cables, Containment and Installation Methods
This domain tests selection and erection: choosing a cable and a containment system that suit the environment, installing them so their rating is not degraded, and keeping the earthing and fire performance intact. Questions often show a route through a building and ask what is wrong with it.
Cable selection
- Match the sheath and insulation to the environment: PVC twin and earth for dry internal work, LSZH where smoke and toxic fume matter, armoured for external and buried runs, fire-resistant cable for life safety circuits.
- Reference method matters: clipped direct, in conduit in a thermally insulated wall, in free air, buried, all give different tabulated capacities.
- Thermal insulation is a common trap: a cable totally surrounded in insulation loses a large part of its rating, so the Ci factor or a derating to the enclosed method must be applied.
- Armour or a separate conductor may serve as the circuit protective conductor only if its cross-sectional area meets the tabulated minimum and continuity is assured by proper glands.
Containment
- Conduit, trunking, tray and basket all have fill limits, typically around 40 per cent for conduit and 45 per cent for trunking, so cables can be drawn in without damage.
- Support at the manufacturer's spacing; commonly adopted values are 300 mm horizontal and 400 mm vertical for small PVC cables, closer where vibration or movement exists.
- Respect minimum bending radius, typically six to eight times the cable diameter for armoured cable.
- Maintain fire compartmentation: seal every penetration with a tested fire-stop system, and keep fire alarm and emergency lighting cables segregated as designed.
- Segregate band one, two and three circuits, and earth metallic containment.
Worked example
A 20 A supply to a detached garage 25 m away, buried in a garden. Select three-core SWA with the armour as the circuit protective conductor, sized so the derated capacity exceeds 20 A after the depth-of-burial factor and any grouping. Route at 0.6 m depth with sand bedding and warning tape or a protective tile, or protect with ducting at reduced depth. Terminate with correct-size brass glands, earthing the armour at the origin with a banjo and ensuring continuity at the remote end. Check voltage drop over 25 m and the earth fault loop impedance for the 20 A device, and record the burial route on the certificate so future digging is warned.
Traps
Ignoring thermal insulation derating; overfilled trunking; unsupported cable; bending below the minimum radius; steel conduit used as a protective conductor without checking continuity and cross-section; missing fire seals; glands not earthed; unprotected cable in a wall outside the permitted safe zones.