Selection and Erection of Wiring Systems
Selection drivers
A wiring system is chosen to suit external influences (water, dust, impact, temperature, corrosion, flora/fauna), the building construction, and maintainability. You must also consider electromagnetic compatibility, fire performance, and the skill available for maintenance. Questions often present an environment (a damp cellar, a farm, a workshop with mechanical damage risk) and ask which system is suitable - match the IP rating and mechanical protection to the influence.
Erection rules that are examined
- Support and spacing: cables must be adequately supported; the spacing depends on cable type and orientation.
- Bending radius: too tight a bend damages insulation and conductors.
- Protection against mechanical damage: conduit, trunking, or armouring where impact is expected.
- Segregation: power and communication cables separated to avoid interference.
- Fire: penetration of fire compartments must be sealed; cables must not compromise fire resistance.
- Concealed cables in walls: in domestic premises, cables buried less than 50 mm deep and without earthed metallic covering require 30 mA RCD protection (and/or mechanical protection). This is a very common exam point.
Traps
Do not confuse "suitable for the environment" with "suitable for the load" - both must be checked. Also, a cable rated for the temperature of the environment may still need derating for grouping. And a wiring system acceptable in one zone of a special location may be prohibited in another.
Worked rule of thumb
Ask three questions in order: (1) What can hit it? (mechanical) (2) What can get into it? (IP) (3) What can it damage or be damaged by? (fire, EMC, chemicals). If any answer is adverse, select a system that resists it - then re-check bend radius and support.