The most common cable found within a domestic electrical installation is the PVC insulated and sheathed Twin and CPC cable – also known as Twin and Earth. A PVC three core and CPC is also used on two-way lighting circuits, smoke detector interlinks, fans with timers and also central heating thermostat connections etc.
Cables should always run within prescribed ‘cable zones’ which are 150mm down from the ceiling, 150mm at each side of any corners within a room and horizontally or vertically to any point or accessory.
When run below the surface of the wall PVC cables won’t react chemically with the plaster so there is no need to provide any protection unless it’s required mechanically (steel conduit, trunking, capping…)
If no mechanical protection is used and the cables are less than 50mm from the surface of the wall, which is usually the case then additional protection with a 30mA RCD must be used – see Regulation 522.6.202.