70ᴼC Thermoplastic PVC/PVC cables will become sticky and soft if they come into contact with expanded polystyrene and eventually become very brittle.
Cables and expanded polystyrene should be kept separate i.e. a non-metallic barrier such as conduit or trunking.
Regulation 522.5.3: “Materials liable to cause mutual or individual deterioration or hazardous degradation shall not be place in contact with each other”.
In the UK, thermoplastic-sheathed cable in "T/E" the design has the Circuit Protective Conductor uninsulated and of reduced csa compared to the main cores. Green and yellow sleeving is provided separately, and then applied.
Mainland UK wiring regulations do not at present (BS7671:AMD3) acknowledge Twin and Earth or Flat TPS with a full sized and insulated (G/Y) earth conductor as a valid cable type, which may be awkward for contractors who work cross-border.
Also available in smaller conductor sizes are versions containing three current-carrying conductors and a circuit protective conductor. These configurations are commonly used for applications such as switched light circuits, battery-backed emergency lighting which requires a switched and unswitched supply, extractor fans with a run-on timer which require a switched and unswitched supply, mains-powered interlinked smoke alarms, and central heating thermostats.
There is an overall sheath of grey PVC (BS 6004), or white for low smoke compound (BS 7211), although prior to 2005 white PVC was also available.