Twin and earth
Twin and earth or 'T and E' is a colloquial name for a type of flat sheathed mains electricity cable. In Australia and New Zealand similar cable is referred to as 'Flat TPS' (Flat Tough Plastic Sheathed). It comprises two individually insulated current carrying conductors and a central circuit protective (CPC = earth) conductor. It is commonly used in the United Kingdom and Ireland for fixed wiring in domestic and commercial premises, but it is not intended to be used where it is subject to regular movement or strong sunlight. In the UK the CPC is uninsulated (bare) and of reduced diameter compared to the main cores. Green and yellow sleeving is sold separately, to be applied at the ends. In the Republic of Ireland the situation is different. IS 201-4:2001 ( I.S. 201 part 4: PVC and Low Smoke Halogen Free Sheathed cables for fixed wiring) permitted both the UK style of twin and earth, and also a version with a CPC with a cross-section equal to that of the main conductors and insulated in green and yellow inside the full length of the cable. However, from 2013, the option for the uninsulated and reduced CPC as been removed from the new standard IS 201-4:2013, and as such is no longer permitted in new Southern Irish installations after a short transitional period. [1] There is a similar situation with the standards for Australian and New Zealand flat 'TPS' (Tough Plastic Sheathed) cable that used to permit an uninsulated CPC core, and now require the CPC to be insulated for the full length.
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 (earth) 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.
Standard UK metric twin and earth cable sizes
1/1 mm² and 1.5/1 mm² have solid conductors and CPC
2.5/1.5mm² has a solid CPC and may have solid or stranded conductors
4/1.5 mm² and 6/2.5 mm² have stranded conductors and a solid CPC
10/4 mm² and 16/6 mm² have stranded conductors and CPC
References
- ↑ "BASEC News". BASEC. Retrieved 2015.