Twin and earth

Twin and earth (often written "T&E" and sometimes "T and E") is a colloquial name in the UK, Australia, New Zealand and other countries for a type of flat sheathed fixed mains electricity cable, containing two insulated current carrying conductors and an Earth connector. In Australia and New Zealand this type of cable is referred to as 'Flat TPS' (Thermo-plastic sheathed), as well as "Twin and Earth" or "Twin with Earth".

Construction

These cables comprise two individually insulated current carrying conductors and a Circuit Protective Conductor (CPC = PE = Earth). It is commonly used for fixed wiring in domestic and commercial premises, but it is not intended to be used where it is subject to regular movement or sunlight, because the sheathing is affected by ultraviolet radiation.

Regional variations

In the UK the CPC is uninsulated (bare) and of reduced diameter compared to the main cores. Green and yellow sleeving, which is sold separately, is required to be used to cover the exposed ends.

In the Republic of Ireland the situation is different. From 2013, Irish wiring rules require 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,from the new standard IS 201-4:2013.[1] [2]

Mainland UK wiring regulations however 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 over sheathing was also available in PVC so care should be taken with older cables of unknown age.

In Australia and New Zealand flat 'TPS' (Thermo-plastic sheathed) "Twin and Earth" cable manufactured prior to 1966 permitted the use of an uninsulated CPC stranded core, requiring the exposed ends of this conductor to be sleeved with Green insulating tubing. This is also illegal in the south of Ireland. (Before the advent of PVC, this tubing was varnished cambric.) Since 1966 it has been required that the CPC be insulated with Green insulation and since 1980 the colour of this insulation has been Green/Yellow.[3]

In Australia and New Zealand Single conductor and Twin conductor TPS cables also exist. These are required because it is not permitted in Australia and New Zealand for the Neutral conductor in a "Twin and Earth" cable to be "re-purposed" as a "Switched Line" conductor, as is done the UK and in North America.

"Three Conductor plus Earth" TPS cables also exist, having uses in multiway switching.

The conductor in a "Single TPS" cable usually has Red insulation and the conductors in a "Twin TPS" cable usually have Red and White insulation - with the White usually used as a "Switched Line" conductor.

1 mm² TPS cables have solid current carrying conductors.

1.5 mm² to 2.5 mm² TPS cables are available with both solid and stranded current carrying conductors but the solid conductors are not usually used since they are more difficult to work with.

TPS cables greater than 2.5 mm² have stranded conductors.

The CPC conductor is always stranded.

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

  1. ↑ "BASEC News". BASEC. Retrieved 2015. Check date values in: |accessdate= (help)
  2. ↑ http://www.meteorelectrical.com/images/6193Y_1.5mm_Twin__Insulated_Earth_Bluebrown_Cable_100M_m.jpg
  3. ↑ Electrical wiring
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