Imperial Twin and Earth (T&W)

PVC twin

Older T&E wiring having similar construction to today’s cables. Several versions exist:

  • 7/.029 T&E
    • imperial 7 stranded version of 2.5mm² T&E
    • used for socket circuits until unstranded 2.5mm T&E was introduced around 1970
    • strands are 0.029″ diameter = 0.74mm
  • 3/.029 T&E
    • imperial 3 stranded version of 1mm² T&E
    • used for lighting circuits
    • 1/.044″ unstranded lighting cable also used
  • Ashathene T&E
    • Precursor to PVC T&E
    • Lasts well
  • PVC outer VIR inner
    • an early T&E cable
    • where the inner rubber has failed, the tails can be sleeved
  • 2 core Twin
    • no earth, used for lighting circuits, or power circuits with a separate (usually uninsulated) single run alongside to provide an earth.

PVC and ashathene versions of this cable last well and are usually in good condition, but rubber does not last well long term.

PVC outer rubber inner cable can have its rubber ends sleeved to make it safe, as its only the ends where exposed to air where the rubber becomes brittle and falls off. Cable with rubber outer insulation can not be made safe this way.

PolyButyl Jute Insulated Cable (PBJ)

  • PolyButyl Jute insulated cable
  • Rubberised Hessian appearance to the sheath.
  • The surface is these cables is quite uneven
  • Commonly used for mains incomer insulation
  • Lots of old PBJ is still safely in service

Copper Clad Aliminium

  • Each conductor has an aluminium core surrounded by copper
  • An attempt to improve the properties of aluminium cable
  • Significantly better than al, the surface oxidation problem is eliminated, creep reduced & the risk from cracking more or less eliminated.
  • Primarily still being produced and used as welding cable

Aluminium

  • Cheaper alternative to copper
  • Used from 1950s to 1970s, and old stock sometimes used into the 1980s.
  • The insulation & sheathing is mostly the same as T&E – PVC for decades, may be rubber on older cable.
  • A known fire risk
  • Aluminium cable creeps, oxidises & fractures, all of which can cause fires.
  • Requires special connections, do not connect to old aluminium cable using connectors intended for copper.
  • Al requires a larger conductor size than Cu for the same current rating
  • Presence of aluminium cable may be considered a material fact for insurance.
  • The main problem with aluminium cable is its thermal expansion coefficient. Repeated temperature cycling causes it to come loose at connection points, and it then oxidises, and aluminium oxide is an insulator. Bad connections generate excessive heat and fire can break out.
  • Having said all that aliminium cable still has its uses and so is still being manufactured today. Aluminum provides a much better conductivity to weight ratio than copper, and therefore is used in power wiring of some aircraft. With the prices of copper going through the roof people are looking at aluminium again and trying to work out better ways of using it such as for car speaker systems.

Cab Tyre Sheathed (CTS)

  • Cab Tyre Sheathed
  • Tinned copper conductors, each core insulated with VIR, & cab tyre outer sheath
  • Single, Flat Twin and Flat Triple
  • Cab tyre was the same rubber formula used for car tyres, making this cable a very tough rubber cable

Tough Rubber Sheathed (TRS)

  • Tough Rubber Sheath
  • Rubber insulated conductors with a rubber outer sheath
  • still used today but more often found used by bands, on stage, on the road and in the theatre.

Vulcanised Indian Rubber (VIR)

  • Vulcanised India Rubber insulated cable.
  • Along with imperial T&E, one of the most common historic wiring cables still in use
  • Comes in 2 forms:
    • Twisted pair, cotton/rubber insulated, with no outer sheath
    • singles drawn into conduit
  • Most VIR wiring doesn’t include an earth wire, which is sometimes run as a separate uninsulated single.
  • Rated to 60C
  • Rubber insulation perishes, cracks & falls off
  • Properties with VIR cable are usually in urgent need of rewiring, and may represent a significant safety risk. However some of the Jute / Hessian reinforced rubber cables that are often seen on consumer unit incomers are still often relatively safe.

A good percentage of the remaining old VIR wiring is now in a dangerous condition, especially at termination points. It is common to see insulation that has fallen off, often leaving live & neutral conductors bare, unsupported and in very close proximity. In the worst cases 2 bare conductors can be found twisted round each other with nothing rigid to support them.


The outer insulation has failed on the black sheath of the cable, and in one place the insulation on the inner live wire has also cracked off. This cable is in a very poor state, and is unsafe.

The rule of thumb with old rubber wiring is

  • replace it as soon as possible
  • don’t move it at all, even small movement sometimes causes shorts.

This situation is quite different to early American rubber wiring, which is usually still in sound condition, due to the use of a different rubber formulation. 

Lead Sheathed


 

  • Common in 1930s for socket circuits
  • Used as exterior farm cable well after that
  • Lead sheath does not make reliable earth connections
  • Rubber inner insulation
  • Tail rubber insulation tends to disintegrate, and muck accumulates on cable ends causing leakage

 
 

Paper Insulated Cable

Paper

  • Paper insulation
  • From the WW1 era
  • Very rare now in domestic wiring
  • Paper is somewhat hygroscopic
  • Pre-war paper cable is still in service in distribution networks, and causes a good deal of downtime
  • The polychlorinated biphenyl (PCB) oil used in old paper insulation is toxic, and the insulation should not be handled with bare hands