Historic Mains Cables

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VIR twisted pair 4841-5.jpg

Before the 1960s several types of cable were used at various times.


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 bipehnyl (PCB) oil used in old paper insulation is toxic, and the insulation should not be handled with bare hands


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


VIR/VRI

Twisted pair 0842-6.jpg
  • 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 60°C
  • 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.

FailingVIRCable.jpg

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.

TRS

  • Tough Rubber Sheath
  • Rubber insulated conductors with a rubber outer sheath


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

Aluminium

  • Cheaper alternative to copper
  • Used from 1950s to 1970s, and old stock sometimes used into the 1980s. The main periods of use were 1950s and early 1970s.
  • The main problem with ali 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.
  • 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 ali 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.

Copper Clad Aluminium

  • CCA
  • Each conductor has an aluminium core surrounded by copper
  • An attempt to improve the properties of ali cable
  • Significantly better than al, the surface oxidation problem is eliminated, creep reduced & the risk from cracking more or less eliminated

PBJ

  • PolyButyl Jute insulated cable
  • Red-brown woven appearance
  • Once commonly used for mains incomer insulation
  • Lots of old PBJ is still safely in service
  • Can harden & crack where bent

Imperial T&E

PVC twin

Older T&E wiring having fairly similar construction to today's cables. Conductors were stranded & tinned to prevent reaction with rubber insulation.

These old cables had a bit smaller earth wires than today's. They were also available with no earth conductor.

Several versions exist:

  • Ashathene T&E
    • Polythene sheath, 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 conductor.

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.

Sizes:

  • 1/.044" T&E = 1 mm²
  • 3/.029 T&E
    • means 3 strands each of 0.029" diameter
    • used for lighting circuits
    • 1.27 mm²
  • 3/.036 = 1.94 mm²
  • 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
    • xsa 2.9 mm², rated 25A continuous in 14th edition (with a rewirable fuse)
  • 7/.036 = 4.5mm²
  • 7/.044 = 6.5mm²
  • unstranded lighting cable also used
  • 7/.052 = 9.35mm²
  • 7/.064 = 14.5mm²

Single insulated PVC

Single insulated PVC

Thin figure of 8 shaped flex is now used as speaker wire, but was used as a standard type of mains flex until the mid 1970s. Less often a 3 core version of this was seen.

Iron wire

From the late 1800s to the 1920s, self contained 32v lighting systems (run by a small generator) were installed using bare iron wire. The wire was in the region of 1/4"-1/2" diameter, with no insulation at all. Very unlikely to be encountered today.


See Also