A Guide To Old Wiring Systems

Electrical Wires
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Wiring: Have you recently become an electrician?

You may come across old wiring and cable types which you may not recognise, it can be hard to get information on these old installations, and find out whether they are unsafe…

A Guide to older cable types…

Tough Rubber Sheath (TRS)

Is easily recognised by its black colouring, I’ve seen this on countless installations, the mechanical protection was pretty non-existent and the rubber easily deteriorated due to overloading or high ambient temperatures.

Easily damaged by direct sunlight which led to the insulation breaking down, leading to the cable becoming dry and inflexible which could easily crack on cable manipulation, certainly at college we were told to leave this well alone and notify the customer for a rewire as removing of accessories (light switch) could crack the insulation leaving exposed live cables.

Lead – sheathed

These had rubber insulation often with a cloth covering, with either tinned or copper conductors. The circuit protective conductor (CPC) was formed by the lead sheath, which was essential to be maintained to ensure the lead sheath could not become live. There was a clamp system at each termination that secured the lead sheath to a metal box. This cable type offered a degree of mechanical protection but the rubber insulation was liable to deterioration over time, often due to excessive heat or overloading at the terminations of the accessories.

If this cable type is found in an installation it can be inspected to check the lead sheath, and rubber insulation condition, but advisable to leave it undisturbed until the wiring can be replaced…

Vulcanised India rubber

One of the earliest wiring systems, this involved cloth covered conductors, encased in a wooden style trunking, which had grooves on the top similar looking to small decking strips. The insulation was liable to break down over time causing cracking due to excessive heat and loading from the terminations of accessories. There was a fire risk with this wooden system, replacement is the only option if this installation is found in a dwelling.

Polyvinyl chloride (PVC)

Early PVC was manufactured to a different formula than today. This tended to produce a plasticiser liquid (or green goo called in the trade), which isn’t corrosive and can be cleaned with methylated spirit, this green goo can be found in accessories and when the cable stripped back a green sticky liquid is present.

PVC cables since the 1970.s have been reformulated to remove this problem, the insulation of the cable shouldn’t be affected because only the plasticiser is removed from the cable.

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