Old electric cables in the UK
It should be noted that in England and Wales from 1st January 2005, Building Regulations Part P requires that only certified persons carryout electrical installation work, or that the work is certified upon completion to satisfy the local Building Control Department.
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Over the last hundred years or so, the design and materials used in the manufacture of electric cables have improved but the older types of cable will still be found installed in some properties.
The older cables typically fall into three types:
- Two inner rubber insulated, multi-strand cores (no earth wire), with a lead outer sheath
- Two or three inner rubber insulated, multi-strand cores, with a rubber outer sheath
- Two inner PVC insulated, multi strand-cores plus an uninsulated multi-strand core (earth), and a PVC outer sheath
Two inner rubber insulated, multi-strand cores (no earth wire), with a lead outer sheath.
Electric cable with a lead outer sheath would be very old indeed. Over the years the insulation around the inner cores would have deteriorated and any installation still using this type of cable should be considered unsafe and isolated immediately as a failure of the inner insulation could cause the outer lead coating to become live.
Two or three inner rubber insulated, multi-strand cores, with a rubber outer sheath.
The risk with this type of cables is that the rubber will have become degraded especially where the cores enter the terminals behind switches/sockets - the cores can become overheated at these points which will have increased the deterioration of the rubber insulation. This type of cable should be considered ready for replacement or, at the very least, tested by an electrician.
Two inner PVC insulated, multi-strand cores plus an uninsulated multi-strand core (earth), and a PVC outer sheath.
Cable using PVC insulated multi-strand cores can still be in good physical condition, although the chances are that the installation will not incorporate modern circuit breakers or a RCD - these can usually be added without having to replace the complete wiring providing that the wiring passes the appropriate tests.
Two points to watch with the cable itself are:
- The Earth wires in junction boxes etc may just be hand twisted together - this was quite a common installation method of the time, it is not a function of the cable itself. However, this could potentially present a hazard if the twisted joint becomes faulty - for safety sake, the cables should be traced through and cables jointed using screw junctions blocks.
- The individual strands of the inner cores can become broken under screw terminals - this will reduce the current carrying capacity of the cable at that point and could cause the wiring to overheat. Generally this will only be a problem when the cable is released from a screw terminal and then reused (such as when adding an additional wire to the terminal); in such an instance, it is better to cut off the ends of the cables so that fresh strands are used in the screw terminal.