Extensions to Old Installations and Temporary Wiring Extensions to old installations

Extensions to old installations are a frequent requirement.

The electricity board reserves the right to be notified of any proposed addition to an existing installation, for a number of reasons. First, they must be satisfied that their service cable and meter will not become overloaded. Secondly, they must ensure that any additional connections do not disturb the service to other consumers.

Extending or repairing old installations may offer many more traps than new work, and the first thing to check is the condition of the existing wiring.

Too often, in old houses, the wiring has severely deteriorated. The rubber insulation has become brittle and broken in places, and where overheating has occurred, the insulation may be in a very poor condition. This may not always be fully revealed with the ‘Megger’ test for insulation testing, but such a test should be accompanied by a visual inspection of the wiring in a typical fitting. In such cases, no extensions should be added to such wiring.

Extensions to existing wiring, where they may be safely carried out, may conveniently be made in sheathed wiring, often run on the surface to avoid disturbance to decorations.

This surface wiring is a legitimate method, providing great care is taken to see that the cables are either protected from damage by means of a suitable metal cover, or else run in such a way that they can be clearly seen. If the cable is subsequently painted to match the woodwork it will scarcely be noticed by the casual eye, yet it is sufficiently obvious to avoid the danger that someone will knock nails right through it.

A point where special care is necessary is where sheathed cable enters a floor. Here it is especially vulnerable. Half round or oval metal channelling should always be fitted at this point, to prevent accidental damage.

Surface-type fittings are usually employed in such work. These fittings have knockouts all round the box, and care must be taken to knock out only the minimum area needed to accommodate the cable, to see that all unused knockouts are left in position, to prevent the intrusion of dust, and that the wires, as made off, are not pinched when the box is screwed to the wall.

When in doubt about the state of an old installation it is better to run the new circuits straight back to the consumer unit, and then to ask the electricity board to connect them up.

Conversion to ring circuit

It is often possible to convert an old wiring system which is still in good order into a ring system, but of course this can only be done if the size of conductor originally used is sufficient to satisfy the Regulations: I.e. it must not be less than 2.5 mm2.

For example, suppose a house has a 2.5 mm2 feed straight from the consumer unit to a 15 A socket-outlet in the sitting room, and another similar feed to another 15 A socket in the dining room. If these two feeds can be clearly traced at the consumer unit, they can be taken as the two ends of a ring, and brought into one 30 A fuse at the consumer unit.

At the remote ends, surface wiring can be taken from the socket-outlet in one room, all round that room, to feed as many 13 A socket-outlets as desired, and then through the dividing wall into the other room, to feed further socket-outlets in that room, terminating at the second original 15 A socket-outlet position.

A problem often facing those called on to extend old installations is the need to provide an earth connection, when the original installation employed only two-pin socket-outlets, with no earth. In this case, there is no alternative, when installing the 13 A socket-outlets that must be used, to running an earth connection right back to the earthing point at the consumer’s terminal. The size of this conductor must be as laid down in the Regulations, but it must not be smaller than 1.5 mm2. The temptation to provide a local earth, near to the socket-outlet concerned, by making a connection to any nearby water pipe must be resisted, since the installation must never have more than one earthing point. If protective conductors are added they must be insulated conductors, coloured green and yellow, and be properly clipped and protected throughout.

The tests that must be made before commencing any work on an old installation may show that the wiring is faulty.

In rectifying a fault on the wiring it may be permissible to restore the faulty wiring to its original condition, with good workmanship and good materials.

If non-standard wiring connections are found in an installation which is supplied on the standard 240 V a.c. phase and neutral system, then one is well advised not to restore the faulty non-standard wiring, but to put in standard connections on a new circuit installed in place of the old. Do not perpetuate incorrect wiring, which is basically unsafe and could give rise to serious troubles, including electrocution incidents.

An example of this aspect of obsolete installation problems was recently mentioned to the author by a contractor. His men went to the house of an elderly widow, who had written to him to say that the electric fire in her bathroom would not work. They found an antique, unguarded bare-element 2kW fire, plugged into a two-pin socket-outlet in the bathroom, obviously with no earth connection. Behind the socket, the rubber insulation on the wiring, installed in wooden troughing, had become so brittle that it had broken away, leaving the live wires bare. Some vibration had made them touch, and the fuse had blown. A nephew of the old lady had replaced the fuse with a much thicker wire, and this had also blown, burning his hand, so he left it alone.

The lady was most indignant when the contractor called on her and said that it would be impossible to effect a simple repair, and the socket-outlet should never have been fixed in the bathroom.

He was, of course, absolutely right in refusing to perpetuate a dangerous situation: but he learned later that a ‘handyman’ had ‘done the job’ for £5, and had simply taped up the insulation for a few inches, where the wire was accessible behind the old two-pin socket-outlet, and replaced the fuse. If the old lady had been electrocuted, he would have had much to answer for.

The contractor advised the electricity board that an unsafe installation existed, and they managed to convince the lady that rewiring was necessary.

Warning points to users of electrical installations

The users of an installation should be warned especially about the following points.

It is very unwise to use adapters if it is at all possible to avoid them. They tend to give rise to radio and television interference troubles through loose contacts, and they may well become overloaded, and in consequence overheated, and possibly dangerous. They give rise to long lengths of flex, which may cause people to trip.

Never extend a flexible lead by simply joining extra flex on to the end and making a taped joint. This can easily pull out and give rise to live wires lying on the carpet for children to touch; and the joint, simply being twisted, is never satisfactory and may itself overheat.

Avoid loose flex wherever possible since it is liable to trip people up or to break or fray when stepped on, and it should never be run under carpets or linoleum, since any damage will not be seen.

Never run an extension by stapling flex on to a skirting board. Flex is not suitable for this type of installation as it has no protection against mechanical damage.

Temporary wiring

Before setting up a temporary installation, it is advisable to study the relevant section of the Regulations for Electrical Installation. In these Regulations, ‘temporary’ is defined as referring to wiring not expected to be in use for more than three months.

Briefly, some of the more important points to be observed include the requirement that all temporary installations shall be dismantled as soon as they are no longer required, and in any case shall be completely overhauled at three-monthly intervals.

Every temporary installation must be provided with protection against excess current and with a switch or other device that disconnects all phases or poles of the supply. Equipment that has been used on temporary installations, particularly those for outdoor use, should be thoroughly checked before being used again; corrosion may have rendered some items unsafe.

Except in private houses, temporary electrical installations must be in charge of a competent person, whose name and address must be clearly shown, close to the main switch.

Apart from these special points, all temporary installations must be properly tested and must have the correct values of insulation resistance, earth continuity, and correctness of polarity exactly as these requirements apply to permanent installations.

For the purposes for which most temporary installations are required, PVC sheathed and insulated cable is usually suitable, but care must always be taken to ensure that temporary wiring is not inadvertently moved by other people so that, for example, wires lie in contact with a hot-steam pipe, or encounter similar hazards.

Temporary outdoor wiring

Outdoor wiring for such purposes as floodlighting must be carried out with special care, since driving rain may well penetrate any of the fittings, and give rise to danger of shock or short circuits and blown fuses.

Extreme care in earthing every metal part of every floodlight or switch is essential. Nothing should be taken for granted – a check should be made with a testing instrument to make certain all metal parts are properly earthed. Temporary earth connections to nearby pipes or other ironwork must not be used. The protective conductor must be continuous back to the earth point of the installation from which the supply is taken.

The switches used on temporary outdoor installations must be of the double-pole type. When assembling the cables, the entry holes in the switches and other appliances should either be provided with weatherproof glands, or else carefully filled with the proper type of semi-plastic compound supplied by fittings manufacturers for this purpose, to prevent the entry of moisture.

Where temporary outdoor cabling runs on the ground, it should be protected against damage by being covered with planking or some similar form of protection. Otherwise persons treading on the cable may drive sharp chippings through the insulation. It is usually better to suspend the cable overhead, if at all possible.

Stage lighting

A special case of temporary lighting often encountered relates to wiring for theatrical purposes in public halls.

If the supply is to be taken from 13 A socket-outlets, care must be taken that double-pole isolating switches are provided for the circuits connected to each separate socket-outlet.


Dimming of stage lighting is often required. Dimmers using variable resistances are available, and must be of the rating needed for each circuit, otherwise they will be overloaded and could become dangerously hot. The easiest method of providing for dimmers is to install an additional two-pin socket-outlet in series with the phase wire and arrange for it to be controlled by a switch. Floodlight and spotlight connections

When supplying floodlights and powerful spotlights, the proper method is to terminate the main wiring 2 m or more away from the floodlamp and then to feed the lamp, via the socket-outlet, with special heat-resistant cable.