Miscellaneous Aspects of electrical Installation Practice Outdoor wiring

Apart from underground wiring connections between buildings, outdoor wiring should employ a catenary, or suspended system.

There are two methods of catenary suspension. In one method, a special cable is employed, using a chlorosulphonated polyethylene sheathed cable with a built-in steel wire for suspension purposes. In the second method, a separate steel wire is employed, and the special outdoor cable is suspended from it by means of straps made from plastic material in ring or tape form.

It is permissible to span a gap of 3 m between adjacent buildings by means of a span of PVC sheathed and insulated cable, carefully clipped at each end to ensure that its own weight, and possible movement in the wind, will not cause the cable to be damaged at the clips.

Bell, television and telephone wiring

It should be clearly noted that wiring for the purposes of bell circuits, telephone circuits, television and radio circuits does not come within the scope of the Regulations for Electrical Installations, except in so far as the Regulations state that this wiring must be kept entirely separate from any wiring connected to circuits which are themselves joined to the mains. This is the first principle to be observed in regard to what we may now call auxiliary circuits.

Bell wiring

Bell wiring calls for little comment. Most bells or chimes are supplied from a bell transformer which must be properly fused with a 1A fuse on the high voltage side. The first test should ensure that both windings are continuous and the second test should ensure that the insulation between the 240 V side, and the low-voltage side is in good order.

Bell-circuit wiring is usually carried out in twin plastic flex, which may well be run in plastic conduit beneath the plaster, or may be neatly tacked with insulated staples on the surface in the case of existing installations. Joints in such wiring should preferably be soldered and taped, since it is desirable to make sure that there is minimum resistance at any jointing point, but ordinary mains-type plastic connectors may of course be used.

Telephone wiring

Telephone wiring may only be installed by the telephone authorities’ own staff, but they like to be advised of new telephone connections that are to be required, as early as possible, and they are willing to provide polythene plastic ducting to be built into the house, and to run to a suitable point outside.

If this procedure is adopted, the telephone wiring can be concealed as unobtrusively as the power wiring, and the appearance of unsightly telephone wires, stapled on top of the skirting board and round the door frames, can be avoided.

Television and radio circuits

It is becoming increasingly common for television and VHF aerial down-lead circuits to be installed as part of the wiring of a building, and to terminate at high-frequency sockets which are themselves mounted on plates that fit standard wiring boxes, so that they may be sunk flush into the wall, enabling the television set aerial lead to be plugged in neatly.

Low-loss television cable may be used for two purposes: for the television connections, and for the VHF connections for FM radio reception. It is common to install such wiring below the plaster in plastic conduit, so that it can be pulled out if trouble develops.

There is no technical reason why this wiring, which is usually plastic covered, could not be installed directly in the plaster, but if so there is always the possibility that it may be damaged by nails driven into the wall. It will then become very difficult to replace, and any fault could necessitate breaking away plaster walls and subsequent redecoration.

There is a most important point to be watched in this connection. The method sometimes adopted is to bring the lead from the aerial on the chimney into the loft or other similar area, and then to join on to it, leads running to high-frequency sockets, in for example, the dining room, the sitting room, and possibly a bedroom as well. This means that the aerial lead has connected to it one socket which is in use for television reception and two other sockets which are not in use, and this frequency causes ? ‘mis-match’, so that reception is impaired.

To avoid this, the lead from the aerial should be brought down to the point most likely to be used regularly, such as the sitting room, and then run to the next room, and from there to the third or other plug points that are to be provided. At the termination in the sitting room a special high-frequency switch can be provided to switch in the additional television cable length into, say, the dining room, or else a neatly arranged lead can be left with a male socket on the end which can be plugged into the wall socket in place of the connection to the television set, and which has the effect of extending the aerial down-lead in series, into the dining room, and on to a bedroom or other receiver point. In this way, the best reception will be obtained without troubles due to mis-matching.

Audio-frequency circuits

In some installations, sound recording enthusiasts, may like to have facilities whereby one room can be used as a studio and the output from the microphone and amplifier taken to other rooms where recording can be carried out without the noise of the instruments disturbing the studio atmosphere.

This facility can be provided very easily if it is foreseen during the installation work. All that is needed is to use the same type of VHF television cable as used for the television circuit itself, and to provide links between the rooms, terminating in each case on a flush-type high-frequency socket-outlet inset in the wall, so that microphone or loudspeaker circuits can be plugged in as required without running very long leads through passages and under doors.

Motor circuits

Until the last decade most small motors, such as those used in domestic premises, and in small workshops, had to be provided with starting devices, but nowadays quite large motors, up to 3.5 kW or above, can be started by simply switching them on to the mains.

The types of motor used in such domestic devices as washing machines, vacuum cleaners, refrigerators and power drills do not involve any special problems. However, for motors approaching 750 W and larger, there is one factor that needs to be taken into account.

When a motor starts, it may well take ten or twelve times its normal operating current. This current rush at starting is of brief duration, but nevertheless may blow fuses unnecessarily, since the installation will not be harmed, nor the wiring overheated, by this excess current of very brief duration. To give an example, a 750 W single-phase motor at full load will take just over 3 A, but when starting it may take considerably more current than this and the makers recommend that a 10 A fuse should be used.

Motors that have to start very frequently may give rise to difficulties as the excess current during the starting period heats up the fuse more and more, and ultimately it may blow, but not because of any defect in the installation.

One method of avoiding this is to install a motor starting unit, which is usually available from the manufacturer of the motor, and comprises a pushbutton-operated starting switch. It is usually equipped with a special overload device to prevent damage to the motor or to the installation. These devices often take the form of a thermal overload. A small heating element, suitably proportioned, is connected into the motor circuit, and operates a bimetal strip in much the same way as the thermostat mentioned earlier, and the bimetal strip when it operates trips out the switch.

For domestic and small workshop applications, single-phase motors are the most commonly used. They present no problems in regard to their connection to the supply.

For larger drives, the 3-phase motor requires a special 3-phase socket, and a proper 3-phase isolation switch must be provided, even if the starting arrangements are incorporated within the machine to be driven.


The transformer is a most useful static device, with no moving parts and therefore no need for maintenance, used to change one voltage to another. Basically, it has remained unchanged since it was first developed by Faraday in 1831.

The transformer consists of iron stampings made from thin plate, arranged in the form of a ring. On one side is a winding having, let us say, 2000 turns, and on the other side a winding having, say, 1000 turns.

If the 2000-turn winding is now connected to a supply of alternating current, then a’ voltage will appear on the 1000-turn winding which is exactly one-half of that applied to other side. The voltage can be stepped up or down at will by changing the ‘turns ratio’, providing the coils are suitable for an application at the voltage desired.

The most common example of the transformer found in domestic use is the bell transformer, giving a low voltage from the 240 V supply. In this case, it would obviously be very wrong to connect 240 V mains to the low-voltage terminal, as not only would the insulation of the low-voltage winding be insufficient for 240 V, but in addition a dangerously high voltage would be produced on the 240 V winding, and this too would flash over to earth and destroy the winding since its insulation would also be insufficient.

There are regulations concerning transformers which insist that the insulation between the two windings should be of at least as high a value as the insulation of the primary or 240 V winding to earth. This prevents the high voltage from penetrating to the low-voltage side.

There are, however, certain types of transformer known as auto-transformers, which must not be used for domestic purposes, although they have many applications in industry. In this type of transformer there is a single winding arranged on one side, as it were, of the iron ring, and the full mains voltage is applied across the winding. A tapping is taken from it, at, say, half-way down, to provide half the mains voltage. This means that there is physical connection between the 120 V circuit and the 240V circuit, and a breakdown in the winding could result in 240 V being applied to the 120 V circuit. This is the reason why the auto-transformer is not permitted in domestic premises.

Continental equipment

The marking of the wires connecting continental equipment to the mains is often different from that used in Britain, and care must be taken to ensure that the earth wire is correctly ascertained. Some continental machinery uses white earth wires, and many domestic appliances manufactured on the continent and in the Far East have no earth connection at all. In such cases, the two-core flex should therefore be removed and a three-core flex substituted, but there a further problem arises. Within the device itself – say, for example, a food mixer – it may not always be possible at first glance to see where to attach an earth connection in such a way that it ensures that all metal parts that can be touched are connected properly to the earth wire. This means some care and thought and perhaps the need to arrange some bonding wires within the appliance to make certain that all metal parts of all kinds within the appliance are in fact joined together and to earth. However, the appliance may be of the double-insulated type, and this is then unnecessary.

Cupboard door switches

To provide an arrangement whereby when a cupboard door is opened a light within it is lit up can be done by means of a normally off switch specially provided for this purpose. These switches have a protruding knob which is arranged that it may be pressed by the back of the cupboard door as the door closes, and this action holds the switch open. As the door is opened the knob springs out and closes the circuit. These switches can be inserted into the frame of the door in a specially made hole, or they may be mounted inside the frame with a wooden block on the door carefully arranged to press on the knob and open the switch as the door closes.

Connections to gas appliances and oil-fired central heating plant There may be a need to provide a supply for the equipment that is associated with central-heating equipment, either for lighting up the flame when a thermostat determines when it should be lit, or for increasing water flow in a central-heating water system circuit.

The supply to such devices may be made by a normal socket-outlet, but the cable used must be of the butyl heat-resistant type.

In certain larger installations, it may be necessary to use flameproof switches and other fittings in the room where the gas-or oil-heated boiler is situated. Consultation with the manufacturers may be necessary.

Connections to underfloor electric heating For bungalows especially, underfloor electric heating is frequently employed. This form of heating may be carried out in several ways, but it often consists of insulated electric wires buried in the floor and covered by a layer of concrete. Some systems employ plastic-coated mineral-insulated copper-sheathed cable, which may be safely operated at high temperatures, others employ special heating wires simply immersed in the concrete, while what is probably the best system uses a conduit buried in the cement of the floor and withdrawable heating cables.

For a room of 5 m by 4 m a floor heating loading of about 3 kW is necessary, and this is better supplied by means of a separate feed run directly back to the consumer unit. This is because a thermostat is needed in the circuit, to prevent undue consumption of current, and this would mean a wiring complication if the floor heating was supplied from the normal ring circuit.

In any case, such supplies should always be taken at off-peak periods, so that a separate feed is automatically needed.

The actual designing and installation of a floor-heating system requires specialist knowledge, and in any case must be carried out over a considerable period in close collaboration with the builder. Such a system can very rarely be installed in any but new buildings.

Warm-air central heating and air conditioning

Some houses are equipped with ducted air systems, so that air is warmed up at a central point by electric heaters, with fans to control the flow of air.

In providing supplies to these heaters, the only point the electrician has to observe is to ensure that the cables are of adequate size and that the necessary thermostat connections, as required by the makers, are incorporated into the wiring layout.

In many houses complete air conditioning is employed. This means that the air taken into the sealed, double-glazed and fully insulated interior of the house is cleaned, humidified if necessary, and heated in winter or cooled in summer, the air-conditioning plant being provided with both heating and refrigerating units.

Some of these air conditioners have complex thermostat and humidity-controlling devices, which need extensive wiring runs; and since the load on many of them is quite high – 12 kW is common for an ordinary three-bedroomed house – obviously special large current-carrying mains cables and appropriate fuses are needed.


To dim a domestic lighting circuit thyristors are widely used. The thyristor blocks the current for part of each cycle of alternation, the amount of blockage being controlled by the application of a control voltage to one of the electrodes of the thyristor by means of a variable resistor. Another commonly used method in the theatres and dance halls is to insert a variable resistance between the mains and the lamp circuits to be dimmed.

This resistance obviously carries the whole current of the lighting system, and so will develop heat. Moreover, until recently most resistance dimmers of this type were fairly bulky, and could not therefore be easily incorporated into the lighting circuits of, say, an ordinary lounge or dining room.

Nowadays, devices have been developed of a size not very much larger than a socket-outlet, which can be safely buried in the wall, and are arranged so that the heat is safely dissipated.

Delay switches

In buildings where there are a number of flats with access by stairways instead of lifts, delay switches are sometimes fitted, to allow for the lights to be switched on when entering the building at basement level, and for them to remain on for a given period of perhaps five minutes. They then switch themselves off to avoid current waste in lighting up the staircases and passages during the night, when no light is needed.

These delay switches take several forms. In some the pressure on the switch winds up a small spring, which then unwinds over a period and switches off the circuit. In another form a pneumatic piston is provided within the switch, which is lowered into its cylinder against the air pressure by the operation of switching on, and then as the air gradually escapes through a controlled orifice, the piston rises with the help of a spring and switches off the lamp. These devices can be purchased from most switch manufacturers.

Security devices

There are several patent burglar-alarm systems on the market, but many of them have the disadvantage that being widely advertised the burglars themselves are fully familiar with their requirements. The best system is undoubtedly one evolved specially for the installation concerned. Obviously all windows and doors need some kind of protection, and small switches will have to be installed on every door and window to operate if the door or window is opened. A simple ‘closed’ circuit system may be wired so that even if the burglar sees the wiring and cuts it, the alarm still operates. Each door or window is fitted with a small switch so that if entry is attempted, the switch contacts will open. This will cause a relay to operate and close a bell circuit, giving an alarm. If wires are cut or break, the alarm will sound, thus the system is self-monitoring, against open circuits.

Emergency lighting

Emergency lighting can be provided in ordinary domestic premises, with moderate cost, by using a trickle charger suitable for a 12 V car battery, the battery itself, and a relay.

The system requires a relay which will remain permanently energised as long as its coil is connected to the 240 V mains circuit. This relay should be properly protected on the mains side with a 2 A fuse, and should be enclosed to prevent damage.

The contacts, which remain permanently open when the relay is energised, may be connected to the battery and to a 12 V lighting circuit run as required in the house. Since this is not mains wiring, it is not subject to the normal regulations, but may be run in any desired method, providing these wires are not brought into contact with the mains. However, twin plastic cable run in plastic conduit will be quite satisfactory, but surface wiring neatly cleated is of course permissible.

Obviously emergency lighting is not required everywhere, but one 36 W car headlamp bulb in the sitting room, another in the kitchen and perhaps a 6W type on the landing and a further 6W bulb in a lavatory will be sufficient.

When the relay drops off through mains failure, all these lamps will be lit. As mains failure is usually of short duration, it may not matter if the lamps remain alight, even if all of them are not needed. In any case, the battery benefits by being discharged on occasion. If no failures occur for long periods, the battery may deteriorate.

When the mains supply is restored, the relay will automatically pick up and break the circuit, and the battery will at once begin to charge up once more.

The type of relay obtained may not necessarily have contacts that will carry the whole current for the emergency lighting circuit if all the lamps are in use, and it may be necessary to employ a second relay, operated from the battery, with contacts of greater capacity. The first, or main failure relay, simply operating a battery circuit for the coil of the second relay, whose contacts will carry the whole of the main current. This depends on the type of relay obtained. The current for a 36W car headlamp bulb at 12 V is 3 A and thus if two of these are in use plus two 6 W lamps, 7 A will be needed, and not all relay contacts will carry this current for any length of time.

Some relays have a number of contacts, designed originally for closing a number of circuits at once, and these contacts can be paralleled up so that the current is shared between them and they are all working within their rating.

Electricity in the greenhouse and the garden Electricity may be used in a greenhouse for heating, for seed propagation, for raising cuttings and for soil sterilisation, and many other purposes. It may also be used in the garden to drive an electrically operated lawnmower or hedgecutter.

Great care should be taken when installing socket-outlets in greenhouses where damp and humid conditions often prevail, to ensure that all fittings are of the weatherproof type, and that the greatest care is taken to ensure proper earthing, since an electric shock obtained under these conditions would be especially dangerous.

Conduit systems, mineral-insulated copper-sheathed or PVC armoured cables, with metallic fittings are the best for this purpose, and if an outdoor socket for the lawnmower is provided, again this must be carefully installed so that corrosion effects are minimised. Weatherproof socket-outlets should be used. These are available with the added protection of earth leakage circuit-breakers.