The consumer unit
Starting at the consumer unit where the installer’s responsibility commences, this component should be chosen with considerable care with two viewpoints in mind: (1) Ease of installation of the original wiring. (2) Ease of adding new wiring when the occasion demands.
For an average domestic installation, the consumer unit might well include eight fuse ways, allocated as follows: two for the two ring circuits, 30 A each; one for the cooker circuit, 30 A; one for the main water-heating immersion heater, 15 A; one for a 3kW fixed radiator in the living room, 15 A; two for lighting circuits, 5 A each; one spare.
But there will be many variations on this.
As mentioned earlier in this post, careful studies concerning diversity of use, carried out over many years in many different types of household using various kinds of appliance, have shown that it is safe to allow for considerable diversity: in other words, to plan the wiring and fuses and other appliances on the basis that not all the potential, or possible, loading will occur at the same time.
For example, in the Regulations for Electrical Installations, for individual domestic installations, the suggested diversity factor that can be applied for example, to fixed heating and power appliances other than motors, cookers and water heaters, is 100% full load up to 10 A, plus 50% of any load in excess of 10A.
Taking this suggestion as a guide, if in a four-bedroomed house there are 2kW fixed electric fires in each bedroom, the total potential loading would be 4 x 2 = 8kW = 33.28 A. But the diversity factor suggested means that the cable size could be proportioned for the first 10 A plus 50% of the remaining current or 11.64A. In total, therefore, the current that should be allowed for is 10 + 11.64 = 21.64 A, instead of 33.28 A. This also applies to the fuses and the other fittings on the mains side of the installation but not to the final circuits.
However, it must be emphasised that this are only guides. The person responsible for the installation must make his own judgement.
For example, while for a cooker the same kind of diversity is suggested, if this cooker is installed, say, in a nursing home where special meals are prepared, or in a large house where a large family lives, or perhaps in a small cafe, then diversity does not necessarily apply.
The worst that can happen if too large a cable and consumer unit are installed is some small, apparently unnecessary expense at the time of installation.
The worst that can happen if too small a cable and consumer unit are installed is overheating, frequent blowing of fuses, and a probable need to embark on a costly and inconvenient programme of rewiring.
Each consumer unit will have its own main switch, fuseholders for each way, and neutral terminals and earth terminals.
There are metalclad and insulated enclosures for consumer units. If a conduit system is being installed, a metalclad type will be used. For sheathed cable systems, the insulated enclosure is more usual, but the metalclad type may be used if desired, care of course being taken to ensure that the metal enclosure is properly earthed.
All types are provided with knockouts. These are either sheet-metal plugs, pressed into holes appropriate for conduit or wire entry, or else clearly defined thin places in the plastic case, which can be tapped out by a careful tap with a hammer.
The Regulations state that all enclosures surrounding live parts must be completely closed. This is required to prevent the ingress of moisture, insects, and dust. So if any knockouts are not required for wiring they must be left intact, or if already knocked out, closed by means of rubber plugs.
Where conduit enters a consumer unit it must be fitted with a smooth brass bush at its end, to prevent the wires being damaged on the rough edges of the pipe. When bringing sheathed wiring in, close-fitting rubber grommets must always be used. These grommets have the dual purpose of protecting the wiring from damage when passing through the rough edges of the hole, and also blocking the hole around the wiring to seal the interior of the box.
Consumer units may be installed straight on to a brick or concrete wall, but there are several disadvantages if this method is used. First, brick walls may be damp, and this will ultimately cause rusting, even on the best galvanised boxes. Secondly, to obtain the neatest wiring layout, particularly with sheathed wiring, it is desirable to bring the wiring in from the back, and this is not possible if the boxes are flush against the wall.
The ideal method is to make up a strong wooden board, of not less than 13 mm timber, supported on battens at either side so that it stands about 50 mm away from the wall. This board is then firmly fixed to the wall at its left and right edges. The sheathed wiring, brought in from above on battens, can then be taken into the consumer unit through holes of generous size drilled in the board at points carefully measured out to coincide with the back knockouts.
With ample space behind the wooden board, the wiring will not be unduly pinched up, so avoiding the possibility of damage and overheating. Moreover, new wiring can easily be added. Further holes can be drilled in the baseboard by removing the fixing screws of any one consumer unit, and – if proper slack has been left in the wiring behind the board – slightly moving it out of place so that the hole can be drilled and the new wire brought through.
It should always be remembered that space should be provided on the baseboard for possible additional consumer units. For example, if off-peak storage heating is to be added, not only will an extra unit be needed, but a time-switch and possibly a contactor will have to be installed.
Again, if the size of the installation grows so that a 3-phase supply mains has to be brought in, there will be the need for separate consumer units for each phase; and if this move has been necessitated by the installation of off-peak storage heating, there will have to be units for the red, yellow and blue phases on-peak, and for the red, yellow and blue phases off-peak, making six in all.
In running out the wiring from the consumer unit, one cardinal principle must always be observed. There should be no joints in the wiring runs other-than those made at proper fittings, using screw-down terminals of suitable size.
Care must always be taken to avoid what is unfortunately a common device for fixing more wires into a terminal than it will properly hold, or wires larger than those for which it was designed. Many amateurs have been known to cut away several of the bared strands of the conductors, to make them fit into the terminal. This practice is entirely wrong, and may well be highly dangerous. The reduced copper section at this one point may well give rise to considerable overheating, without blowing any fuse, and it could therefore happen that a fire is caused through a switch or socket-outlet becoming overheated.
With sheathed wiring systems, round joint boxes are available.
The electrician must be certain that the terminals are large enough to contain the required number of wires of the size to be used.
It should be mentioned, however, that many of these boxes are only suitable for use with the smaller sizes of cables, usually less than the 2.5 mm2 size. Some boxes are specially made to take three 2.5 mm2 conductors, for example where a tee-off is required.
The best practice is to avoid boxes wherever possible. Lighting circuits may be looped in, using four-plate ceiling roses, to avoid the necessity for joints and, on ring circuits, any spur connections ought if possible to be made at socket-outlets, since the terminals on these units are capable of carrying three 2.5 mm2 conductors.
Joint boxes are designed with knockout sections, and these should be broken out in such a way that the cable fits snugly and does not allow empty spaces around it where dust and insects may penetrate.
Where there is considerable likelihood that moisture or fumes or dust will penetrate, the proper box to use is a conduit type with sealing glands so that the cable is properly sealed in to the box entry holes.
Metal boxes for switches, socket-outlets, etc., should be used with sheathed wiring systems, although plastic-type boxes are available. The protective conductor brought in with the cable must make proper connection with the box itself. An earthing screw is sometimes provided for this purpose, but the socket-outlet or other device usually has an internal arrangement whereby the earth connection terminal on the socket-outlet body has a metal strap connecting it to the screws used to fix the device on to the box, thus providing an earth connection when the socket-outlet is assembled.
A short cut sometimes adopted by amateurs, where they wish to earth a metal box, is to trap the earth wire beneath the lid of the box, holding it tight by means of the fixing screws. This means that a gap is left, through which moisture may penetrate. This short cut should not be employed.
Where plastic boxes are used, care must be taken to see that holes are not left in such a way that subsequent plastering will fill the box with liquid plaster.
Whatever type of box is used, it should be chosen so that there is ample room for all the wiring, and that when the switch or other fitting is applied, the wiring will not be squeezed up so tight that there is danger of a sharp point on the back of the switch pressing the wires against the metal part, with the possibility of ultimate breakdown of the insulation.
There is another danger that must be carefully avoided, and that is of cutting off the ends of the cable a fraction too short, so that when the switch is pressed home and the retaining screws are tightened, one or other of the connections is being gradually strained either to breaking point or is pulled out of the terminal.
Only experiment and practice on a dummy circuit, using some scrap cable, will show how much cable should be left.
All fittings – boxes, surface switches, socket-outlets – must allow for the outer sheath of the cable, in sheathed cable systems, to be brought well inside the protection of the box – in other words, the unprotected cores of the cable, even with their normal insulation, must never be accessible outside an area protected by some form of box.
Ceiling roses and connectors
Ceiling roses, for pendant lamps, are plastic with non-flammable back plates, called patresses.
These ceiling roses often have four terminals instead of the three that might be expected. This is because many electricians use the loop-in system, which uses a little more wire but involves less labour than the alternative.
The loop-in system has the advantage of not requiring a joint box as was necessary with older systems. If, however, there is some good reason for using the joint box, care should be taken to ensure that no bare conductors are exposed. The conductors must ‘fill’ the terminals, which may require the conductors to be doubled back on themselves and that the terminal screws are tight.
Connection of fixed appliances
Most fixed appliances, with sheathed cable systems, are connected via a fused spur to the ring.
Other appliances such as fluorescent lamp fittings and towel rails are generally provided with bases of such a type that either they will fit directly on to the standard galvanised steel conduit termination box, or else they fit on to the wall or ceiling in such a way that they form a complete enclosure over the properly bushed end of the conduit, or of the protected end of the sheathed cable, and at the same time are provided with proper means for securing the protective conductor connections. Fixed floor standing heaters are wired to a connection unit via flexible heat resisting cable.
On ring circuits, the number of fused spurs is unlimited, but the number of unfused spurs must not exceed the total number of socket-outlets and items of stationary equipment connected directly in the circuit. Both types of spur are connected to the ring via connection units or into the backs of socket outlets. The fused connection unit is available in flush or surface type, and can be mounted in standard conduit boxes.
For clocks, where the current consumption is very small indeed, special fused clock connectors should be used. These moulded plastic accessories are neat and small, and are often connected into a convenient lighting circuit. A 2 A fuse is incorporated.
Outlets for fixed storage heaters
As socket-outlets are not permitted on off-peak circuits for storage heaters a switch unit with a flex outlet is used, and may be obtained with a flush or surface mounting.
A special problem may arise in connection with those storage heaters that are fitted with fans to boost the heat output when needed. Since the boost is needed at on-peak times as well as during off-peak hours these fans must obviously operate from a normal, or on-peak circuit. Special fittings are available, whereby the heater and fan can be connected to a single unit, designed to allow two circuits to be used, each isolated from each other. This type of fitting is not now required when White Meter 8 and Economy 7 tariffs are used.
But for installations that are fed on the 3-phase system, it is necessary to ensure that two different phase connections are not brought near to each other in the same room. This would mean that a voltage above low voltage could exist, and the possibility of a fatal shock would be increased.
This requirement means that when the off-peak storage heating is installed the electrical layout of the installation as a whole must be considered. It may not be found very difficult to rearrange the main circuits at the consumer unit so that, for example, the two storage heaters that are situated on the ground floor are both fed from the red phase off-peak circuit, while the ground floor ring for on-peak socket-outlets is also fed from the red phase. On the first floor, the blue phase may be used both for the first floor ring and for the off-peak storage heaters.
Many manufacturers provide a special type of floor socket-outlet, fitted with a strong cover that may be screwed into place when the plug is not in use, so that no dust or moisture can find its way into the fitting, and furniture can be confidently allowed to stand on it. Such a special fitting should always be used when the circumstances make it necessary.
To prevent ingress of rain, and condensation inside, specially designed weatherproof switches, socket-outlets, lampholders and all other fittings should always be used outdoors.
Galvanised steel conduit boxes should be used, even if conduit is not being employed, since sheathed cable can be brought into these boxes, in a watertight fashion, by means of the special glands supplied by the manufacturers. If there is any doubt about watertightness, most cable manufacturers supply a type of plastic compound that remains flexible indefinitely to plug the intake ends of the cable coming into outdoor-mounted fittings.
Switches should be weatherproof, mounted in metal boxes. Great care must be taken in bringing in either conduit or sheathed cable to see that earthing continuity is carefully preserved, and that no moisture can enter, whatever the direction of the rain.
It is contrary to the Regulations that there should be any portable electrical appliance or socket-outlet or switch other than that of the ceiling type in a bathroom, and all electrical fittings must be of the all-insulated type: that is, the outer body must contain no metal parts at all. The two most commonly used special fittings for bathrooms are the shaver socket and the ceiling switch.
To ensure that no user of an electric shaver could receive a shock while shaving in wet conditions, specially designed shaver sockets must be used in bathrooms.
For bathrooms one way of complying with the Regulations is to place the lighting switches outside the room altogether, and this practice is often adopted. A single light may be switched in this way, and another light, say over a mirror, may well be switched by means of a ceiling switch. This type of switch is mounted on the ceiling and operated by means of a pull cord, usually of nylon. Some such switches have an indicator light, so they may be found in the dark.
Apparatus installed inflammable atmospheres If electrical apparatus is to be brought anywhere near an area where petrol, paraffin, butane gas, or any flammable substance is used, or where the vapour that rises from such substances may persist, then various regulations made by local authorities must be obeyed, and these regulations, added to the Regulations for Electrical Installations, impose a very high standard of practice in such situations.
To start with, specially designed flameproof gear must be used. It is rigidly tested to ensure not only that sparks cannot escape and cause explosions, but in addition it is so arranged that if flammable gases are trapped inside the conduits or boxes, and an internal explosion occurs, the metal parts will resist it and will not cause bare electrical wires to be exposed.
The beginner is advised not to contemplate the installation of flameproof gear unless he has professional advice available, and in addition he must be sure that he has studied – and complied with – the stringent regulations that apply.
We shall see later that all extra low voltage circuits, television and sound radio, aerials, loudspeaker connections, and telephone wiring – must be kept quite separate from power wiring, and must not be run in the same conduit or trunking.
There is however one point where the extra low voltage and standard systems meet. Electrical bells and chimes are often supplied at 12 V by means of small transformers, fed from the mains.
Such transformers are usually supplied by fused spur connections from a convenient lighting circuit, and may be fused at 2 A. Some bell transformers have their own fuses inside the case. Care must be taken to ensure that the low-voltage circuit wires cannot possibly come into contact with the 240 V mains circuit unless the bell circuit is insulated for the higher voltage. Auto-transformers must not be employed.
We have seen earlier that with the phase and neutral supply system the switches used should be of the single-pole type – that is, they break the phase conductor only, and not the neutral.
But there are some exceptions to this rule. In the case of immersion heaters the Regulations lay down that double-pole switches shall be used; and for any fixed heating appliance where the heating elements could possibly be touched, both conductors feeding the appliance must be interrupted by a switch placed near the appliance.
Double-pole switches are available from all manufacturers for these special applications, and will mostly fit into the same boxes as their single-pole counterparts.