Working with Cables and Plugs

Cables used for home wiring are mainly twin and earth PVC flat sheathed cables containing one red and one black PVC insulated conductor and one uninsulated earth conductor.

The red conductor is used in the live pole of the circuit, the black in the neutral pole. (The black conductor is also used in the live pole for the switch wire of a lighting circuit linking the switch to the light. However, to identify it as the live, red PVC sleeving is placed over each end within the switch and lighting point.)

Also used in lighting circuits is three-core and earth PVC flat sheathed cable, for linking the two switches of a two-way switching circuit and for the cable running to a two-gang switch where two lights on the same fitting are switched independently from one position. The core colours of this cable are red, yellow, and blue. The colours have no significance in home wiring but identify the different wires at the switches.

At the ends of all cables where they terminate at switches, socket outlets, ceiling roses or other wiring accessories the bare earth conductors should be enclosed in green-yellow PVC sleeving. Sizes of the conductors of house wiring cables range from 1.0mm2 to 16mm2, the size chosen depending upon the current rating of the circuit.

Flexible cords

Flexible cords are made in numerous sizes and types. The most widely used is circular PVC sheathed — three-core for earthed electrical appliances, two-core for double-insulated appliances and for insulated lighting pendants.

Other types used are parallel-twin flex; unkinkable flex for electric irons and percolators; and braided circular flex for heaters as an alternative to circular PVC sheathed.

Plugs

Plugs used in the home are: (A) three-pin (flat) 13A fused plugs; (B) three-pin and two-pin (round) non-fused 2A, 5A and 15A plugs; and (C) moulded-on plugs such as those fitted to electric shavers.

The 13A fused plug is the standard plug. Round-pin plugs generally are obsolescent and the two-pin versions, except those fitted to shavers, are potentially dangerous as they provide no earthing for appliances required to be earthed.

Wiring a 13Alused plug

The three terminals of a fused plug are

marked L for the live core of the flex, N for the neutral core, and E for earth.

When connecting a plug to old flex having the former colour coding red (live), black (neutral) and green (earth) take especial care that the wires are connected to the correct terminals of the plug. For safety always connect the green/yellow (or green) wire to the E terminal first followed by the brown (or red) to the L terminal and finally the blue (or black) to the N terminal.

Double check that the sheathing of the flex and not the unsheathed ends of the wires are clamped by the cord grip.

The flex connections to a three-pin round-pin plug are the same as for a fused plug. When connecting the two-core flex of a double-insulated appliance, the E (earth) terminal is left blank.

When a moulded-on plug needs replacing either replace the flex complete with a new moulded-on plug or cut off the old plug and fit a conventional plug. Fixings except where cables are likely to be disturbed.

Cables in the loft

In a loft the cables can rest on the ceiling alongside joists. Where they cross joists they must be fixed to the tops of the joists but should be routed well away from access points and walkways. Where expanded polystyrene granules instead of glass fibre or vermiculite is used for loft insulation, the cables should be either run in PVC conduit or covered with PVC channel, since direct contact with the polystyrene may harm the PVC sheathing.

Cables down walls

PVC sheathed cables may be run down the cavities of hollow partition walls without the need for intermediate fixings for drops of up to 3m.

Plug adaptors

The function of a multi-plug adaptor is to enable more than one portable appliance or portable lamp to be used from a socket outlet. To prevent overloading a socket outlet use only one adaptor and preferably one which has only two outlets and will not accept another adaptor which, with its numerous plugs and flexes, produces a potentially dangerous ‘Christmas-tree’.

If you buy an adaptor which accepts plugs of lower rating than that of the socket, ensure it contains a protective 2A or 5A fuse.

Cable sizes Circuit Cable (mrn2)

Lighting 1 .0 or 1.5

Immersion heater 2.5

Ring circuit 2.5

Radial multi- I 3A-outlets (20A) 2.5

Radial multi-13A- 4.0

outlets (30A)

Shower unit 4.0 or 6.0

Cooker 6.0 or 10.0

Meter leads 16.0

Running cables

 

PVC sheathed house wiring cables can be run under floors, in voids between ceilings and floors, in the loft, buried in the plaster of walls, fixed to wall and ceiling surfaces or in mini-trunking.

Cables under floors

Cables run in a ceiling void may rest on the ceiling but where they cross joists they are threaded through holes drilled in the joists at least 50mm below the tops of the joists. Under a suspended ground floor the cables may rest on the concrete screed, the earth or building structure without the need for any

On solid walls the cables may be fixed to the wall surface or be buried in the plaster.

Surface fixings

The cables are fixed to the wall surface with plastic cable clips fitted with single steel pins. The maximum permitted distances between fixings are 400mm for vertically run cables and 250mm for horizontally run cables.

The clips are of different sizes to suit the various cables and are available in white for use with white sheathed cable.

Cables buried in plaster

PVC sheathed cables may be buried direct into the plaster of walls and plastered over without the need for any additional protedtion from mechanical damage in the form of conduit or channel. If conduit or channel is used the wider and deeper chases are likely to weaken the wall without protecting the cables from drilling, plugging or driving nails.

To minimise the risk of damage the cables must be run vertically above and/or below the respective switch or other wiring accessory where any person fixing shelves or other structures will expect the cable to be situated.

Solid floors

Where the floor is solid and it is not possible to run cables beneath to socket outlets in those rooms, in some instances the cables can be run behind the skirting boards and behind door frame architraves where they cross doorways. A more satisfactory method is to enclose the cable in mini-trunking which is fixed to the walls on the tops of skirting boards and around door frames. Special boxes and elbow fittings are used with the trunking. Moulded skirting is also available which can be used to conceal a number of cables.

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