May 26, 2011 at 4:22 pm
filed under Electrical
Tagged domestic, electrical, supply
Electricity comes into the house from the street via a service cable. In the street the service cable is connected to one of the phases and the neutral of a four-wire distribution cable.
The phases are coloured red, yellow and blue respectively. Some houses are supplied from the red phase; some from the yellow phase; others from the blue phase or, where electricity demand is very high, it is divided equally between the three phases. The phase colour is indicated by a disc on the electricity company’s apparatus in the house.
The energy company’s service cable terminates in the house in a sealed box containing a substantial fuse for each phase and a terminal block for the neutral. The phase wire and terminals have a voltage of 240 between the phase wire and the neutral terminal and to earth. The neutral wire, connected to an earthing plate at the electricity sub-station in a nearby street, is at zero volts to earth and is therefore termed an earthed neutral. The earthing of the neutral is for safety and has an important bearing on installation and appliances.
In the house the service unit containing the fuse and terminals is mounted on a board alongside the electricity meter and connected to it by two heavy duty cables which carry the whole current of the installation. Adjacent to the meter, but not on the same board, are fixed the consumer’s main switches and fuses. Where the off-peak tariff or white meter tariff is in force, the electricity company also fits a time switch and sometimes other apparatus.
The network of wires and cables comprising the home installation is divided into a number of individual circuits. The circuits have different current ratings according to their function (e.g. lighting, heating, water heating, cooking). Current ratings of circuits are standardised at 5-amp, 15-amp, 20-amp, 30-amp and 45-amp.
Splitting up an installation into circuits enables the smallest possible size of cable to be used in a circuit, provides individual fuse protection and enables any one circuit to be isolated from the mains when required. The whole installation is protected by the Board’s service fuse.
The consumer unit is the heart of the home installation and the point of origin of all circuits. A modern unit consists of a single-pole fuse distribution board fitted with fuses or miniature circuit-breakers and a double-pole main switch.
Fuses in a consumer unit have four standard ratings and are colour coded: 5-amp (white), 15-amp (blue), 20-amp (yellow) and 30-amp (red). Most units have facilities for one 45-amp (green) fuse, needed for the circuits of larger size cookers.
Miniature circuit-breakers, used as a better alternative to fuses, are for all current ratings of the same physical size and can be fitted in any combination of current ratings. It is possible to place the fuses and miniature circuit-breakers in any order but preferably the fuse of the highest current rating should be next to the main switch, with the remainder in descending order so that the lowest rating is farthest away from the main switch. At least one spare fuse unit in excess of immediate requirements should be included for any future extension to the installation.
Instead of a composite consumer unit supplying all circuits many older installations have a number of main switch and fuse units. These are replaced by one unit upon re-wiring.
An earth wire must run throughout every circuit. It is the bare wire in every PVC sheathed cable. The wire is attached to an earthing terminal at every light, switch socket-outlet, fused connection unit and other accessory, so that all fittings and appliances having exposed metal parts can be earthed.
The purpose of earthing is to provide a path for earth fault current of sufficient magnitude to blow a fuse so that the electric current to a faulty appliance or faulty wiring is out off and the faulty section made safe against fire or electric shock. Without effective earthing a fuse cannot blow in the event of an earth fault. Thus the metalwork remains “live” at mains voltage and is a source of danger. Where effective earthing is not possible, it is advisable to install an earth leakage circuit-breaker in conjunction with an earth rod electrode driven deep into the garden soil. This is a professional job and should be undertaken only by a qualified electrician in consultation with the Iota’ electricity company.
All metal work in a house likely to come into contact with electric cables and equipment has to be bonded electrically and connected to the installation earthing terminal. Examples are hot and cold water pipes and radiators, metal baths,’ sinks and wastes, cold and hot water storage tanks. Mains gas and water service pipes must also be bonded to earth. Green insulated earth wire is used for all bonding and for all earthing conductors not enclosed in the sheathed circuit cables.
Cable employed in home wiring for all circuits is usually flat twin and earth PVC sheathed cable. It has two PVC insulated current-carrying conductors, one red, the live wire, and one black, usually the neutral. Between the two insulated current-carrying conductors is a bare or non-insulated earth conductor. The flat sheath is of standard grey but cable having white sheath is also available. In some sections of a lighting circuit, especially in a two-way switch circuit, flat three-core and earth PVC sheathed cable is used. This has three PVC insulated current-carrying conductors — red, yellow and blue respectively. A bare earth conductor lies between the yellow and blue conductors.
The colours are used for identification in two-way and intermediate switch circuits — red for live and yellow and blue for the strap wires.
Cables used to connect main switches, consumer units and other apparatus to the electricity company’s meter are single-core PVC insulated and PVC sheathed. One, the live or phase wire, has red insulation; the other, the neutral, has black insulation.
A flexible cord is a cable the conductors of which comprise a number of fine copper wires twisted together to provide adequate flexibility when fitted to portable electrical appliances. Flexible cords are made in a range of types and sizes for the various uses and loadings of appliances and for pendant lighting fittings. The majority have three cores or conductors, coloured brown (live), blue (neutral) and green-yellow striped (earth). Formerly the core colours were red (live), black (neutral) and green (earth). Many existing flexes on appliances are in the old colours. This is a point to watch when renewing a flex or fitting a plug.
For double-insulated and all-insulated appliances two-core sheathed flexible cord is used. Core colours are brown (live) and blue (neutral). No earth core is required. Older flex colours are red (live) and black (neutral).
PVC sheathed cables used in home wiring do not normally have to be enclosed in conduit or trunking. The cables may be fixed to the surface of walls and ceilings, run behind skirting boards and door architraves, between and across joists in the roof space and voids between floors and ceilings and beneath joists of suspended floors on the ground floor.
Where the cables traverse joists under floorboards they must pass through holes drilled in the joists at a depth of not less than 2 in. (5 cm) below the tops of the joists so as to be well clear of floorboard fixing nails. Cables run on exposed surfaces of walls and ceilings must be fixed by clips spaced not more than 9 in. (23 cm) in horizontal runs and not more than 15 in. (38 cm) in vertical runs. For neatness spacings should be less than these required maxima. Fixing is not essential in positions to which there is normally no access and where the cables are unlikely to be disturbed (e.g. under floors and walls, where they can lie on the building structure, and down hollow partition walls).
Wiring accessories are mostly the visible parts of an electrical installation, so their appearance is an important factor in selection. They include: switches, fuses, plugs, socket outlets, ceiling roses, lamp-holders, fused connection units, flex connectors, plug adaptors and joint boxes.
Switches, socket outlets, fused connection units, flexible cord outlets and similar accessories are usually mounted on boxes which are moulded plastic for surface mounting and metal for flush mounting. A switch or socket outlet may be surface-mounted initially; then, during re-decoration, a flush box can be sunk in the wall and the switch or socket mounted flush.
Boxes are essential. The box, with the accessory itself, forms a non-combustible enclosure for the insulated ends of cables where the sheath has been removed for connections, for otherwise exposed terminals and for any joints in connectors which have to be made. Boxes have an earth terminal for the earth conductor of the circuit; this is necessary for plastic switches ad other accessories without an earth terminal.
The modern ceiling rose or cord-operated ceiling switch needs no box because it has an enclosed base enabling it to be mounted direct on the ceiling. The same applies to the modern batten lamp-holder
A lighting circuit consists of a limited number of lighting points and switches. With a current rating of 5 amps it may supply up to 1200 watts of lighting. This is a nominal value because each lampholder carrying a bulb up to and including 100W is assumed to have a rating of 100W. Larger bulbs are assessed at their actual rating. Where a lighting circuit has no bulbs larger than 100W it may supply up to 12 lamp-holders. Allowing, therefore, for one or two multilight fittings and one or more bulbs of 150W, a lighting circuit should not supply more than eight lighting points. This is an important point to observe when adding lights to an installation.
Irrespective of size, a house should have a minimum of two lighting circuits, so that when a circuit fuse blows only a section of the lighting will be affected and total darkness will be avoided.
no comments
RSS / trackback