Electrical Company & Contractors in London

The various circuits in a dwelling are detailed below.

Lighting circuits
A lighting circuit is wired in either 1.0mm2 or 1.5mm2 cable using either of two methods. One is the loop-in system where the cable from the 5A fuse runs to each ceiling rose or other lighting fitting and terminates at the last on the circuit. From each light a cable is run to its switch which if a 2-way switch, a 3-core and earth cable is run from this switch to the second 2-way switch. The other method is the joint-box system where each light and its switch requires a separate 4-terminal joint-box. The cable from the 5A fuse runs to each joint-box in turn and terminates at the last joint-box in the circuit. From each joint-box a cable is run to the light and another cable to the switch. A lighting circuit may supply up to a maximum of 10 lighting points which is the equivalent of about 1200 watts.

Ring circuits
A ring circuit is wired in 2.5mm2 cable. The cable starts from the terminals of a 30A fuse, runs to each socket outlet it is to supply and returns to the consumer unit where it is connected to the same terminals as the first end of the cable. Cables termed spurs may branch off the ring cable at socket outlets or joint-boxes to supply remotely positioned outlets. A ring circuit may supply an area of up to 100 square meters or 7200 watts maximum load.

Socket outlet circuit
The maximum load for this type of wiring is 3000 watts.

Spur on a ring circuit
This type of wiring is effectively a branching off a ring circuit and the maximum load is the again 7200 watts.

Single appliance circuit
This type of wiring is necessary for power hungry appliances, for example electrical showers, cookers, electrical heaters.

The fixed wiring in a house or flat supplying the various circuits is usually flat, twin-core and earth PVC sheathed with some sections of a lighting circuit such as 2-way switching being wired in flat 3-core and earth PVC sheathed cable. The sheath is either grey or white. Within the sheath of twin-core and earth cable are two PVC insulated, current-carrying, copper conductors: one red, one black and an uninsulated copper conductor positioned between them.

The red conductor is normally used as the live and the black as the neutral, but the return wire from a switch to the light of a lighting circuit although live when the switch is’ON’is a black wire which needs a red sleeve slipped over each end. The uninsulated wire is the earth conductor and its bare end within a switch or any other accessory must be enclosed in a green yellow PVC sleeve, so that it does not make contact with a current carrying terminal. Core colours of a 3-core and earth cable are red, yellow and blue, and when used in a switch circuit the,ends of the blue and yellow conductors must be enclosed in red sleeving. Sizes of cables used in home wiring are 1.0mm2, 1.5mm2, 2.5mm2, 4mm2, 6mm2 and 10mm2.

Always have a card of fuse-wire available, these cover the most popular current ratings: 5, 15 and 30 amps. Make sure you rewire a fuse with the correct size fuse wire using the colour code as a guide to the size required. For cartridge fuses, always have at least two spares of each current rating in the consumer unit. Locate the ‘blown’ fuse and remove the fuse holder. Take out the bits of old fuse wire or the old cartridge and insert the new one, taking care not to stretch the fuse wire on tightening the terminal screw, as this would decrease the current rating, lead to heating of the wire and premature failure. Before replacing the fuse holder in the consumer unit try and find out why the fuse ‘blew’. If the new fuse blows immediately there is a fault in the circuit which requires attention.

Miniature circuit breakers

(MCBs) An MCB is simply a single pole switch which automatically trips (switches-off) when there is a fault in the circuit or serious overloading. To restore the current you simply switch on the MCB but if there is a fault in the circuit it is impossible to close the circuit breaker until the fault is rectified. MCBs can also be used to switch off individual circuits when required, or leave a circuit supplying a freezer or refrigerator switched on when leaving the house for an extended period.

In older electrical installation, the wiring is divided into a number of circuits each having a specific current rating. The current ratings are determined by the current ratings of the circuit fuses or MCBs in the consumer unit and protecting the circuits. There are five old standard ratings: 5, 15,20,30 and 45 amps but rarely does a consumer unit include all five ratings. Some homes only have 5A circuits, for lighting only, but most have 5A. and 30A fuses: the 30A being for ring circuits supplying 13A socket outlets, and a separate 30A fuse for the cooker and another 30A fuse for the instantaneous shower unit. Immersion heaters are usually supplied from either a 15A or 20A fuseway and night storage heaters from 20A fuse usually from a separate consumer unit under times witch control set for the overnight cheap rate. Where the cooker is of extra large size it is supplied from a 45A fuse.

Circuit fuses
Circuit fuses are in two types: rewirable, using fuse wires, and cartridge type where the fusing element is enclosed in a cartridge similar to but physically larger than the fuse in a 13A plug. The cartridges are of different dimensions according to their current rating. This, unlike a rewirable fuse makes it impossible to uprate a fuse by inserting one of higher rating which could lead to overloading the circuit wiring, which in turn could result in a fire. The fuse units of both types are colour coded according to their current rating. The old colours are: white (5A); blue (15A); yellow (20A); red (30A);and green (45A).

Although there are numerous terms used in electrical engineering those which really concern the householder are volts, amps and watts. A volt is the unit of electrical pressure which as already explained is standard at 240Y in the UK from the mains supply.
Apart from being lethal this comparatively high voltage makes it essential that all ‘flexible cords, cables, switches, socket outlets and other accessories used in home installations are of the correct type designed for 24OV , working without any likelihood of leakage of dangerous currents which would result in fire and shock. In addition it is imperative that electrical work is done properly and is correctly maintained by qualified electricians to prevent leakage of dangerous currents,

The amp is the unit of current, and this determines the sizes of flexible cords and cables and the current capacity of electrical wiring accessories, that is, switches and socket outlets. The watt is the unit of power and represents the electrical loading of a lamp or appliance and therefore the current consumption. The watt is the product of the voltage and current; multiply the volts by the current and the result is the wattage. 1000 watts equal 1 kilowatt. A 1KW appliance switched on for one hour consumes 1KWH (1 unit) of electricity. Electrical appliances are usually rated in watts (or KWs). To calculate the current in amps when the wattage is known, so that the correct size of flexible cord or cable is chosen, divide the watts by the voltage. For example, a 2-bar electrical fire having a loading of 2000 watts consumes 2000W / 240Y =8.4 A.

Electricity comes into the house from the mains electricity system via a 2-wire service cable usually laid underground, but in rural areas it often comes via an overhead service cable. The cable terminates in a sealed unit containing the Electricity Board’s service fuse, and mounted on the same board is the electricity meter which registers the electricity consumption in units (KWHs). Fixed close to the meter is the householder’s consumer unit containing a double pole main switch and the circuit fuses, though in some installations miniature circuit breakers (MCBs) are fitted instead of fuses. The consumer unit is linked to the meter by a pair of large size PVC insulated and sheathed cables, one red, the other black in colour. There is also a green, or green yellow PVC insulated earth conductor running from an earth terminal block in the consumer unit to an earth terminal which is usually an earthing clamp secured to the metallic sheath of the Electricity Board’s underground service cable. In some older installations the earthing clamp is on the mains water pipe (which may no longer be used as the sole means of earthing).

Where the Board is unable to provide earthing facilities an earth leakage circuit breaker (ELCB) is fitted in conjunction with an earthing rod driven into the ground outside the house. This circuit breaker is a double pole main switch which automatically trips and cuts off the electricity to all circuits when a line/earth fault (a live conductor coming into contact with earthed metalwork) occurs in the installation. This ELCB is normally wired into the mains between the consumer unit and the meter but some consumer units incorporate this as the main switch. The electricity supply throughout the UK is standardised at 240 volts AC 50 Hz and is a 2-pole service. One pole is termed the live (or line) the other is termed the neutral. As the neutral is solidly connected to earth in the mains distribution system the live pole has a voltage of 240 volts above earth and the neutral is at potential which means zero volts between neutral and earth.

It is because the neutral is earthed that anyone who is standing on a concrete floor or on the earth itself or in contact with earthed metalwork and touches a live wire or contact will receive an electric shock of up to 240 volts which can be fatal. Or if metalwork such as the casing of an electric kettle or heater is not properly earthed and through a fault becomes live with mains electricity, a person touching it will receive a serious electric shock. Normally, should a live conductor come into contact with earthed metalwork a heavy surge of current results and this blows the fuse or trips a circuit breaker, so cutting off the electricity feeding the fault.

London seen from the air

Shale oil and gas in the United States is rapidly increasing as a source of cheap natural gas and oil. The cost of energy in Europe is almost 40% higher than in the USA, making European and British manufactures less competitive.

Fears over EU-US energy price divide BBC

Heating and ventilating a garden home office can be a problem. But do not neglect them; their importance is usually underestimated when building a garden office. Irritability when you are too warm, and lack of control when your hands are cold, will lead to bad working conditions and can be unproductive. Whatever type of electric heating you use, it will be cheaper in the long run if you insulate old buildings. You can opt for an electric radiator or electric underfloor heating.

A common mistake is to fit a free-standing electric fan, which just moves stale air around the room, instead of a proper extractor set in an outside wall. Ventilation ducts such as airbricks should also be fitted to let in fresh air to replace the extracted stale air.

Handy power drills do a lot more than just drill. A huge number of fittings can be attached to the basic drill unit, enabling it to do almost anything that can be done by a specialised power tool-perhaps not quite so fast or accurately, but certainly well enough for general use. Basically, a power drill is a compact electric motor fitted with a projecting shaft at one end on which is mounted a chuck-a revolving clamp that grips and drives drill bits or other attachments.

The motor unit is held in the hand by a pistol grip, and the motor is started by pressing a ‘trigger’ at the top of the grip. For safety reasons, the motor stops if pressure is released on the trigger, but most drills have a locking pin that can be engaged to hold the trigger in the ‘on’ position. Electric power is supplied to the drill by a cable that enters the machine through the bottom of the handle or by a battery. On all modern drills, a complex system of insulation is built in to keep the user from getting an electric shock. The motor is cooled by a built-in fan that draws air through slots in the side of the drill. These slots must be kept uncovered and free of sawdust, or the motor may overheat and burn out. Many drills can be adjusted to run at different speeds.

The normal type is a multi speed drill geared to run at up to 1,000 rpm and 2,500-3,000 rpm. The speeds are suitable for most household jobs, and a two speed machine is probably the best type for the householder to buy. Variable-speed drills, where the speed can be infinitely varied by an electrical device, are also made, but these are specialist tools. It is false economy to buy a single-speed drill and a separate electrical speed reducer, because although single-speed machines are cheaper, the cost of the two units together will be higher than that of a good two-speed dri1l. The most suitable speeds for various woodworking and other operations are shown in the table below. Drills come in various sizes, which are graded by the capacity of their chuck-i.e. the largest drill bit that can be fitted into it.

A medium-sized machine-say, should be adequate for all ordinary jobs, though the smallest may be too light for some. An indispensable accessory that every drill user will need is an extension cable. This enables the tradesman to use the tool in places remote from a power outlet. Cables are available in standard lengths from 8m to 100m, or you can make up your own. The longer the cable, the thicker it needs to be to prevent power loss. Heavier machines also need thicker cables. You must also have an isolating RCD if using a power tool out of doors; the shop that supplies the drill can advise you.

Drill bits and fittings

Many types of bit are sold for cutting different sizes and shapes of hole in different materials. The ‘everyday’ sort are twist bits, used for drilling all sizes of hole in metal, and holes in wood . Larger holes in wood are drilled with Jennings bits, which have a wide spiral to remove the surplus wood, and a centre spur or spike to keep the cut accurate when it is being started. They cut neat, flat-bottomed holes, but they have to be cleaned out more often than auger bits. Dowel bits are like twist bits with a wood-drill-shaped point for extra accuracy. Very large holes are drilled with flat bits. The flat bit has a flat, spadeshaped cutter with a central spur to hold it in place. The hole saw has a revolving toothed ring attached to a central twist bit the ring removes wood like a revolving pastry-cutter.

Different sizes of ring are available. Very long holes, such as those up the shaft of standard lamps, are drilled with shell or parrot-nosed augers. They are generally used on a lathe, and not in a hand-held drill. Other types of bit include countersink bits, for countersinking screw holes, and drill countersinkers, which are specially shaped to drill and countersink (or counterbore) a hole for a particular size of screw.

For drilling hard masonry, a hammer attachment to a drill is useful. This makes the bit vibrate up and down as it revolves.

Glass and tiles are drilled with a spear point drill, which also has a hardened tip.