Electricity, quite rightly, should be treated with respect. Danger arises with carelessness and lack of knowledge. Most domestic electrical work can be tackled with safety and assurance – but keep to the rules and put safety first.
Electricity is probably the most significant single force in modern life. It provides light, warmth and power from a whole range of modern appliances, from the domestic household appliance to TV and radio.
All matter contains tiny molecules of electrons, or, both negative and positive. The generation of electrical forces by power stations causes these electrons in wire to move along at incredible speeds, rather like a train shunting wagons. These atoms are harnessed in the home to provide the essentials of modern life.
The turbines move the negative particles along wires and build up an excess of negative atoms at one end. These are drawn back to the generating source, creating a flow of atoms or electrons along the supply wire.
Electricity is commonly regarded as a fuel. It should properly be regarded as energy, generated by large turbines at power stations, operated by steam, atomic energy or hydro-electric water power. Water power provides half the daily elec-tricity of the world.
Electricity is never consumed; it flows back to the power station along the wire. All that happens is that its energy is harnessed.
Electricity is generated at 11,000 volts and transmitted from the power station at 132,000 volts, 275,000 volts or 400,000 volts, and at a fairly low current or amperage.
It is transformed down to a lower voltage by local sub-stations or transformers, and fed into homes at 240V. In many countries the voltage is lower-110V. This is safer but needs heavier cable to be able to provide higher amperages for heating appliances.
Types of current
There are two types of current-direct current, (DC) and alternating current, (AC). Electricity cannot be stored easily and has to be generated to meet the needs of a community or industry as it is needed.
The turbines at the power stations, rotating at some 3,000 revolutions a minute, create a flow of electrons first in one direction and then in the other, some 50 times or cycles per second, providing alternating current.
Flow of electrons
The flow of electricity may be compared with the flow of water from the tap. When the tap is closed, flow ceases. Opening the tap is comparable with switching on elec tricity. Similarly, with electricity, water pressure may be compared with voltage.
The rate of water flow, similar to the flow of current, is determined partly by pressure and partly by the size of outlet. A narrow pipe allows less water to flow than a wide one; a thin wire restricts or resists the flow of electricity.
Electricity flows through water, earth or air. It flows best through metal. Copper, and, to a lesser extent, aluminium are common conductors of electricity. Materials which resist electrical flow are called insulators. The most common insulators are rubber, PVC and porcelain.
The earth wire is a ‘safety’ wire, designed to take current safely to earth in the event of a short circuit.
Earth connections are traditionally made to water pipes or to other metal fixtures. With the growth of plastics in domestic plumbing, connection to metal surfaces may be distinctly dangerous, for plastic is a good insulator. Very often, a device called an earth-leak circuit breaker is fitted between the electricity company’s fuse box and the meter, which records electrical consumption.
This detects the presence of voltage on the domestic earth circuit, or unbalanced or earth-leakage current, and automatically switches off supply. With this method, all the domestic earth wires are connected to an earth terminal on the circuit breaker.
Electrical values are worked out by a formula called Ohms Law. The ohm is the unit of electrical resistance encountered in a circuit. Some examples of Ohms Law are given as follows.
The higher the electrical resistance, the lower the amperage. To find the strength of electricity required, divide voltage of the household supply by the resistance of the equipment. If an appliance has a re- sistance of 16 ohms and the voltage is 240V, the amperage is 15A.
The rate at which a piece of equipment uses electricity is calculated in watts. The rating of equipment is in amperes or amps. To find amperage divide theby the supply voltage. A 2000W fire, divided by 240V, gives 8.3 amps. The nearest standard fuse is 13A. Up to 750W you can use a 3A fuse and between 750 and 3000W13A fuses.
To determine watts, multiply amps by voltage. An appliance rated at 12.5A multiplied by 240V will have a consumption of 3000W or 3kW. It is usual to use the kilowatt (kW), which is 1000W, as the unit for higher wattages.
To find out the current needed by a 2.5kW heater, divide wattage (2500W) by 240V, giving 10.4A. Therefore, use a 13A or 15A circuit and fuse.
The number of amps depends on the re-sistance or ohms. To find wattage, multiply amps by volts to obtain the power; the answer is in watts.
A light bulb with a resistance of 1000 ohms will have a current of 1/4A, found by dividing 240V by 1000 ohms.
10. November 2011 by admin
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