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Mains Electricity |
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Voltage and Current
Mains voltage is approximately 230 V. It is an alternating current (a.c.) supply.
This means the current is constantly changing direction.
Current usually has a frequency of 50 hertz (Hz), which means it changes direction and back again 50 times per second.
The live terminal of the mains supply alternates between a positive and a negative voltage, with respect to the neutral terminal.
The neutral terminal stays at a voltage close to zero, with respect to earth. Cells and batteries supply a direct current (d.c.). This means it always flows in the same direction. |
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Safety
Mains electricity can kill if not used safely.
- Never overload mains sockets
- Never touch electrical connections with wet hands
- Replace frayed cables
- Don't use dirty plugs – they have a high resistance and may overheat
- Switch off TVs at night
Electrical appliances are connected to the mains using a cable containing two or three copper wires that are covered in flexible plastic.
Copper is a good electrical conductor and plastic is an insulator.
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Plugs
This diagram illustrates how a plug should be wired.
To make plugs safe to use they have:
- A plastic or rubber case which does not conduct electricity
- Brass pins, because brass is a good conductor
- The correctly sized fuse, which will blow if too much current flows
- An earth pin that is connected to the appliance casing and prevents electric shock
- A cable grip that holds the cable in place and prevents the three wires inside from becoming disconnected
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Fuses and Circuit Breakers
A fuse is a piece of wire which gets hot and eventually melts if too much current flows through it, due to a fault in the appliance. This breaks the circuit.
The fuse in a circuit should have a value higher than, but as close as possible to, the current through the appliance when it is working normally.
All metal appliances must be earthed. If a fault in the appliance connects the case to the live wire, and the supply is switched on, a very large current flows to earth and overloads the fuse.
A circuit breaker performs a similar function to a fuse. If too much current flows through the circuit, a switch is flicked to break the circuit.
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