Mains Electricity


Direct and alternating currents

Wet cell, dry-cell, non-rechargeable, and dry-cell rechargable batteries supply a current that always passes in the same direction - called a direct current (d.c.). The trace for d.c. on a cathode ray oscilloscope is a straight line.

Alternating current (a.c.) us one that is constantly changing direction. The trace for an a.c. on a cathode ray oscilloscope is a wave. The period and amplitude of the wave form determines the nature of the a.c. supply.

Mains electricity is an a.c. supply.

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The three-pin plug

Most electrical devices are connected to mains electricity supply using a cable & a 3-pin plug. Plug is insterted into a socket on the ring main circuit. The materials used for the plug & cable designed to reduce risk of electrocution. Main properties of cable & plug:

  • Inner cores of the wires made of copper - excellent conductor
  • Outer layers of wires made from flexible plastic - good insulator
  • Wires comprise of either two-core or three-core cable. Three-core carries earth wire
  • Pins of plug made from brass - good conductor, strong & stiff.
  • Casing made from plastic or rubber - both good insulators
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Circuit breakers and fuses

If an electrical fault occurs there's an increase in current flow. A fuse or circuit breaker in circuit provides a disconnection in live wire, switching off circuit.

Depending on type of electrical appliance, the plug will be fitted with fuses that have different ratings. When current in fuse wire exceeds rating of fuse - will melt, breaking circuit.

Thicker the cable, the higher the rating of fuse value. Fuses replaced each time circuit is overloaded. Some modern circuits protected by using circuit breakers, which automatically break an electric circuit if it becomes overloaded. Circuit breakers easily reset by pressing a button.

Some circuits protected by residual current circuit breakers. Operate by detecting a difference in current between live & neutral wires. Operate much faster than a fuse.

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Devices that have outer metal cases usually earthed. Outer case of an electrical appliance is connected to earth pin in plug through earth wire. Earth wire & fuse work together to protect appliance & user

If a fault occurs:

1. Case will become live

2. Current will then 'flow to earth' through earth wire - least resistance

3. Overload of current causes fuse to melt, breaking circuit

4. Appliance & user protected

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Energy transfer

When an electrical charge flows through a resistor, resistor gets hot. Some of electrical energy is used, but a lot of energy is waster, usually heating surroundings.

In a filament bulb only 5% of energy goes into light, remaining 95% wasted as heat energy. Less energy is waster in power-saving lamps such as compact fluorescent lamps.

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