Electrical Safety (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award)): Revision Note

Mains Circuits

• All electrical appliances are connected to the mains supply

• In order to protect the user or the device, there are several safety features built into domestic appliances, including:

  • double insulation

  • earthing

  • fuses

  • circuit breakers

Double insulation

• The conducting part of a wire is usually made of copper or some other metal

  • If this comes into contact with a person, this poses a risk of electrocution

• For this reason, wires are covered with an insulating material, such as rubber

The conducting part of a wire is covered in an insulating material for safety

• Some appliances do not have metal cases and so there is no risk of them becoming electrified

• Such appliances are said to be double insulated, as they have two layers of insulation:

  • Insulation around the wires themselves

  • A non-metallic case that acts as a second layer of insulation

• Double insulated appliances do not require an earth wire or have been designed so that the earth wire cannot touch the metal casing

Earthing

• Many electrical appliances have metal cases

• This poses a potential safety hazard:

  • If a live wire (inside the appliance) came into contact with the case, the case would become electrified and anyone who touched it would risk being electrocuted

• The earth wire is an additional safety wire that can reduce this risk

• If this happens:

  • The earth wire provides a low resistance path to the earth

  • It causes a surge of current in the earth wire and hence also in the live wire

  • The high current through the fuse causes it to melt and break

  • This cuts off the supply of electricity to the appliance, making it safe

Fuses & trip switches

• Fuses and trip switches (circuit breakers) are safety devices designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or a surge)

Fuses

• Fuses are used to protect individual appliances

• Fuses are located in the plug

• Fuses usually consist of a glass cylinder containing a thin metal wire

• A fuse without an earth wire protects the circuit and the cabling for a double-insulated appliance

• If the current in the wire becomes too large:

  • The wire heats up and melts

  • This causes the wire to break, breaking the circuit and stopping the current

Circuit symbol of a fuse

The circuit symbol for a fuse has a wire running through it (not to be confused with a resistor)

• Fuses have ratings, which signify the maximum current that can flow through the fuse wire without it melting

• If the current exceeds that value, the fuse wire melts and the individual appliance is disconnected from the mains supply

• Fuses come in values of 3 A, 5 A and 13 A

• The correct fuse to use is the value just above the current required for the appliance

Choosing the correct fuse

• Suppose an appliance uses 3.1 amps

  • A 3 amp fuse would be too small - the fuse would blow as soon as the appliance was switched on

  • A 13 amp fuse would be too large - it would allow an extra 10 amps to pass through the appliance before it finally blew

  • A 5 amp fuse would be the most appropriate choice, as it is the next size up

Trip switches

• The current enters the house at the consumer unit (sometimes referred to as a 'fuse box')

• The consumer unit consists of a series of trip switches (or circuit breakers) which control the amount of current supplied to each circuit within the house

  • When the current is too high the switch 'trips' (automatically flicks to the off position)

  • This stops the current flowing in that circuit

Example of a domestic circuit

The consumer unit distributes current to all the circuits in the house

• The main advantages of trip switches are:

  • they provide protection from current surges or faults

  • they can be reset when the problem is fixed