Electrical Safety (Oxford AQA IGCSE Combined Science Double Award)

Earthing

• Many electrical appliances have metal cases

• This poses a potential safety hazard:

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

  • This is because metal is a good conductor of electricity

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

• If the live wire does touch the metal casing:

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

  • The current is harmlessly conducted away

  • This is because the route to Earth via the earth wire has a much lower resistance than through a person's body

Earth wire connected to metal casing

Fuses & Circuit Breakers

Fuses

• Fuses are used to protect individual appliances

• Fuses are located in the plug of an appliance

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

• If the current in the wire becomes too large (e.g. in a surge from the national grid):

  • The wire heats up and melts

  • This breaks the circuit and stops the current

Circuit symbol of a fuse

• 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

Circuit breakers

• Similar to fuses, 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)

  • This is sometimes referred to as the circuit breaker tripping when the current is too high

• Domestic electricity is supplied to homes with a set potential difference and set maximum current

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

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

Example of a domestic circuit

• Circuit breakers stop excess current flowing faster than fuses

• Circuit breakers can be reset, unlike fuses which must be replaced

Safety Features of Appliances

• Earth wires are connected to appliances with metal casings to conduct current to the Earth when the live wire breaks and touches the casing

• However, when this happens, a large current flows

  • This is because the resistance of this path to Earth is low

• A fuse is connected along the live wire in this circuit such that, when this large current flows, the fuse melts and stops any current flowing