Dangers of Mains Electricity (AQA GCSE Combined Science: Trilogy: Physics): Revision Note

Potential Difference of Live, Neutral and Earth Wires

• The live wire carries most of the power to the circuit and, as a result, it is the most dangerous wire

  • It has a voltage of around 230 V

• The neutral wire is much safer than the live wire, although it can deliver a small shock

  • It has a voltage close to 0 V

• The earth wire only carries a current to the ground if there is a fault in the appliance, causing a surge of current

  • It has a voltage of 0 V

Dangers of Mains Electricity

• Normally, the earth wire does not carry any electricity – it is there for safety only

• As a result, in most circumstances, its potential difference is 0 volts

• Many electrical appliances have metal cases that pose a potential safety hazard:

  • If a live wire (inside the appliance) came into contact with the metal casing, the casing would briefly become electrified (live)

  • If anyone touched it at this moment, they would get an electric shock

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

• If the casing becomes live:

  • The earth wire completes the circuit, providing a low resistance path to the Earth

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

  • This surge of current passes through the fuse in the circuit, causing it to melt and break

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

Signs, like the above, warn of the risk of electrocution

• Because of the large potential difference between the live (230 V) and the earth (0 V), if the two are connected together, a very large current can be created

• If a person provides the connection between live and earth then a large current can pass through them, providing a potentially lethal shock

• Electricians will always switch off the mains electricity supply to the whole house, or section of a house when they are working with electrical appliances

  • This is because they will come into contact with live wires when they are working

  • The potential difference of the live wire is 230 V and the potential of the electrician is 0 V

  • Therefore, there is a large potential difference between the live wire and the electrician, so, a current would pass through the electrician's body to reach the earth

  • Even if a device is switched off but the mains supply is on, the live wire can still cause an electric shock