Mains Cables (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

Mains Electricity

• Mains electricity is the electricity generated by power stations and transported around the country through the National Grid

• Everyone connects to the mains when plugging in an appliance such as a phone charger or kettle

• Mains electricity is an alternating current (a.c.) supply

• In the UK, the domestic electricity supply has a frequency of 50 Hz and a potential difference of about 230 V

  • A frequency of 50 Hz means the direction of the current changes back and forth 50 times every second

• Mains electricity, being an alternating current, does not have positive and negative sides to the power source

  • The equivalents to positive and negative are called live and neutral, and these form either end of the electrical circuit

Three Core Cables

• All electrical appliances are connected to the mains using at least a live wire and a neutral wire

• In the UK, most electrical appliances are connected to the mains using a three-core cable consisting of

  • A live wire

  • A neutral wire

  • An Earth wire

• The insulation covering each wire is colour-coded for easy identification:

  • Live wire – brown

  • Neutral wire – blue

  • Earth wire – green and yellow stripes

Live, Neutral and Earth Wires

• The three wires have distinct purposes

The Live Wire

• The live wire:

Carries the alternating potential difference from the mains supply to a circuit

• It is the most dangerous of the three wires

• If it touches the appliance without the Earth wire, it can cause electrocution

The Neutral Wire

• The neutral wire:

Forms the opposite end of the circuit to the live wire to complete the circuit

• Because of its lower voltage, it is much less dangerous than the live wire

The Earth Wire

• The earth wire:

Acts as a safety wire to stop the appliance from becoming live

• This prevents electric shocks from occurring if the appliance malfunctions or the live wire breaks off and touches the case of the plug

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

• 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