Mains Electricity
Electrical appliances are usually connected with a THREE-CORE CABLE, consisting of:
- LIVE WIRE – BROWN: Carries the ALTERNATING POTENTIAL DIFFERENCE from the supply and is always connected to the FUSE. This wire has an electric potential of 230V.
- NEUTRAL WIRE – BLUE: Completes the circuit and is approximately at the EARTH POTENTIAL: an electric potential of 0V.
- EARTH WIRE – GREEN AND YELLOW: Is used for SAFETY, to stop the appliance from becoming live. This also has an electric potential of 0V, and it only carries a current if there is a FAULT.
SAFETY ASPECTS
The potential difference between the wire can be calculated by finding the difference between electric potentials:
E.g. The potential difference between the LIVE WIRE (230V) and EARTH WIRE (0V) is:
230 V – 0V = 230V.
This means connecting the LIVE WIRE to the EARTH WIRE can pose serious dangers due to the LARGE potential difference of 230V. This can result in a LARGE CURRENT which can create a FIRE.
E.g. The potential difference between the LIVE WIRE and A HUMAN.
The human body has an electric potential of 0V so:
230 V – 0V = 230V.
The live wire may be dangerous even when a switch in the mains circuit is open as it has an potential of 230V so there will be a POTENTIAL DIFFERENCE of 230V across your body, which can cause CURRENT to flow through you, leading to DEATH.
THE EARTH WIRE
- The EARTH WIRE in an electrical system is crucial for SAFETY. It normally carries no electricity during regular appliance operation.
- The ELECTRIC POTENTIAL of the EARTH WIRE is 0V because it's connected to the ground, making it the same electric potential as the Earth.
- METAL CASES on electrical appliances can be a safety hazard. If the LIVE WIRE inside the appliance touches the metal case, the case could become LIVE.
- Touching a LIVE metal case can lead to ELECTROCUTION. The EARTH WIRE helps to prevent this by providing a safe path for the electricity.
- The EARTH WIRE offers a LOW RESISTANCE PATH to the Earth, which is important if a fault occurs.
- When a fault happens, the EARTH WIRE allows current to flow to the ground, creating a surge of current in the EARTH WIRE and potentially also in the LIVE WIRE.
- This surge can trigger a FUSE to melt and break the circuit, which stops the flow of electricity and makes the appliance safe to touch.