The earth wire is a safety wire in a three-pin plug that provides a low-resistance path to the ground (earth). It is coloured green and yellow (striped). Under normal conditions, no current flows through it.
How the earth wire protects
If a fault causes the live wire to touch the metal casing of an appliance:
- A large current flows through the earth wire to the ground (because it has very low resistance).
- This large current blows the fuse (or trips the circuit breaker), which breaks the circuit.
- The appliance is disconnected from the mains before anyone can receive an electric shock.
Without the earth wire, the metal case would become live. Touching it would send current through your body to the ground, causing electrocution.
The three wires
| Wire | Colour | Voltage | Role |
|---|---|---|---|
| Live | Brown | 230 V (AC) | Carries the current to the appliance |
| Neutral | Blue | 0 V | Completes the circuit, carries current back |
| Earth | Green/yellow | 0 V | Safety wire, only carries current during a fault |
Double insulation
Some appliances (such as hairdryers and power tools with plastic casings) are double-insulated. They have two layers of insulation and no exposed metal parts. These appliances do not need an earth wire because the user cannot touch any conducting part. They are marked with the double-insulation symbol (a square inside a square).
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