# Earth wire

A safety wire that connects a metal case to the ground. If the live wire touches the case, a large current flows down the earth wire, blows the fuse and cuts the power.

Physics (school) · Part 07: Mains electricity and magnetism · Card 4 of 8 · Last updated 2026-10-19

## Analogy

An emergency exit that makes a dangerous fault harmlessly switch the power off.

## In practice

Metal-cased appliances like toasters need an earth wire. Plastic-cased double insulated ones do not.

## Practice

### Flashcards

- **What does the earth wire do?** It lets a fault current escape so the fuse blows and the case does not stay live.

## Questions people ask

### Why do some appliances not need an earth wire?

Appliances with plastic cases are double insulated, so the live wire cannot touch any metal a person might handle. With no way for the casing to become live, an earth wire is unnecessary. These appliances show a symbol of two squares, one inside the other.

### What happens if the live wire touches a metal case?

Without an earth wire, the case becomes live and a person touching it could be shocked. With an earth wire, a large current surges through the earth wire, which has very low resistance. The surge melts the fuse and cuts off the power, so the case becomes safe.

## Related concepts

- [Mains electricity](https://studyvisually.com/learn/science/physics-school/physics-school-part-7/physics-school-part-7-concept-2)
- [Plug wires and fuses](https://studyvisually.com/learn/science/physics-school/physics-school-part-7/physics-school-part-7-concept-3)

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