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.
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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.