Many appliances have a metal case. If a fault lets the live wire touch the case, the case becomes live and could give someone a fatal shock. The earth wire prevents this. It connects the metal case to the ground. Its insulation is green and yellow. In normal use it carries no current.
When the live wire touches the case in a fault, the earth wire gives the current an easy path to the ground. A very large current flows. This large current melts the fuse, which is a thin wire that melts if the current is too high, and the appliance is disconnected from the supply.
A circuit breaker does the same job in another way. It is a switch that trips when the current is too large. Unlike a fuse, a circuit breaker can be reset after it has tripped, so it does not need replacing.
