Appliances must have a fuse. Otherwise if the earth wire is not in a metal case and a fault happens the live wire touches the metal case and becomes live giving a shock to anyone who touches it.
If a fault develops in an earthes appliance a large current flow to earth and melts the fuse disconnecting the supply.
A fuse must be put in the live wire so is that if it melts it cuts of the supply. The rating of a fuse must be slightly higher than the normal working current of the appliance. If it is much higher it will not melt soon enough is it is not enough the normal current will melt it.
A circuit breaker can be used i place of a fuse. This is an electromagnetic switch that opens and cuts off the supply if the current is bigger than a certain value.
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