Electrical safety — Explain

Why must the outer casing of an electrical appliance be either non-conducting or earthed?

This protects the user from electric shock. A non-conducting (double-insulated) casing cannot become live even if an internal fault occurs, since no conducting path reaches the outside. An earthed metal casing is connected via the earth wire to the ground, so if a fault causes the live wire to touch the casing, a large current flows to earth, blowing the fuse or tripping the circuit breaker and disconnecting the supply before the casing can become dangerously live.