three pin plugs

[lizzierose]

why is the earth pin longer in three pin plugs?

[thecooler]

For safety. It's the first one in and the last one out, so if there is any fault in the wiring, you should be safe as your appliance is earthed.

[Hippy]

An additional safety feature is that the earth pin opens a shutter in the socket allowing the other two pins to enter. The shutter is there to prevent tiny fingers prodding nails and knitting needles into the live contacts, or, indeed, wiser folk from having electrifying accidents when there is no plug in the socket.

[cath16]

It is a matter of safety.
The longer earth pin acts as the opening rod for the shutters on the socket, and connects to earth before either the live or neutral pins connect. This way, if there is a fault with the equipment being plugged in, with the earth already connected, the fuse or circuit breaker would blow, preventing injury to the person holding the plug. Also, should the plug get knocked or accidentally pulled out when the power is on, the first pins to disconnect are the power pins, followed by the earth pin which also closes the shutters on the socket.