1.) The earth pin is longer because it is the first pin to make contact when all three pins (earth, active and neutral) are inserted into a socket. It is also the last pin to lose contact. The longer earth pin provides a return path before the live pin engages. This way an appliance with a short to its case will simply trip a circuit breaker instead of electrocuting the user.
2.) Many wall sockets have shutters on the live and neutral lines to prevent children from inserting conducting materials which may result in electric shock. Insertion of a longer pin helps in opening the shutters, making possible the insertion of other two pins.
3.) More the thickness of the conductor, less is the resistance path for the current that is to be grounded. This is for the safety against an electric shock. If there s any leakage current, it would take the lesser resistant path through the earth pin to ground rather than through the person operating the appliance.
It is also observed that a longer earth pin provides a tighter connection with the socket and prevents a loose connection.
Which instruments require three-pins?
There are earthed instruments and unearthed instruments. The Earthed instruments require the third pin to provide the Earthing connection in case of insulation failures, while unearthed instruments use safeguards such as double insulation. As already stated above, appliances such as AC, refrigerator and washing machines do require an Earthing connection, as it is not easy to design these instruments without the chance of safety hazards in case of any failures, while instruments such as mobile phone chargers do not require a third Earthing pin because they have simple construction and possess double insulation in their design.
Three-pin vs. Two-pin:
A three pin socket is superior to a two pin socket because it protects the user from receiving electric shocks.The bigger pin is the earth connector which does the job of protecting the user.