In an ac power plug, the earth pin is longer and of larger diameter as compared to the live and neutral pins. We use such type of plugs daily but have you ever thought why this is so?
This arrangement performs mainly three functions:
Since the earth pin is longer, it is the first pin to establish contact when all three pins (earth, active and neutral) are inserted into a socket. It is also the last pin to be disengaged. 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.
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, facilitating the insertion of other two pins.
More the thickness of the conductor, less is the resistance path for the current to be grounded. This is for the safety against shock. The leakage current, if any, will take the less resistant path through the earth pin to ground rather than through the person operating the appliance.
One more reason is it fixes the plug tightly so it also prevents loose connection.
Is something missing here?