By "plug top" do you mean a regular electrical plug, like this:
If so then the reason the ground (or earth) prong is longer is simply so that it is connected before the live prongs. That way if there is a short you know you are grounded before there is a circuit.
Sorry if I misunderstood and you meant something more complex.
A byproduct of this is the use of shuttered sockets. Here in the UK (presumably in other countries too) many wall sockets have shutters on the live and neutral connectors to prevent children (well, electricians too!) from sticking various objects into the socket and shocking themselves. By taking advantage of the longer pin and using it to open the shutters, you ensure that the live and neutral ports are only open just before the relevant pins are inserted.
Of course there's still a flaw with this. Electricians amonst others are prone to holding open the shutters through the ground port with a screwdriver; if they can do it so surely can children! Plus as the shutters open before the pins are fully engaged, you can fit small objects such as paper clips into them.
Ok, I think you know little Electrical stuff. In Electrical, current always flow through the easiest path that is lesser resistance. if you study about resistivity you can understand that resistivity is inversely proportional to the area (if area increases - resistivity will decrease). When you have short circuit the resistivity of the earth pin is lesser (area is more) so the current will flow through this pin and will reach the ground. you can avoid shock.