Why is three pin plug more safe than a two pin plug?

Answer 1

It is less likely to partially uncouple and arc, while drawing full load. Such an arc could ignite a flammable material in close proximity.

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ITS PURPOSE, AND ITS ONLY PURPOSE is to ensure the proper operation of the overcurrent device, usually a breaker but sometimes a fuse, if a fault occurs internal to the equipment being powered.

Consider the metal housing of your typical washing machine. If a fault occurs and the hot wire comes into contact with the metal housing, the housing becomes energized. The properly connected and UNDISTURBED third pin assures that if this happens, the breaker or fuse trips. Without it, the housing is as energized as you would find if you put your finger into a light socket. If you then touch the washing machine with one hand and reach over and touch a metal water pipe, or are standing on damp concrete, you die.

But a washing machine will work for years with such a fault unlikely. So another example is...Sometimes you must troubleshoot a faulty light fixture with the power still on. If while removing the fixture from the wall or ceiling the hot wire comes into contact with the metal housing of the fixture, it becomes energized. You could energize the fixture in this way without knowing it and reach down and touch something else metal, maybe a water faucet, and you die. The properly connected and UNDISTURBED third wire assures the breaker trips as soon as the fault occurs, not waiting for when your hair catches fire.