The longer of the three feet of the socket is grounded, which can be called the grounding foot, and the other two shorter feet are to connect household appliances to the circuit, which can be called them conductive feet. In the design of the power plug, in order to take into account the safety of the user, consciously designed the grounding foot than the conductive foot a few millimeters long. So it is the way to plug into the ground is indicated.
The metal casing of household appliances is electrified due to damage to the insulator and other reasons, when the grounding foot will form a grounded short-circuit current, so that the metal casing of the household appliances is grounded and discharged to the ground, so that people are not electrocuted, and to play a role in safety protection.