The small electrode plate of the unmanned vending machine shipping gearbox plays a safety protection role in the circuit. When the human body touches the electrical equipment whose shell has been electrified, because the contact resistance of the grounding body is much smaller than the resistance of the human body, most of the current enters the earth through the grounding body, and only a small part flows through the human body, which will not cause harm to human life. The grounding pin first touches the grounding wire in the socket, so that the grounding protection can be formed first, and then the power is turned on.

Conversely, when pulling out the three-pin plug, the conductive pin is first separated from the conductive end in the power socket, and the grounding pin is disconnected. If the metal shell of household appliances is charged due to insulator damage and other reasons, then the grounding foot will form a ground short-circuit current, so that the metal shell of household appliances is grounded and discharged to the ground, so that people are not electrocuted and play a role in safety protection.

When this reference potential is not connected to earth, it is considered to be relative to zero potential. However, this relative zero potential is unstable, it will change with the change of the external electromagnetic field, so that the parameters of the system change, resulting in the unstable operation of the circuit system. When this reference potential is connected to the earth, the reference potential is regarded as the zero potential of the earth.