In order to prevent the risk of electric shock due to insulation damage, the grounding of the metal casing or bracket insulated from the live parts of the electrical equipment is called protective grounding. For example, the outer casing (frame) of the motor is connected to the grounding body to achieve direct grounding. Once a phase winding and the casing leak (insulation damage), the current is generated by the grounding resistance Rd of the casing grounding body, and the magnitude is
Ud=IdRd
Where: Id-----leakage current, or ground current.
After the protective grounding, when the motor leaks and the human body touches the motor casing, the grounding current passes through both the grounding device and the human body. Because in general, the body resistance is much larger than the grounding body resistance Rd, so the current flowing into the human body is very small, and most of the current flows into the earth through the grounding body. If the Rd value is very small, the current through the human body can be zero. Therefore, as long as Rd does not exceed the specified safety value, personal safety can be guaranteed.
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