The grounding method of China's power distribution system has been IEC-based, and its classification is still based on the grounding combination of power distribution system and electrical equipment, generally divided into TN, TT, IT systems and so on. The meaning of the above letters: The first letter indicates the relationship of the power ground point to the ground. Where T indicates direct grounding; I indicates no grounding or grounding through impedance. The second letter indicates the relationship between the exposed conductive portion of the electrical device and the ground. Where T represents a separate direct ground connection that is not connected to the power ground point; N indicates a direct connection to the power system ground point or to the conductor from that point.
According to whether the neutral line and the protection line are combined, the TN system is further divided into TN-C, TN-S and TN-C-S systems. TN-C system: The protection line and the neutral line are combined into a PEN line. TN-S system: The protection line is separated from the neutral line. TN-C-S system: The protection line and the neutral line on the side close to the power supply side are merged into a PEN line, which is divided into a protection line and a neutral line from a certain point.