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TN-S grounding system (the neutral wire and the protective wire are separate in the entire system); TN-C grounding system (the neutral wire and the protective wire are combined in the entire system); TT grounding system (this system has a direct grounding point, with the exposed conductive parts of electrical equipment being grounded); TN-C-S grounding system (in this system, the neutral wire and the protective wire are combined in some sections); IT grounding system (in this system, the live parts are not directly connected to the ground, while the exposed conductive parts of electrical equipment are grounded). The first letter indicates the relationship between the power system and the ground: T-----single-point grounding; I-----all live parts are insulated from the ground, or one point is grounded through an impedance. The second letter indicates the relationship between the exposed conductive parts and the ground: T-----the exposed conductive parts are directly electrically connected to the ground, regardless of any grounding points in the power system; N-----the exposed conductive parts are directly electrically connected to the grounding points of the power system (in AC systems, these grounding points are usually the neutral point). If there is another letter after that, it indicates the combination of the neutral wire and the protective wire: S-----the neutral wire and the protective wire are separate; C-----the neutral wire and the protective wire are combined (i.e., PEN wire). In systems of 110 KV and above, a single-point grounding system (i.e., a high-current grounding system) is commonly used. 35KV and 10KV systems generally adopt a neutral-ungrounded system or a system with high impedance grounding (i.e., a low-current grounding system). The 380V/220V low-voltage distribution systems can be classified into IT systems, TT systems, and TN systems depending on the type of protective grounding used. The power neutral point of IT systems is insulated from ground or grounded through a high impedance, while the metal enclosures of electrical equipment are directly grounded. That is: it was previously referred to as the protective grounding of three-phase three-wire power supply systems. The power neutral point of the TT system is directly grounded ； The metal casing of electrical equipment is also directly grounded, independent of the grounding of the power supply neutral point. That is, the protective grounding in the previous three-phase four-wire power supply system. In a TN system, which is used in 380/220V three-phase four-wire low-voltage power grids where the neutral point of the transformer or generator is directly grounded, the metal casings of electrical equipment that are not charged under normal operating conditions are directly electrically connected to the neutral point of the power supply through a common protective wire. That is, the protective earth connection in the previous three-phase four-wire power supply system. The power neutral point of the TN system is directly grounded, with a neutral wire led out. Based on the form of its protective conductor, the TN system is further divided into three types: TN-C system, TN-S system, and TN-C-S system. (1) TN-C system (three-phase four-wire system): In this system, the neutral wire (N) and the protective wire (PE) are combined; this wire is also known as the protective neutral (PEN) wire. Its advantage is that it saves one wire, but the disadvantage is that an unbalanced three-phase load or a disconnected protective neutral wire can cause the metal enclosures of all electrical devices to develop a dangerous voltage. (2) The TN-S system is a three-phase five-wire system; in this system, the N wire and the PE wire are separate, and power supply is provided via five wires starting from the transformer. Its advantage is that no current flows through the PE wire under normal conditions, so it does not cause electromagnetic interference to other devices connected to the PE wire. Furthermore, since the N wire is separated from the PE wire, a break in the N wire does not affect the protective function of the PE wire. ③The TN-C-S system (a hybrid system of three-phase four-wire and three-phase five-wire systems); in this system, the configuration from the transformer to the customer’s distribution box is four-wire, with the neutral wire and the protective earth wire being combined ； From the distribution box, the neutral and protective earth wires of the users are separated; therefore, it combines the characteristics of both the TN-C system and the TN-S system. It is commonly used in areas at the end of distribution systems where conditions are poor or where high requirements exist regarding electromagnetic interference resistance.