In the renovation of the two networks, some units have some problems in the design and installation of grounding systems for low-voltage electrical devices, which will bring unnecessary drawbacks to future operation. The following are the details: 1. The TT grounding system should not require repeated grounding of the neutral line. The People's Republic of China Electric Power Industry Standard DL 499-92 "Technical Specifications for Rural Low-Voltage Electricity" (hereinafter referred to as the Specifications) stipulates that the following requirements should be met when using the TT system: except for the direct grounding of the neutral point on the low-voltage side of the transformer, the neutral line shall not be grounded again and shall maintain the same level of insulation as the phase line. However, some units require repeated grounding of the neutral line in the TT system during the renovation of the two networks, on the grounds that it is to prevent the three-phase voltage imbalance caused by the drift of the neutral point after the neutral line is broken. This is a direct violation of the Specifications. In fact, this practice has limited effect and many problems. (1) The residual current operated protective device cannot be put into use: After the neutral line is repeatedly grounded, part of the normal load current will flow through the ground, forming a residual current on the residual current operated protective device and causing it to malfunction. Repeated grounding of the neutral line in a TT system can cause maloperation of the residual current operated main protection. Standards stipulate that low-voltage power grids using TT systems should be equipped with residual current operated main protection and final-stage protection. However, in a TT system with repeated grounding of the neutral line, main protection cannot be installed. In the event of a single-phase ground fault or electric shock, the power supply cannot be disconnected, potentially causing personal injury or death. Some power supply units, in an attempt to solve the problem of main protection device operation, have even disconnected the working ground of the transformer neutral line, which is absolutely prohibited. Direct grounding of the neutral point on the low-voltage side of the distribution transformer aims to suppress the rise in low-voltage side voltage when the high- and low-voltage windings of the distribution transformer are broken down due to insulation damage; in a single-phase ground fault, it prevents the voltage of the non-faulty phases to ground from rising; and it facilitates the implementation of single-phase ground fault protection. (2) The TT system is transformed into a TN-C system. In the TT system, if the neutral line is repeatedly grounded, the formal TT system is transformed into a substantive TN-C system. After the neutral line in the TT system is repeatedly grounded, it becomes a TN-C system. If the N line repeated grounding point is close to the grounding of the user equipment, the two grounding resistors are in parallel circuits, which means that the equipment casing is connected to the neutral line, forming a TN-C system. 2 Measures should be taken in the TT system to prevent the neutral line from breaking. (1) The neutral line must have sufficient mechanical strength. The N line should be the same as the conductor cross-section of the phase line. (2) Ensure the construction quality of the N line connection. (3) Try to achieve three-phase load balance. (4) Low-voltage lines should be inspected and repaired regularly. Any defects should be dealt with immediately. 3 The TN-C system should not be required . In the transformation of low-voltage power lines, some units require that the casing of the energy meter be connected to the neutral line, forming a TN-C system. However, the TN-C system is only suitable for factories and mines with independent transformers and professional electrical maintenance personnel. The regulations stipulate that rural low-voltage power grids should use the TT system; general users should not use the TN-C system because: (1) It cannot be equipped with residual current operated protective devices to effectively prevent indirect contact electric shock, grounding arc fire and direct contact electric shock caused by grounding faults in electrical equipment; (2) It cannot disconnect the PEN line, so it is difficult to prevent electric shock accidents and electrical fires caused by fault voltage during electrical maintenance; (3) In a single-phase circuit of the TN-C system, if the PEN line is interrupted, the casing of the electrical equipment can carry a voltage to ground of up to 220V, threatening personal safety; (4) In a three-phase circuit of the TN-C system, if the PEN line is interrupted, not only will the equipment lose its equipotential connection and grounding, but it will also cause single-phase equipment to burn out due to the break in the neutral when the three phases are unbalanced; (5) The voltage generated by the unbalanced current of the PEN line in the TN-C system will generate potential difference and stray current in the electrical equipment, which can easily cause arcing and interference with electronic equipment. In the transformation of the two networks, the author found that the grounding system of some units was inappropriate, and its wiring is shown in Figure 3. It is a TN-C system. The meter box is bolted to the brick wall of the household. The meter reader feels an electric shock when reading the meter. The reason is that the three-phase load is unbalanced and the N line is energized, which causes the outer shell of the electricity meter box to be energized, resulting in electric shock to the meter reader. 4 Selection principles of low voltage power grid protection grounding system (1) Factories and mines that are not powered by independent transformers should not use the TN-C system. (2) Scattered residential or rural users should use the TT system. (3) Civil buildings should use the TN-S system or TN-CS system. (4) Commercial, hotels, entertainment venues, office buildings, etc. should use the TN-S system and make equipotential bonding. (5) In places with explosion and fire hazards, the TN-C system is prohibited. Instead, the TN-S, TN-CS, TT or IT system should be used. (6) The TT system should be used at construction sites. (7) Computer rooms or electronic information equipment should use the TN-S system. (8) Coal mines or other mines should use the IT system.