According to the current national standard "Code for Design of Low-Voltage Power Distribution" (GB50054), low-voltage power distribution systems are divided into three types: TN, TT, and IT. The first capital letter T indicates that the neutral point of the power transformer is directly grounded; I indicates that the neutral point of the power transformer is not grounded (or grounded through high impedance). The second capital letter T indicates that the casing of the electrical equipment is directly grounded, but not connected to the power grid's grounding system; N indicates that the casing of the electrical equipment is connected to the system's grounding neutral line. TN system: The neutral point of the power transformer is grounded, and the exposed parts of the equipment are connected to the neutral line. TT system: The neutral point of the power transformer is grounded, and the casing of the electrical equipment is protectively grounded. IT system: The neutral point of the power transformer is not grounded (or grounded through high impedance), while the casing of the electrical equipment is protectively grounded. 1. TN System The neutral point grounding of the power transformer in a power system can be further divided into three categories based on the different ways in which the exposed conductive parts of the electrical equipment are connected to the system: TN-C system, TN-S system, and TN-C-S system. These will be described below. 1.1 The characteristics of the TN-C system are: the neutral point of the power transformer is grounded, and the protective neutral line (PE) and the working neutral line (N) are shared. (1) It uses the neutral line (neutral line) of the neutral point grounding system as the return conductor of the fault current. When the phase line of the electrical equipment touches the shell, the fault current returns to the neutral point through the neutral line. Since the short circuit current is large, the power supply can be cut off by the overcurrent protection device. The TN-C system generally adopts zero-sequence current protection; (2) The TN-C system is suitable for three-phase loads that are basically balanced. If the three-phase loads are unbalanced, there will be unbalanced current in the PE line. In addition, the harmonic current caused by some load equipment will also be injected into the PE line, so the neutral line N will be energized and is very likely to be higher than 50V. This not only makes the equipment shell energized, causing safety hazards to people, but also makes it impossible to obtain a stable reference potential; (3) The TN-C system should repeatedly ground the PE line. Its function is to effectively reduce the voltage of the neutral line to ground when the phase of the equipment connected to the neutral line comes into contact with the shell. As can be seen from the above, the TN-C system has the following defects: (1) When the three-phase load is unbalanced, an unbalanced current appears on the neutral line, and the neutral line presents a voltage to the ground. When the three-phase load is severely unbalanced, touching the neutral line may lead to an electric shock accident. (2) The neutral line through the leakage protection switch can only be used as the working neutral line, not as the protective neutral line of electrical equipment. This is determined by the working principle of the leakage protection switch. (3) For single-phase electrical equipment connected to a two-pole leakage protection switch, such as the protective neutral line of its metal casing in the TN-C system, it is strictly forbidden to connect it to the working neutral line of the circuit, nor is it allowed to connect it to the PEN line in front of the leakage protection switch. However, it is very easy to make a mistake in use. (4) The connection line of the repeated grounding device is strictly forbidden to be connected to the working neutral line through the leakage protection switch. The TN-S power supply system completely separates the working neutral line and the protective neutral line, thereby overcoming the defects of the TN-C power supply system. Therefore, the TN-C system is no longer used on construction sites. 1.2. In the TN-S system, the neutral line (N) and the protective earth (PE) are separate. (1) When the phase line of the electrical equipment touches the casing, causing a direct short circuit, an overcurrent protection device can be used to cut off the power supply. (2) When the N line is disconnected, if the three-phase load is unbalanced, the neutral point potential rises, but the casing has no potential, and the PE line also has no potential. (3) The PE line of the TN-S system should be repeatedly grounded at both ends to reduce the danger caused by the PE line being broken. (4) The TN-S system is suitable for industrial enterprises and large civil buildings. At present, most construction sites that use a single transformer for power supply or whose substations are close to the construction site have adopted the TN-S system. In conjunction with the step-by-step leakage protection, it has indeed played a role in ensuring the safety of construction electricity. However, the TN-S system must pay attention to several issues: (1) The protective earth line must never be disconnected. Otherwise, if a live part of the grounding equipment touches the casing or leaks current, a single-phase circuit cannot be formed, and the power supply will not be automatically cut off, resulting in two consequences: first, the grounding equipment will lose its safety protection; second, the casings of other intact grounding equipment will become live, causing a large range of electrical equipment casings to become live, resulting in a terrible electric shock threat. Therefore, the "JGJ46-88 Safety Technical Specification for Temporary Power Supply at Construction Sites" stipulates that the dedicated protective line must be repeatedly grounded at both ends. (2) Electrical equipment in the same power system is absolutely not allowed to be partially grounded or partially connected to neutral. Otherwise, when the protective grounding equipment leaks current, it will cause the potential of the neutral grounding wire to rise, causing all equipment casings using protective grounding to become live. (3) Material and connection requirements for protective grounding PE wire: The cross-section of the protective grounding wire should not be less than that of the working neutral wire, and a yellow/green double-color wire should be used. The protective grounding wire connected to the electrical equipment should be an insulated multi-strand copper wire with a cross-section of not less than 2.5mm2. The protective neutral wire should be reliably connected to the electrical equipment using copper lugs, and hinges are not allowed. The terminals of the electrical equipment should be galvanized or coated with anti-corrosion grease. The protective neutral wire should be connected through a terminal block in the distribution box, and there should be no joints in other places. 1.3 TN-C-S system It consists of two grounding systems. The first part is the TN-C system and the second part is the TN-S system. The interface is at the connection point of the N line and the PE line. (1) When a single-phase short circuit occurs in the electrical equipment, it is the same as the TN-S system. (2) When the N line is disconnected, the fault is the same as the TN-S system. (3) In the TN-C-S system, the PE line should be repeatedly grounded, while the N line should not be repeatedly grounded. The equipment casing connected by the PE line will never be energized during normal operation, so the TN-C-S system improves the safety of operators and equipment. The TN-C-S system is generally adopted when the transformer is far from the site or there is no special transformer for construction. 2. TT Power Supply System The neutral point of the power supply is directly grounded, and the exposed conductive parts of electrical equipment are connected to the grounding electrode via PE wires (this grounding electrode has no electrical connection with the neutral point grounding). When this system is used for protection, if a leakage fault occurs in a device, the fault voltage on the metal casing of the device is relatively large, while the current is relatively small, which is not conducive to the operation of the protective switch and is harmful to people and equipment. To eliminate the defects of the TT system and improve the reliability of power safety, a technical innovation to improve the TT system is proposed based on the principle of parallel resistance. The technical innovation is: using a green/yellow bicolor wire (PT wire) with a cross-section not less than that of the working neutral wire, in parallel with the protective grounding wires of 4-5 sets of grounding resistances buried under the main distribution box, distribution boxes, and main mechanical equipment as the protective grounding wire, and using the green/yellow bicolor wire to connect the metal casing of the electrical equipment. It has the following advantages: 1) The voltage to ground at the fault point of a single-phase grounding is relatively low, and the fault current is relatively large, which allows the leakage protection device to quickly operate and cut off the power supply, which helps to prevent electric shock accidents. 2) The PT line is not connected to the neutral line, making the line installation clear and intuitive, eliminating the risk of incorrect wiring. For large construction sites with multiple construction units working simultaneously, PT lines can be installed in sections and by unit, which is beneficial for safe electricity management and saving on conductor usage. 3) It eliminates the need for redundant grounding wires under each electrical device, saving on grounding wire installation costs and improving grounding quality, ensuring a grounding resistance ≤10Ω, resulting in more reliable electrical safety protection. TT systems are widely used abroad, but in China, they are limited to localized electronic equipment with high grounding requirements and are generally not used on construction sites. However, if it is a public transformer and other users are using a TT system, then this system should also be used on the construction site. 3. IT System The energized parts of the power system are not directly connected to the ground (or are grounded via a resistor), while the exposed conductive parts of the receiving equipment are directly grounded through a protective conductor. This system is mainly used for 10KV and 35KV high-voltage systems and some low-voltage power supply systems in mines and underground mines. It is not suitable for construction site applications and will not be analyzed further here. The newly issued "Standard for Safety Inspection of Building Construction" (JGJ59-99) issued by the Ministry of Construction stipulates that the TN-S grounding protection system must be used in the power system with direct grounding of the neutral point dedicated to the construction site. Therefore, the TN-S grounding protection system has been widely used in the construction site. However, if the PE line is broken or not properly connected to the electrical equipment, the repeated grounding resistance value will not meet the safety requirements, and electric shock accidents will also occur. In order to improve the safety of the TN-S grounding protection system, the concept of equipotential bonding is proposed here. Equipotential bonding is to make a substantial electrical connection between the exposed conductive parts of the electrical equipment and the conductive parts outside the system (such as the main reinforcement in the concrete, various metal pipes, etc.) through the protective neutral line (PE line), so that the potentials of the two tend to be equal. It should be noted that there is no current passing through the equipotential bonding line under normal conditions, only the potential is transmitted, and there is current passing through when there is a fault. The function of equipotential bonding. (1) The total equipotential bonding can reduce the expected contact voltage; (2) The total equipotential bonding can eliminate the electric shock hazard caused by the conduction of fault voltage along the PE line outside the device. Therefore, this technology should be gradually promoted on construction sites. Of course, no grounding method is foolproof or absolutely safe. Temporary power supply on construction sites must be strictly installed according to the requirements of JGJ46-88 standard, and residual current devices must be used. Strict adherence to construction power supply design and acceptance procedures and standardized management are essential to prevent accidents.