Specifications for construction sites of low voltage power distribution systems

According to the definition of the current national standard "Low Voltage Distribution Design Code" (GB50054), low voltage distribution systems are divided into three types, namely TN, TT, and IT. Among them, the first capital letter T means that the neutral point of the power transformer is directly grounded; I means that the neutral point of the power transformer is not grounded (or grounded through high impedance). The second capital letter T means that the shell of the electrical equipment is directly grounded, but has no connection with the grounding system of the power grid; N means that the shell of the electrical equipment is connected to the grounding neutral line of the system.

TN system: The neutral point of the power transformer is grounded, and the exposed part of the equipment is connected to the neutral line.
TT system: The neutral point of the power transformer is grounded
, and the shell of the electrical equipment is protected by grounding. IT system: The neutral point of the power transformer is not grounded (or grounded through high impedance), and the shell of the electrical equipment is protected by grounding.

1. TN system
The neutral point grounding of the power transformer of the power system can be divided into three categories according to 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. The following are introduced respectively.
1. TN-C system
Its characteristics are: the neutral point of the power transformer is grounded, and the protective neutral line (PE) is shared with the working neutral line (N).
(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 midpoint through the neutral line. Due to the large short-circuit current, an overcurrent protector can be used to cut off the power supply. The TN-C system generally uses zero-sequence current protection;
(2) The TN-C system is suitable for situations where the three-phase load is basically balanced. If the three-phase load is unbalanced, there will be unbalanced current in the PEN line, and the harmonic current caused by some load equipment will also be injected into the PEN, so that the neutral line N is charged, and it is very likely to be higher than 50V. It not only makes the equipment shell charged, causing unsafe conditions for people, but also cannot obtain a stable reference potential;
(3) The TN-C system should repeatedly ground the PEN line. Its function is to effectively reduce the neutral line to ground voltage when the phase of the zero-connected equipment contacts the shell.
From the above, we can see that the TN-C system has the following defects:
A. When the three-phase load is unbalanced, unbalanced current appears on the neutral line, and the neutral line presents voltage to the ground. When the three-phase load is seriously unbalanced, touching the neutral line may cause electric shock accidents.

B. The neutral line through the leakage protection switch can only be used as the working neutral line, not as the protective neutral line of the electrical equipment. This is determined by the working principle of the leakage protection switch.
C. The protective neutral line of the metal shell of the single-phase electrical equipment connected with the two-pole leakage protection switch, such as the TN-C system, is strictly prohibited from being connected to the working neutral line of the circuit, nor is it allowed to be connected to the PEN line in front of the leakage protection switch, but it is very easy to be misconnected during use.
D. The connection line of the repeated grounding device is strictly prohibited from being connected to the working neutral line through the leakage protection switch.
The TN-S power supply system completely separates the working neutral line from the protective neutral line, thereby overcoming the defects of the TN-C power supply system, so the TN-C system is no longer used on construction sites.
2. TN-S system
The neutral line (N) and the protective line (PE) of the entire system are separated.
(1) When the phase line of the electrical equipment touches the shell and short-circuits directly, the overcurrent protector can be used to cut off the power supply;
(2) When the N line is disconnected, such as the three-phase load is unbalanced, the neutral point potential rises, but the shell has no potential and the PE line 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 breaking.
(4) The TN-S system is suitable for industrial enterprises and large civil buildings.
At present, the construction sites that use a single transformer for power supply or the substation is close to the construction site basically use the TN-S system. In combination with the step-by-step leakage protection, it does play a role in ensuring the safety of construction electricity use. However, the TN-S system must pay attention to several issues:
A. The protective neutral line is absolutely not allowed to be disconnected. Otherwise, when the live part of the zero-connection device touches the shell or leaks electricity, a single-phase circuit will not be formed, and the power supply will not be automatically cut off, which will have two consequences: first, the zero-connection device loses its safety protection; second, the shells of other intact zero-connection devices behind it will be charged, causing the shells of a large range of electrical equipment to be charged, causing a terrible threat of electric shock. Therefore, the "JGJ46-88 Technical Specifications for Temporary Electricity Safety at Construction Sites" stipulates that the dedicated protection line must be repeatedly grounded at the beginning and end.

The TT system is widely used abroad, but in China it is limited to local electronic equipment with high grounding requirements. At present, this system is generally not used on construction sites. However, if it is a public transformer and other users use the TT system, this system should also be used on the construction site.
3. IT system
There is no direct connection between the live parts of the power system and the earth (or grounding through resistance), and the exposed conductive parts of the receiving equipment are directly grounded through the protection line. This system is mainly used for 10KV and 35KV high-voltage systems and some low-voltage power supply systems in mines and underground. It is not suitable for application on construction sites, so it is not analyzed here.
The "Construction Safety Inspection Standard" (JGJ59-99) newly issued by the Ministry of Construction stipulates that the TN-S zero protection system must be used in the power system with direct neutral point grounding dedicated to the construction site. Therefore, the TN-S zero protection system has been widely used on construction sites, but if the PE line is broken or the electrical connection with the electrical equipment is not done well, the repeated grounding resistance value does not meet the safety requirements, and electric shock accidents will also occur. In order to improve the safety of the TN-S zero protection system, the concept of equipotential connection is proposed here. The so-called 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 potential of the two tends to be equal. It should be noted that when the equipotential bonding line is normal, no current passes through it, only the potential is transmitted, and the current passes through it when there is a fault. The role of equipotential bonding: A) The total equipotential bonding can reduce the expected contact voltage; B) The total equipotential bonding can eliminate the risk of electric shock caused by the fault voltage conducted by the PE line outside the device.
Therefore, this technology should also be gradually promoted on the construction site. Of course, no matter what form of grounding is adopted, it is by no means foolproof and absolutely safe. Temporary power supply on the construction site must strictly follow the requirements of JGJ46-88 specification for system setting and use of leakage protection device, strictly implement the construction power design, acceptance system, and standardize management to prevent accidents.