Explain in detail three types of TN power supply systems

Maintenance electricians and other power workers encountered the most in the operation process is the TN-type power supply system, according to whether the protection zero line is separated from the working zero line, TN-type power supply system can be divided into three categories: TN-C power supply system, TN -S power supply system, TN-CS power supply system. Below we explain in detail these three types of TN power supply system

First, TN-C mode power supply system

It uses the working zero line as the zero-protection line, which can be called the protection neutral line and can be represented by NPE.

TN-C mode power supply system features

1, because the three-phase load is unbalanced, there is current on the working zero line, there is voltage on the ground, so the metal casing of the electrical equipment connected to the protection line has a certain voltage.

2. If the working neutral line is disconnected, the outer casing of the leakage protection device that is connected to zero is charged.

3. If the phase line of the power supply hits the ground, the potential of the device's casing rises, causing the dangerous potential on the neutral line to spread.

4. When the leakage protector is used on the trunk line of the TN-C system, all repeated grounding behind the working zero line must be removed, otherwise the leakage switch cannot be closed; and the working neutral line must not be disconnected under any circumstances. Therefore, the working neutral line can only make the upper side of the leakage protector have repeated grounding.

5, TN-C mode power supply system is only suitable for the basic balance of three-phase load.

Second, the TN-S mode power supply system is a power supply system that strictly separates the working neutral line N from the dedicated protection line PE, called the TN-S power supply system.

TN-S mode power supply system features

1. When the system is running normally, there is no current on the dedicated protection line, but there is an unbalanced current on the working zero line. There is no voltage on the PE line to the ground, so the zero protection of the metal casing of the electrical equipment is connected to the special protection line PE, which is safe and reliable.

2. The working zero line is only used as a single-phase lighting load circuit.

3. The special protection line PE is not allowed to be disconnected, nor is it allowed to enter the leakage switch.

4. The leakage protector is used on the main line. The working zero line must not have repeated grounding. The PE line has repeated grounding, but it does not pass through the leakage protector. Therefore, the leakage protector can also be installed on the TN-S system power supply main line.

5, TN-S mode power supply system is safe and reliable, suitable for low-voltage power supply systems such as industrial and civil buildings. In the “three links and one leveling” (electricity, water, road and level) before the construction work, the TN-S power supply system must be used.

Third, the TN-CS mode power supply system in the temporary power supply for building construction, if the front part is TN-C mode power supply, and the construction specification stipulates that the construction site must adopt TN-S mode power supply system, then the total power distribution in the back part of the system can be The box separates the PE line. This type of system is called the TN-CS power supply system.

The characteristics of the TN-CS system are as follows

1. The PE line cannot enter the leakage protector under any circumstances, because the leakage protector at the end of the line will cause the front leakage protector to trip and cause a large-scale power outage.

2. For the PE line, except that it must be connected to the N line at the main box, the N line and the PE line must not be connected to each other. No switches or fuses should be installed on the PE line. PE line.

Through the above analysis, the TN-CS power supply system is temporarily modified on the TN-C system. When the working grounding of the three-phase power transformer is good and the three-phase load is relatively balanced, the TN-CS system is still feasible in the practice of construction electricity. However, when the three-phase load is unbalanced and there is a dedicated power transformer at the construction site, the TN-S power supply system must be used.