According to the terms and concepts of various protective grounding modes stipulated by the International Electrotechnical Commission (iec), low-voltage distribution systems are called tt system, tn system and it system according to different grounding modes. Tn system is divided into tn-c, tn-s and tn-c-s systems.

Here is a brief introduction of various power supply systems.

1, tt grounding power supply system

Tt grounding mode refers to the protective system that directly grounds the metal shell of electrical equipment, which is called protective grounding system, also known as tt system. The first symbol T indicates that the neutral point of the power system is directly grounded; The second symbol T indicates that the exposed metal conductive part of the load equipment that is not connected with the charged body is directly connected with the earth, no matter how the system is grounded. All grounding of the load in tt system is called protective grounding, as shown in figure 1- 1. The characteristics of the power supply system are as follows.

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

(2) The working zero line is disconnected, and the shell of leakage equipment connected to zero is charged.

(3) If the phase line of the power supply touches the ground, the shell potential of the equipment will rise and the dangerous potential on the neutral line will spread.

(4) When the leakage protector is used on the trunk line of TN-C system, the repeated grounding after the working zero line must be completely removed, otherwise the leakage switch will not be closed; In addition, the working zero line must not be disconnected under any circumstances. Therefore, in actual operation, the working zero line can only make the upper side of the leakage protector have repeated grounding.

(5)tn-c power supply system is only suitable for the basic balance of three-phase load.

4.tn-s power supply system

It is a power supply system that strictly separates the working zero line N from the special protection line pe, which is called tn-s power supply system, as shown in figure 1-4. The characteristics of tn-s power supply system are as follows.

(1) During normal operation of the system, there is no current on the special protection line, but there is unbalanced current on the working zero line. There is no voltage between pe line and ground, so the zero protection of the metal shell of 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 loop.

(3) The special protection line pe is not allowed to disconnect or enter the leakage switch.

(4) The leakage protector is used on the trunk line, and the working zero line cannot be repeatedly grounded, while the pe line has repeated grounding, but it does not pass through the leakage protector, so the leakage protector can also be installed on the power supply trunk line of tn-s system.

(5)tn-s power supply system is safe and reliable, and suitable for low-voltage power supply systems such as industrial and civil buildings. "Three links and one leveling" (electricity supply, water supply, access and site leveling) before construction-tn-s power supply system must be adopted.

5.tn-c-s power supply system

In the temporary power supply of building construction, if the front part is tn-c power supply, and the construction code stipulates that the construction site must adopt tn-s power supply system, the pe line can be separated by the construction distribution box at the back part of the dotted line, as shown in figure 1-5. This system is called tn-c-s power supply system. The characteristics of tn-c-s system are as follows.

(1) When the metal shell of electrical equipment is charged (the phase wire collides with the shell or the insulation of the equipment is damaged and leaks), the risk of electric shock can be greatly reduced due to the grounding protection. However, the low-voltage circuit breaker (automatic switch) may not trip, resulting in the leakage equipment shell voltage higher than safe voltage, which is a dangerous voltage.

(2) When the leakage current is relatively small, the fuse may not be blown, and the leakage protector is needed for protection, so tt system is difficult to popularize.

(3) Because tt system grounding device consumes a lot of steel, it is difficult to recycle it after installation, which is time-consuming and laborious. At present, some construction units use special protective wires when using electricity to reduce the amount of steel used to install grounding devices, as shown in figure 1-2.

The connection mode at the back of the dotted line in the figure adopts the construction distribution box, which separates the newly added special protection line pe from the working zero line N. Its characteristics are as follows: First, there is no electrical connection between the * * * ground wire and the working zero line; Second, during normal operation, the working zero line can have current, and the special protection line has no current.

From the above analysis, it can be seen that tt system is suitable for places where grounding protection is very scattered.

2.tn power supply system

This kind of power supply system is a protection system that connects the metal shell of electrical equipment with the working zero line, which is called zero connection protection system and denoted by tn. The characteristics of the power supply system are as follows.

(1) When the equipment shell is charged, the zero protection system can increase the leakage current to the short-circuit current, which is very large, several times that of tt system. In fact, it is a single-phase short-circuit fault, the fuse of the fuse will blow, and the release of the low-voltage circuit breaker will trip immediately, so that the faulty equipment is cut off, which is safer.

(2)tn system saves materials and man-hours, and is widely used in China and many other countries, so it has many advantages over tt system. Tn power supply system is divided into tn-c and tn-s according to whether the protective zero line is separated from the working zero line.

3.tn-c power supply system

It uses working zero line as neutral protection line, which can be called protective neutral line.