International Electrotechnical Commission (IEC) regulations for each grounding power supply system - Database & Sql Blog Articles

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According to the terminology of various protective grounding methods specified by the IEC, low-voltage distribution systems are called TT systems, TN systems, and IT systems according to different grounding methods. The TN system is further divided into TN-C, TN-S, and TN-CS systems. The following is a brief introduction to various power supply systems.
1. TT mode grounding power supply system TT grounding mode refers to the protection system that directly grounds the metal casing of electrical equipment, called protective earthing 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 metal conductive portion of the load device that is not exposed to the charged body is directly connected to the earth, regardless of how the system is grounded. All grounding of the load in the TT system is called protective grounding, as shown in Figure 1-1. The characteristics of this power supply system are as follows.

(1) When the metal casing of the electrical equipment is charged (phase line is in contact with the casing 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) does not necessarily trip, causing the voltage of the earth leakage device to be higher than the safe voltage, which is a dangerous voltage.
(2) When the leakage current is relatively small, the fuse may not be able to be blown, so the leakage protector is also required for protection, so the TT system is difficult to promote.
(3) Because the TT system grounding device consumes a lot of steel, it is difficult to recycle after installation, and it takes time and labor. When some construction units use electricity, a special protection wire is used to reduce the amount of steel used to install the grounding device, as shown in Figure 1-2.


In the figure, the wiring method of the dotted line is used to construct the power distribution box, and the newly added special protection line PE line is separated from the working zero line N. The characteristics are: first, the common ground line has no electrical connection with the working zero line; Second, during normal operation, the working neutral line can have current, while the dedicated protection line has no current.
From the above analysis, it can be seen that the TT system is suitable for places where the grounding protection is very scattered.
2. TN mode power supply system This power supply system is a protection system that connects the metal casing of the electrical equipment to the working zero line. It is called the zero-connection protection system and is represented by TN. The characteristics of this power supply system are as follows.
(1) When the equipment is charged, the zero-protection system can increase the leakage current to the short-circuit current. This current is very large, which is several times that of the TT system. In fact, it is a single-phase short-circuit fault, and the fuse of the fuse will be The fuse, the trip unit of the low-voltage circuit breaker will immediately act and trip, so that the faulty equipment is powered off, which is safer.
(2) The TN system saves materials and man-hours and is widely used in China and many other countries. It can be seen that it has more advantages than the TT system. In the TN mode power supply system, it is divided into two types, TN-C and TN-S, according to whether the protection zero line is separated from the working zero line.
3, TN-C mode power supply system It uses the work zero line as the zero-protection line, which can be called the protection neutral line, as shown in Figure 1-3. The characteristics of this power supply system are as follows.


(1) Due to the unbalanced three-phase load, there is an unbalanced current on the working zero line, and there is a 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 wire is broken, 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 casing of the device 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 neutral 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) The TN-C mode power supply system is only applicable to the basic balance of the three-phase load.
4, TN-S mode power supply system It is a power supply system that strictly separates the working zero line N and the special protection line PE, called the TN-S power supply system, as shown in Figure 1-4, the characteristics of the TN-S power supply system are as follows: .


(1) When the system is in normal operation, 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) The TN-S mode power supply system is safe and reliable, and is 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.
5. In the TN-CS mode power supply system, if the front part is TN-C mode power supply, and the construction specification stipulates that the TN-S mode power supply system must be used on the construction site, the construction power can be used in the back line of the dotted line. The distribution box is separated from the PE line, as shown in Figure 1-5. This type of system is called the TN-CS power supply system. The characteristics of the TN-CS system are as follows.

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