What are TT, IN and IT systems?

What are TT, IN and IT systems?

1. Construction Engineering Power Supply System
The basic power supply systems used in construction engineering power supply include three-phase three-wire system and three-phase four-wire system, but the connotations of these terms are not very strict. The International Electrotechnical Commission (IEC) has made unified regulations for this, called TT system, TN system, and IT system. Among them, the TN system is divided into TN-C, TN-S, and TN-CS systems. The following content is a brief introduction to various power supply systems.
TT system TN-C
power supply system → TN system → TN-S
IT system TN-CS
(I) Basic methods of engineering power supply
According to the various protection methods and terminology concepts stipulated by IEC, low-voltage distribution systems are divided into three categories according to different grounding methods, namely TT, TN and IT systems, which are described as follows.

(1) TT power supply system The TT method refers to a protection system that directly grounds the metal shell of electrical equipment, which is called a protective grounding system, also known as a 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 in contact with the live body is directly connected to the earth, regardless of how the system is grounded. In the TT system, all grounding of the load is called protective grounding, as shown in Figure 1-1. The characteristics of this power supply system are as follows.
1) When the metal shell of the electrical equipment is live (phase line touches the shell or the equipment insulation is damaged and leaks), the risk of electric shock can be greatly reduced due to grounding protection. However, the low-voltage circuit breaker (automatic switch) may not trip, causing the shell voltage of the leakage equipment to be higher than the safe voltage, which is a dangerous voltage.
2) When the leakage current is relatively small, even if there is a fuse, it may not melt, so a leakage protector is still needed for protection, so the TT system is difficult to promote.
3) TT system grounding devices consume a lot of steel, and are difficult to recycle, time-consuming, and wasteful.
Some construction units now use the TT system. When the construction unit borrows its power supply for temporary power use, a dedicated protection line should be used to reduce the amount of steel required for the grounding device, as shown in Figure 1-2.

The dotted line frame in the figure is the main distribution box for construction power, which separates the newly added special protection line PE line and the working neutral line N. Its characteristics are: ① The common grounding line has no electrical connection with the working neutral line; ② During normal operation, the working neutral line can have current, while the special protection line has no current; ③ The TT system is suitable for places where the grounding protection is very scattered.
(2) TN power supply system This power supply system is a protection system that connects the metal shell of the electrical equipment to the working neutral line. It is called the zero connection protection system and is represented by TN. Its characteristics are as follows.
1) Once the shell of the equipment is energized, the zero connection protection system can increase the leakage current to a short-circuit current. This current is very large, 5.3 times that of the TT system. In fact, it is a single-phase short-circuit fault. The fuse of the fuse will melt, and the release of the low-voltage circuit breaker will immediately operate and trip, so that the faulty equipment is powered off, which is safer.
2) The TN system saves materials and labor hours. It 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 power supply system, it is divided into two types, TN-C and TN-S, depending on whether the protective neutral line is separated from the working neutral line.
(3) TN-C power supply system It uses the working neutral line as the neutral protection line, which can be called the protective neutral line and can be represented by NPE, 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 neutral line, and there is a voltage to the ground, so the metal shell of the electrical equipment connected to the protective line has a certain voltage.
2) If the working neutral line is broken, the shell of the leakage protection equipment will be energized.
3) If the phase line of the power supply touches the ground, the shell potential of the equipment will rise, causing the dangerous potential on the neutral line to spread.
4) When using a leakage protector on the main line of the TN-C system, all repeated grounding behind the working neutral line must be removed, otherwise the leakage switch will not be closed; and the working neutral line must not be broken under any circumstances. Therefore, in practice, the working neutral line can only have repeated grounding on the upper side of the leakage protector.
5) The TN-C power supply system is only applicable to the basic balance of the three-phase load.
(4) TN-S power supply system It is a power supply system that strictly separates the working neutral line N and the dedicated 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 operating normally, there is no current on the dedicated protection line, but there is an unbalanced current on the working neutral line. There is no voltage between the PE line and the ground, so the metal shell of the electrical equipment is connected to the dedicated protection line PE for zero protection, which is safe and reliable.
2) The working neutral line is only used as a single-phase lighting load circuit.
3) The dedicated protection line PE is not allowed to be disconnected, nor is it allowed to enter the leakage switch.
4) When using a leakage protector on the main line, the working neutral line must not be repeatedly grounded, while the PE line is repeatedly grounded, but it does not pass through the leakage protector, so the leakage protector can also be installed on the TN-S system power supply main line.
5) The TN-S power supply system is safe and reliable, and is suitable for low-voltage power supply systems such as industrial and civil buildings. Before construction work begins, the "three connections and one leveling" (electricity, water, roads and ground leveling) must be done using the TN-S power supply system.
(5) TN-CS power supply system In temporary power supply for construction, if the front part is TN-C, and the construction specification stipulates that the construction site must use the TN-S power supply system, the PE line can be separated from the on-site main distribution box at the rear of the system, as shown in Figures 1-5 and 1-6. This system is called a TN-CS power supply system. The characteristics of the TN-CS system are as follows.

Figure 1-5 TN-CS power supply system 1-6 PE line from the main distribution box on the construction site
1) The working neutral line N is connected to the dedicated protection line PE, as shown in Figure 1-5. When the unbalanced current of this section of ND line is relatively large, the zero protection of the electrical equipment is affected by the neutral line potential. There is no current on the PE line from point D to the back, that is, there is no voltage drop on this section of the wire. Therefore, the TN-CS system can reduce the voltage between the motor housing and the ground, but it cannot completely eliminate this voltage. The magnitude of this voltage depends on the load imbalance of the ND line and the length of the ND line. The more unbalanced the load is and the longer the ND line is, the greater the voltage deviation of the equipment housing from the ground. Therefore, it is required that the load unbalanced current should not be too large, and repeated grounding should be performed on the PE line, as shown in Figure 1-6.
2) The PE line cannot enter the leakage protector under any circumstances, because the leakage protector at the end of the line will trip the previous leakage protector and cause a large-scale power outage.
3) Except for the PE line that must be connected to the N line at the main box, the N line and the PE line must not be connected at other sub-boxes. Switches and fuses are not allowed to be installed on the PE line, and the large wire cannot be used as the PE line.
Through the above analysis, the TN-CS power supply system is a temporary workaround 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 construction power supply practice. 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.
(6) IT power supply system I indicates that there is no working grounding on the power supply side, or it is grounded through high impedance. Every two letters T indicate that the electrical equipment on the load side is grounded, as shown in Figure 1-7.

When the power supply distance is not very long, the IT power supply system has high reliability and good safety. It is generally used in places where power outages are not allowed or where strict continuous power supply is required, such as electric steelmaking, operating rooms in large hospitals, underground mines, etc. The power supply conditions in underground mines are relatively poor and cables are easily affected by moisture. When using the IT power supply system, even if the neutral point of the power supply is not grounded, once the equipment leaks, the single-phase leakage current to the ground is still small and will not destroy the balance of the power supply voltage. Therefore, it is safer than the system with the neutral point of the power supply grounded.
However, if it is used when the power supply distance is very long, the distributed capacitance of the power supply line to the earth cannot be ignored. As can be seen from Figure 1-8, when a short circuit fault occurs in the load or leakage causes the equipment casing to be charged, the leakage current forms a line through the earth, and the protection device may not be activated, which is dangerous. It is only safer when the power supply distance is not too long. This power supply method is rarely seen on construction sites.
(II) Summary of power supply line symbols
1) In the power supply symbols specified by the International Electrotechnical Commission (IEC), the first letter indicates the relationship between the power (power) system and the ground. For example, T indicates that the neutral point is directly grounded; I indicates that all live parts are insulated.
2) The second letter indicates the relationship between the exposed conductive part of the electrical device and the ground. For example, T indicates that the equipment shell is grounded, which has no direct relationship with any other grounding point in the system; N indicates that the load adopts zero connection protection.
3) The third letter indicates the combination relationship between the working neutral line and the protection line. For example, C indicates that the working neutral line and the protection line are combined, such as TN-C; S indicates that the working neutral line and the protection line are strictly separated, so the PE line is called a dedicated protection line, such as TN-S.