What is TT The TN The IT System?

1、 Construction power supply system

The basic power supply systems used in construction projects include three-phase three wire system, three-phase four wire system, etc., but the connotations of these terms are not very strict. The Electrotechnical Commission (IEC) has made unified regulations on this, known as TT system, TN system, IT system. The TN system is further divided into TN-C, TN-S, and TN-C-S 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-C-S

（1） Basic methods of engineering power supply

According to various protection methods and terminology concepts specified by IEC, low-voltage distribution systems are divided into three categories based on different grounding methods, namely TT, TN, and IT systems, as described below.

(1) TT power supply system refers to a protection system that directly grounds the metal casing of electrical equipment, known as a protective grounding system or TT system. The first symbol T represents the direct grounding of the neutral point in the power system; The second symbol T represents the metal conductive part of the load equipment that is exposed and not connected to the charged body, and is directly connected to the ground, 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 electrical equipment is electrified (due to contact between phase lines or damage to equipment insulation causing leakage), the risk of electric shock can be greatly reduced due to grounding protection. However, low-voltage circuit breakers (automatic switches) may not necessarily trip, causing the shell of the leakage equipment to have a ground voltage higher than the safe voltage, which is considered a dangerous voltage.

2) When the leakage current is relatively small, even if there is a fuse, it may not necessarily melt, so a leakage protector is still needed for protection, making it difficult to promote this TT system.

3) The TT system grounding device consumes a lot of steel and is difficult to recycle, time-consuming, and material intensive.

Some construction units now use TT systems. When the construction unit borrows its power supply for temporary use, a protective wire should be used to reduce the amount of steel required for grounding devices, as shown in Figure 1-2.

The dotted line box in the figure is the main distribution box for construction electricity, which separates the newly added protective line PE from the working neutral line N. Its characteristics are: ① there is no electricity between the shared grounding wire and the working neutral line; ② During normal operation, the working neutral wire can have current, while the protective wire has no current; ③ The TT system is suitable for places with scattered grounding protection.

(2) The TN power supply system is a protection system that connects the metal casing of electrical equipment to the working neutral line, called the neutral protection system, represented by TN. Its characteristics are as follows.

1) Once the casing of the equipment becomes electrified, the neutral protection system can increase the leakage current to a short-circuit current, which is very large, 5.3 times that of the TT system. In fact, it is a single phase to ground short-circuit fault. The fuse of the fuse will melt, and the release of the low-voltage circuit breaker will immediately trip, causing the faulty equipment to be powered off, which is relatively safe.

2) The TN system saves materials and labor hours, and is widely used in China and many other countries, indicating that it has many advantages over the TT system. In the TN power supply system, it is divided into two types: TN-C and TN-S, based on whether the protective neutral line is separated from the working neutral line.

(3) The TN-C power supply system uses a working neutral line as a neutral protection line, which can be referred to as a 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 imbalance of three-phase loads, there is unbalanced current on the working neutral line and voltage to ground, so there is a certain voltage on the metal casing of the electrical equipment connected to the protection line.

2) If the working neutral wire is disconnected, the casing of the leakage protection device connected to the neutral wire will be energized.

3) If the phase line of the power supply touches the ground, the potential of the equipment's casing will rise, causing the dangerous potential on the neutral line to spread.

4) When using a leakage protector on the TN-C system main line, all repeated grounding behind the working neutral line must be removed, otherwise the leakage switch cannot be closed; Moreover, the working neutral wire must not be disconnected under any circumstances. Therefore, in practical use, the working neutral wire can only allow for repeated grounding on the upper side of the leakage protector.

5) The TN-C power supply system is only suitable for basic balance of three-phase loads.

(4) The TN-S power supply system is a power supply system that strictly separates the working neutral line N and the protective 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 running normally, there is no current on the protection line, only an unbalanced current on the working neutral line. The PE line has no voltage to ground, so the metal casing of electrical equipment is connected to the protective line PE for zero protection, which is safe and reliable.

2) The working neutral wire is only used as a single-phase lighting load circuit.

3) The protective wire PE is not allowed to break or enter the leakage switch.

4) Leakage protectors are used on the main line, and the working neutral line must not have repeated grounding, while the PE line has repeated grounding but does not pass through the leakage protector. Therefore, leakage protectors can also be installed on the TN-S system power supply main line.

5) The TN-S power supply system is safe and reliable, suitable for low-voltage power supply systems in industrial and civil buildings. The TN-S power supply system must be used for the "three connections and one leveling" (electricity, water, road, and ground) before the construction of the building project.

(5) If the TN-C-S power supply system is used for temporary power supply in building construction, and the construction specifications stipulate that the TN-S power supply system must be used on the construction site, a PE line can be branched out from the main distribution box at the back of the system, as shown in Figures 1-5 and 1-6. This system is called TN-C-S power supply system. The characteristics of TN-C-S system are as follows.

Figure 1-5 TN-C-S power supply system 1-6 PE line branching out of the main distribution box at the construction site

1) The working neutral line N is connected to the protective line PE, as shown in Figure 1-5ND. When the unbalanced current in this section of the line is relatively large, the grounding protection of electrical equipment is affected by the potential of the neutral line. There is no current on the PE line from point D to the rear, which means there is no voltage drop on this section of the wire. Therefore, the TN-C-S system can reduce the voltage of the motor casing to ground, but it cannot 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 and the longer the ND line, the greater the voltage deviation of the equipment casing to ground. So it is required that the unbalanced current of the load should not be too large, and repeated grounding should be done on the PE line, as shown in forehead 1-6.

2) Under no circumstances should the PE line enter the leakage protector, as the action of the leakage protector at the end of the line will cause the previous stage leakage protector to trip, resulting in a widespread power outage.

3) Except for the main box where the PE line must be connected to the N line, the N line must not be connected to the PE line at any other sub box location. Switches and fuses are not allowed to be installed on the PE line, and it is also not allowed to use a large wire as the PE line.

Based on the above analysis, the TN-C-S power supply system is a temporary modification of the TN-C system. When the grounding condition of the three-phase power transformer is good and the three-phase load is relatively balanced, the TN-C-S system is still feasible in construction power practice. However, when three-phase loads are unbalanced and there are power transformers on construction sites, the TN-S power supply system must be used.

(6) The 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 represent the grounding protection of electrical equipment on the load side, as shown in Figure 1-7.

The IT power supply system has high reliability and good safety when the power supply distance is not very long. 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 prone to moisture. By using IT power supply system, even if the neutral point of the power supply is not grounded, once the equipment leaks, the single phase to ground leakage current is still small, which will not damage the balance of the power supply voltage, so it is safer than the system with the neutral point of the power supply grounded.

However, if used for a long power supply distance, the distributed capacitance of the power supply line to the ground cannot be ignored. From Figure 1-8, it can be seen that when a short circuit fault or leakage occurs in the load, causing the equipment casing to be charged, the leakage current forms a circuit through the ground, and the protective equipment may not necessarily operate, which is dangerous. It is only safer when the power supply distance is not too long. This power supply method is rare on construction sites.

（2） Summary of symbols for power supply lines

1) In the power supply symbol specified by the Electrotechnical Commission (IEC), the first letter represents the relationship between the power (source) system and the ground. If T represents a neutral point directly grounded; I represents the insulation of all live parts.

2) The second letter represents the relationship between the exposed conductive part of the electrical device and the ground. If T represents the grounding of the device casing, it has no direct relationship with any other grounding point in the system; N indicates that the load adopts neutral protection.

3) The third letter represents the combination relationship between the working neutral line and the protective line. If C represents that the working neutral wire and the protective wire are integrated, such as TN-C; S means that the working neutral line and the protective line are strictly separated, so the PE line is called the protective line, such as TN-S.