(1) TT mode power supply system refers to the protection system that directly grounds the metal casing of electrical equipment, called the protective earthing 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 grounds that are loaded in the TT system are called protective grounds.
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, even if there is a fuse, it may not be able to be blown. Therefore, the leakage protector is also required for protection, which makes it difficult to promote the TT system.
3) TT system grounding device consumes a lot of steel, and it is difficult to recycle, labor, and materials.
Separate the newly added special protection line PE line from the working zero line N, which is characterized by: 1 the common ground line has no electrical connection with the working zero line; 2 during normal operation, the working zero line can have current, and the special protection line does not have Current
(2) The TN mode power supply system is a protection system that connects the metal casing of the electrical equipment to the working neutral line, called the zero-connection protection system.
1) Once the equipment is charged, the zero-protection system can increase the leakage current to the short-circuit current. This current is very large. In fact, it is a single-phase short-circuit fault, the fuse of the fuse will be blown, and the trip of the low-voltage circuit breaker. It will immediately act and trip, making the faulty device power off, which is safer.
2) The TN system saves materials and man-hours. 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, which uses the working zero line as the zero-protection line, which can be called the protection neutral line, represented by NPE.
(4) TN-S mode power supply system It 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.
1) When the system is in normal operation, there is no current on the dedicated protection line PE, 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, and 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.
(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, it can be matched in the back part of the system. The electrical box is separated from the PE line. The characteristics of the TN-CS system are as follows.
1) The working neutral line N is connected to the dedicated protection line PE and should be grounded repeatedly on the PE line.
2) 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.
3) For the PE line, except for the N-line at the header, the N-line and the PE line must not be connected to each other. The switch and fuse are not allowed on the PE line, and the switch must not be used. PE line.
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 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.
(6) IT mode Power supply system I indicates that there is no working ground on the power supply side or grounded through high impedance. Each two letter T indicates that the load side electrical equipment is grounded.
The TT mode 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 steel making, operating rooms in large hospitals, underground mines, etc. The power supply conditions in the underground mine are relatively poor, and the cable is susceptible to moisture. Using the IT power supply system, even if the neutral point of the power supply is not grounded, once the equipment leaks, the single ground leakage current is still small and 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.
However, if the power supply distance is long, the distributed capacitance of the power supply line to the earth cannot be ignored. When a short-circuit fault occurs in the load or the electric leakage causes the equipment casing to be charged, the leakage current forms a path through the earth, and the protection device does not necessarily operate, which is dangerous. It is safer only when the power supply distance is not too long. This type of power supply is rare on the job site.
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