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The previous article “Understanding the Differences Between TN, IT, and TT Low Voltage Distribution Systems: Essential for Electrical Safety (1)” shared some content about the TN system, and today I will continue to share more.
The ungrounded neutral point system (IT) is a three-phase three-wire system without an N line, where the exposed conductive parts of electrical equipment are directly grounded.。
The IT system can have a neutral line, but the IEC strongly recommends not to set a neutral line. If a neutral line is set, and a grounding fault occurs at any point in the N line, the system will no longer be an IT system.。
The shell of this electrical equipment is grounded separately,Characteristics of the IT system:1) When a grounding fault occurs, the capacitive current relative to ground in the non-faulty phase is very small, and it is not necessary to immediately disconnect the fault circuit, ensuring the continuity of power supply, the voltage relative to ground in the non-faulty phase increases by 1.73 times;
A 220V load requires an additional step-down transformer or a dedicated power supply from an external source; commonly used in places with high requirements for power supply continuity, such as emergency power supplies, hospital operating rooms, etc.
2) When the power supply distance is not very long, the reliability and safety of the power supply are high. Generally used in places that require strict continuous power supply, such as electric steelmaking, hospital operating rooms, underground mines, etc.
3) Using the IT power supply system, even if the neutral point of the power source is not grounded, once the equipment leaks, the single-phase leakage current relative to ground is still small, which will not disrupt the balance of the power supply voltage, making it safer than systems with grounded neutral points.
However, if used in cases where the power supply distance is very long, the distribution capacitance of the power supply line to the ground cannot be ignored.
The directly grounded neutral point system (TT) is a three-phase four-wire system.
In the TT system, these two groundings must be independent of each other. The grounding of the equipment can be that each device has its own independent grounding device, or several devices can share one grounding device.
The main advantages of the TT system are:
1) The protective effect of grounding the shell of low-voltage electrical appliances is not as good as that of the IT system.
2) Compared to ungrounded low-voltage electrical appliances, in the event of a shell collision accident, it can reduce the voltage of the shell relative to ground, thus reducing the degree of electric shock hazard.
3) The voltage relative to ground at the fault point of single-phase grounding is lower, and the fault current is larger, causing the leakage protection device to quickly act to cut off the power supply, which is beneficial to prevent electric shock accidents.
4) There is no need to bury repeated grounding wires for each electrical device, which can save the cost of burying grounding wires, and also helps improve the quality of grounding wires and ensure that the grounding resistance is less than 10 ohms, ensuring electrical safety.
5) When the metal shell of electrical equipment is live (due to phase line collision or insulation damage causing leakage), grounding protection can greatly reduce the risk of electric shock.
Low-voltage circuit breakers (automatic switches) may not necessarily trip, causing the voltage of the shell of the leaking equipment to be higher than the safe voltage, which is considered dangerous voltage.
6) When the leakage current is relatively small, even with a fuse, it may not blow, so a leakage protection device is also needed for protection, making the TT system difficult to promote.
7) Since the grounding device is near the equipment, the probability of the PE line being broken is very small, and even if the line is broken, it is easy to detect.
8) When the equipment is operating normally, the shell is not live, and in the event of a fault, the high potential of the shell will not be transmitted to the entire system along the PE line.
——–Wishes from the River Master————–
Man Jiang Hong – The Great Path of Electricity
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