At present, China has five power supply systems: IT, TT, TN-c, TN-c-s and TN-S. They have their own advantages and disadvantages, and we should choose sensitively according to the actual situation in life.
Five power supply systems
1.IT power supply system
In all power supply systems, IT power supply systems are safe and reliable. Since the power supply of the IT system is not grounded, the current flowing to the ground is very small, and the voltage balance of the power supply will not be damaged. Therefore, even if the IT system is leaking, the electrical equipment can still be used normally; Even if it comes into contact with a leakage device, it will not be electrocuted.
However, its flaw is obvious, that is, it is only suitable for a small range of power supplies. Therefore, the IT power supply system is mainly used in places that require strict continuous power supply (not easy to lose power), such as hospital operating rooms, underground mine ventilation equipment, cable cars, etc.
2. Tn-s power supply system
For large-scale power supply, we can use the remaining four power supply systems according to the actual situation. Among the remaining four power supply systems, the TN-S power supply system is safe and reliable, and it is also widely used. TN-S power supply system, also known as three-phase five-wire power supply system, is composed of three front + one neutral wire + one ground wire.
Although the TN-S power supply system is safe and reliable, it requires a large number of wires and a high capital investment. Since the equipment only needs the front and neutral line to operate normally, it has a grounding wire for personal safety. In order to save costs, the TN-S power supply system is only used when the power load interval transformer is not far away or there is a special transformer.
The reason is very simple, the transformer power load is relatively close, the required cable is shorter, and the cost of 4 and 5 wires is not much different. However, if the transformer is far away from the power load and the cable required is very long, the cost difference can be significant.
3. Tn-c-s power supply system
As mentioned earlier, the TN-S power supply system is used when the power supply interval is short or there is a special transformer. What should I do if the load is loose due to the long interval between power supplies? In order to save costs, we can choose the front-end 4-wire and back-end 5-wire power supply system, that is, the front-end TN-c power supply system and the back-end TN-S power supply system.
4. Tn-c power supply system
The power supply interval is long, and the load is scattered. If you want to continue to save money, then simply don't connect the ground wire and connect the neutral wire to the shell, is this good? In some cases, of course. This power supply system is called TN-c power supply system, which is often referred to as zero protection system.
However, this system is only suitable for three-phase balance, and there are no flammable and explosive occasions. If the three phases are unbalanced, the neutral PEN will be charged, and then the chassis will be charged, which is not safe. Generally, factories and small areas do not meet the requirements, so this power supply system is rarely used.
5.TT power supply system
What if the power supply is far apart, the three phases are unbalanced, and the load is particularly loose? You can also use the TT power system. The power supply leads three (three front) or four (three front + one neutral wire) to supply power to the equipment as required. Then make a grounding device near the electrical equipment, lead the ground wire, and connect the equipment shell with the ground wire.
When the equipment leaks, most of the current flows to the ground along the ground, as long as a small part of the current passes through the human body, which greatly reduces the risk of human contact with the equipment shell leakage. Although the grounding wire of this power supply system can reduce the risk of electric shock, it cannot ensure safety completely, so all electrical equipment needs to be equipped with leakage switches.
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