Five Power Supply Systems
Release time:
2022-06-30
Source:
Among all power supply systems, the IT power supply system is safe and reliable. Because the IT system’s power supply is ungrounded, the leakage current to earth is very small and does not disrupt the voltage balance of the power supply. Therefore, even if a leakage fault occurs in the IT system, electrical equipment can still operate normally; and even if a person touches a leaking device, no electric shock will occur.
China currently has five types: IT, TT, TN-C, TN-C-S, and TN-S. Power supply systems. Each has its own advantages and disadvantages, and in daily life we should make flexible choices based on the specific circumstances.
Five Power Supply Systems
1.IT Power supply System
Among all power supply systems, the IT power supply system is safe and reliable. Because the IT system’s power supply is ungrounded, the leakage current to earth is very small and does not disrupt the voltage balance of the power supply. Therefore, even if a leakage fault occurs in the IT system, electrical equipment can still operate normally; and even if a person touches a leaking device, no electric shock will occur.
However, its drawback is obvious: it is only suitable for small-scale power supplies. Therefore, IT power supply systems are primarily used in applications that demand strict, uninterrupted power supply—where power outages are undesirable—such as hospital operating rooms, underground mine ventilation equipment, and cable cars.
2. TN-S power supply system
For large-scale power supply, we can select among the remaining four power-supply systems based on actual conditions. Among these four systems, the TN-S system is both safe and reliable, and it is also widely used. The TN-S system, also known as the three-phase five-wire system, consists of three phase conductors, one neutral conductor, and one protective earth conductor.
Although the TN-S power supply system is safe and reliable, it requires a large number of conductors and entails high capital investment. This is because the equipment can operate normally with only the phase and neutral lines; however, a protective earth conductor is mandatory for personal safety. To reduce costs, the TN-S system is typically employed only when the electrical load is located close to the transformer or when a dedicated transformer is available.
The principle is straightforward: when the transformer’s power load is relatively close, the required cable length is shorter, and the cost difference between a 4-wire and a 5-wire system is minimal. However, if the transformer is located far from the power load, the cable length increases significantly, leading to a substantial cost disparity.
3. TN-C-S power supply system
As mentioned earlier, the TN-S power supply system is typically used when the supply distance is relatively short or when a dedicated transformer is available. But what if the supply distance is long and the load is widely dispersed? To reduce costs, we can adopt a hybrid power supply system that uses a 4-wire configuration at the front end and a 5-wire configuration at the back end—specifically, a TN-C system at the front end followed by a TN-S system at the back end.
4. TN-C Power Supply System
The power supply distance is long, and the load is dispersed. If the goal is to further reduce costs, some might suggest eliminating the protective earth conductor altogether and instead connecting the neutral conductor directly to the equipment enclosure. Is this a good approach? Of course, in certain circumstances. Such a power supply system is known as a TN-C system, commonly referred to as a combined neutral-and-protective-earth system.
However, this system is only suitable for balanced three-phase power and must not be used in flammable or explosive environments. If the three phases are unbalanced, the PEN neutral conductor will become energized, which in turn will cause the enclosure to be energized, posing a safety hazard. In most factories and small-area applications, the required balance cannot be achieved, so this type of power supply system is rarely used.
5.TT Power supply System
What if the power supply distance is very long, three-phase imbalance occurs, and the loads are highly dispersed? In such cases, you can also adopt a TT power system. The power supply is routed to the equipment as required—either three conductors (three phase lines) or four conductors (three phase lines plus one neutral line). A grounding device is then installed near the electrical equipment, a grounding conductor is connected, and the equipment enclosure is bonded to this grounding conductor.
When equipment experiences a leakage current, most of the current flows to ground through the earth, with only a small portion passing through the human body, thereby significantly reducing the risk of electric shock from contact with the equipment’s metal enclosure. Although the grounding conductor in such power supply systems can mitigate the danger of electric shock, it cannot provide absolute safety; therefore, all electrical equipment must be equipped with residual-current devices (RCDs).
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