tn-s tn-c-s tn-c tt it等接地系统的区别

6tn-s tn-c-s tn-c tt it等接地系统的区别（）

TN-S, TN-C-S, TN-C, TT, IT and other ground system differences

Analysis of the grounding mode of low voltage power supply system

I. power supply system for engineering construction

The basic power supply system of power supply for construction is

three-phase (), three wire and (380/220V) three-phase four wire system, but these terms are not very strict. The International Electrotechnical Commission (IEC) has made uniform regulations, which are called TT system, TN system and IT system. Among them, TN system is divided into TN-C, TN-S and TN-C-S systems. Here is a brief analysis of the various power supply systems mentioned above.

(1) the basic mode of engineering power supply;

According to the various protection modes and terms defined by IEC, the low-voltage distribution system is divided into three kinds according to the grounding mode, namely, TT, TN and IT systems, as follows.

(1) TT mode power supply system TT means the direct protection system of the metal casing of the electrical equipment, called the protective earthing system, also known as the TT system. The first symbol T denotes the direct connection of the neutral point of the power system; the second symbol T represents the load

equipment; the metal case and the normal uncharged metal part are directly connected to the earth and are independent of how the system is grounded.

1) when the metal casing of the electric equipment is charged (the phase line meets the shell or the equipment is damaged and the electricity is out of circulation), because of the earthing protection, the risk of electric shock can be greatly reduced. However, the low-voltage circuit breaker (automatic switch) does not necessarily trip, resulting in leakage of equipment of the earth's surface voltage is higher than the safety voltage, belonging to the dangerous voltage.

2) when the leakage current is relatively small, even if the fuse does not necessarily fuse, so the leakage protector is needed for protection, so the TT system is not suitable to be used in the 380/220V power supply system.

3) TT system grounding device of steel consumption, and difficult recovery, labor and material costs.

Now some construction units is the use of TT system, the construction unit specially installed a grounding device, leads to a special grounding wire, grounding device to reduce the amount of steel, as shown in figure 1-2.

Figure 1-2

Separate the special protection line of the newly added PE line and zero line N work, its characteristics are: A. common ground and the zero line contact without

electricity; the normal operation of the B., the zero line of work can be current, and no special protection line current; C. TT system suitable for electrical equipment and very small capacity dispersion situation.

(2) TN mode power supply system. This power supply system is a protective system which connects the metal shell of the electrical equipment with the normal uncharged metal part and the working zero line, called the zero protection system, and is represented by TN. Its features are as follows.

1) once the equipment shell charged, zero connection protection system can be increased as the leakage current (220V) short-circuit current, this current is large, many times of the TT system, is actually a single relatively short circuit fault, the fuse fuse will fuse, release of low voltage circuit breaker will act immediately and make fault tripping power equipment, safety.

2) TN system saves material and working hours. It is widely used in our country and many other countries. It can be seen that it has more advantages than TT system. In the TN mode power supply system, is divided into two kinds, such as TN-C and TN-S, according to whether the protective zero line is separated from the work zero line.

(3) TN-C power supply system which is used as a working zero line zero connection protection line, can be called the protection of neutral line, NPE) due to the unbalance of three-phase load, working zero line unbalanced current, have a certain potential difference on the line, so the metal electrical equipment

connected with the line protection the voltage on the earth.

2) if the work zero line is broken, then protect the zero connected leakage equipment shell charged (to the ground 220V!).

3) if the line of power touches the ground, the outer potential of the device rises, causing the potential danger to spread on the neutral line.

4) when using the earth leakage protector on the main line of the TN-C system, all the repeated earthing behind the leakage protector must be removed,

otherwise the leakage switch cannot be closed. Moreover, the zero line of the work shall not be disconnected under any circumstances. Therefore, the utility line can only make repeated earthing on the upper side of the leakage protector.

5) the TN-C mode power supply system is only suitable for the three-phase load basic balance (no 220V load).

(4) TN-S mode power supply system, which is the power supply system which separates the work zero line N and the special protection line PE strictly, is 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 special protection line, but there is unbalanced current on the working zero line. The PE line has no voltage to the ground, so the metal casing of the electric equipment is

connected with zero, and the protection is connected to the special protective line PE. The safety is reliable.

2) the working zero line is only used as the single-phase lighting load loop.

3) special protective line PE shall not be disconnected, nor shall the leakage switch be allowed to work as a zero line.

4) use leakage protector line, there shall be no repeat grounding leakage protector, and the PE line has repeatedly grounding, but without leakage protector, so the TN-S system of power supply lines can also install leakage protection.

5) TN-S mode power supply system is safe and reliable. It is suitable for low voltage power supply system, such as industrial and civil building. Before the construction of the project, \"three links and one leveling\" (electric, water, road and horizon) must be powered by the TN-S mode.

(5) TN-C-S power supply system in the construction of the temporary use of electricity, if the first part is (no 220V load) TN-C mode of power supply, and the construction specification construction site must adopt TN-S power supply system, can be in the system after the part of the site distribution box from the PE line, as shown in figure 1-5. The system is called the TN-C-S power supply system. The features of the TN-C-S system are as follows.

Figure 1-5 TN-C-S mode power supply system

1) work zero line N and special protection line PE Unicom, as shown in Figure 1-5 after the main switch box unbalanced line current is relatively large, the electrical equipment zero connection protection by zero line potential influence. The general switch box behind the PE line without current, the section of wire no voltage drop, therefore, the TN-C-S system can reduce the voltage of the electrical equipment shell, but can not completely eliminate the voltage, the voltage depends on the load on the N line of unbalanced current size and the N line in the switch box front line the length of. The higher the load imbalance current and the longer the N line is, the greater the deviation of the equipment enclosure to the ground voltage. Therefore, the load imbalance current should not be too large, and repeated earthing should be done on the PE line.

2) PE line in any case can enter the leakage protector, because leakage protection action will make before the end of the line leakage protection tripping caused by large-scale power outages, specification: zero line zero connection protection shall not be connected to any switch and fuse.

3) the PE line must not be connected with the N line except in the main box, and the other parts are not allowed to connect the N line with the PE line. No switches and fuses are allowed on the PE line, and the connection must be firm.

Through the above analysis, the TN-C-S power supply system is a temporary adaptation on the TN-C system. When the three-phase power transformer works well and the three-phase load is balanced, the TN-C-S system is feasible in the practice of construction power consumption. However, in the unbalanced

three-phase load and the construction site has dedicated power transformers, TN-S mode power supply system must be adopted.

(6) the IT mode power supply system I indicates that the power supply side has no work ground or is connected to the ground through high impedance. The second letter T represents the load side electrical equipment for ground protection, as shown in figure 1-6.

The IT mode power supply system has high reliability and good security when the power supply distance is not very long. It is generally used in places where power cuts are not permitted, or places requiring strict continuous power supply, such as continuous production units, operation room in large hospitals,

underground mines and so on. The power supply condition in underground mine is poor, and the cable is easy to damp. The use of IT power supply system, even if the power supply is neutral, once the equipment leakage, single phase leakage current is still small, will not damage the balance of power supply voltage, so the system grounding power than the neutral point is safe.

However, if the power supply distance is very long, the distribution

capacitance of the power line to the earth can not be ignored. As can be seen from Fig. 1-6, when the load is short circuited or the electric leakage causes the equipment case to be electrified, the leakage current forms a loop through the earth, and the protection device does not necessarily act, which is dangerous. It is safer only when the power supply is not too long. This kind of power supply is very rare on the construction site. Our company's 35KV, 10KV and 6KV systems adopt

this IT mode. The disadvantage of the IT mode is obvious. When the line is in single phase grounding, the remaining two relative voltage reaches the line voltage, and the overvoltage of the power equipment is very high. Last year the construction line three overhead construction units are broken, two power plant 6KV system connected with B, A, C two relative voltage increased 1.732 times, causing two catalytic 6KV voltage transformer due to overvoltage burned, the main fan trip, influence the normal production of the device.

Two) brief summary of power supply line symbols

1) the power supply (power) system ground relation is defined by the International Electrotechnical Commission (IEC) as the symbol for the mode of supply. If the T indicates neutral point direct grounding, I represents all live part insulation (ungrounded).

2) the second letters represent the relation of the metal part exposed by the electric device to the ground. If the T indicates that the device enclosure is grounded, it is not directly related to any other ground point in the system; the N indicates that the load is connected with zero protection.

3) third letters represent the combination of the work line and the protective line. If C means the work, the zero line and the protection line are one (we call zero ground one), for example TN-C; S indicated that the work zero line and the protection line are strictly separated, therefore PE line is called the special protection line, like TN-S.

In fact, I understand, to put it plainly, is the metric value - hop number similar to the distance vector routing protocol.

This definition of hops is the number of network segments needed to reach the destination network, i.e., how many routers do you need to pass through?. (more than 16 considered unreachable)

The router has two interfaces can reach you want to access the network, then it will give the metric routing table value that ignored traffic to the metric value of relatively small interface, because he believes that this path is the fastest.

PC similarly, PC actually has routing tables.

For example, if you have two network cards, you can reach the same

destination address, one bandwidth is 10M, and the other is 100M. Well, if you set the 10M NIC's interface hops to 2100M to 1, then the data will be forwarded from the 100M network card, because the metric of this network card is smaller. (on the premise that you are a WIN2000 or WinXP system), when this network card is connected to down, it will forward data from the 10M network card. Play the role of floating routing.

How do you come up with a new problem? Well, what do you mean by floating route? Do you understand?.

If the number of hops for the two cards is the same, then I would expect a load

balancing.

The proposal is set to automatic, allowing the system to help you select.