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.
tn-s tn-c-s tn-c tt it等接地系统的区别
