接电系统（Power supply system）.docVIP

接电系统（Power supply system） (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 said that the neutral point of power system is directly connected to the ground; second T mark indicates that the load device is not connected with the charged body exposed metal conductive part is directly connected with the earth, and how the system grounding. In the TT system, all the ground of the load is called a protective ground, as shown in figure 1-1. The features of this power supply system are as follows. 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 earths 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 it is difficult to popularize the TT system. 3) TT system grounding device of steel consumption, and difficult recovery, labor and material costs. Now some construction units are using TT system, construction units borrow their power for temporary use of electricity, the application of a special protective line, in order to reduce the need for grounding equipment steel consumption. Separate the special protection line of the newly added PE line and zero line N work,