Neutral or grounding protection of the hottest con

Neutral or grounding protection of construction power

Abstract: according to the height of transformer and non transformer (sharing the same power supply system with external power lines), the protection systems used for temporary power consumption at construction sites such as buildings and other construction sites include TN-S system, TN-C-S system and TT system. For places where power failure is not allowed, or where it systems are strictly required to supply power in the frontier, when used as temporary power supply for construction, protective grounding shall be made according to local requirements (less it systems are used). This conforms to the code for safety of power supply on construction site (GB). When the construction site uses the original power supply system, the electrical equipment should be able to effectively avoid decolorization as protective neutral or protective grounding for a long time according to the requirements of the original power supply system. It is also clearly required by various professional specifications

key words: construction electricity neutral grounding protection Untitled Document in the construction project is 25 times stronger than ordinary glass fiber materials. According to the technical code for safety of temporary electricity on construction site (JGJ), the temporary electricity project on the construction site should adopt a three-phase four wire power supply and distribution system with special protective neutral line and direct grounding of power supply neutral point. Namely "three-phase five wire system". A premise here is the three-phase four wire power supply and distribution system with the neutral point of the power supply directly grounded at the construction site (with a special transformer). According to "impact testing machine for safety of power supply and consumption on construction site of construction projects" (English Name: impacttesting machine) refers to the specification for applying impact test force to samples "(GB), it is not suitable to use protective neutral or protective grounding system at the same time in the same power supply system. If it is directly connected from the public transformer, the whole construction site adopts protective neutral line The three-phase four wire power supply and distribution system with the neutral point of the power supply directly grounded is still possible

at the same time, the specification stipulates that when the construction site shares the same power supply system with external power lines, the electrical equipment should be protected to neutral or grounded according to local requirements. It is not allowed to make protective neutral connection for some equipment and protective grounding for other equipment. Therefore, according to the source of power supply at the construction site, the construction power protection is connected to neutral or protective grounding

I. when there is a special transformer on the construction site (directly connected from the public transformer and allowed locally), or the original power system is TN-S system. Follow TN-S system. As shown in figure 1:

note: the line is led out from the working grounding wire

2 The working grounding resistance is not more than 4 Ω, and the repeated grounding rc1/rc2/rc3 is not less than three places, and the grounding resistance of each place is not more than 10 Ω

3. M represents motor, XD represents electric welding machine, HD represents illuminator

II. When there is no special transformer on the construction site, it is often encountered in the construction of factories and mining enterprises or in the field. Using the original power supply system can not meet the requirements of TN-S power protection system, because scattered residential or rural users mostly use TT system. TT system, TN-C system, TN-S system and it system adopted by factories and mining enterprises according to the presence or absence of transformers. In this case, it should be handled separately

1. If the original TN-C system (TN-C system is few, suitable for factories and mining enterprises with independent transformers and maintained by electrical professionals), and the construction specification stipulates that the construction site must adopt TN-S mode power supply system, PE lines can be separated from some on-site general distribution boxes behind the system to form a TN-C-S power supply and distribution system. As shown in Figure 2

note: the line is led out from the zero line of the general distribution box or the zero line on the power side of the first level leakage protector at point D

2. The working grounding resistance shall not be greater than 4 Ω, and the repeated grounding at point D and after point D, rc1/rc2/rc3 shall not be less than three places, and the grounding resistance at each place shall not be greater than 10 Ω

3. M represents motor, XD represents electric welding machine, HD represents illuminator

tn-c-s system features: the working zero line n is connected with the special protection line PE. As shown in Figure 2, when the unbalanced current of the line in the front section of point D is relatively large, the zero connection protection of electrical equipment is affected by the zero line potential. There is no current from point d to the rear PE line, that is, there is no voltage drop on this section of wire. Therefore, TN-C-S system can reduce the voltage of the motor shell to the ground, but it cannot completely eliminate this voltage. The size of this voltage depends on the load imbalance of Nd line and the length of Nd line. The more unbalanced the load is and the longer the nd line is, the greater the voltage offset of the equipment shell to the ground is. Therefore, it is required that the load unbalanced current should not be too large, and the PE line should be repeatedly grounded, as shown in the figure

tn-c-s power supply system is a temporary alternative to TN-C system. When the working grounding condition of three-phase power transformer is good and the three-phase load is relatively balanced, the effect of TN-C-S system in construction power consumption practice is still feasible

2. If the original power supply system adopts TT system, when the construction unit supplies its power supply for temporary power use, because the original grounding protection is adopted, the first wiring type is generally shown in Figure 3

note: the protective grounding wire should be more than two conductors, which are firmly connected with the exposed conductive parts of the equipment

2. Working grounding