Protective grounding of the power supply system of

Protective grounding of residential power supply system

Abstract: I have published about protective grounding of residential power supply, such as "what is TT, in, it system?" Wait for the manuscript. However, with the implementation of the mandatory standard "code for design of residential buildings" gb50096-1999 on June 1, 1999, D. the length, pneumatic units and display digits are implemented in a dynamic exchange manner, and the TN-C system that previously met the requirements of the code is no longer applicable to residential low-voltage distribution systems. Now there is a clear basis for the industrial system and installation of residential electrical design

key words: Untitled Document on protective grounding of residential power supply system I have published about protective grounding of residential power supply, such as "what is TT, in, it system?" Wait for the manuscript. However, with the implementation of the mandatory standard "code for design of residential buildings" (gb50096-1999) on June 1st, 1999, TN-C system, which previously met the requirements of the code, is no longer applicable to residential low-voltage distribution system. Now there is a clear basis for the design and installation of residential electricity. The new code is "people-centered", and puts forward quite strict requirements for residential safety on the premise of ensuring "applicability, safety, hygiene and beauty". The new specification clearly stipulates that houses should adopt the grounding form of low-voltage distribution systems such as it, TN-C-S or TN-S, and carry out total equipotential connection

the reasons why TN-C system should not be used are introduced below, as well as the grounding forms and fault prevention of three low-voltage distribution systems in the new specification

I. TN-C system

grounding of electrical equipment should not be used, which is generally divided into protective grounding and functional grounding. Protective grounding is divided into two forms: grounding and neutral grounding. The so-called "grounding" refers to the direct electrical connection of the exposed conductive part of the electrical equipment to the ground. Neutral connection refers to the direct electrical connection between the exposed conductive part and the grounding point of the power supply system through the protective wire (PE) or pen wire (in AC system, the grounding point is the neutral point)

tn-c system is called three-phase four wire system. The neutral line (n) and protection line (4) of the whole system are integrated, which is called four n lines (Figure 1). Because the TN-C system adopts protective neutral connection, that is, the exposed conductive part of the electrical equipment is well connected with the pen. In case of grounding fault of electric equipment, due to the small impedance of four n lines and large short-circuit current, the protection device acts quickly and has high response sensitivity. However, TN-C system needs to rely on the unbalanced current in the concave N line to maintain the balance of three-phase voltage, so TN-C system is generally suitable for the situation where the three-phase load is relatively balanced. At present, the vast majority of residential users are single-phase users, which is difficult to achieve the balance of three-phase loads. There will be a large and unstable unbalanced current flowing through the four n lines, and the high-order harmonics generated in the use of a large number of household appliances (such as fluorescent lamps) are also superimposed on the neutral line n, causing the neutral point grounding potential to shift. Once the pen line breaks down or the contact resistance of the pen line increases, the neutral point potential will seriously shift, making the exposed conductive metal shell of household appliances live, causing electric shock accidents. Moreover, grounding fault is most likely to cause electrical fire. Therefore, the new specification has clearly stipulated that TN-C system is no longer used for residential power supply

II. The power supply system adopted in the specification

1, TT system is also a three-phase four wire system, with one point directly grounded, and the system has no PE line. The protective (PE) wire of the exposed conductive part of the electrical equipment is connected to the grounding electrode (Figure 2) that is not directly related to the grounding point of the power system. TT system is characterized by no electrical connection between neutral point n and protective grounding wire PE, that is, N and PE lines are separated, which is suitable for residential buildings with public power supply. Generally, each residential building has a separate grounding electrode and PE line. Therefore, whether the three-phase load of the wooden pipe is balanced, whether the neutral line is charged, whether the PE line will not be charged, and the exposed conductive parts of the electrical equipment will not be charged, ensuring the safety of use. When single-phase grounding fault occurs to electrical equipment, because the sensitivity of single-phase short-circuit protection of TT system is lower than that of IW System (TT system takes the earth as the fault current path, and the grounding electricity of power supply and PE line is mainly related to wdw-yaw series automatic cement pressure test resistance, with small fault current), fuses and circuit breakers often cannot act immediately, resulting in the electrification of equipment shell. Therefore, leakage protector must be used to cut off the power supply in order to improve the sensitivity of TT system electric shock protection and make TT system more safe and reliable

2: TN-S system, also known as three-phase five wire system, is a three-phase four wire plus PE wire grounding system. The neutral line (n) and protective line (PE) of the whole system are separated (Figure 3), and the exposed conductive part of the electrical equipment is connected to the PE line. Generally, TN-S system is adopted when there is an independent substation in the residential building. Since the neutral line N and protective grounding wire PE of TN-S system are grounded together at the neutral point of the transformer, the two lines are separated from the low-voltage bus of the substation. As with TT system, no matter whether the neutral line n is charged or not, the PE line is not charged, and the equipment shell connected with the jumper is also not charged. Moreover, in TN-S system, in case of electrical fault, the four wire grounding current is large, and one circuit fuse and circuit breaker can act to cut off the power supply (high sensitivity). Therefore, TN-S grounding system significantly improves the safety of use. In the user's distribution box, the PE line is connected to the main grounding terminal board of the grounding strip

3.tn0-c-s system this system has a point of direct grounding. The exposed conductive part of the electrical equipment is connected to the grounding point through the protective wire. The neutral wire n of the first part of the line in the system is integrated with the protective wire PE, and the second part is the TN-S system, that is, the N and PE wires are separated (Fig. 4). When TN-C-S system is adopted, when protection 1 After the power line is connected and the neutral line is separated (usually at the entrance of the house), it can no longer be combined (the insulation level of neutral line n should be the same as that of phase line). Therefore, adopting TN-C-S system in residence actually becomes TN-S system. That is, after PNE line enters the user's distribution box, N terminal board and PE terminal board are set separately in the distribution box. When n and PE lines enter the house, they are separated from each other and there is no electrical connection

III. Prevention of grounding fault

grounding fault is different from general electrical short-circuit fault. It is a short-circuit fault between the live conductor and the earth through metal materials. Because the grounding fault is relatively hidden, it is often the cause of many fires, and often accompanied by grounding fault, electric shock and personal injury events occur. Therefore, for the safety of residents, it is necessary to strengthen the prevention of grounding fault

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