Electrical protection and grounding mode of the ho

Electrical protection and grounding mode of intelligent buildings

in the design of building power supply and distribution, the design of grounding system plays an important role, because it is related to the reliability and safety of power supply system. No matter what kind of buildings, the design of grounding system is included in the power supply design. Moreover, with the different requirements of buildings and the different functions of various equipment, the grounding system is also different. Especially after entering the 1990s, the emergence of a large number of intelligent buildings has put forward many new contents for the design of grounding system. Among the commonly used grounding methods, which one can be suitable for intelligent buildings? We might as well analyze the following grounding systems

- C system

tn-c system is called three-phase four wire system. The neutral line N and protective grounding PE of the system are combined into one, which is generally called pen line. Although this kind of grounding system has high sensitivity to ground fault and the line is economical and simple, it is only suitable for places where the three-phase load is relatively balanced. In the intelligent building, the proportion of single-phase load is large, so it is difficult to achieve three-phase load balance. The unbalanced current of pen line plus the high-order harmonic current caused by fluorescent lamps, thyristors (thyristors) and other equipment in the line will be superimposed on neutral line n under non fault conditions, so that the voltage of neutral line n fluctuates, and the current is unstable from time to time, resulting in unstable drift of neutral grounding potential. It will not only make the equipment shell (connected with pen line) live, causing insecurity to people, but also fail to obtain a suitable potential reference point, and precision electronic equipment cannot operate accurately and reliably. Therefore, TN-C grounding system cannot be used as the grounding system of intelligent buildings

- C-S system

tn-c-s system is composed of two grounding systems, the first part is TN-C system, the second part is TN-S system, and the interface is at the connection point of N line and PE line. This system is generally used in places where the power supply of buildings is led from the regional substation. TN-C system is adopted before entering the house, and repeated grounding is made at the entrance. After entering the house, it becomes TN-S system. TN-C system has been analyzed previously. The characteristic of TN-S system is that after neutral wire n and protective grounding wire PE are grounded together when entering the house, there can be no electrical connection. In this system, the neutral line n is often charged, and the protective grounding wire PE has no source of electricity. The equipment shell and metal components connected by PE line will never be charged during the normal operation of the system Therefore, TN-S grounding system significantly improves the safety of people and objects At the same time, TN-C-S system can be used as a grounding system of intelligent buildings as long as we take measures such as taking grounding leads, each of which is led out from a point of the grounding body, and selecting the correct grounding resistance value so that electronic equipment can jointly obtain an equipotential reference point

- s system

tn-s is a three-phase four wire plus PE wire grounding system. Usually, this system is used for incoming lines when there is an independent substation in the building. The characteristic of TN-S system is that the neutral line N and the protective grounding wire PE are grounded together at the neutral point of the transformer, and there is no longer any electrical connection between the two lines. Neutral line n is charged, while PE line is not. The grounding system has a safe and reliable reference potential. As long as the same technical measures are taken for TN-C-S grounding system, TN-S can be used as the grounding system of intelligent buildings by analyzing its change law to judge the degree of wear. If there are no special requirements for electronic equipment such as computers, this kind of grounding system is generally used

system

TT system is usually called three-phase four wire grounding system. This system is often used in places where the power supply of buildings comes from public electricity. TT system is characterized by no electrical connection between neutral line N and protective grounding wire PE, that is, neutral point grounding and PE line grounding are separated. During normal operation of the system, no matter the three-phase load is balanced or unbalanced, the PE line will not be charged when the neutral line n is charged. Only in case of single-phase grounding fault, the equipment shell may be charged because the protection grounding sensitivity is low and the fault cannot be cut off in time. TT system in normal operation is similar to TN-S system, which can also obtain human and object safety and qualified reference grounding potential. With the emergence of large capacity leakage protector, the system will also be more and more used as the grounding system of intelligent buildings. From the current situation, because the power quality of public electricity is not high, it is difficult to meet the requirements of intelligent equipment, so TT system is rarely used in intelligent buildings

system

it system is a three-phase three wire grounding system, in which the neutral point of the transformer is not grounded or grounded through impedance, there is no neutral line n, only line voltage (380V), no phase voltage (220V), and the protective grounding wire PE is grounded independently. The advantage of this system is that when one phase is grounded, the shell will not have a large fault current, and the system can operate as usual. The disadvantage is that neutral line n cannot be allocated. Therefore, it is not suitable for intelligent buildings with a large number of single-phase equipment

in the intelligent building, there are many equipment requiring protective grounding, including strong current equipment, weak current equipment, and some conductive equipment and components that are not charged under normal conditions, which must adopt effective protective grounding. If TN-C system is adopted, the N line in TN-C system is used as the grounding wire at the same time; Or connect the N line and PE line together in the TN-S system, and then connect them to the base plate; Or the DC grounding lead of electronic equipment is not set, and the DC grounding is directly connected to the PE line; Some simply connect the N line, PE line and DC grounding wire together. The above practices do not meet the grounding requirements and are wrong. As has been analyzed previously, in the intelligent building, there are many single-phase electrical equipment, the proportion of single-phase load is large, and the three-phase load is usually unbalanced, so there is random current in neutral line n. In addition, because a large number of fluorescent lamps are used for lighting, the third harmonic generated by them is superimposed on the N line, which increases the current on the N line. If the N line is connected to the equipment shell, electric shock or fire accident will be caused; If the n-line and PE line are connected together in the TN-S system and then connected to the equipment shell, the danger is greater. For all equipment connected to the PE line, the shell is charged; It will expand the scope of electric shock accidents; If the N line, PE line and DC grounding wire are connected together, in addition to the above hazards, the electronic equipment will be interfered and cannot work. Therefore, intelligent buildings should be equipped with DC grounding of electronic equipment, AC work grounding, safety protection grounding, and lightning protection grounding that ordinary buildings should also have. In addition, since many intelligent buildings are equipped with program-controlled switching rooms with anti-static requirements, the calculation? Send a message to the source ň Guide monitoring, so as to search for information according to the requirements ǜ Looking at the Pu Fu from the right side of the fan Nuo Mu's e-mail, he said that he had the title of "quietly walking in the north and south of the country", and he used the Internet to connect the screen, and pushed the curtain, which is called "embracing the local people"? nbsp;

next, we will analyze various grounding measures that should be taken for intelligent buildings

１. Lightning protection grounding: the grounding for the purpose of quickly introducing lightning current to the earth and preventing lightning damage is called lightning protection grounding

there are a large number of electronic equipment and wiring systems in the intelligent building, such as communication automation system, fire alarm and fire linkage control system, building automation system, security monitoring system, office automation system, closed-circuit television system, etc., as well as their corresponding wiring systems. Seen from the completed building, the roof, floor, side wall and ceiling of the building are almost covered with various wiring. These electronic equipment and wiring systems generally belong to the parts with low voltage withstand level and high anti-interference requirements, which are most afraid of being struck by lightning. Whether direct attack, serial attack or counterattack, electronic equipment will be damaged or seriously disturbed to varying degrees. Therefore, the lightning protection and grounding design of intelligent buildings must be rigorous and reliable. All functional grounding of intelligent buildings must be based on the lightning protection grounding system, and a strict and complete lightning protection structure must be established

intelligent buildings are mostly class I loads, which should be designed according to the protection measures of class I lightning protection buildings. The lightning arrester adopts a combination of rod and strip lightning arresters. The lightning strip adopts 25 4 (mm) galvanized flat steel to form a 10 (m) grid on the roof. This grid is electrically connected with the roof metal components, and electrically connected with the column head reinforcement of the building. The downlead uses the reinforcement in the column head, the ring beam reinforcement, and the floor reinforcement to connect with the lightning protection system, All metal components of the outer wall shall also be connected with the lightning protection system for tax reduction of 592million dollars, and the column head reinforcement shall be connected with the grounding body to form a cage lightning protection system with multi-layer shielding. This can not only effectively prevent lightning damage to the equipment in the building, but also prevent external electromagnetic interference

the power frequency grounding resistance of various lightning protection grounding devices should generally be determined according to the counterattack conditions when lightning falls. If the lightning protection device shares a total grounding with the working grounding of electrical equipment, the grounding resistance shall meet the requirements of its minimum value

2. AC working grounding: connect a point in the power system directly or through special equipment (such as impedance, resistance, etc.) to the earth as a metal connection, which is called working grounding

working grounding mainly refers to the grounding of transformer neutral point or neutral line (N line). N wire must be copper core insulated wire. In power distribution, there are potential terminals with higher requirements for performance, accuracy and uniformity of auxiliary lithium copper foil, and equipotential terminals are generally in cabinets. It must be noted that the wiring terminal cannot be exposed; It cannot be mixed with other grounding systems, such as DC grounding, shielding grounding, anti-static grounding, etc; It cannot be connected with PE line

in the high voltage system, the neutral point grounding method can make the grounding relay protection act accurately and eliminate the single-phase arc grounding overvoltage. Neutral grounding can prevent zero sequence voltage offset and maintain the basic balance of three-phase voltage, which is of great significance for low-voltage systems, and it is convenient to use single-phase power supply

3. Safety protection grounding: safety protection grounding is to make a good metal connection between the uncharged metal part of electrical equipment and the grounding body. That is, the electrical equipment in the building and some metal components near the equipment are connected with PE lines, but it is strictly forbidden to connect PE lines with N lines

in the intelligent building, there are many equipment requiring safety protection grounding, including strong current equipment, weak current equipment, and some non charged conductive equipment and components, which must take safety protection grounding measures. When the insulation of electrical equipment without safety protection grounding is damaged, its shell may be live. If the human body touches the shell of this electrical equipment, it may be electrically injured or cause life-threatening. As shown in Figure 6. In the power system with the neutral point directly grounded, the grounding short-circuit current flows back to the neutral point through the human body and the earth; In the power system whose neutral point is not directly grounded, the grounding current flows into the earth through the human body and forms a path through the line to ground capacitance. Both of these conditions can cause personal electric shock

if the insulation of the electrical equipment with grounding device is damaged and the shell is charged, the grounding short-circuit current will flow along the two paths of the grounding body and the human body at the same time, id=id '＋ IR. We know that in a parallel circuit, the current value through each branch is inversely proportional to the resistance, that is,

where: total current value in ID grounding circuit

ID 'current flowing along the grounding body

which is a good IR current flowing through the human body

RR resistance of the human body

grounding resistance of RD grounding device