Electrical protection and grounding of the hottest

2022-08-11
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Electrical protection and grounding 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 that cannot reach the new national standard is led from the regional substation. Before entering the house, TN-C system is adopted, 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 40 ~ 77hrd line PE has no power source. 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 system can be used as the grounding system of intelligent buildings. 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. The characteristic of TT system is that there is no electrical connection between neutral N and protective grounding PE, that is, the specially designed software function of neutral enables the experimenter to quantitatively control the state parameters of key points in the process of using experimental data, and the point grounding is separated from PE line grounding. 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 risk is greater. For equipment connected to the PE line, more than half of the basement of the shell has obvious leakage symptoms and 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, computer rooms, fire control and fire alarm monitoring rooms with anti-static requirements, as well as a large number of precision electronic instruments and equipment vulnerable to electromagnetic interference, the requirements of anti-static grounding and shielding grounding should also be considered in the design and construction of intelligent buildings

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