Construction electricity is involved in a wide range of areas and is highly professional. It is easy to cause electrical accidents and accidents when it is slightly inadvertent. Therefore, temporary power management is an important part of project safety management. Grounding and zero-connection protection are weak links in power management. Based on 8 years of experience in on-site construction, I will talk about the difference between protective grounding and protection zero in construction electricity.
1. Main problems of temporary power connection and grounding in construction
In order to prevent accidents of electric shock on the charged body, the construction site must take protective measures according to different situations. Protective grounding and protection to zero are the basic technical measures to prevent electric shock caused by accidental electrification of electrical equipment. Due to the vague concept of protection grounding and protection zeroing of individual site management personnel, the practice of grounding and zero-connection protection has caused many violations. The main performance is that zero-connection protection is not in place, imperfect, and grounding protection is abused and misused, especially It is to use protective ground instead of protection to zero, which greatly harms the safety of electricity.
2, the concept and role of protective grounding, protection and zeroing measures
In the temporary power system at the construction site, the protection effects of safety technical measures such as protective grounding, protection zero and repeated grounding are quite different.
The working grounding is in the TT or TN power supply system. The neutral point of the transformer is directly grounded. The grounding resistance should be less than 4Ω. In the working grounding, the system voltage is stabilized to prevent the high-voltage side power supply from entering the low-voltage side and avoiding the low-voltage system. The destruction of electrical equipment does not work properly. Generally, the construction site management personnel are not easy to contact the working ground. In the construction power management, the working grounding is only for general inspection. Repeated grounding can provide additional protection after protecting the neutral wire breakage, and can also reduce the ground resistance of the leakage device and shorten the fault duration. In a construction site power supply system, the setting of repeated grounding is not less than two, which are the beginning, the middle end and the end of the power supply line, and a set of repeated grounding should be done at the equipment concentration or high equipment.
The protective grounding is to connect the metal casing of the electrical equipment to the earth. The grounding resistance should be less than 4Ω. Its function is to protect the human body from leakage and electrical safety and prevent electric shock.
The protection zero is to connect the electrical equipment casing to the neutral line of the power grid (in the TN system, connected to the special protection neutral line). The principle is to change the fault of the electrical equipment into a single-phase short-circuit fault due to single-phase short-circuit. The current is very large, so it can quickly cut off the insurance or the automatic load switch trip, so that the equipment is disconnected from the power supply, so as to avoid the occurrence of electric shock. Here, it is quite important that the protection cut-off can be matched with the protection zero.
3. Comparison of protection grounding protection and zero matching
3.1 Effectiveness analysis of protection grounding in TT system
Under normal circumstances, in the TT system, the transformer neutral grounding resistance and protective grounding resistance are not more than 4Ω. For example, the human body resistance is 1700Ω. In the 380-220V power grid, when the phase line is short-circuited, the fault is calculated. The current is 27.5A, the voltage applied to the human body reaches 110V, and the current flowing through the human body reaches 65mA. The electric current flowing through the human body is still greater than the safe current, and the faulty power supply is only 27.5A. When the electrical equipment is greater than 1.5KW, it is not enough to make the circuit's overcurrent device (such as fuse, automatic switch release, etc.) fast. Action, the motor casing will be charged for a long time, which is very dangerous to the human body. Although it is theoretically not difficult to find a solution to the problem, such as reducing the protective earthing resistance to less than 0.78 Ω, the voltage applied to the human body can be reduced to a safe voltage of 36 V or less, but this will greatly increase the cost of the grounding device and Difficulty.
3.2 Analysis of the effectiveness of protection and zero connection
In the low-voltage power distribution system where the electrical equipment is directly grounded by the neutral point, after the protection is connected to zero, when the electrical equipment is faulty, the metal casing of the electrical equipment directly connects the phase line to the neutral line. The phase-to-earth fault becomes a single-phase short-circuit, because the zero-line impedance is small, and the short-circuit current can reach several times or even several times of the rated current of the powered device. Such a large short-circuit current can usually cause a fuse mounted on the line. Or other current protection devices act quickly to shut off the power. However, in the event of a shell failure, the voltage applied to the human body reaches more than 140 volts. Obviously, this voltage value is still dangerous to the human body, so the effectiveness of the protection zero is whether the line and short circuit protection device can be in the "shell". After a short-circuit fault occurs, it operates sensitively and quickly cuts off the power.
3.3 Economic comparison between protective grounding and protection zeroing
When the TT power supply system with protective grounding is used, the protective grounding resistance is required to be less than 4 Ω, that is, each device requires a certain amount of steel to enter the underground, costing materials, and when the TN power supply system with protection zero is used, the zero is laid. The line can be used repeatedly for many times, saving labor and materials, so the zero protection is economically reasonable. In the special power supply system where the electrical equipment is quite dispersed, the use of TT power supply (protective grounding) can reduce one power supply line and save money, which also has its rationality. However, in the construction site, it is necessary to enforce the special protection zero line. TN-S power supply system (three-phase five-wire system).
3.4 Fault Analysis of Electrical Fittings for Protective Earthing and Protection and Zero Connection
In the same system, the protection grounding and protection zero can not be mixed, that is, some equipments are protected by zero, while others are protected by grounding. It can be seen from the calculation that after adopting this hybrid method, not only the protective grounding shell housing has a dangerous voltage of 110V, but in the case that the line protection device is not operated, the equipment casing will be charged for a long time, which in turn expands the understanding of the electric hazard range. .
4, the conclusion
From the above analysis, it can be seen that the protection zero and the protective grounding are two important safety technical measures for the construction power supply system. There are also obvious differences between the two, which must be strictly distinguished:
(1) In the power supply system where the neutral point of the transformer is grounded, the protection zero is more secure and better economical than the protective ground. Therefore, in the temporary power supply system for construction, the protection must be strictly adopted. The TN-S power supply system should not use a TT power supply system with protective grounding.
(2) At the construction site, it is forbidden to use protective grounding instead of protective grounding for some equipments, resulting in protective grounding and protection zero-mixing and misconnection; if the equipment is protected and connected to zero, the equipment is grounded again. This practice is equivalent to repeated grounding, and its necessity depends on the specific situation.
(3) Protection and zero connection must be sensitive and reliable short-circuit protection device to cooperate. Therefore, it is strictly forbidden to replace the metal fuse that meets the requirements with metal materials such as copper wire, otherwise the protection zero will lose the protection effect.
(4) The wire diameter and thread color of the protection neutral wire must be selected strictly according to the requirements, and a good and reliable line connection must be made. It is forbidden to install a fuse or a separate current-cutting switch on the protection zero line, and repeat the grounding as required. The effectiveness of protection against zeroing.
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