Construction technology for electrical installation grounding
1, two ways of grounding
(1) Connected to the earth
Usually, the human body cannot be separated from the earth, so various electrical systems used by people use the potential of the earth as a reference potential. In order to obtain the earth potential, it is necessary to connect with the earth. For this purpose, the grounding pole is used as a terminal connected to the earth, and the ground potential is taken through the conduction of the grounding wire. This is our commonly used connection.
Ground.
(2) Connected to the conductor instead of the earth
With the advancement of electrical technology, the current grounding has given a new meaning, that is, the connection with the conductor instead of the earth is also grounded. For example, the tire of the automobile is an insulating material, so the grounding of various electrical systems on the automobile is also the metal that replaces the earth. Bodywork. Many electrical accidents such as electric shocks and electrical fires that occur in electricity use are caused by excessive potential differences. For example, the electrical safety on an airplane is based on the replacement of the ground with the fuselage, and the low-impedance equipotential bonding is supplemented by other safety measures. Therefore, the connection to the conductor instead of the earth to achieve grounding is actually a low-impedance equipotential bonding with the conductor. In the same way, the grounding of the grounding pole is the equipotential bonding with the earth, but the grounding resistance of the grounding pole is measured in Ω, the value is very large, the potential difference caused is also large, and the equipotential effect is not ideal.
2. Equipotential bonding reduces the requirement for grounding resistance
An equipotential environment similar to that in an aircraft can also be created in a building on the ground to increase the level of electrical safety. That is the equipotential bonding in the building, that is, the exposed conductive part of the electrical device and the conductive part (metal structure, pipe, etc.) outside the electrical device are connected in the building to make the potential equal or close to reduce the electrical disaster. Equipotential bonding is a basic electrical safety measure that must be implemented in a building electrical installation. Therefore, due to the action of the equipotential, in many cases, it is not necessary for the international electrical standard to specify the resistance requirement of the grounding resistance.
It should be noted that the above is the protection grounding of the exposed conductive part of the electrical equipment of the low-voltage power distribution system. For the system grounding of the neutral point of the low-voltage side of the substation, China's regulations have been too large, such as 4Ω to 10Ω, which is not safe enough, but it has not been put forward. Need to know
In terms of electrical safety, the smaller the grounding resistance of the system grounding, the safer it is to use electricity. Therefore, the international electrical standards have requirements for calculation formulas.
3. Discussion on the requirement that the grounding resistance of the common grounding is not more than 1Ω.
At present, there are often multiple electrical systems in a building. To avoid electrical accidents such as personal electric shock caused by potential differences between different system grounding devices, the International Electrotechnical Standard stipulates that only one common grounding device can be installed in a building. This requirement has been basically for our electrical professionals.
Accepted. However, in some of China's specifications, the grounding resistance of the common grounding is not more than 1Ω, but the description and basis are not seen. This is a problem worth discussing. In order to reduce the potential difference, we hope that the impedance of the grounding and equipotential bonding Z=(R2 X2) 1/2 is as small as possible. At 50 Hz
The influence of the reactance X in the gas device is negligible, and the resistance of the PE wire is less than the grounding resistance, so people often only specify the requirements for the grounding resistance value. Now that we have entered the information age, the operating frequency of information technology equipment is measured in MHz, so that not only the influence of the resistance R but also the impedance R needs to be considered in the impedance Z.
The effect of reactance X under high frequency action needs to be considered.
Therefore, the international electrical standards do not stipulate the grounding resistance of the information technology device to ensure the normal operation of the information equipment, but stipulate many measures to minimize the high frequency coupling impedance in the equipotential bonding system.
4. Protection technology for building electrical grounding
(1) Lightning protection grounding
There are a large number of electronic equipment in the 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 respective wiring systems. From the completed building, the tops of the building
The plates, bottom plates, side walls, and ceilings are almost covered with various wires. These electronic devices and wiring systems generally belong to the parts with low withstand voltage, high anti-interference requirements, and the most fear of lightning strikes. Whether it is direct, string, or counterattack, the electronic device may be damaged or seriously interfered to varying degrees. Therefore, right
The lightning protection grounding design of buildings must be strict and reliable. All functional grounding of building buildings must be based on lightning protection grounding systems and establish a strict and complete lightning protection structure.
(2) AC working ground
The working ground mainly refers to the transformer neutral point or neutral line (N line grounding) must use copper core insulated wire. There are auxiliary equipotential terminals in the power distribution, and the equipotential terminals are generally in the cabinet. It must be noted that this terminal must not be exposed and cannot be connected to other grounding systems such as DC.
Ground, shield grounding, anti-static grounding, etc., can not be connected to the PE line.
In the high-voltage system, the neutral grounding method can make the grounding relay protection operate accurately and eliminate the single-phase arc grounding overvoltage. Neutral grounding prevents zero-sequence voltage offset and maintains a three-phase voltage balance, which makes sense for low-voltage systems and makes it easy to use single-phase power supplies.
(3) Safety protection grounding
In the building, there are many equipments that require safety protection and grounding. Strong electrical equipment, weak electrical equipment, and some non-charged conductive equipment and components must be secured and grounded. When the insulation of the electrical equipment that is not grounded by the safety protection is damaged, the outer casing may be charged. If the human body touches the outer casing of the electrical equipment, it may be electrically injured or life-threatening. In a power system where the neutral point is directly grounded, the grounding short-circuit current flows back to the neutral point through the human body and the earth; in the power system where the neutral point is not directly grounded, the grounding current flows into the earth through the human body and is formed by the line-to-ground capacitance. Pathways, both of which can cause electric shocks.
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