Type and construction of electrical grounding
First, the type of building electrical grounding
1, lightning protection grounding
Lightning protection grounding, as the name suggests, is to introduce the electric lightning generated by the lightning of nature into the earth, in order to prevent strong current damage to electrical equipment. Due to the needs of modern office and management, modern architecture, electronic monitoring, elevators, computer networks, office automation appliances, and him
The power lines we need are all over the floor, and the pressure rating of these equipment is relatively low, and modern high-rise buildings are particularly prone to attract lightning in nature during thunderstorms. Powerful currents can cause irreparable damage through electrical equipment. There are breakdown capacitors, display tubes, etc.
Therefore, in accordance with the relevant provisions of the construction industry, in the process of installing electrical equipment, comprehensively estimate the lightning protection level, and design the corresponding lightning protection grounding for the electrical.
2, AC work ground
AC working grounding is a grounding measure that connects the neutral point in the appliance or circuit that uses AC power by connecting a resistor or directly connecting it. This kind of grounding is very common in household appliances. Generally, the large-scale household electrical, the electrician principle is to use the transformer neutral point in the electrical appliance or
The neutral wire is grounded. Generally speaking, the material of the neutral wire is a copper core insulated wire. At the same time, this method is also applied to high-voltage systems. For example, grounding the neutral point of the relay in the high-voltage system can eliminate the single-phase arc to maintain the three-phase voltage stability.
3, DC working ground
DC grounding is primarily designed for the normal operation of computers, electronic communications and signal conversion equipment within buildings. Due to the information input and output of the above electronic communication devices, the conversion of wireless signals into electrical signals, energy conversion, signal amplification, etc., are based on electron tubes or other needs.
Microcurrent device. DC working grounding is to provide these devices with an external DC power supply that can supply power accurately.
4, safety protection grounding
Safety protection grounding is a protective measure for connecting an electrically uncharged conductor to a grounding body. Safety protection grounding, generally used in weak electrical boxes, high-power electrical equipment. If this type of electrical is not properly grounded, it can be injured if it comes into contact with these electrical enclosures.
Second, the construction of 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 electric shocks, electrical fires
Electrical accidents 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 between the conductors, etc.
Potential junction. 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.
And make the potential equal or close to reduce electrical disasters. 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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