Views:5 Author:Site Editor Publish Time: 2021-08-26 Origin:Site
How much do you know about electrical grounding?
1. What is grounding?
Grounding In order to prevent electric shock or protect the safety of equipment, connect the metal chassis or shell of electrical and telecommunications equipment to the ground wire; use the earth as the current loop ground wire. In the power system, a good electrical connection between the neutral point of the equipment and the electrical device, the shell or the support and the conductor of the grounding device is called grounding.
2. The role of grounding
We often only know that grounding can prevent personal shocks. In fact, in addition to this function, grounding can also prevent damage to equipment and lines, prevent fires, prevent lightning strikes, prevent electrostatic damage, and ensure the normal operation of the power system.
2.1 Prevent electric shock
The impedance of the human body has a great relationship with the conditions of the environment. The more humid the environment, the lower the impedance of the human body, and the more vulnerable it is to electric shock. For example, almost all people who have installed AC radios have received electric shocks, but almost all of them can get rid of the power source because the environment is dry and the skin is also dry. Grounding is an effective way to prevent electric shock. After the electrical equipment is grounded through the grounding device, the potential of the electrical equipment is close to the ground potential. Due to the existence of grounding resistance, the potential of electrical equipment to ground always exists. The greater the grounding resistance of electrical equipment, the greater the potential of electrical equipment to ground when a fault occurs, and the greater the risk of being touched in time. However, if the grounding device is not provided, the voltage of the faulty equipment casing is the same as the phase-to-ground voltage, which is still much higher than the grounding voltage, so the risk is correspondingly increased.
2.2 Ensure the normal operation of the power system
The grounding of the power system, also known as working grounding, is generally grounded at the neutral point in the substation or substation. The grounding resistance of the working ground is very small, and a grounding grid is required for large-scale substations to ensure that the grounding resistance is small and reliable. The purpose of working grounding is to make the potential between the neutral point of the power grid and the ground close to zero. The low-voltage power distribution system cannot avoid the phase line touching the shell or touching the ground after the phase line breaks. If the neutral point is insulated from the ground, the ground voltage of the other two phases will rise to 3 times the phase voltage, which may result in work Electrical equipment with a voltage of 220 burns out. For a system with a neutral point grounded, even if one phase is short-circuited to the ground, the other two phases can still be close to the phase voltage, so the electrical equipment connected to the other two phases will not be damaged. In addition, it can prevent the system from oscillating. As long as the insulation level of electrical equipment and lines is considered according to the phase voltage
2.3 Prevent lightning strikes and static electricity hazards
When lightning occurs, in addition to direct lightning, induction lightning is also produced. Inductive lightning is divided into electrostatic induction lightning and electromagnetic induction lightning. The most important method of all lightning protection measures is grounding.
3. Grounding type
The common grounding types are as follows:
Repeated grounding, protective grounding, working grounding, lightning protection grounding, shielding grounding, anti-static grounding, etc.
3.1 Repeated grounding
Repeated grounding means that in a system where the neutral point is directly grounded, a metal wire is used to connect the grounding device at one or more places on the neutral main line.
In the low-voltage three-phase four-wire neutral point directly grounded line, the construction unit should ground the terminal of the zero trunk line and branch line of the distribution line during installation, and ground the zero trunk line every 1 km.
For power distribution lines that are more than 50 meters away from the grounding point, the neutral line connected to the user should still be grounded repeatedly, and the repeated grounding resistance should not be greater than 10 ohms.
3.2 Protective grounding
The metal shell and metal bracket of the electrical equipment that are not charged under normal conditions are electrically connected to the earth, which is called protective grounding. Protective grounding is mainly used in power supply systems where the neutral point is not grounded.
If the protective grounding measures are not used, when the human body touches the live shell, a loop is formed due to the distributed capacitance between the transmission line and the ground, so that the human body has an electric current and an electric shock accident occurs.
If the electrical equipment adopts protective grounding measures, when the human body touches the live shell, the human body is connected in parallel with the resistance of the protective grounding device. Since the ground resistance is smaller than the resistance of the human body, it can be considered that the current passing through the human body is very small, and the current hardly passes through the human body, thus avoiding electric shock accidents.
3.3 Working ground
The schematic ground of the grounding grid is to make the system and the connected instruments operate reliably and to ensure the accuracy of measurement and control. It is divided into machine logic ground, signal loop ground, shield ground, and intrinsically safe ground in petrochemical and other explosion-proof systems.
3.4 Lightning protection grounding
Lightning protection grounding is a part of lightning protection measures, and its function is to introduce lightning current into the earth.
The lightning protection of buildings and electrical equipment mainly uses lightning arresters (including lightning rods, lightning belts, lightning nets and lightning arresters, etc.).
One end of the arrester is connected to the protected equipment, and the other end is connected to the ground device. When a direct lightning strike occurs, the lightning arrester will lead the lightning to itself, and the lightning current will enter the earth through its down conductor and grounding device.
In addition, due to the side effect of electrostatic induction caused by lightning, in order to prevent indirect damage, such as house fire or electric shock, metal equipment, metal pipes and steel structures in the building are usually grounded; lightning waves will follow low-voltage overhead lines, The TV antenna invades the house, causing the insulation breakdown of the electrical equipment in the house, which may cause a fire or personal electric shock. Therefore, the iron feet of the insulating porcelain bottle on the line and before entering the house must be grounded.
3.5 Shield ground
It is an effective measure to eliminate the harm of electromagnetic field to the human body, and it is also an effective measure to prevent electromagnetic interference. High-frequency technology has been widely used in electric heating, medical treatment, radio broadcasting, communications, television stations and navigation, radar, etc. Under the action of the electromagnetic field, the human body will have biological effects on the absorbed radiant energy, causing harm to the human body, such as slight tremor of fingers, skin scratches, loss of vision, etc. Setting a shielding device for the equipment shell that generates a magnetic field and grounding the shielding body can not only reduce the intensity of the electromagnetic field outside the shielding body, and achieve the purpose of reducing or eliminating the harm of the electromagnetic field to the human body, but also protect the equipment in the shielding grounding body from the external electromagnetic field. Interference influence.
3.6 Anti-static grounding
In order to prevent the harmful effects of static electricity and discharge it, it is the most important part of static electricity protection.
4. Grounding device
The grounding device is composed of a grounding body and a grounding wire.
4.1 Grounding wire
Due to poor insulation performance or humid use environment of household appliances, the shell will be charged with static electricity, and electric shock accidents may occur in serious cases. In order to avoid accidents, a wire can be connected to the metal shell of the electrical appliance, and the other end of the wire can be connected to the ground. Once the electrical leakage occurs, the grounding wire will bring static electricity into the ground and release it. In addition, it is more important for electrical maintenance personnel to use electric soldering iron to solder the circuit, sometimes because the electric soldering iron is charged and break down and damage the integrated circuit in the electrical appliance. Friends who use computers sometimes ignore the grounding of the mainframe. In fact, connecting a ground wire to the mainframe of the computer can prevent the occurrence of crashes to a certain extent. Grounding wire in the power system: It is an important tool to ensure the staff when there is unexpected voltage on the equipment and lines that have been powered off. According to the regulations issued by the Ministry, the grounding wire must be made of bare copper flexible wire over 25mm2. In electrical appliances, the grounding wire is a line that is connected to the housing of electrical equipment and other parts to timely lead out unsafe electric charges or leakage currents generated due to various reasons.
4.2 Grounding body
A conductor or a group of conductors that are in close contact with the earth's soil and provide electrical connection with the earth.
5. Grounding method
The modern power system itself is a strong source of electromagnetic interference, which mainly interferes with communication equipment in this frequency band through radiation. In order to suppress the interference effects of external high-voltage transmission lines, grounding measures are adopted. There are two commonly used grounding methods, which are discussed as follows:
5.1 Decentralized grounding method
The grounding of the shell grounding circuit is to connect the lightning protection grounding of the communication building, the grounding of the power system, the grounding of various communication equipment and the grounding of other equipment to separate grounding systems. Due to the increasing number of grounding systems, there is a potential coupling between the grounding wires. The impact is often difficult to avoid, and scattered grounding can easily cause interference. At the same time, the height of the main building continues to increase, and the unsafe factors brought by its grounding method are also increasing. When a facility is struck by lightning, it is easy to form an underground counterattack and damage other equipment.
5.2 Joint grounding method
The joint grounding method is also called the single-point grounding method, that is, all grounding systems share a common "ground". Joint grounding has the following characteristics:
(1) The grounding system of the entire building forms a cage-type equalizer. For direct lightning strikes, the points on the same floor in the building are relatively uniform; for induced lightning, the cage-type equalizer and the frame structure of the building will also interfere with external electromagnetic fields. Can provide 10-40dB shielding effect;
(2) The grounding resistance of the general joint grounding method is very small, and there is no coupling effect between various grounding bodies, which is beneficial to reduce interference;
(3) The grounding mine in dry weather saves metal materials and occupies less land.
It is not difficult to see from the above that the joint grounding method can effectively suppress the interference of external high-voltage transmission lines.
The ground wire for anti-static grounding should be connected in series with a 1 MΩ current-limiting resistor, that is, connected to the grounding device through the current-limiting resistor. Isn't the grounding resistance as small as possible? Why do you need a series of resistors?
Computer grounding aims at the easy flow of ground current, and the smaller the grounding resistance, the better. The grounding of the calculation center should minimize potential fluctuations caused by noise. At the same time, it should be noted that the signal circuit and power circuit, high-level circuit and low-level circuit cannot use the same common ground loop. For network wiring that requires high transmission bandwidth, isolated shielding and grounding should be used to prevent interference from electrostatic induction. Strive to be simple, economical and effective in design. If grounding can be effectively combined with shielding, it will be able to better solve interference and suppress noise.
6. Grounding wire connection method
The DC ground connection can be divided into three types: series grounding, parallel grounding and mesh grounding.
6.1 Series grounding
The DC ground wire of the equipment in the computer room is connected to the copper skin of the DC ground in series. Although this connection method has different potentials at individual points, it is often used in simple grounding systems because of its very small resistance. The disadvantage is that when a higher configuration is required, from the point of view of noise prevention, due to the series grounding, the resistance of each series makes the potential of each point deviate, and it is easy to generate noise.
6.2 Parallel grounding
In this method, the ground potential of each circuit is only related to the ground current and ground wire impedance of the circuit. The potential difference between the points is relatively balanced, and a better low-frequency grounding can be obtained, so it is used more frequently. Due to the low DC voltage of the computer, the ground current between the racks is not easy to form a coupling, but this connection method requires a lot of ground wires, and the wiring is complicated.
6.3 Reticulated ground
In large computer rooms, the ground requirements are relatively strict. At present, mesh ground wires are widely used as DC grounds, which are called mesh grounds. The DC mesh ground uses a copper strip of a certain cross-sectional area to be arranged in a 600mm×600mm grid under the raised floor. The intersections and the supporting points of the raised floor are staggered. The feet are welded or crimped on the raised floor. Together.
In order to insulate the DC mesh ground from the earth, insulating materials such as 2 to 3mm thick insulating rubber or polyvinyl chloride board should be placed under the copper tape, and the ground resistance is required to be above 10MΩ. The DC mesh ground system not only helps to better ensure the consistency of the logic circuit potential reference point, but also greatly improves the internal and external anti-interference ability of the machine. However, the network system is relatively large, complex in construction, and expensive, so it is only suitable for application in large computer rooms.
7. Need to pay attention to the problem when dealing with the ground wire of the common ground
(1) Grounding resistance-the grounding resistance of the shared grounding pile should meet the minimum grounding resistance requirements of various grounding.
(2) In order to prevent the mutual interference of the grounding system and ensure the insulation of the building, a shielded wire with an insulating sheath should be used for the grounding bus, and one end of the shielding sleeve should be grounded.
(3) Although the DC ground, AC ground and safety ground are finally connected to the ground piles, it does not mean that various grounds can be connected at will. They should also be connected to each other before they are connected to the same ground pile according to the above requirements. Maintain strict insulation.
(4) In the computer equipment where the DC ground and the chassis are safely grounded separately, because the DC ground is strictly insulated from the rack, each is connected to the system ground stakes. However, the chassis and the DC ground of some computers are connected to the electrical appliances. At the same time, the working place of the AC equipment is strictly insulated from the chassis.
8. Principles of electrical equipment grounding technology
(1) In order to ensure the safety of persons and equipment, various electrical equipment should be grounded for protection in accordance with the national standard GB14050 "Forms of System Grounding and Safety Technical Requirements". The protective grounding wire should not be used for other purposes except to achieve the specified work grounding or protective grounding requirements.
(2) In addition to special requirements for electrical equipment with different uses and voltages, a general grounding body should generally be used. According to the equipotential connection requirements, the metal components of the building and the metal pipes (transporting flammable and explosive materials) Except for metal pipes) connected to the general grounding body.
(3) The artificial general grounding body should not be installed in the building. The grounding resistance of the general grounding body should meet the minimum grounding resistance requirements among various grounding.
9. Technical requirements for grounding devices
9.1 Grounding of electrical equipment in substation (distribution)
(1) The grounding body of the grounding device of the substation (distribution) should be laid horizontally. The grounding body is made of round steel with a length of 2.5m and a diameter of not less than 12mm, or angle steel with a thickness of not less than 4mm, or a steel pipe with a thickness of not less than 4mm, and connected by flat steel with a cross-section of not less than 25mm×4mm to form a closed ring. The corners should be curved.
(2) The grounding body should be buried outside the wall of the substation (distribution) with a distance of no less than 3m. The buried depth of the grounding grid should exceed the thickness of the local frozen soil layer, and the minimum buried depth should not be less than 0.6m.
(3) The working grounding and protective grounding of the main transformer of the substation (distribution) shall be connected to the artificial grounding grid respectively.
(4) The lightning rod (line) should be equipped with an independent grounding device.
9.2 Protective grounding of electrical equipment in flammable and explosive places
(1) Electrical equipment, mechanical equipment, metal pipes and metal structures of buildings in flammable and explosive places should be grounded, and jumper wires should be laid at the pipe joints.
(2) In the neutral grounded line below 1kV, when the line overcurrent protection is a fuse, the safety factor of the protection device is not less than 4, and when it is a circuit breaker, the safety factor of the action is not less than 2.
(3) The connection points of the grounding trunk line and the grounding body shall not be less than two, and they shall be connected to the grounding body at both ends of the building respectively.
(4) In order to prevent accidents caused by sparks when measuring grounding resistance, the measurement should be carried out in a place where there is no risk of explosion, or the terminal button for measurement should be led to a place outside of flammable and explosive places.
9.3 Grounding of DC equipment
Due to the effect of DC current, the metal is corroded seriously and the contact resistance is increased. Therefore, when installing a grounding device on the DC line, the following measures must be carefully considered:
(1) For the grounding of DC equipment, the natural grounding body cannot be used as the PE wire or the grounding body and the grounding wire that are repeatedly grounded, and it cannot be connected to the natural grounding body.
(2) The thickness of the artificial grounding body of the DC system should not be less than 5mm, and the corrosion should be checked regularly.