Grounding In order to prevent electric shock or protect the safety of equipment, the metal chassis or shell of equipment such as power and telecommunications is connected to the ground wire; The earth is used as the current loop grounding wire. In the power system, the neutral point, shell or bracket of the device and the grounding device with a good electrical connection with the conductor is called grounding.
1 Grounding types---------- common grounding types are as follows:
Repeated grounding, protective grounding, working grounding, lightning protection grounding, shielding grounding, antistatic grounding, etc.
2 Repeat grounding
Repeated grounding is the connection of the grounding device with metal wires at one or more places of the zero trunk in a system where the neutral point is directly grounded.
In the low-voltage three-phase four-wire system neutral point direct grounding line, the construction unit should ground the terminal of the zero trunk line and the branch line of the distribution line when installing, and ground the zero trunk line every 1 kilometer on the zero trunk line.
For distribution lines that are more than 50 meters away from the connection point, the zero line at the access to the user should still be repeatedly grounded, and the repeated grounding resistance should not be greater than 10 ohms.
Protect the ground
Under normal circumstances, the metal shell and metal bracket of the electrical equipment are electrically connected to the earth, which is called protective grounding. Protective grounding is mainly used in power supply systems where neutral points are not grounded.
If the protective grounding measures are not adopted, then when the human body touches the live shell, the circuit is formed due to the distribution capacitance between the transmission line and the earth, so that the human body has an electric shock accident when the current passes through.
If the electrical equipment adopts protective grounding measures, then when the human body touches the live housing, the human body is connected in parallel with the resistance of the protective grounding device. Since the grounding resistance is smaller than the human body resistance, it can be considered that the current through the human body is very small at this time, and the current almost does not pass through the human body, avoiding electric shock accidents.
Work grounded
Grounding network schematic grounding is a grounding designed to enable the system and the instruments connected to it to operate reliably and to ensure measurement and control accuracy. It is divided into machine logic, signal loop grounding, shielding grounding, and intrinsically safe grounding in petrochemical and other explosion-proof systems.
Lightning protection grounding
Lightning protection grounding is part of the formation of lightning protection measures, and its role is to introduce lightning currents into the earth.
Lightning protection for buildings and electrical equipment is mainly made of lightning arresters (including lightning rods, lightning belts, lightning nets and lightning removal devices, etc.).
One end of the arrester is connected to the protected device and the other end is connected to the ground device. When a direct lightning strike occurs, the lightning arrester directs the lightning to itself, and the lightning current enters the earth through its downline 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, etc., it is usually necessary to ground the metal equipment, metal pipes and steel structures in the building; lightning waves will invade the house along the low-voltage overhead line and TV antenna, causing the insulation breakdown of the electrical equipment in the house, resulting in fire or personal electric shock casualties, so it is also necessary to ground the insulating porcelain bottle iron feet on the line and before entering the house.
Shield ground
It is an effective measure to eliminate the harm of electromagnetic fields 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 and so on. Under the action of electromagnetic fields, the absorbed radiation energy will have a biological effect, causing harm to the human body, such as slight trembling of fingers, skin scratches, and vision loss. The shielding device is set up for the shell of the equipment that generates the magnetic field, and the shielding body is grounded, which can not only reduce the strength of the electromagnetic field outside the shield, 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 interference of the external electromagnetic field.
Anti-static grounding
In order to prevent the impact of electrostatic hazards and release them, it is the most important part of electrostatic protection.
2 Grounding wires in the power system: An important tool to ensure that the staff is in the event of an unexpected voltage on equipment and lines that have been powered out. According to the regulations issued by the ministry, the grounding wire must be made of bare copper soft wire above 25mm2.
In electrical appliances, the grounding wire is a line that is connected to the electrical equipment shell and other parts of the unsafe charge or leakage current that is generated for various reasons in a timely manner.
Grounding
A conductor or group of conductors that come into close contact with the earth's soil and provide an electrical connection to the earth.
Grounding device
The grounding device consists of a grounding body and a grounding wire.
Ground wire
Household appliances and equipment due to poor insulation performance or use of humid environment, will lead to their shell with a certain degree of static electricity, serious electric shock accidents. In order to avoid accidents, a wire can be connected to the metal shell of the appliance, and the other end of the wire is connected to the earth, and once the electrical appliance leaks, the ground wire will bring static electricity into the earth and release it. In addition, it is more important for electrical maintenance personnel to use soldering irons to weld circuits, sometimes because the soldering iron is charged and break through the integrated circuits in the damaged electrical appliances. Friends who use computers sometimes ignore the grounding of the main chassis, in fact, the ground wire of the computer maincase can prevent the occurrence of the crash phenomenon to a certain extent.
Grounding mode
The modern power system itself is a strong source of electromagnetic interference, mainly through radiation interference in the communication equipment in this frequency band. In order to suppress the interference influence of external high-voltage transmission lines, grounding measures are adopted, and there are two commonly used grounding methods, which are discussed as follows:
Decentralized grounding method
Shell grounding circuit grounding is the communication building lightning protection grounding, power system grounding, communication equipment of various types of grounding and other equipment grounding separately connected to the grounding system of each other, due to the increasing number of ground system, the potential coupling effect between the ground wire is often difficult to avoid, dispersed grounding is easy to cause interference. At the same time, the height of the main building is increasing, and the unsafe factors brought by its grounding method are also increasing. When a certain facility is struck by lightning, it is easy to form an underground counterattack and damage other equipment.
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 pressure equalization body, and for direct lightning strikes, the points on the same floor in the building are relatively uniform; For induction thunder, cage-type pressure equalizers and the frame structure of the building can also provide a shielding effect of 10-40dB for external electromagnetic field interference;
(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 conducive to reducing interference;
(3) Dry day grounding mines save metal materials and occupy less land.
It is not difficult to see from the above that the use of joint grounding can effectively suppress the interference of external high-voltage transmission lines.
The ground wire of the antistatic ground should be connected to the ground device by a 1 megohm current limiting resistor in series, that is, connected to the grounding device by means of the current limiting resistor. Isn't the smaller the ground resistance, the better? Why string resistors?
Computer grounding is aimed at the easy flow of grounding current, requiring the smaller the grounding resistance as much as possible. The grounding of the computing center should minimize potential changes caused by noise, and it should be noted that the signal circuit and the power supply circuit, the high-level circuit and the low-level circuit cannot use the same common ground loop. For network wiring with high transmission bandwidth requirements, isolated shielding grounding should be used to prevent interference from electrostatic induction. In the design strive for simple, economical and effective grounding and shielding effective combination, will be able to better solve the interference, suppress noise.
4 Grounding wire connection method
In the connection method of DC ground, it can be divided into three types: series grounding, parallel grounding and mesh grounding.
Ground in series
The DC ground wire of the equipment in the machine room is connected to the copper sheet of the DC ground in a series manner, although the individual potentials of this connection method are different, but because the resistance is very small, it is more used in simple grounding systems. The disadvantage is that when a higher configuration is required, from the perspective of preventing noise, due to the series grounding, the resistance of each series makes the potential of each point deviate, and it is easy to produce noise.
Parallel grounding
In this method, the ground potential of each circuit is only related to the ground current and ground impedance of the circuit, and the potential difference between each point is more balanced, and a better low-frequency ground can be obtained, so it is more applied. 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 many ground wires, and the wiring is more complicated.
Meshed
In the large computer room, the requirements for the ground are relatively strict, and the mesh ground wire is widely used as a DC ground, called the mesh ground. DC mesh is a certain cross-sectional copper belt under the movable floor cross-arranged into a 600mm × 600mm square, its intersection and the position of the support point of the movable floor staggered arrangement, the foot point with solder welding or crimping together.
In order to make the DC mesh ground and earth insulated, under the copper belt should be padded 2 ~ 3mm thick insulation rubber or polyvinyl chloride plate and other insulation materials, the ground resistance is required to be above 10MΩ. The DC mesh ground system not only helps to better ensure the consistency of the potential reference point of the logic circuit, but also greatly improves the internal and external interference resistance of the machine. However, the mesh ground system is relatively large, the construction is complex, and the cost is high, so it is only suitable for application in the mainframe computer room.
5 Problems that need to be paid attention to when dealing with co-ground lines
(1) Grounding resistance - the grounding resistance of the common grounding pile should meet the requirements of the minimum grounding resistance in various grounding.
(2) In order to prevent mutual interference of the grounding system and ensure the insulation of the building, the grounding busbar should use a shielding wire with an insulating skin, and one end of the shielding sleeve should be grounded.
(3) Although DC ground, AC ground and safety are finally connected to the ground pile, it does not mean that the various ground can be connected at will, and they should also be strictly insulated from each other before they are connected to the same pile in accordance with the above requirements.
(4) In the computer equipment where the DC ground and the chassis are safely grounded, because the DC ground and the frame are strictly insulated, each is connected to the pile of the system, but some computer cases and the DC ground are connected together on the electrical appliance, and the working place of the AC equipment is strictly insulated with the casing
Source: Network
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