Xin Li (Xinjiang Institute of Architectural Planning and Design, Xinjiang Kuitun 833200) Good electrical connection. The part that is grounded in the power system is usually a neutral point, or it can be a point on the phase line. The grounding part of the electrical equipment is a normally uncharged metal conductor, usually a metal shell. The grounding device of the electrical equipment consists of a grounding body and a grounding wire. The metal body in direct contact with the soil is called a grounding body; the wire (or conductor) between the connecting electrical equipment and the grounding body is called a grounding wire.
1.1 Safety Grounding. It mainly includes the protection grounding set up to prevent the insulation damage of electrical facilities or electrical and electronic equipment and endangering personal safety; to eliminate static electricity accumulation in the production process and to cause static electricity grounding due to electric shock or explosion; to prevent electromagnetic induction and equipment The shielding of the metal shell, shield or shield sheath is grounded. Among them, the protective earthing is the most widely used, and it grounds the machine (outer) casing. The purpose of this grounding is for safety.
1.2 System grounding. This grounding provides the circuit system with a reference potential (potential), which can also lead away the interference. The purpose of this grounding is to resist external interference.
1.3 lightning grounding. In order to prevent the lightning overvoltage from harming people or equipment, the grounding of the overvoltage protection equipment set is called lightning protection grounding, such as grounding of lightning rods and arresters.
1.4 Repeatedly grounding. In the system of low-voltage power distribution system, in order to prevent loss of grounding protection due to neutral line failure, resulting in electric shock hazard and damage to equipment, repeated grounding of the neutral line. The repeated grounding points in the system are: the terminal of the overhead line and the appropriate point in the line; the neutral line of the four-core cable; the incoming line of the cable or overhead line at the building or workshop.
1.5 Anti-static grounding. Grounding for the purpose of eliminating the hazards of static electricity to humans and equipment, such as the grounding of metal pipes or vehicles of certain liquids or gases.
1.6 Shield grounding. In order to prevent electrical equipment from being disturbed by electromagnetic interference, it may affect the grounding of shielded equipment that may cause work or electromagnetic disturbance to other equipment.
We often only know that grounding can prevent people from getting an electric shock. In addition to this role, grounding can also prevent equipment and lines from being damaged, prevent fires, prevent lightning strikes, prevent electrostatic damage, and ensure the normal operation of the power system.
2.1 to prevent electric shock. The body impedance has a great relationship with the environment. The more humid the environment is, the lower the body's impedance is, and the more vulnerable it is to electric shock. For example, people who have been equipped with AC radios have received electric shocks, but almost all of them can get rid of the power supply, because the people are in a dry environment and the skin is dry. Grounding is an effective method to prevent electric shock.
After the electrical equipment is grounded through the grounding device, the potential of the electrical equipment is brought close to the ground potential. Due to the existence of the grounding resistance, the electrical equipment always exists to the ground potential. The greater the grounding resistance of the electrical equipment is, the higher the ground potential of the electrical equipment is when the failure occurs, and the greater the risk of human touch. However, if no grounding device is provided, the voltage f of the faulty device housing is the same as the phase-to-ground voltage, which is much higher than the grounding voltage, and therefore the danger is also increased accordingly.
2.2 Ensure the normal operation of the power system. The grounding of the power system, also known as the working ground, is generally grounded at the neutral point of the substation or substation. The grounding resistance of the working ground is very small, and a large grounding station requires a grounding net to ensure that the grounding resistance is small and reliable. The purpose of the working ground is to make the potential between the neutral point of the grid and ground close to zero.
2.3 to prevent lightning and electrostatic hazards. When a lightning strike occurs, in addition to direct lightning, inductive lightning is also produced. Inductive lightning is divided into electrostatic induction lightning and electromagnetic induction lightning. The most important method for all lightning protection measures is grounding.
3 Electrical equipment grounding technical principles 3.1 To ensure personal and equipment safety, all kinds of electrical equipment should be protected and grounded according to the national standard GB14050 system grounding form and safety technical requirements. The protective grounding line should not be used for any purpose other than to achieve the required working grounding or protective grounding requirements.
3.2 For electrical equipment of different purposes and different voltages, except for special requirements, a general grounding body should be used. According to the requirement of equipotential bonding, metal components of buildings and metal pipelines (metal pipelines conveying flammable and explosive materials) should be provided. Excluded) Connected to the total grounding body.
3.3 The total manual grounding body should not be located in the building. The grounding resistance of the total grounding body should meet the minimum grounding resistance requirements of various groundings.
3.4 Grounding with special requirements, such as weak current systems, computer systems and medium voltage systems, shall be carried out in accordance with the relevant regulations when the neutral point is directly grounded or grounded by a small resistance.
4 Electrical Equipment Grounding Methods Safety Grounding 4.1.1 Protection Zero. The neutral line in the three-phase four-wire power supply system is to protect the neutral line, which is an important part of the circuit loop. In the three-phase four-wire grid where the neutral point is directly grounded, the electrical and electronic equipment should be protected by zero. When the electrical and electronic equipment is in normal operation, the uncharged metal shell is connected to the neutral wire of the power grid. When the leakage occurs or the shell is touched, the metal shell is connected to the neutral wire to form a single-phase short circuit and the current is very large, so that the circuit protection device Move quickly and cut off the power. In the use of zero protection, the power line is not allowed to disconnect, if the middle line is disconnected, it will lose its protective effect. Normally the system adopts zero-line repeated grounding method to achieve protection.
4.1.2 Protective grounding. Grounding installed to prevent electric shock is called protective grounding. Protective earthing is only applicable to grids that are not grounded at the neutral point. All metal enclosures, brackets and connected metal parts of the electrical equipment in this grid should be grounded. The neutral grounding circuit system should not use protective grounding.
The system grounding line is not only the static and dynamic current channel in each circuit, but also the mutual coupling of each level circuit through the common grounding impedance, thus forming the weak link between the circuits. Therefore, all anti-interference technology in electrical and electronic equipment is related to grounding. The grounding of the system in the electrical and electronic equipments and equipments should be connected to the earth and how to connect the earth. It has a close relationship with the stability of the system. There are usually four ways.
4.2.1 Floating mode. Floating ground is groundless. It is a floating way. Its purpose is to isolate the circuit or equipment from the public ground or the common wire that may cause circulation, thereby suppressing the interference from the grounding wire. The disadvantage of this grounding method is that the device is not directly connected to the earth and is prone to electrostatic accumulation. After the accumulated charge reaches a certain level, electrostatic breakdown with a strong discharge current will occur between the device and the earth. A destructive source of disturbance.
4.2.2 single point grounding method. Because the 2-point grounding is easy to form a ground loop, the function of the point grounding is to eliminate and prevent the formation of a ground loop. Single point grounding is available in series and in parallel. Single point grounding provides common ground for many connected systems.
When current flows through the ground wire, the wire has more or less impedance. The series-grounded circuit currents 11 and 2IN all pass through the impedance Z1. Z1 is the common impedance common to the circuits 1 and 2N. Therefore, the potentials of the circuits 1 and 2N are affected by the 11 and 2IN, and they are mutually restrained. The parallel connection mode has no common impedance, and the circuits 1 and 2N do not interfere with each other. Therefore, the parallel connection is the most simple and practical.
4.2.3 Multi-point grounding method. For high-frequency circuits (signal frequency is 10MHz or more, since the inductance of the components and the layout of the circuit itself will increase the impedance of the ground wire, the one-point grounding method is no longer applicable. In order to reduce the impedance of the ground line and reduce the ground The mutual coupling between the circuits caused by the stray inductance and distributed capacitance should be connected to the ground terminals of each component at short distances.
4.2.4 mixed grounding. When the circuit system has both low frequency circuits and high frequency circuits or digital circuits, hybrid grounding should be used in the system. The low-frequency part of the circuit system adopts single-point grounding, while the high-frequency part needs multi-point grounding. This grounding method not only includes the characteristics of single-point grounding, but also includes the characteristics of multi-point grounding, so as to achieve the best suppression of interference. the goal of.
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