Ground protection of electrical equipment
Grounding Protection of Electrical Equipments Wang Jie (Hebei Energy Institute of Technology, Tangshan 063004, Hebei, China) The importance of grounding protection is highlighted in the step-by-step approach to explore the correct and effective safety issues related to grounding and electric shock.
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A certain part of the power system and electrical equipment is connected to the grounding electrode via a grounding wire, which is called grounding. It can also be said that any part of the electrical equipment is in good electrical connection with the earth (soil). 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.
Grounding is divided into two types according to different functions: protective grounding and functional grounding.
Protective grounding. Protective grounding In order to prevent the insulation damage of the electrical equipment, the voltage of the metal shell to the ground is limited to a safe voltage, so as to avoid the personal electric shock accident. The exposed conductor part of the electrical equipment is grounded, which is called protective grounding.
Lightning grounding. The introduction of lightning into the earth to prevent lightning currents from causing electric shock or damage to property is called grounding for lightning protection. The grounding resistance of the lightning protection grounding is lower than that of the protection grounding, but it must pay special attention to the counterattack caused by the lightning current. Therefore, the design and construction of the lightning protection grounding should be especially careful.
Anti-static grounding. The electrostatic charge is introduced into the earth to prevent the static electricity from accumulating on the human body and equipment. This is called antistatic grounding.
Anti-corrosion grounding. The metal body is buried in the ground as a sacrificial positive or a sacrifice negative, and the metal body connected to it is called an earth ground.
2 Functional grounding can also be divided into four types: work grounding. In order to ensure the normal operation of the power system, grounding is performed in an appropriate place in the power system, which is called a working ground. In the communication system, this point is generally a neutral point.
Logical ground. In order to obtain a stable potential, the appropriate metal part in the electronic device is considered as a "zero" potential and needs to be obtained as a logical ground.
Shield ground. The metal shell or the metal mesh is grounded, the electronic equipment in the protective shell or the net is not interfered with by the outside, or the electronic equipment in the shell or the net does not cause interference to the external electronic equipment.
Signal grounding. The ground that is set to protect the signal with a stable reference potential is called signal ground.
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.
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 almost all electric shocks, but almost all of them can get rid of the power supply because the environment in which people are living is dry and the skin is relatively 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 the grounding device is not provided, the voltage of the faulty equipment enclosure is the same as the phase-to-ground voltage, which is much higher than the grounding voltage, so the danger is also increased.
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. The low-voltage power distribution system cannot avoid hitting the ground after hitting the shell or breaking the phase line. If the neutral point is insulated from the earth, the voltage of the other two phases will increase to the phase voltage of /3. The result may be that the work Electrical equipment with a voltage of 220V burns out.
For systems with neutral grounding, even if one phase is shorted to ground, the other two phases can still approach the phase voltage, so the electrical equipment connected to the other two phases will not be damaged. In addition, the system can be prevented from oscillating, and the electrical equipment and lines need only consider the insulation level according to the phase voltage.
3 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.
Directly thunder. The so-called direct lightning is a lightning strike that directly hits a building, a person, a tree, or a domestic animal. The lightning protection of a building is to install lightning rods and lightning protection tapes on the top of the building, or install a lightning rod next to the spherical tank body.
Static induction mine. Metal spherical tanks and other equipment, even if there is no electrical equipment above, grounding is still very important, if the lightning directly hit the metal ball cans, lightning current through the grounding device can quickly spread in the ground. In addition to the lightning directly hitting the metal ball jar, when the metal ball jar is in the electric field formed between the thundercloud and the earth, the metal ball jar will induce an electric charge with the thundercloud. If the ball tank is not grounded, after the thundercloud discharge, the induced charge on the ball tank is difficult to release, resulting in a high electrostatic induction overvoltage to the ground, which may cause fire or explosion. If the ball jar is well grounded, the static charge generated by the induction will not accumulate on the ball jar and it will be quickly introduced into the ground.
Electromagnetic induction. A conductor in a powerfully changing electromagnetic field induces a very high potential. Lightning strikes produce currents with amplitudes and steepnesses that are much larger than those of the high-voltage lines, thus forming a powerful changing electromagnetic field in the lightning strike zone. If there are metal objects close to each other in this zone and a closed loop with a small gap is formed, this loop A high potential will be induced and spark discharge will occur at the gap. If the conductor in the area forms a closed loop, an induced current will be generated in the conductor.
1. For the protection of personal and equipment safety, all electrical equipment should be protected and grounded in accordance with the national standard GB405O "System grounding form and safety technical requirements. The protective grounding wire should not be used for any purpose other than to achieve the specified working grounding or protective grounding requirements.
2 For electrical equipment of different purposes and different voltages, in addition to special requirements, a general grounding body should be used to meet the equipotential bonding requirements. Metal components of buildings and metal pipelines (excluding metal piping for flammable and explosive materials) should be used. ) Connect to the total grounding body.
3 The total manual grounding body should not be installed in the building. The grounding resistance of the total grounding body should meet the minimum grounding resistance requirements of various groundings.
4 There are special requirements for grounding, such as weak current system, computer system and medium voltage system. If the neutral point is grounded directly or grounded by small resistance, it shall be carried out according to relevant special items.
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