Grounding protection for electrical equipment

Grounding protection of electrical equipment Wang Jie (Hebei Energy Vocational and Technical College, Tangshan 063004, Hebei Province) emphasizes the importance of grounding protection and discusses the correct and effective grounding and electric shock related safety issues.

―), female, double specialist, lecturer of the training department of Hebei Energy Vocational and Technical College.

Connect a part of the power system and electrical equipment to the grounding pole via a grounding wire, called grounding. It can also be said that any part of the electrical equipment is well connected to the earth (soil). The grounded part of the power system is generally a neutral point or a point on the phase line. The grounding part of the electrical equipment is a metal conductor that is normally uncharged, usually a metal casing.

Grounding is divided into two categories according to their functions: protective grounding and functional grounding.

Protective grounding. Protective grounding In order to prevent the insulation damage of electrical equipment, the voltage of the metal casing to the ground is limited to the safety voltage to avoid personal electric shock accidents. Grounding the exposed conductor part of the electrical equipment is called protective grounding.

Lightning protection grounding. Grounding that draws lightning into the earth to prevent lightning currents from causing electric shock or damage to property is called lightning protection grounding. The grounding resistance of lightning protection grounding is not high compared with the protective grounding, but it should pay special attention to the counterattack caused by lightning current. Therefore, the design and construction of lightning protection grounding should be especially careful.

Anti-static grounding. Introduce static charge into the ground to prevent damage to the human body and equipment due to static electricity accumulation. It is called anti-static grounding.

Anti-corrosion grounding. The metal body is buried in the ground as a sacrificial positive or sacrificial negative, and the metal body connected to it is called an anti-corrosion ground.

2 Functional grounding can also be divided into four types: working ground. In order to ensure the normal operation of the power system, grounding in the appropriate place of the power system is called working ground. In an AC system, this point is generally a neutral point.

Logic ground. In order to obtain a stable potential, a suitable metal part in the electronic device is taken as a "zero" potential, which needs to be obtained as a logic ground.

Shield grounding. Ground the metal shell or metal mesh, protect the electronic equipment inside the shell or net from external electrical interference, or cause the electronic equipment inside the cabinet or the network to not cause interference to external electronic equipment.

Signal ground. The ground set to protect the signal from a stable reference potential is called signal ground.

We often only know that grounding can prevent personal electric shock. In addition to this effect, grounding can also prevent damage to equipment and lines, prevent fires, prevent lightning strikes, prevent static damage and ensure the normal operation of the power system.

1. Prevent electric shock. The impedance of the human body has a great relationship with the condition of the environment. The more humid the environment, the lower the impedance of the human body and the more susceptible it is to electric shock. For example, almost everyone who has installed an AC radio has been subjected to an electric shock, but almost all of them can get rid of the power supply because the environment in which the person is exposed is dry and the skin is 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 the grounding resistance, the ground potential of the electrical equipment always exists, and the grounding resistance of the electrical equipment is larger. When the fault occurs, the ground potential of the electrical equipment is also larger, and the risk of human touch is greater. However, if the grounding device is not provided, the voltage of the faulty device casing is the same as the phase-to-ground voltage, which is much higher than the grounding voltage, so the danger is also increased accordingly.

2 to ensure the normal operation of the power system. The grounding of the power system, also known as the working grounding, is generally grounded at the neutral point of the substation or substation. The grounding resistance of the working ground is very small. A large grounding grid is required for the large substation to ensure that the grounding resistance is small and reliable. The purpose of working grounding is to bring the potential between the neutral point of the grid and ground to near zero. The low-voltage power distribution system cannot avoid the phase line hitting the shell or the phase line is broken and then hit the ground. If the neutral point is insulated from the ground, the voltage of the other two phases will be raised to the phase voltage of /3, and the result may work. Electrical equipment with a voltage of 220V burned out.

For systems where the neutral point is grounded, 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 electrical equipment and wiring only need to consider the insulation level according to the phase voltage.

3 to prevent the danger of lightning and static electricity. When lightning occurs, in addition to direct lightning, inductive lightning is also produced, and the inductive lightning is divided into electrostatic induction lightning and electromagnetic induction lightning. The most important method of all lightning protection measures is grounding.

Direct lightning. The so-called direct lightning is that lightning strikes directly hit buildings, people, trees or livestock. Building lightning protection is to install lightning rods and lightning protection belts at the top of the building, or install lightning rods beside the spherical tanks.

Static induction lightning. Metal ball and other equipment, even if there is no electrical equipment on it, grounding is still very important. If the lightning hits the metal spherical tank directly, the lightning current can quickly scatter in the ground through the grounding device. In addition to the lightning directly hitting the metal spherical tank, when the metal spherical tank is in the electric field formed between the thundercloud and the earth, the metal spherical tank will induce a charge opposite to the thundercloud. If the spherical tank is not grounded, after the thundercloud is discharged, the charge induced on the spherical tank is difficult to release, forming a high electrostatic induction overvoltage to the ground, which may cause a fire or explosion. If the spherical tank is well grounded, the static charge generated by the induction cannot accumulate on the spherical tank, and it will be quickly introduced into the ground.

Electromagnetic induction lightning. A conductor in a strongly changing electromagnetic field induces a very high potential. The lightning strike produces a magnitude and steepness that is much greater than the current of the high-voltage line, so the lightning strike zone forms a strong changing electromagnetic field. If there is a metal object close to each other in this area and a closed loop with a small gap is formed, the loop It will induce a very high potential, and spark discharge will occur at the gap. If the conductors in the area form a closed loop, an induced current is generated in the conductor.

1. In order to protect the personal and equipment safety, all electrical equipment should be protected and grounded according to the national standard GB405O <system grounding form and safety technical requirements. The protective grounding wire shall not be used for any purpose other than to meet the specified working grounding or protective earthing requirements.

2 For electrical equipment of different purposes and different voltages, in addition to special requirements, generally use a total grounding body according to the requirements of equipotential bonding, metal components of buildings, metal pipes (except for metal pipes conveying inflammable and explosive materials) ) is connected to the main grounding body.

3 The artificial total 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 grounding.

4 Grounding with special requirements, such as weak current system, computer system and medium voltage system, should be carried out according to the relevant special items when the neutral point is directly grounded or grounded through a small resistance.

When the grounding device is running, the grounding wire and grounding turn down on page 58) When the insured person's personal account is swiped, the hospital front-end workstation uses the magnetic card number as a parameter, and calls the insured person's identification function encapsulated in the DL file to apply to the social security center. After the server issues a request, the social security center application server receives the service request, and after processing, returns the relevant data of the insured to the function to achieve the purpose of identification. For a networked hospital without an HS system, a set of straight-through procedures that have been embedded in the dynamic link library file will be provided to realize the payment of the medical insurance fee settlement of the designated hospital.

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