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As you know, if the secondary neutral point of transformer is classified, there are two kinds of low-voltage power grids with line voltage below 1 kV voltage: the grounding grid and the ungrounded power grid. National standards (GB5306-85), in the grounding grid, transformer low voltage side neutral point must be grounding directly, and the grounding resistance is not greater than 4 Ω. In the ungrounded power grid, the neutral point of the transformer is not grounded. In order to reduce the danger of high pressure string into low pressure, a breakdown fuse is generally installed between the neutral point and the earth. When the electrical equipment is used in different power grids, its ground lines shall be connected in different ways according to the corresponding requirements in the national standards. The details are as follows:
1. Connection method when used in ungrounded power grid.
In the ungrounded power grid, the ground wire on the electrical equipment should be closely connected with the earth, and the equipment shall be grounded, as shown in figure 1. According to the figure, the human resistance Rr is in parallel with the ground resistance Rb. The grounding current Id passes (Rr/Rb) and the power grid to the ground insulation impedance to form a loop when the shell is encountered. All the relative insulation resistance is equal to Z, and the voltage of the power grid is U, and the voltage of the leakage device can be obtained by using the electrical engineering method:
Normally | Z | is very big, Rb < < | Z |, so after the protective earthing device of voltage is reduced greatly, as long as the equipment appropriately control the size of the Rb can limit of voltage in the security. The national standard requirements in the general case Ω Rb is not greater than 4.
Protective earthing must have a grounding device. The grounding device is made up of ground and ground wire (including ground wire mesh). The grounding body is divided into natural grounding and artificial grounding. It is an important part of the grounding device, and it must be made strictly according to the standard installation. The following is briefly introduced.
Any metal conductor with reliable contact with the earth may be used as a natural grounding body unless otherwise specified. Such as buried metal pipes, Wells, etc. The artificial earthing body usually adopts steel pipe with diameter of 40 ~ 50 mm or 40 mm x 40 mm x 4 mm ~ 50 mm x 50 mm x 5 mm Angle steel vertical embedding. For cost and performance, the length is generally 2. 5 m is appropriate, the quantity is not less than two. They can be arranged in rows or in a circular or radial distribution, as shown in figure 2, as shown in figure 3. Whether the natural grounding body or the artificial ground body, after the installation, the ground resistance should be measured, the standard can be put into use. It is also very dangerous to use a water pipe as an earthing or to "insert a wire into the local line".
2. Connection method in grounding grid.
In the grounding grid, the ground wire on the electrical equipment should be closely connected with the protection of the zero line, and the device protection shall be connected to zero, as shown in figure 4. When a phase of a charged part touches the shell of a device, a single phase short circuit of the relative zero line is formed through the device's shell, that is, the contact shell short circuit. The short circuit current Id can prompt the protection device on the line (such as the fuse RD) to move quickly, cut off the power of the fault part, and eliminate the risk of electric shock.
We must note that the grounding system is not allowed to be protected in the grounding grid, as shown in figure 5. A grounding grid phase voltage for U, and low voltage transformer neutral point grounding resistance for the Ro, equipment shell of voltage of Ud, if the device has protective grounding device, when a certain phase touch shell short-circuit occurs, due to the human body resistance grounding resistance Rd Rr and protection in a parallel state, and Rd < < Rr, Rd material Ro 4 Ω material, human withstand voltage of:
Obviously, this voltage is dangerous to the human body, which means that the protection of the grounding device in the grounding grid itself is not safe.
For the same reason, at this point the zero line voltage is changed from the original Uo = 0 to:
This makes the whole system no longer safe, and other protective zeros also carry dangerous voltages.
At the same time, because Rd and Ro limit the size of the fault current, the protective device on the line may not move to make this dangerous state exist for a long time.
Therefore, it is not correct to emphasize "it is the ground to earth" in the grid environment without looking at the equipment work.
1. Connection method when used in ungrounded power grid.
In the ungrounded power grid, the ground wire on the electrical equipment should be closely connected with the earth, and the equipment shall be grounded, as shown in figure 1. According to the figure, the human resistance Rr is in parallel with the ground resistance Rb. The grounding current Id passes (Rr/Rb) and the power grid to the ground insulation impedance to form a loop when the shell is encountered. All the relative insulation resistance is equal to Z, and the voltage of the power grid is U, and the voltage of the leakage device can be obtained by using the electrical engineering method:
Normally | Z | is very big, Rb < < | Z |, so after the protective earthing device of voltage is reduced greatly, as long as the equipment appropriately control the size of the Rb can limit of voltage in the security. The national standard requirements in the general case Ω Rb is not greater than 4.
Protective earthing must have a grounding device. The grounding device is made up of ground and ground wire (including ground wire mesh). The grounding body is divided into natural grounding and artificial grounding. It is an important part of the grounding device, and it must be made strictly according to the standard installation. The following is briefly introduced.
Any metal conductor with reliable contact with the earth may be used as a natural grounding body unless otherwise specified. Such as buried metal pipes, Wells, etc. The artificial earthing body usually adopts steel pipe with diameter of 40 ~ 50 mm or 40 mm x 40 mm x 4 mm ~ 50 mm x 50 mm x 5 mm Angle steel vertical embedding. For cost and performance, the length is generally 2. 5 m is appropriate, the quantity is not less than two. They can be arranged in rows or in a circular or radial distribution, as shown in figure 2, as shown in figure 3. Whether the natural grounding body or the artificial ground body, after the installation, the ground resistance should be measured, the standard can be put into use. It is also very dangerous to use a water pipe as an earthing or to "insert a wire into the local line".
2. Connection method in grounding grid.
In the grounding grid, the ground wire on the electrical equipment should be closely connected with the protection of the zero line, and the device protection shall be connected to zero, as shown in figure 4. When a phase of a charged part touches the shell of a device, a single phase short circuit of the relative zero line is formed through the device's shell, that is, the contact shell short circuit. The short circuit current Id can prompt the protection device on the line (such as the fuse RD) to move quickly, cut off the power of the fault part, and eliminate the risk of electric shock.
We must note that the grounding system is not allowed to be protected in the grounding grid, as shown in figure 5. A grounding grid phase voltage for U, and low voltage transformer neutral point grounding resistance for the Ro, equipment shell of voltage of Ud, if the device has protective grounding device, when a certain phase touch shell short-circuit occurs, due to the human body resistance grounding resistance Rd Rr and protection in a parallel state, and Rd < < Rr, Rd material Ro 4 Ω material, human withstand voltage of:
Obviously, this voltage is dangerous to the human body, which means that the protection of the grounding device in the grounding grid itself is not safe.
For the same reason, at this point the zero line voltage is changed from the original Uo = 0 to:
This makes the whole system no longer safe, and other protective zeros also carry dangerous voltages.
At the same time, because Rd and Ro limit the size of the fault current, the protective device on the line may not move to make this dangerous state exist for a long time.
Therefore, it is not correct to emphasize "it is the ground to earth" in the grid environment without looking at the equipment work.
