The metal housing of the electrical equipment must be grounded. Its main function is: once the charged body collides with the equipment housing, the grounding resistance of the equipment determines the voltage of the housing to the ground. Once a hazardous electrostatic voltage is generated, static electricity can be quickly eliminated by grounding. When electrical equipment on the roof or outdoors is struck by lightning, the grounding of the electrical equipment can quickly spread the lightning current into the ground. Although the grounding of electrical equipment is important, it is not uncommon for it to be used during construction. This article discusses this issue from the following aspects.
1 Bolt and ground connection connection. However, in actual operation, when the metal shell of the equipment is welded with galvanized bolts, the galvanized layer is completely destroyed and the rust prevention effect is lost. Especially for open-air equipment, the bolts rust quickly and affect the reliability of the grounding; There are some construction units that weld a flat section of galvanized flat steel on the metal shell of the equipment, and then connect the galvanized flat steel as ground wire with galvanized bolts. When this method is used to weld galvanized flat steel, because the galvanized flat steel is very short, the galvanized layer of the galvanized flat steel is also damaged, which affects the reliability of the grounding. The correct method: In the metal casing of the electrical equipment, a copper grounding terminal is welded by chemical exothermic welding. The connection to the grounding wire can achieve reliable grounding. The so-called chemical exothermic welding is the use of graphite investment molds, the copper terminal and the electrical equipment shell fixed together, in the investment mold into the copper alloy powder and pyrotechnic powder, using a gun to ignite the pyrotechnic powder, the copper alloy powder A chemical reaction takes place, forming a high-temperature copper melt, which melts the shallow surface of the metal housing of the electrical equipment and melts with the copper ground terminal.
In the project, welding problems between flat steel and flat copper are also often encountered. At this time, brazing can meet the requirements of electrical connection. However, brazing flux is corrosive, and it is not easy to clean after welding, so it is prone to chemical corrosion. However, chemical exothermic welding does not contain corrosive flux, which is called fluxless welding and therefore does not cause chemical corrosion. There is a potential difference between copper and iron, and this potential difference will produce electrical corrosion. In the case of the chemical welding method, an alloy layer is formed between the copper and the iron, so that the electric corrosion due to the potential difference is avoided. Chemical exothermic fusion welding has been widely used in the grounding construction of power plants and other important engineering projects. With the gradual emphasis on grounding reliability, the application of chemical welding will inevitably become more and more extensive.
2 There are currently 3 methods for earthing the roof device housing: 1) connect the metal shell of the electrical equipment above the daughter wall lightning protection zone with the lightning protection zone, and connect the electrical device shell below the lightning protection zone with the grounding line of the power supply; After the metal shell of the electrical equipment of the parapet is connected with the lightning protection strip, it is connected with the PE wire of the power supply; 3) The metal shell of the electrical equipment of the roof is only connected with the lightning protection strip and no longer connected with the PE wire of the power supply.
According to regulations, lightning rods or lightning protection metal nets on buildings should be connected with other metal objects on the top of a building as a whole. In the above mentioned practice, it is an error to connect the metal shell of the electrical equipment below the lightning protection strip to the lightning protection strip. Because the lightning protection device may still cause lightning strikes, the possibility of a lightning strike is greatly reduced because of the presence of the lightning protection device. After the electrical equipment on the roof is connected to the lightning protection zone, is it possible? Connected to the PE line of the power supply, the relevant national regulations explicitly stated this, but the power cable for the lighting lamp on the frame equipped with the lightning arrester and the lightning protection line must use the cable with metal sheath buried directly in the soil or worn. Wire into the metal tube.
The metal sheath or metal pipe of the cable must be grounded, and the length buried in the soil should be more than 10m before it can be connected to the grounding network of the power distribution device or connected to the power line and the low-voltage power distribution device. Therefore, from a safety point of view, the metal casing of the roof electrical equipment can not be connected with the PE wire. If it is connected with the PE wire, once the electrical equipment is struck by lightning, part of the lightning current will enter the ground along the lightning deflector, and the other part will enter the indoor along the wire. At this time, people will be subject to lightning current damage when they touch the PE wire. The last method in the previous practice is correct, that is, the metal housing of the electrical equipment on the roof is only connected to the lightning protection strip.
For the TT system, the metal casing of the electrical equipment on the roof is connected to the grounding device through the lightning conductor and the lightning deflector. At this time, the lightning protection ground also serves as the protection grounding, and the grounding resistance is not more than 4. The grounding device for lightning protection and the working ground of the power supply The distance of the device in the ground shall not be less than 3m. The TN system adopting the joint grounding body is used. Although the electrical equipment shell of the roof is connected to the lightning protection zone only, it is connected to the PE line at the end, but the lightning protection trip leads to the underground and the grounding grid. After the integration, the lightning deflector is equivalent to the PE line and reaches the grounding requirement. The specification has a distance requirement for the connection point between the lightning conductor and the grounding grid, ie, the underground connection point between the lightning rod and the main grounding grid to the underground connection point of the 35KV and below equipment and the main grounding grid, and the length along the grounding body shall not be less than 15m. 3 Leading out of the PE line in the busway The distribution of busbars in high-rise buildings is quite common. The number of busway slots is divided into three lines, four lines and five lines. For three-phase power distribution, some designs use three-wire busway, some use four-wire busway; three-phase power supply lighting distribution, some designs use four-wire busway, and some use five-wire busway; Three-wire power distribution or four-wire lighting distribution system, the PE line is laid separately outside the busway, for four-wire power distribution or five-wire lighting distribution system, PE line and phase line (neutral) in the same bus Slot.
In the case of a busway with a PE wire, if a pin leads out the PE wire in the busway, then pin insertion or insufficient elasticity of the elastic piece of the pin will cause grounding to be unreliable. Pull out the phase line with the pin, once the lack of phase, will soon be discovered; but the situation is not easy to find the lack of land, if found, it is often too late, an electrical accident has occurred. Therefore, PE lines should not be led out with pins. At present, some manufacturers do not use lead wires for busbars of 400A and above, including phase lines, but use argon arc welding to weld joints and connect them with bolts. Obviously, the reliability of grounding is better than that of pins. If the PE wire is laid separately outside the busway, it can be bolted, which can reduce the project cost and facilitate inspection.
4 Conclusion Earthing protection is recognized by all kinds of electrical specifications in China, but the serious defects exposed in the site construction can not be ignored. Practice has proved that only in strict accordance with the requirements of the electrical specification construction, the use of scientific welding process can ensure reliable grounding safety Sex.
Korean style. Building Electrical Design Handbook, M. Beijing: China Building Industry Press, 1991. Beijing: China plans to publish, 1999. (Editor: Liu Cunying)
Changzhou Longfu Knitting Co., Ltd. , https://www.czcircularmachine.com