The actual temperature rise in the switch cabinet, especially the bus connection, is usually higher than the data measured in the type test.
The main reasons are as follows:
(1) The data measured in the type test are usually completed in the laboratory, and the duration is not long, usually not more than 8 hours. It does not have the cumulative effect of temperature rise and cannot be equivalent to equipment that operates for a long time and continuously heats.
(2) Different metals have different expansion effects. The metal expansion coefficient of steel bolts is far less than that of copper and aluminum buses. Especially for bolted equipment joints, with the change of load current and temperature during operation, the expansion and contraction degree of aluminum or copper and iron will be different and creep will occur, that is, the metal will slowly plastic deformation under the effect of stress. The creep process is also closely related to the temperature at the joint. It has been proved in practice that when the working temperature at the joint exceeds 80 ° C, the joint metal will expand due to overheating, which will stagger the positions of the contact surfaces, form tiny voids and oxidize. When the load current decreases and the temperature drops back to the original contact position, due to the coverage of the oxide film on the contact surface, it cannot be a direct contact between metals during the original installation process. The increase of contact resistance in each temperature change cycle will increase the heat in the next cycle, and the increase of temperature will further deteriorate the working state of the connector, thus forming a vicious cycle.
(3) The fastening bolt pressure at the connection part is improper. Some installation or maintenance personnel believe that the tighter the connecting bolts, the better the conductor connection. In fact, that is not the case. In particular, the elasticity coefficient of aluminum bus is small. When the pressure of the nut reaches a certain critical pressure value, if the strength of the material is poor and the pressure continues to increase improperly, it will cause deformation and lifting of the contact surface, rather than reducing the contact area and increasing the contact resistance, thus affecting the conductor contact effect.
(4) The conductivity of the selected conductor materials does not meet the requirements, mostly due to the insufficient purity of the conductor raw materials.
(5) Other factors on the site, such as improper installation and maintenance process, such as improper treatment of the contact surface of the bus, unevenness and unsmooth, and failure to apply special electric grease during the processing, connection and installation of the bus, result in the reduction of the effective contact area, the increase of the contact resistance, and heating.
Low voltage switchgear is a low-voltage distribution assembly that involves low-voltage switchgear and is completely assembled according to a certain wiring scheme. It is used for power supply and lighting distribution in power supply systems below 1000V. It is also divided into two categories: fixed installation and withdrawable installation.
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