Junhe--A Brief Analysis of Low-Voltage Pull-Out Switchgear

Thread Content

A brief analysis of low-voltage draw-out switchgear: 400V drawer-type switchgear refers to low-voltage draw-out switchgear and control equipment designed for an AC voltage level of 400V. It consists of cabinet-based or panel-based assemblies that include busbars and draw-out functional units, and may have fixed or movable components. Electrical power can be distributed to various functional units through busbars or branch circuits, making it an appropriate type of power distribution equipment. The development of drawer-type switchgear has a history of nearly 40 years. In our country, mature application cases already existed in the early 1970s. With the innovation of low-voltage electrical components, the size of the primary components has decreased rapidly; as a result, the structure of drawer-type switchgear has undergone significant changes over the past 30 years. Overall, drawer-type switchgear is becoming smaller in size, with an increasing number of circuits per cabinet. IEC60050 provides strict definitions for withdrawable components, specifying some of their characteristics: Withdrawable components mainly refer to the primary circuit and may not include auxiliary or secondary circuits. Therefore, there are also some relatively special pull-out switchgear available on the market, such as strip-type plug-in switchgear. Here, the common drawer-type switchgear is taken as the focus to introduce relevant information. Common drawer-type switchgear uses steel plates to form a closed enclosure, with the electrical components of the incoming and outgoing circuits installed in removable drawers, thus creating functional units capable of performing specific power supply tasks. Between the functional units and the busbars or cables, a function board made of grounded metal or plastic serves as a separator, creating three areas: the busbars, the functional units, and the cables. There are also isolation measures between each functional unit. The internal partition format is generally 4b, but 3b can also be used. The drawer-type switchgear has 4 positions in total: the connected position, the testing position, the disconnected position, and the removed position. Due to their high reliability, safety, and interchangeability, drawer-type switchgear can be used to form centrally controlled power distribution centers, and are widely applied in industrial and mining enterprises as well as high-rise buildings where power supply reliability is important. It should be noted that the current main standards applicable to 400V drawer-type switchgear in our country are GB 7251.1-2013, GB/T7251.12-2013, and GB/T24274-2009. Among them, GB/T7251.1-2013 and others adopt IEC61439-1:2011, while GB/T7251.12-2013 and others adopt IEC61439-2:2011. They were revised based on the original GB7251.1-2005 (IEC 61439-1:1999), separating the general provisions and the product-related provisions that were combined in the original standard, thereby making GB7251.1-2013, as a mandatory standard, more authoritative and applicable. Categories: (1) Classified by purpose: distribution centers, control centers; (2) Classified by enclosure type: insulating material type, metal material type, insulating and metal hybrid type; (3) Classified by installation method: wall-mounted, wall-separated. Some general structural requirements for drawer-type switchgear: The cabinets, enclosures, and drawable components of the complete set of equipment must possess sufficient mechanical strength and stiffness, and must be able to withstand mechanical stress, electrical stress, thermal stress, as well as the effects of humidity that may occur during normal use. The enclosures of the complete set of equipment shall meet the requirements of GB/T20641-2006. Taking a shell made of steel plate material as an example, the thickness of the steel plate used for the support structure should not be less than 2.5 mm, while the thickness of the steel plate used for the outer shell should not be less than 2.0 mm. To ensure corrosion resistance, the complete set of equipment should be made of corrosion-resistant materials, or its exposed surfaces should be coated with a corrosion-resistant coating. The coating should have a uniform color, good adhesion, and must pass corrosion resistance tests; at the same time, the conditions under which the equipment will be used and maintained must also be taken into account. Again, taking a shell made of steel plate material as an example, there are three surface treatment methods for the supporting structural components: galvanizing/powder coating/painting. Regardless of the surface treatment method used, it must meet the anti-corrosion requirements. The external dimensions of the frame should preferably be selected from the following values. Height: 1800mm, 2000mm, 2200mm; Width: 400mm, 600mm, 800mm, 1000mm, 1200mm; Depth: 600mm, 800mm, 1000mm, 1200mm. Although the drawer-type units themselves constitute isolated units, the isolation method of the busbar compartments and cable compartments cannot be ignored either. The specifications do not explicitly define the isolation method for drawer-type switchgear; theoretically, it seems that all methods are suitable. However, in past practices, there have been numerous accidents caused by improper isolation. Therefore, for applications with high reliability requirements, it is recommended to use the 4b type of isolation ； If both the construction quality and the operation and maintenance quality are high, isolation in the 3b format can be adopted. Other forms of isolation are not recommended. The protection rating of the cabinet should meet the requirements of the users and the installation site. The opening angle of the door must be at least 90°; the coating layer must not be damaged during opening and closing. The direction in which the door opens must comply with the design requirements. The hinges on the door must be capable of withstanding sufficient load (for example, not less than 10 kg, or 4 times the mass of the door itself).