First, the selection of low voltage distribution box protection type The protection type of the low-voltage distribution box should be selected according to the conditions of the workplace. (1) In places where the relative humidity does not exceed 75%, and there is no dust, no corrosive gas or vapor, and no flammable or explosive substances, the low-voltage distribution box can be opened. (2) Protected type should be used in particularly humid places where the relative humidity is close to 100% and where there is corrosive gas or vapor. (3) Dust-proof distribution boxes should be used for dusty places that can generate a large amount of dust. (4) In the H-1 and H-2 fire hazard areas with flammable liquids and suspended or accumulated combustible dust or combustible fibers, dustproof type shall be used; in the case of H-3 fire with solid combustible materials Dangerous, optional protection type. (5) Selection in explosive places. (1) Where there is a gas or vapor explosive mixture 1 In the Q-1 class where the explosive mixture can be formed under normal conditions, a low-voltage distribution box with explosion-proof, explosion-proof ventilation and inflation type should be used. 2 Under normal conditions, it can not be formed, and in the abnormal situation, the Q-2 level of explosive mixture can be formed. Any type of explosion-proof type can be selected. 3 A Q-3 grade site that cannot form under normal conditions and can form an explosive mixture under abnormal conditions, and is less likely to be sealed. (2) Where there is an explosive mixture of dust or fiber. 1 In the G-1 class where the explosive mixture can form under normal conditions, any one of the explosion-proof type, explosion-proof ventilation type and inflatable type can be selected. 2 G-2 grades that cannot form under normal conditions and form explosive mixtures under abnormal conditions may be dust-proof. Second, the basic safety requirements for low voltage distribution box protection devices and installation In addition to the reasonable selection of protection type for low-voltage distribution boxes, all electrical components and installations must also comply with the relevant safety regulations in the "Low-Voltage Power Distribution Devices and Circuit Design Specifications" and "Code for Construction and Acceptance of Electrical Installation Engineering". (1) The working voltage, current, frequency and grade of the selected electrical components must meet the requirements of their work. The installation and wiring should be neat and the connection should be reliable. (2) Each branch line in the distribution box or a place where protection needs to be selective shall generally be provided with protective appliances. When a fuse or an automatic switch is used as the protection device, the fuse rated current of the fuse or the setting current of the automatic switch overcurrent release should be as close as possible but not less than the load calculated by the protected line, and should be guaranteed. During normal short-duration overload (such as motor starting, etc.), the protection device will not be disconnected by the protection circuit. (1) When using a fuse to protect the distribution line, the fuse should be installed on each pole of each phase that is not grounded (zero). When the fuse is short-circuit protected, the cross-section selection of the melt or conductor should meet the following requirements: 1 The rated current of the melt shall not be greater than 2.5 times the allowable current carrying capacity of the cable or the insulated conductor of the pipe or 1.5 times the allowable current carrying capacity of the insulated conductor. 2 When a single-phase short circuit occurs at the end of the protected line (referring to the neutral point directly connected to the grid), the short-circuit current should not be less than 4 times the rated current of the fuse. (2) When an automatic switch is used to protect the distribution line, its overcurrent release shall be installed on each phase of each phase that is not grounded. When the automatic switch is used for short circuit protection, the setting current should meet the following requirements: 1 When using an automatic switch with only instantaneous or short-delay overcurrent release, the setting current of the trip unit should avoid the short-time overload current, and the setting current and the allowable current carrying capacity of the conductor can meet the working requirements. 2 When using an automatic switch with a long-delay overcurrent release or a short-delay overcurrent release, the setting current of the long-delay overcurrent release is generally not greater than the allowable current of the insulated conductor or cable. 1.1 times, its action time should avoid the duration of short-time overload current. 3 It should be noted that when a single-phase short circuit occurs at the end of the protected line (referring to the neutral point directly connected to the power grid), the short-circuit current should not be less than 1.5 times the setting current of the automatic switch instantaneous or short-delay overcurrent release. (3) In the place of fire and explosion hazard and the distribution line that may be overloaded for a long time, the load protection device shall be installed. The allowable current carrying capacity of the insulated wire and cable of the distribution line shall not be less than the fuse rating of the fuse. The current or automatic switching is 1.25 times longer than the long-delay overcurrent trip setting current. (4) All metal parts of the distribution box that are not energized under normal conditions must be reliably grounded (zero). (5) The socket wiring in the distribution box should be correct. AC/DC or different voltage outlets should have obvious areas. Each switch in the box should have a logo indicating the object to be controlled. (6) The floor-standing low-voltage distribution box installed in the workshop should be such that its bottom is higher than the ground, and the minimum distance between the front and back of the box is not less than 0.8 m. (7) When the open low voltage distribution box is used in the place, when the height of the unshielded bare conductive part is less than 2.3 meters, the fence shall be set, and the clear distance from the fence to the bare conductive part shall not be less than 0.8 meters, the height Not less than 1.2 meters. Other exposed components that may be touched within the operating range are also screened intact. Third, the safety matters that should be paid attention to during use Strict management and proper use of low voltage distribution boxes are essential measures to reduce electrical accidents. In order to do a good job in its management and use, it is necessary to implement the principle of “grading management, line managementâ€, the system is clear, and each is responsible. (1) The equipment power department shall number, register, and file each low-voltage distribution box. Regular technical inspections and measurements are performed. (2) The maintenance electrician should inspect the distribution box. If there is serious heating of the components, the cause should be ascertained. The looseness of the wiring should be tightened. The dust and debris in the box should be cleaned in time to keep it clean. (3) The operator shall not stack or hang the workpiece and other items within 1.2 meters in front of the distribution box door. Do not collect water around the distribution box. Non-electrician personnel are not allowed to disassemble electrical components. (4) It is strictly forbidden to bring the load to pull and close, so as to avoid the formation of excessive arc burning of power distribution components or causing accidents such as burns and burns of operators.
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The low-voltage distribution box is the lowest level of control and protection facilities in the power system, and is the most basic part of ensuring the safe operation of the power system. It has a great chance of contact with workers, but because of its large quantity, wide distribution, and low value products, it is often ignored by the equipment power department, which makes many low-voltage distribution boxes out of control, resulting in unsafe factors. Produced, increasing the risk of personal electric shock and electrical fire. Therefore, the safe selection and use of low-voltage distribution boxes must be taken seriously.
May 03, 2023