As the main switch is separated from the whole distribution box, it can also be used as the power supply terminal (including bus bar) for the protection function of switching, and can also be used as the backup protection circuit of feeder. The power cabinet is a combination of electrical control cabinets that provide power for the normal operation of the whole machine, including fuses, circuit breakers, contactors, frequency converters, high-voltage cabinets, transformers, etc. Switch cabinet is an electrical equipment. The external line of the switch cabinet enters the main control switch in the cabinet first, and then enters the sub control switch. Each branch circuit is set according to its needs. For example, instruments, automatic control, motor, magnetic switch, various AC contactors, etc., some of them are equipped with high-voltage room and low-voltage room switch cabinet, with high-voltage bus, such as power plant, and some are also equipped with low cycle load shedding to ensure the main equipment. The power distribution cabinet distributes the electric energy of a circuit of the upper power distribution equipment to the nearest load. This equipment shall provide protection, monitoring and control of the load. Although there is obvious visible part of the fracture, but the switch knife, knife fusion switch volume is large, the degree of protection is poor, so it is generally no longer selected from the distribution box as the main switch. Can have isolation function, low-voltage electrical apparatus, it has been required, if there is no obvious break point, and only need to break enough gap, meet the requirements of impulse withstand voltage, leakage current does not exceed the preset value, and reliable can be displayed on the opening and closing state of mechanical operating mechanism. At present, most of the low-voltage circuit breakers have been fencing. The master switch selects the circuit breaker or load switch (e.g. to remove the protection circuit breaker trip unit), which requires specific conditions. If the relay type power distribution box, that is, the return line is connected to multiple distribution boxes through t, the main switch of the distribution box should be used for each circuit breaker with isolation function; otherwise, each distribution feeder has a short circuit, it must jump to the main switch of the power line, thus affecting the normal operation of the line and its connection of other distribution boxes. If through the protection switch, the radial distribution box of the higher level distribution unit of this switch shall have the interval function always loading switch. Some designers believe that when the radial power supply, main distribution box breaker switch is installed, and the switch power supply of the distribution box or feeder bus terminal is short circuited, the switch should always jump to this box, and the higher level protection switch should be selective, so that the maintenance personnel do not have to look for a higher level switch in the above-mentioned accident shutdown operation, which saves a lot of trouble. The author disagrees. The reason is very simple. In the same cycle, the first end of the circuit breaker and the end are installed. If it is selective, the first end of the circuit breaker needs a short time delay function, which will increase the investment, or the end-to-end cross circuit breaker tripping setting current ratio is more than 2.5 times. When the latter reaches selective chimerism, this can reduce the protection sensitivity of the head end protection circuit breaker The overload circuit breaker can be overloaded only by the terminal. If the first trip, it belongs to different drug administration. For example, the main switch of the household distribution box and the switch box of the distribution floor are the end of the box, so the selectivity between the lower circuit breakers is reasonable because of the user's operating authority of the home ground power distribution. I think: the first radial distribution of circuit breakers at the plant end does not need selective protection, because they are all under the management of the same department and are close to each other. Besides the power distribution box, the bus switch circuit of the upper feeding device may fail once for ten years. In case of the above fault, the operation at the end of the power circuit breaker will not take any effort.