With the rapid development of China's social economy, high-rise buildings are lined up in various cities. While the function of the city is becoming more and more perfect and complex, the demand for reliability of power supply is also higher, and many users require two or more power supplies. In developed coastal cities, for example, the density of modern buildings in the city has increased, along with the difficulty factor of line construction, and the power grid has also become more complex, and once a circuit fault occurs, it is difficult to identify and repair it in a short time. In this situation, the contradiction between the traditional power distribution system and the increasingly complex circuit needs is formed, and the improvement of the traditional power distribution system becomes inevitable. The existence of ring main units provides a more ideal solution to this contradiction.
Ring main unit is a group of transmission and distribution equipment installed in metal or non-metallic insulated cabinet or made into interval type ring network power supply unit of electrical equipment, the core part of which adopts load switch and fuse, with the advantages of simple structure, small volume, low price, can improve the power supply parameters and performance and power supply safety. It is widely used in distribution stations and box-type substations in load centers of urban residential communities, high-rise buildings, large public buildings, factories and enterprises. At present, the most used ring main units in China are mainly the combination of load switch and fuse cabinets, air insulated load switch cabinets, followed by vacuum load switch cabinets. In the past, the domestic load switch had poor contact extinguishing performance, three phases were not synchronized for breaking operation, fault breaking capacity became smaller, operation was not easy, short life defects these shortcomings, the installation in the circuit breaker cabinet was restricted, and the ring main units has replaced the switchgear cabinets with only circuit breakers.
In the load switch and fuse combination ring main unit, the fuse is a current-limiting fuse that can withstand high breaking current, and the arc caused by the melting of the fuse in the opening process is interrupted by quartz sand to break the circuit, while the fuse firing pin to start the spring storage impactor of the load switch to synchronize the three pole contacts of the load switch, so that the whole opening process is unfolded according to the time characteristics of the inverse time limit. Within 10ms, the faulty power supply is quickly cut off, and it is not easy to appear the phenomenon of missing phase power supply. Due to the short action time, the transfer current (the fuse of the fuse melts so that the spring storage impactor will be the fault current borne by the line during the whole process of the load switch breaking) is much smaller than the peak fault current, effectively protecting its lines and equipment. Therefore, the load switch and fuse ring main unit protection is safer and more reliable, and the line design does not require short-circuit current calibration, but also saves the cost of the line. The structure of Ring main unit is compact, and its heat dissipation performance is better than the cart-type cabinet. Its frame size is relatively small, meanwhile it can be installed against the wall or away from the wall. Its small footprint also saves civil construction costs.
