Ring main unit is a set of power distribution electrical equipment assembled into sectional ring main units or power supply units of electrical equipment within metal or non-metal insulation. Its core part uses load switches and fuses, featuring simple structure, small size, low cost, and advantages such as improved power supply parameters, performance, and safety. It is widely used in urban residential communities, high-rise buildings, large public buildings, factories, enterprises, and other load center distribution substations and box-type substations. A ring main refers to a ring-shaped distribution network, where the power supply relay forms a closed ring, delivering power to the ring relay, and from the relay, distribution is carried out through a high-voltage switch. The benefit of this is that each distribution branch can receive power from its left and right main trunks. If the main line on the left or right breaks, the main power supply will continue; when there is a problem with the main line, power from the left and right main sources will continue. Although the total power is from a single source, each branch is distributed similarly to dual power supply, improving the reliability of power supply.

　　The so-called "ring main unit" means that each distribution branch is equipped with a switchgear cabinet (outgoing switchgear), and the switchgear busbar is also part of the ring main relay. In other words, it is connected by the busbars of each outgoing cabinet. Each outgoing cabinet is called a "ring main unit." In fact, it is impossible to see the meaning of "ring main" by looking at just one ring main unit.

　　The rated current of these ring main units is not large, so the high-voltage switches in ring main units generally use simple high-voltage load switches with high-voltage fuses, rather than complex circuit breakers. That is to say, the high-voltage switch in the ring unit is generally a load switch. The ring main unit uses a combination of load switches to carry normal current and uses fuses to cut off fault current, replacing circuit breakers. Of course, this is limited to a certain capacity.

　　This type of switchgear can also be applied in distribution systems without ring main structures. Therefore, with the widespread application of this switchgear, the term "ring main unit" has gone beyond the scope of ring main distribution and generally refers to high-voltage switchgear with load switches as the main switch.

　　Ring main units have not been around for long but have already been widely used, mainly due to the new development of buildings in industrial enterprises and institutions. From the perspective of power supply and distribution, the capacity of their smaller distribution transformers generally does not exceed 1250KVA, with long-term stable power supply and infrequent operation of high-voltage switches. For such users, the performance of ring main units is not perfect but sufficient. With large-scale urban construction, a large number of large high-rise buildings have sprung up like mushrooms after rain, and distribution is often set downstairs. According to fire protection requirements, the transformer and distribution equipment must be "oil-free." Based on reasonable use of building area, the transformer and distribution equipment should be "small." Thus, ring main units or vacuum circuit breakers, dry-type transformers, and dry-type transformers have entered buildings. Ring main units have a simple structure, reliable and safe operation, low maintenance costs, and low operating costs. Compared with switchgear equipped with circuit breakers, their advantages are very prominent. Ring main units require load switches with higher performance and smaller size. New types of load switches have emerged, which in turn promote the technological progress of ring main units. Today's ring main units not only have excellent performance but have also become standardized, further expanding their range of use.