The ring main unit has not been in existence for a long time, but it has been widely used in recent years, which is mainly due to the newly developed industrial enterprises and public institutions, from the perspective of power supply and distribution, with medium and small capacity for most of its distribution transformers generally do not exceed 1250KVA, long-term stable power supply, high-voltage switch infrequent operation. For this type of user, the performance of the ring main unit is not perfect, but it is sufficient and competent. With the large-scale urban construction, a large number of large high-rise buildings rise from the ground, substations are often located downstairs, according to the requirements of fire protection, substation and distribution equipment must be "oil-free", according to the rational use of the construction area, substation and distribution equipment should be "miniaturized", so ring main unit or vacuum circuit breaker, dry-type transformer, dry-type transformer has entered the building. The ring main unit has a simple structure, reliable and safe operation, a small amount of maintenance, and low operating costs, which has outstanding advantages compared with the switch cabinet equipped with circuit breakers. The ring main unit requires the load switch to have higher performance and smaller volume, and the new type of load switch came into being, which in turn led to the technological progress of the ring main unit, and today's ring main unit not only has excellent performance and serialization, which expands the use range of the ring main unit.