A ring main unit (RMU) is a type of switchgear used in ring networks to connect two power sources or sections. Its main function is to separate two segments, featuring a single function and compact size. In contrast, a mid-mounted switchgear is designed for end-users, offering comprehensive functions and a larger volume.
There are differences in the structural design and component selection between RMUs and mid-mounted switchgears, and their applications also vary significantly. RMUs are mainly used as interconnection switchgear. Mid-mounted switchgears, however, come in various types, including incoming and outgoing line switchgear, metering cabinets, current transformer cabinets, station transformer cabinets, and more.
Generally, RMUs are not equipped with protective devices. In the event of a fault, the internally installed fuses melt due to overheating to cut off the circuit. The switches commonly used in RMUs are indoor AC high-voltage load switch-fuse combinations or sulfur hexafluoride (SF₆) load switches. On the other hand, the switches in mid-mounted switchgears can be equipped with automatic protective devices. When a fault occurs, the protective device sends a signal to trigger the switch to trip and cut off the power supply. Most switches used in mid-mounted switchgears are VS1-type vacuum circuit breakers.
Due to their single function, RMUs are generally used to connect different power supply sections rather than to control, protect, or meter specific electrical equipment.
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