Ring Main Unit

A ring main unit, or RMU, is a factory assembled, metal enclosed set of switchgear used at the load connection points of a ring type electricity distribution network. It comprises two switches that can connect a load to either or both main conductors of the ring, together with a fusible switch or circuit breaker that supplies a distribution transformer, and the enclosed cabinet connects to that transformer through a standardized bus throat or cables. Ring main units are used for medium voltage distribution from roughly 7,200 up to about 36,000 volts, were first introduced in the United Kingdom and are now used internationally; in North America the same function is often built into pad mounted transformers that combine switches and transformer in a single cabinet. They may be air, oil or gas insulated, use fusible switches or circuit breakers with vacuum or gas insulated interrupters, and can include protective relays that trip the circuit breaker during a fault.