A Ring Main Unit (RMU) is an electrical device used in secondary distribution networks to provide a continuous power supply to the customer by shifting sources in the event of any emergency or repair on the primary source. It is a factory-assembled, metal-enclosed set of switchgear used at the load connection points of a ring-type distribution network. The RMU includes two switches that can connect the load to either or both main conductors, and a fusible switch or circuit breaker and switch that feed a distribution transformer. The metal enclosed unit connects to the transformer either through a bus throat of standardized dimensions or through cables and is usually installed outdoors. RMUs can be characterized by their type of insulation: air, oil, or gas. The switch used to isolate the transformer can be a fusible switch or may be a circuit breaker using vacuum or gas-insulated interrupters. The unit may also include protective relays to operate the circuit breaker on a fault. RMUs are used for medium-voltage power distribution, from 7200 volts to about 36000 volts.