In an electricity distribution grid, a ring main unit (RMU) is a factory assembled, metal enclosed set of switchgear used at the load connection points of a ring-type distribution network. It includes in one transformer protection units, two line 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.

Ring main units 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.