A ring main unit (RMU) is a compact medium-voltage switchgear assembly used to distribute electricity safely and reliably in ring-network or radial-network systems. It is widely used in urban distribution networks, commercial buildings, industrial facilities, renewable-energy projects and compact substations.
This guide explains what an RMU is, its main components, typical applications and the key points to consider when selecting one for a project.
What is an RMU switchgear?
An RMU combines several medium-voltage functions in one metal-enclosed unit. Depending on the configuration, it can include load break switches, circuit breakers, fuses, earthing switches, busbars, protection relays and cable compartments. By combining these functions, an RMU provides compact distribution, isolation and protection without the space required by a conventional switchgear lineup.
Most RMUs are designed for 12 kV systems, while 24 kV and 36 kV solutions are also available. The choice between air-insulated and SF6 gas-insulated construction depends on the application, footprint, environmental requirements and maintenance strategy.
Main RMU components
- Incoming and outgoing feeders: connect the ring network and distribution cables.
- Load break switch or circuit breaker: provides switching and fault-protection functions.
- Fuse or relay protection: protects transformer and feeder circuits.
- Earthing switch and mechanical interlocks: support safe isolation and maintenance.
- Metering and automation interface: supports SCADA, fault indication and remote operation when required.
Where are RMUs used?
RMU switchgear is especially suitable where a reliable compact distribution point is needed. Common applications include city power networks, industrial plants, commercial complexes, wind and solar projects, rail infrastructure, data centres and prefabricated substations.
For urban projects, the compact footprint and fully enclosed construction of an RMU can simplify installation in substations with limited space. For industrial users, the correct protection and automation options help isolate faults and keep critical loads operating.
How to choose an RMU
Start with the system voltage, rated current and prospective short-circuit level. A typical 12 kV RMU may be configured at 630 A, but the final selection must match the network study, cable specification and transformer capacity. Confirm the required fault level, protection scheme, number of feeder ways, metering requirement and control voltage before requesting a quotation.
Also decide whether the project needs a fixed or extensible configuration, manual or motorized operation, and local or remote control. For outdoor or prefabricated substation projects, enclosure rating and environmental conditions should be included in the specification.
RMU options from Enertro
Enertro supplies medium-voltage RMU configurations for power-distribution projects, including HXGN17-12 metal-enclosed RMU, RM6-C 12kV SF6 gas-insulated RMU and XGN15-12 SF6 RMU. You can also explore our medium-voltage switchgear range for related distribution solutions.
Need help selecting an RMU? Send your single-line diagram, voltage rating, feeder quantity and protection requirements to our team. We will recommend a suitable configuration for your project.