A ring main unit (RMU) is compact, factory-assembled medium-voltage switchgear used in secondary distribution networks. It sits where cables from a ring-shaped network meet a distribution transformer, so that power can reach every transformer from two directions and a faulty cable section can be isolated without cutting supply to the rest of the ring.
How an RMU works
A typical RMU has two cable switches for the incoming and outgoing ring cables and one transformer feeder. The cable switches are load-break switches. The transformer feeder is protected either by a fuse-switch combination or by a vacuum circuit breaker with a protection relay. Units are named by their functions: CCF means two cable switches and one fuse-switch, and CCV means two cable switches and one circuit breaker.
Typical ratings
- Rated voltage: 12 kV, 24 kV, or 36/40.5 kV for wind and solar collection networks
- Rated current: commonly 630 A for the ring switches
- Short-time withstand current: commonly 20 kA or 25 kA
- Indoor units for substations and buildings; outdoor units in weatherproof enclosures
Insulation types
| Type | How it works | Points to consider |
|---|---|---|
| SF6 gas-insulated | Live parts sealed in a stainless-steel tank filled with SF6 | Most compact; SF6 is a greenhouse gas, restricted in new MV equipment in some markets |
| Solid-insulated | Live parts encapsulated in epoxy resin | SF6-free and widely made in China; check partial discharge test results |
| Air-insulated | Air between live parts, often with SF6 or vacuum switches | Lower cost, larger footprint; common in older Chinese designs |
What to check when buying an RMU from China
- Configuration and number of ways match your network design
- Insulation medium accepted by your utility, and any SF6 restrictions in your market
- Type test reports to IEC 62271-200, including internal arc classification
- Motorised operation and remote terminal unit (RTU) support if the network is automated
- Extensibility, in case more ways are needed later
- Utility approval: many utilities keep a list of approved RMU suppliers
RMUs are also the core of most prefabricated (box-type) substations. Ranked lists of Chinese RMU and prefabricated substation manufacturers are linked below.