What is a ring main unit (RMU)?
A ring main unit is compact, factory-built medium voltage switchgear used in distribution substations. A typical 11 kV RMU has two load break switches for the ring cables coming in and going out, and one transformer feeder that supplies the substation transformer. Everything that carries high voltage sits in a sealed tank, filled with SF6 gas or, in newer designs, dry air or solid insulation. That makes the unit small, low maintenance and safe to stand in front of.
Urban MV networks are laid as rings but run open at one point, the normally open point (NOP). If one cable section fails, the RMUs either side are switched to isolate it, and the NOP is closed to feed customers from the other end. You can try this on the MV Hub network simulator.
PractitionerRing switches are usually rated 630 A, with a short-time rating of 16 kA, 20 kA or 25 kA for 1 s or 3 s at 12 kV. Transformer feeders are fuse-switch combinations to IEC 62271-105 (about 200 A rated) or vacuum circuit breakers to IEC 62271-100 with a self-powered or auxiliary-powered relay.
ExpertCheck the switch class in IEC 62271-103 (E1/E2 electrical endurance, M1/M2 mechanical endurance) and the earthing switch class in IEC 62271-102 (E0, E1, E2 for short-circuit making). E2 earthing switches can close onto a live circuit more than once without needing maintenance, which matters for the operating error you can simulate above.
What are the functional units in an RMU?
| Letter | Functional unit | Main use |
|---|---|---|
| C | Cable switch (load break switch + earth switch) | Incoming and outgoing ring cables |
| F | Fuse-switch combination | Transformer feeder, typically up to about 1000 to 1250 kVA at 11 kV |
| V | Vacuum circuit breaker + relay | Larger transformers, MV feeders, or where selectivity or SCADA tripping is needed |
| D | Direct cable connection (no switch) | Some utilities use it for a solid cable tee |
| M | Metering unit (CTs and VTs) | MV metering for bulk supply customers |
How do RMU interlocks keep people safe?
Each cable switch in an RMU is usually a three-position switch: closed, open and earthed. Because it is one mechanism, it physically cannot be closed and earthed at the same time. On top of that, mechanical interlocks control the order of operation:
- The earth position can only be reached from open, never straight from closed.
- The cable compartment cover can only be removed when the earth switch is closed.
- The switch cannot leave the earth position while the cable cover is off.
- On a fuse-switch, a blown fuse striker trips all three phases and the fuses can only be reached with both sides earthed.
- On a breaker unit, the disconnector can only be operated with the circuit breaker open.
Interlocks do not know whether the far end of a cable is live. That is why you always check the voltage presence indicators (VPIS) before earthing, and why switching programs coordinate both ends of a cable.
What do LSC2, PM and IAC AFL mean?
These are IEC 62271-200 classifications that belong on every RMU datasheet:
- LSC2 (loss of service continuity): when you open one unit's cable compartment, the busbar and the other units can stay in service. Older documents also refer to LSC2A and LSC2B.
- PM (partition class): metallic partitions and shutters between live parts and the open compartment. PI means insulating partitions.
- IAC AFL 20 kA 1 s: internal arc classified for accessibility A (authorised staff), tested on the Front and Lateral sides at 20 kA for 1 s. Add R if the rear is accessible. Use B where the public can reach the enclosure, as with many kiosks.
ExpertThe internal arc test assumes a defined room layout and gas exhaust path. If your substation room is smaller, has a lower ceiling or vents differently from the tested setup, the IAC rating may not apply as installed. Ask the manufacturer for the minimum room dimensions, the pressure relief direction and the pressure rise calculation for the room.
SF6 vs SF6-free RMUs: what is changing?
SF6 is an excellent insulating gas but a very strong greenhouse gas. Regulation (EU) 2024/573 bans SF6 in new MV switchgear up to 24 kV from 2026, and up to 52 kV from 2030, with limited exceptions. Most major manufacturers now offer SF6-free RMUs that use vacuum interrupters with clean air, dry air or solid insulation. Outside the EU, utilities in the GCC, Australia, New Zealand and Asia are at different stages, so check whether the utility has approved the SF6-free model you want to specify.
Fuse-switch or circuit breaker for the transformer feeder?
Fuses are simple, fast and cheap, and they limit fault energy well. Circuit breakers with relays give adjustable protection, earth fault sensitivity, remote tripping and easier coordination with the LV main breaker. A practical rule:
- Up to about 1000 to 1250 kVA at 11 kV, a fuse-switch is common and usually accepted by utilities.
- Above that, or where you need selectivity, SCADA tripping, or transformer temperature and Buchholz trips, use a vacuum circuit breaker with a relay.
- Pick the fuse rating from the manufacturer's selection table so it rides through the transformer inrush (often about 10 to 12 times full-load current for 0.1 s) and still clears an LV terminal fault quickly.
Regional notes
GCC
Expect high ambient temperatures, dust and humidity. Check the derating at 50 °C, specify anti-condensation heaters, and only use RMU models on the utility's approved list (for example DEWA, ADDC/AADC, SEWA, KAHRAMAA or MEW). Saudi Arabia runs at 60 Hz with 13.8 kV and 33 kV networks under SEC specifications.
Australia
RMUs are specified to AS 62271.200 (identical to IEC 62271-200), with substation design to AS 2067 and the network operator's own standards. Pad-mount substations are common, so check the IAC class for public access.
New Zealand
The lines company (Vector, Orion, Wellington Electricity and others) sets the approved equipment and switching rules. Work under the Electricity (Safety) Regulations 2010 and the EEA Safety Manual - Electricity Industry (SM-EI).
Asia
Utilities such as TNB, SP Group, the Indian DISCOMs and K-Electric publish their own RMU specifications. 22 kV is the main voltage in Singapore, and 11 kV, 22 kV and 33 kV are all used in Malaysia and India.
Checks before energising an RMU
- Factory routine test certificates are reviewed, and the SF6 or gas pressure gauge is in the green zone.
- Cable tests are complete (for example VLF withstand or tan delta), with phasing checked end to end.
- Cable terminations are torqued, cable screens are earthed, and cable covers are fitted.
- Interlocks have been operated through every position, and the VPIS is tested with a phase comparator.
- Protection settings are applied and secondary injection tested (V units), or the fuse ratings match the approved design (F units).
- The substation earth is tested, and the room is clean and dry, with the pressure relief path clear.
Full test procedures are coming in MV-509 MV Testing & Commissioning Lab.