Ring Main Units (RMUs)
A ring main unit is a compact MV switchgear panel, usually three-way, that loops a medium voltage ring between substations so a fault on one section can be isolated without cutting power to the rest of the loop. NLS Power sources and supplies RMUs built to IEC 62271-200 for compound, industrial park and campus distribution networks across Saudi Arabia.
A ring main unit, almost always shortened to RMU, is a compact switchgear assembly that sits at each substation along a medium voltage ring distribution network, typically with two ring switches to accept incoming and outgoing ring cable and one switch, fused or with a circuit breaker, feeding the local transformer. The ring topology is the point: if a cable fault occurs anywhere on the loop, the two RMUs either side of the fault open, isolating that section, while every other substation on the ring stays supplied from the other direction, cutting outage scope and duration compared with a simple radial feed.
IEC 62271-200 covers the switchgear assembly, and the switching medium is usually SF6 gas, vacuum interrupters with air or solid insulation, or increasingly SF6-free designs using vacuum switching with clean air or another lower-impact insulating gas, a specification point worth raising early given the direction environmental regulation is moving. Interlocking between the ring switches and the transformer feeder switch prevents an operator from creating an unsafe switching sequence, and SEC connection approval typically specifies particular interlock and earthing requirements for RMUs feeding a utility-connected transformer.
RMUs suit any distribution layout with multiple substations that benefit from ring rather than radial supply: industrial parks, large compounds, university and hospital campuses, and master-planned developments with several buildings sharing one incoming MV supply. Extensible RMU designs let a project add a bay later as the development phases in more buildings, without replacing the whole switchgear assembly.
NLS Power sources and supplies RMUs sized to the ring configuration, switching medium and number of ways a project's distribution network design calls for.
Specifications
- ✓Configuration: typically three-way (two ring switches, one transformer feeder), extensible designs available
- ✓Switching medium: SF6 gas, vacuum with air/solid insulation, or SF6-free vacuum/clean-air designs
- ✓Standards referenced: IEC 62271-200 for the switchgear assembly
- ✓Interlocking: mechanical interlocks between ring switches and transformer feeder prevent unsafe switching sequences
- ✓Feeder protection: fused switch-disconnector or circuit breaker on the transformer way, per fault level and transformer rating
- ✓Typical use: industrial parks, large compounds, university and hospital campuses, master-planned multi-building sites
Frequently Asked Questions
What does a ring main unit actually do?+
It sits at each substation on a medium voltage ring and lets a fault on one cable section be isolated by opening just the two RMUs either side of it, while the rest of the ring keeps running supplied from the other direction. That's a materially faster and less disruptive recovery than a radial network offers.
Is SF6 gas still used in ring main units?+
It's still common, but SF6-free designs using vacuum interruption with clean air or another lower-impact insulating medium are increasingly specified given the direction environmental regulation is moving. Both options are available; the choice usually comes down to the project spec and long-term maintenance philosophy.
Can a ring main unit be extended later if a development adds more buildings?+
Extensible RMU designs let a project add another bay to the assembly as new buildings come online, without replacing the whole switchgear unit. It's worth specifying an extensible type upfront on any phased master-planned development.