What Is a Ring Main Unit?
A ring main unit (RMU) is a compact medium voltage switchgear assembly designed for secondary distribution substations. RMUs are the standard switching solution in underground cable distribution networks — the type used in cities, industrial parks, commercial zones, and residential developments worldwide.
The term "ring main" refers to the network topology: cables form a closed loop (ring) with power available from both directions. This means a fault on one cable section can be isolated while the rest of the ring continues to receive power, significantly improving supply reliability.
RMU Functional Configurations
RMUs are specified by their functional units. Each unit contains either a load break switch (designated "R" for ring) or a circuit breaker (designated "T" for transformer or "V" for vacuum breaker):
| Configuration | Description | Application |
|---|---|---|
| RRT | 2 ring switches + 1 transformer feeder | Standard secondary substation with one transformer |
| RRTT | 2 ring switches + 2 transformer feeders | Secondary substation with two transformers |
| RRTV | 2 ring + 1 transformer + 1 VCB feeder | Substation feeding important loads requiring protection |
| RR | 2 ring switches only | Cable junction point (no transformer) |
RMU Insulation Types Compared
SF6-Insulated RMU
SF6 gas provides excellent insulation and arc-quenching properties in the smallest possible enclosure. SF6 RMUs have been the industry standard for over 30 years. However, SF6 is a potent greenhouse gas (GWP = 23,500) and faces increasing regulatory pressure under the EU F-Gas Regulation and similar legislation worldwide.
Solid-Insulated Switchgear (SIS) RMU
SIS RMUs use solid epoxy resin insulation for the primary components and vacuum interrupters for switching. No gas is used — making them completely gas-free and truly maintenance-free. SIS technology has matured rapidly and now matches SF6 performance in the 12-24kV range. The trade-off is a slightly larger footprint compared to SF6.
Air-Insulated RMU
Air-insulated RMUs use air at atmospheric pressure for insulation. They are the largest option but have the lowest environmental impact and simplest construction. Air-insulated designs are gaining interest as a straightforward SF6-free alternative, particularly in regions with strict environmental regulations.
Comparison Table
| Parameter | SF6 | Solid Insulated (SIS) | Air Insulated |
|---|---|---|---|
| Footprint (RRT config) | Smallest | 10-20% larger | 40-60% larger |
| Environmental impact | High (GWP 23,500) | None | None |
| Maintenance | Annual gas check | None | Periodic cleaning |
| Design life | 30 years | 30 years | 25-30 years |
| Cost | Medium | Medium-High | Low-Medium |
| Max voltage | Up to 40.5 kV | Up to 24 kV (expanding) | Up to 36 kV |
| Altitude derating | Minimal | None | Significant above 1000m |
How to Choose
For new installations in regions with F-gas regulations (EU, UK, Australia), solid-insulated RMUs are the future-proof choice. For retrofit projects replacing existing SF6 equipment in the same footprint, SF6 RMUs may still be necessary due to dimensional constraints. For cost-sensitive projects with available space, air-insulated RMUs offer the simplest solution.
NAIJI Electric manufactures both SF6 and solid-insulated RMUs for 12kV and 24kV distribution networks. Our XGN-12 series provides flexible configurations from RR to RRTTVV. Request a quote for your distribution project.