What is a Ring Main Unit?

A ring main unit (RMU) is a core switching device in medium-voltage distribution networks, specifically designed for ring network power supply systems to achieve segmentation, protection, isolation, and switching of cable feeders. Its typical application voltage levels range from 12 kV to 35 kV, and it is widely deployed in urban distribution networks, industrial parks, commercial complexes, data centers, and new energy access points.

Important Definition: According to IEC 62271-200, an RMU is a metal-enclosed switchgear, typically containing two incoming units, used to form a distribution ring network operating in closed or open loop mode.

The Core Function of Ring Main Unit

The RMU performs three core functions in the power distribution system

Power supply continuity guarantee

• With a dual-power ring network structure, when a section of the line fails, the faulty section can be quickly isolated, and power can be restored to the non-faulty area from the other power source
• The switching time can be as short as 30–60 seconds, significantly better than traditional radial networks

Equipment protection and isolation

• Integrated load switches (LBS) or circuit breakers (CBs), in conjunction with high-voltage current-limiting fuses, provide full-range protection for transformers and cables
• Provides visible breaks and mechanical interlocks to meet operational safety regulations

Flexible expansion and intelligent access

• The modular design supports plug-and-play expansion
• A communication interface is reserved to support remote monitoring, fault location, and self-healing control

Types and Technical Architectures of RMUs

Classification by main switch type

Type	Full Name	Key Features	Typical Applications
LBS + Fuse	Load Break Switch–Fuse Combination	Low cost; transformer protection up to 1250 kVA	Commercial buildings, residential complexes
Vacuum CB	Vacuum Circuit Breaker	Capable of interrupting short-circuit currents; suitable for frequent switching operations	Industrial plants, data centers, PV step-up substations

Selection principles

• If the downstream is a single transformer with a capacity ≤1250 kVA, LBS+Fuse (economical and efficient) should be preferred
• If multiple outgoing lines, frequent switching, or high reliability requirements are needed, a vacuum circuit breaker type RMU must be selected

Classification by insulation method（IEC 62271-200）

	Type	Insulation Medium	Advantages	Limitations
	Air-Insulated (AIS)	Dry air	Low cost; easy to inspect and maintain	Large footprint; affected by environmental conditions such as humidity and pollution
	Gas-Insulated (GIS)	SF₆ or eco-friendly gases (e.g., Clean Air)	Extremely compact (up to 70% space savings); maintenance-free; IP67-rated enclosure	Higher upfront cost; SF₆ requires strict environmental management
	Solid-Insulated Switchgear (SIS)	Epoxy resin / silicone rubber	Gas-free and environmentally friendly; excellent resistance to condensation	Limited heat dissipation; mainly used in high-altitude areas with thin air

Key Technical Parameters

Core performance parameters

	Parameter	Typical Values (12 kV Class)	Applicable Standard	Engineering Significance
	Rated Voltage	12 kV / 24 kV	IEC 62271-200	Must be ≥ the system’s maximum operating voltage (Um = 12/√3 × 1.1 ≈ 7.2 kV)
	Rated Current	630 A / 1250 A	—	Determines cable sizing and permissible temperature rise
	Short-Circuit Interrupting Capacity	LBS: Not applicable / CB: 20–25 kA	—	Circuit breaker type must be verified against the system short-circuit level
	Internal Arc Classification (IAC)	AFLR	IEC 62271-200 Annex AA	Ensures personnel safety (test pressure ≥ 1.5 bar)
	Loss of Service Continuity (LSC)	LSC-2B	—	Allows maintenance on one compartment while keeping the remaining feeders energized

Typical Engineering Application Scenarios

Application Scenario	Key Challenges	RMU Solution
Urban Underground Substations	Severe space constraints; high humidity	SF₆ GIS RMU with IP67 protection; maintenance-free design
Industrial Parks	Multiple transformers; high reliability requirements	Vacuum circuit breaker–type RMU with automation terminal (FTU)
PV / Energy Storage Plants	Outdoor installation; frequent switching operations	Solid-insulated or SF₆ RMU with C3 anti-corrosion coating; remote control capability
Commercial Complexes	Aesthetic requirements; low noise levels	Compact AIS RMU, wall-integrated installation, operating noise below 55 dB

Conclusion

Ring main units (RMUs) are the physical foundation for the transformation of modern power distribution networks from “passive response” to “active self-healing.” For engineers and EPC teams, selecting the appropriate switch type, insulation method (and level of intelligence) is key to achieving the optimal balance between cost, reliability, and sustainability.

Driven by new power systems and “dual-carbon” goals, RMUs are rapidly evolving towards SF₆-free, digital (state-aware), and modular (rapid deployment), becoming indispensable intelligent terminals for building resilient power distribution networks.

Appendix: Commonly Used Standards

IEC 62271-200: AC metal-enclosed switchgear for rated voltages above 1  kV and up to 52 kV

GB/T 11022: Common technical requirements for standards on high-voltage switchgear and controlgear

DL/T 593: Common technical requirements for standards on high-voltage switchgear and controlgear

FAQ

1. Can the load switch in the RMU interrupt the short-circuit current?

Absolutely not! Load switches (LBS) can only interrupt rated load current (e.g., 630 A) and small multiples of overload current. Short-circuit current (e.g., 20 kA) must be interrupted by a high-voltage current-limiting fuse connected in series. Using an LBS to interrupt a short circuit will result in equipment explosion.

2. Is a GIS RMU necessarily superior to an AIS RMU?

This is not the case,a comprehensive assessment is required:

Dimension	AIS RMU	GIS RMU
Initial Cost	Low (approx. RMB 80,000–120,000 per unit)	High (approx. RMB 150,000–250,000 per unit)
Footprint	Large (about 1000 mm wide)	Compact (about 600 mm wide, ~40% space savings)
Maintenance	Easy to inspect; on-site maintenance possible	Factory return typically required; high dependence on supplier
Environmental Impact	No greenhouse gases involved	SF₆ requires strict management (GWP = 23,500)

Recommendation: GIS is recommended for areas with limited space or high humidity, AIS is more economical for conventional industrial plants.

3. How to verify the “five protections” function of RMU?

Five protectionsrefers to:

• Prevent accidental opening/closing of switches
• Prevent opening/closing of disconnecting switches under load
• Prevent connecting grounding wires while the circuit is energized
• Prevent closing the circuit with the grounding wire connected
• Prevent accidental entry into energized compartments

Verification method

• Simulate each violation during the FAT (Factory Acceptance Test)
• Check whether the mechanical/electrical interlocks are forcibly blocked
• Require the supplier to provide a five-proof logic diagram and sign for confirmation

4. Can an RMU be used for generator output protection?

Not recommended. Generator short-circuit current decays slowly and has a high DC component, placing extremely high demands on the breaking capacity of switchgear. RMUs are typically designed for the distribution side (feeder protection), not the power supply side.

Recommendation: Use a dedicated generator circuit breaker at the generator outlet.

5. What communication protocols does a smart RMU need to support?

Mainstream protocols include：

• Modbus RTU/TCP: Used for SCADA data acquisition;
• IEC 61850 (GOOSE/MMS): Used for advanced applications in smart substations
• DNP3: Commonly used in the North American market

6. What are the environmentally friendly alternatives for SF₆RMU？

The industry is rapidly shifting towards SF₆-free RMUs：

• Dry air (N₂/O₂): CHINT and Siemens offer commercially available products
• Clean air (80% N₂ + 20% O₂): GWP=0, suitable for 12–24 kV
• g³ gas (3M Novec™): Dielectric strength close to SF₆, but at a higher cost
• The EU F-gas regulation will restrict the use of SF₆in new equipment from 2026

--- END ---