RMU Selection Guide: SF6 vs Solid-Insulated 12–24kV

Selecting a ring main unit (RMU) requires matching five interdependent variables: system voltage, insulation technology, module configuration, short-circuit rating, and environmental protection class. An error in any one of these can result in equipment that is undersized, non-compliant with local regulations, or over-specified at unnecessary cost. This guide provides a structured approach to RMU selection for project engineers and procurement managers.

Step 1: Confirm System Voltage and Insulation Level

The first and most important selection criterion is the system nominal voltage. RMU rated voltage must be equal to or greater than the highest system voltage (Us) of the network.

Key rule: Never specify the rated voltage based on nominal voltage alone. Use the highest system voltage. A 10 kV distribution system with a 10.5 kV voltage tap must be treated as a 12 kV system for equipment selection.

Step 2: Select Insulation Technology

Modern ring main units use three insulation technologies. The choice is driven by environmental regulations, installation environment, and project lifecycle requirements.

SF6 Gas-Insulated RMU

The dominant technology for the past 40 years. All live parts are enclosed in a sealed stainless-steel tank filled with SF6 gas at low pressure (0.03–0.05 MPa). SF6 provides both dielectric insulation and arc quenching.

Advantages:

• Most compact design — footprint typically 400–500 mm wide per module
• Proven 25+ year track record in thousands of installations worldwide
• IP67 gas tank rating — suitable for flooding, high humidity, coastal environments
• Lowest capital cost per kW of distribution capacity

Disadvantages:

• SF6 GWP = 23,500 — the EU F-Gas Regulation (EU 2024/573) progressively restricts use and requires registration of all SF6-filled equipment from 2026
• Requires certified SF6 gas handling for decommissioning
• Gas density monitoring required for early leak detection

Specify SF6 when: Project is in a market with no current F-Gas restrictions; footprint is the critical constraint; budget is constrained.

Solid-Insulated (Vacuum) RMU

The fastest-growing RMU technology. Live parts are encapsulated in solid epoxy or silicone resin, with vacuum interrupters for arc quenching. Eliminates SF6 entirely.

Advantages:

• Zero greenhouse gas — fully F-Gas regulation compliant, no EU restrictions
• No gas monitoring or handling equipment required
• Comparable footprint to SF6 designs in current generation units
• Lower total lifecycle cost (no gas management, no gas disposal)
• IEC 62271-200 compliant; compatible with existing ring network architectures

Disadvantages:

• Slightly higher capital cost than SF6 (typically 10–20% more)
• Less field service infrastructure in some developing markets

Specify solid-insulated when: Project is in the EU, UK, or any market with F-Gas regulations; project has a 25+ year lifecycle requiring future-proof specification; green building certification (BREEAM, LEED) requires low-GWP equipment.

Air-Insulated RMU (AIRMU)

Uses atmospheric air as the insulating medium, with vacuum interrupters. Largest physical footprint of the three types but lowest environmental impact.

Specify air-insulated when: Space is not a constraint; SF6-free specification is required but solid epoxy encapsulation is not acceptable; specific markets (Australia, New Zealand, Nordic countries) with strong preference for air-insulated designs.

Insulation Technology Comparison Table

Step 3: Select Module Configuration

The module configuration defines the number and type of switching functions in the RMU. Modules are designated as:

• L — Load break switch: can make and break load currents (not fault currents); used for ring incoming/outgoing connections
• V — Vacuum circuit breaker: can make and break fault currents; replaces F modules when overcurrent protection without fuse is required
• F — Fuse-switch: load break switch with HV HRC fuse; used for transformer feeder protection
• T — Transformer control module: combined switch and fuse for compact transformer substations
• C — Cable connection module: no switching function; metering or additional cable termination only

Common Configurations and Their Applications

Selection rule: Start with the minimum configuration that meets your switching requirements. The 2L+1F covers the majority of urban ring network nodes. Add F modules if multiple transformers are fed. Upgrade L to V (circuit breaker) modules only if the protection scheme requires fault-breaking at the RMU level — this adds cost and complexity.

Step 4: Specify Short-Circuit Ratings

Two short-circuit parameters must be specified and verified against the network:

Rated short-time withstand current (Ik): The maximum fault current the bus bars and switching components can carry for 1–3 seconds without damage. Common values are 12.5 kA, 16 kA, and 20 kA. Match this to the maximum prospective fault current at the installation point, calculated from the network impedance.

Rated short-circuit making current (Ip): The peak current the switching device can close onto (IEC 62271: Ip = 2.5 × Ik for X/R = 12). For a 20 kA system: Ip = 50 kA peak.

Underspecifying the short-circuit rating is a safety hazard — it can cause the RMU enclosure to rupture under fault conditions. If the network fault level is uncertain, specify 20 kA as a conservative default for urban MV networks in most countries.

Step 5: Verify Environmental Protection Requirements

IEC 62271-200 Type Test Checklist

Before accepting a ring main unit from any supplier, verify that the equipment has been type-tested to IEC 62271-200 (and GB 3906 for China-manufactured units) at an accredited third-party laboratory. The minimum required type tests are:

• Dielectric tests: Power frequency withstand (28 kV/1 min for 12 kV; 50 kV/1 min for 24 kV) and lightning impulse withstand (75 kV for 12 kV; 125 kV for 24 kV)
• Short-circuit tests: Rated making current test (50 kA peak for 20 kA equipment); rated breaking current test for circuit breaker modules
• Load break switching tests: 200 operations at rated load current for L modules
• Temperature rise test: Rated continuous current at ambient temperature
• Mechanical endurance test: 1,000 operations for VCB modules; 200 for LBS modules
• IP classification test: Per IEC 60529 for the relevant IP rating
• Internal arc test (IAC): Increasingly required for urban installations — confirms the enclosure contains an internal arc fault without injury to personnel

Ask for the full type test report from a recognized laboratory (CESI, KEMA, NOJA Power Lab, SGS), not just the type test certificate. The report shows actual test values compared to pass criteria — a certificate alone does not confirm the test was passed at the required ratings.

NAIJI Electric XGN15-12 RMU: Technical Overview

NAIJI Electric's XGN15-12 ring main unit is a 12 kV SF6 gas-insulated RMU manufactured to IEC 62271-200 and GB 3906 standards. Key parameters:

NAIJI also manufactures a 24 kV solid-insulated RMU for F-Gas-free projects and export to EU markets. Annual production capacity: 10,000+ RMU units. Type test reports available from CESI and national grid utility approved laboratories. [Contact NAIJI for RMU specifications →](/contact/)

Selection Checklist Summary

1. Confirm system nominal voltage → select Ur (12, 24, or 36 kV)
2. Check local F-Gas regulations → select SF6 or solid-insulated
3. Count ring connections + transformer feeders → define module configuration (2L+1F, 2L+2F, etc.)
4. Calculate maximum network fault level → specify Ik (12.5, 16, or 20 kA)
5. Assess installation environment → specify IP rating
6. Require IEC 62271-200 type test report from accredited laboratory
7. Verify IATF / ISO 9001 certification and production capacity