What Factors Should Engineers Consider When Selecting a Ring Main Unit?

Selecting the right Ring Main Unit is a critical decision that impacts the reliability, safety, and operational efficiency of medium voltage distribution networks. For engineers, the choice goes beyond basic specifications—it involves evaluating short-circuit withstand capabilities, insulation technology, environmental conditions, and long-term maintenance strategies. A poorly chosen Ring Main Unit can lead to unplanned outages, increased arc flash risks, and higher total cost of ownership. Therefore, understanding the interplay between load break switches, fault interruption mechanisms, and enclosure ratings becomes essential for any robust power system design.

At Lugao Power Co.,Ltd., we have spent decades refining our approach to RMU engineering. In this article, we break down the technical factors that should drive your selection process. From dielectric properties to future scalability, we provide a checklist that aligns with both IEC and IEEE standards. Whether you are designing a secondary distribution network for a wind farm, a data center, or an industrial plant, these insights will help you make a confident, data-driven decision. Our factory produces more than 10,000 units annually, and we have learned that successful projects always start with asking the right questions about the Ring Main Unit.

Table of Contents

1. What Are the Key Electrical Parameters That Define a Ring Main Unit’s Performance?

When engineers specify a Ring Main Unit, the primary electrical parameters form the backbone of every design calculation. These parameters determine whether the RMU can handle normal load currents, transient overvoltages, and short-circuit conditions without compromising the network. At Lugao Power Co.,Ltd., our factory insists on verifying each parameter through type tests, and we recommend that engineers request third-party test reports. Below are the critical electrical parameters you must analyze.

• Rated Voltage (Ur): Typically 12kV, 24kV, or 36kV for secondary distribution. The RMU must withstand continuous operating voltage plus temporary overvoltages (e.g., 1.2x Ur for 8 hours). Our factory’s Ring Main Unit models cover 12kV to 40.5kV, with a 1.5 times lightning impulse withstand voltage of 95kV for 24kV versions.
• Rated Current (Ir): Common values are 630A or 1250A for feeder and loop applications. But consider: is the RMU expected to handle future load growth? We often advise engineers to select a 1250A main busbar even if initial load is 400A, because our factory data shows that 70% of substations expand within five years.
• Short-Time Withstand Current (Ik): Usually 20kA/1s, 25kA/3s, or 31.5kA/1s. This is the ability to tolerate short-circuit current without mechanical or thermal damage. For industrial grids with high fault levels, we recommend 25kA/3s minimum. Our Lugao Power Co.,Ltd. Ring Main Unit achieves 31.5kA/3s on all busbar connections.
• Peak Withstand Current (Ip): 2.5 times Ik (e.g., 50kA for 20kA RMU). This electrodynamic withstand ensures that conductors do not repel or deform during the first peak of a fault.
• Rated Short-Circuit Making Capacity: For load break switches, this is the ability to close onto a fault. Our factory’s units are rated for 63kA peak making current, ensuring robust closure under worst-case scenarios.
• Internal Arc Classification (IAC): IEC 62271-200 defines IAC ratings such as AFL/AFLR 12kV 20kA/1s. We always advise engineers to specify IAC with accessibility (e.g., AFL for restricted access). Lugao Power Co.,Ltd. provides IAC A FL 31.5kA/1s on all Ring Main Unit enclosures.

Beyond these, consider the power frequency withstand voltage (dry and wet) and partial discharge levels. For a 24kV RMU, the 1-minute power frequency withstand should be 50kV. Partial discharge inception voltage must be above 1.1 Ur. Our factory uses automated PD testing for every Ring Main Unit before shipping. The following table summarizes our standard parameters for the LGU-RMU series, which engineers can use as a benchmark.

Finally, engineers must verify the rated short-circuit breaking capacity for any integrated circuit breakers. In our experience, a common oversight is ignoring the transient recovery voltage (TRV) characteristics. Our factory ensures that every Ring Main Unit with a vacuum breaker meets TRV class C2 or E2. Remember: electrical parameters are not independent; for example, higher Ir often reduces short-circuit withstand due to thermal constraints. Always cross-check using manufacturer-provided derating curves. Lugao Power Co.,Ltd. provides free simulation reports upon request.

2. How Does the Insulation Medium (SF6 vs. Dry Air vs. Solid) Impact Reliability and Environmental Compliance?

The insulation medium inside a Ring Main Unit is more than just a dielectric—it dictates maintenance intervals, leakage risks, and even regulatory acceptance. Most engineers are familiar with SF6 (sulfur hexafluoride) due to its excellent arc-quenching properties, but growing environmental regulations (F-gas regulations, EU 517/2014) push for alternatives. Our factory has developed both SF6-insulated and eco-friendly dry air + solid insulated Ring Main Unit lines. Here is how each medium affects your selection.

• SF6-insulated RMU: Offers compact size (up to 70% smaller than air-insulated), high dielectric strength (3x that of air), and sealed-for-life construction. However, SF6 has a global warming potential (GWP) of 23,500. Leakage below 0.1% per year is acceptable, but regular gas density monitoring is required. Our Lugao Power Co.,Ltd. SF6 Ring Main Unit uses laser-welded stainless steel tanks, achieving less than 0.01% annual leakage.
• Dry Air / N2-insulated RMU: Uses purified air or nitrogen at 1.2–1.5 bar. GWP=0, no reporting obligations, but requires larger enclosure dimensions (20-30% larger) and lower dielectric strength. Our factory’s dry air RMU meets 24kV ratings with a 1.4 bar absolute pressure, and we include active pressure monitoring as standard.
• Solid Insulated RMU: Epoxy resin encapsulation eliminates gas leakage completely. Extremely robust for vibration-prone environments like offshore platforms. The downside: higher weight and limited repairability. For urban substations, our solid-insulated Ring Main Unit offers 100% green credentials and partial discharge free performance.

From a reliability standpoint, consider the operating environment. In coastal areas with high humidity, SF6 and solid insulation are superior because dry air systems may require additional drying cartridges. For cold climates (below -25°C), SF6 might liquefy unless heaters or gas mixtures (SF6/N2) are used. Our factory solves this with a special gas blend in our Arctic-grade Ring Main Unit. Also, evaluate the manufacturer’s ability to perform on-site refilling. Lugao Power Co.,Ltd. provides global service teams trained for both SF6 and eco-friendly media.

Environmental compliance is now non-negotiable. Many utility tenders require a lifecycle assessment (LCA). Our factory has conducted LCA for our RMU portfolio, showing that a dry air Ring Main Unit reduces carbon footprint by 98% over 30 years compared to SF6. For engineers working on Green Building certifications (LEED, BREEAM), selecting a low-GWP RMU can contribute to credits. However, SF6 still offers unmatched compactness for underground substations. The decision matrix should include: space constraints, leakage monitoring capability, and local environmental regulations. At Lugao Power Co.,Ltd., we offer a hybrid solution: SF6 main tank with dry air cable compartments, balancing performance and sustainability.

3. Why Should Engineers Evaluate Fault Making and Breaking Capacities Before Selection?

Fault making and breaking capacities are the safety nets of any Ring Main Unit. While a load break switch can interrupt load currents up to rated current, a fault situation demands that the RMU can close onto a short circuit (making capacity) and then break the fault current (breaking capacity) – often in coordination with upstream protection. Our factory has witnessed failures where engineers assumed the RMU's switch-disconnector could handle fault duties similar to a circuit breaker. This is a dangerous misconception.

• Fault making capacity (peak value): For a Ring Main Unit’s load break switch, the making capacity is typically 2.5 to 3 times the rated short-time current. For a 25kA/3s RMU, the making capacity should be at least 62.5kA peak. Our factory tests every batch with a synthetic test circuit to verify making performance under worst-case asymmetry.
• Fault breaking capacity (RMS): Standard load break switches are not designed to break short-circuit currents; they rely on fuses or upstream circuit breakers. If your design uses the RMU as a switching device only, ensure that the upstream breaker coordinates with fuse characteristics. However, a Ring Main Unit equipped with a vacuum circuit breaker can have breaking capacities up to 31.5kA symmetrical. Lugao Power Co.,Ltd. offers an RMU with an integrated VCB, breaking capacity 25kA at 24kV.
• Transfer current breaking: In loop configurations, when one side is open and a fault occurs, the RMU must break the transfer current (typically up to 1.2x Ir). Our factory’s SF6 three-position switch (closed, open, earthed) is tested for 1000 operations at transfer current 850A.

Another critical factor is the earth switch’s making capacity. Earth switches must be capable of closing onto a live fault – a requirement often ignored. According to IEC 62271-102, the earth switch should have a rated short-circuit making capacity equal to the peak withstand current. Our factory’s Ring Main Unit includes an interlocked earth switch tested at 62.5kA peak. We always advise engineers to request test certificates for both making and breaking duties, not just the thermal withstand.

Furthermore, consider the number of fault-breaking operations before maintenance. For vacuum interrupters, the electrical endurance can be 100 full fault operations at rated current. Our factory uses copper-chromium contacts to achieve 30 full fault interruptions at 31.5kA. In contrast, SF6 rotary switches have lower fault breaking capability – they rely on the pressure rise from the arc. In our experience, specifying a Ring Main Unit with a combination of fuse-switch for feeder protection and VCB for incomer gives the best balance. Lugao Power Co.,Ltd. provides a coordination study as part of our engineering support, ensuring that your fault capacities align with relay settings and transformer inrush.

4. What Mechanical and Environmental Endurance Tests Validate a Ring Main Unit for Harsh Conditions?

Even the best electrical parameters mean nothing if the Ring Main Unit fails due to corrosion, vibration, or extreme temperatures. Engineers must look beyond datasheets and demand proof of mechanical and environmental endurance. Our factory operates an ISO 17025 accredited lab where we subject every RMU design to the following validation tests. As a rule of thumb, if a manufacturer cannot provide these reports, do not specify their product.

• Mechanical endurance (M1/M2 class per IEC 62271-1): M1 = 1000 operations, M2 = 5000 operations for load break switches. For Ring Main Units in frequently switched networks (e.g., capacitor banks or wind turbines), we recommend M2 class. Our Lugao Power Co.,Ltd. Ring Main Unit achieves 10,000 operations on the three-position switch without maintenance.
• IP protection rating: For outdoor RMU (kiosk type), IP54 or IP65 is mandatory. For indoor, IP3X minimum. Our factory offers IP67 for submersion-prone areas – tested in 1m water for 30 minutes.
• Corrosion resistance (C5-M high corrosivity): According to ISO 12944, enclosures used in industrial or marine zones require C5-M coating. Our factory uses nanoceramic pretreatment + polyester powder coating, passing 1000 hours salt spray test per ASTM B117.
• Temperature rise test: At 1.1 x rated current, the temperature rise on contacts should not exceed 65K for silver-plated contacts. Our factory performs fiber-optic monitored temperature rise tests, ensuring our Ring Main Unit runs 15% cooler than IEC limits.
• Short-circuit and internal arc test: Beyond the standard 1s arc, we test for pressure relief effectiveness. Our factory’s RMU features a rear arc duct that directs gases away from the operator, verified with full-scale test at 31.5kA/1s.

Seismic qualification is often overlooked. In earthquake-prone zones, the Ring Main Unit must withstand acceleration levels (0.5g to 1.0g) without losing functionality. Our factory has qualified our RMU for 0.8g horizontal and 0.4g vertical per IEEE 693. Also, consider altitude derating: above 1000m, the dielectric strength reduces by about 1% per 100m. For installations at 2000m, our factory provides a modified insulation design with increased clearances.

Finally, ask for type test reports from an independent laboratory like KEMA, CESI, or IPH. These reports should include the thermal, dielectric, and mechanical sequences. At Lugao Power Co.,Ltd., we make our full type test reports available to engineers via a secure portal. We have also introduced a digital twin service: we simulate the mechanical life of your specific Ring Main Unit configuration using finite element analysis. This proactive validation ensures that no hidden weakness compromises your network.

5. How Can Lifecycle Costs and Modularity Influence Long-Term Substation Planning?

Engineers often focus on initial procurement cost, but the total cost of ownership (TCO) for a Ring Main Unit spans 20–40 years. Our factory has analyzed data from over 500 substations, and we found that maintenance, energy losses, and expansion costs account for 60% of TCO. Therefore, selecting an RMU solely on upfront price leads to higher long-term expenses. Here’s how to adopt a lifecycle perspective.

• Maintenance intervals and access: An SF6 sealed RMU requires no gas handling for 20 years if leak rate is below 0.1% per year. Our factory’s Ring Main Unit has a maintenance-free design for the switching chamber. However, for dry air units, periodic pressure checks are needed. We train local partners to perform 5-year inspections.
• Energy losses (load losses): Copper losses in busbars and contacts can be significant. A 630A RMU with 150W losses at full load over 20 years (assuming $0.12/kWh) costs $3150 in losses alone. Our factory optimizes contact design using silver-nickel alloy, reducing losses by 18% compared to standard designs.
• Modularity and expandability: A future-proof Ring Main Unit should allow adding feeder modules without gas handling or busbar disassembly. Our factory offers a modular “plug-and-bus” system: you can extend a 2-feeder RMU to 6 feeders in 2 hours. This modularity avoids costly substation rebuilds.
• Spare parts availability: Ensure the manufacturer guarantees spare parts for at least 25 years. Lugao Power Co.,Ltd. stocks all components for our RMU models, and we assign a unique QR code to each unit for instant part identification.

Additionally, consider the cost of unplanned outages. According to our factory’s reliability database, a Ring Main Unit with internal arc classification and fault-tolerant mechanisms reduces outage duration by 80%. We incorporate self-diagnostic sensors (temperature, partial discharge, gas density) that connect to a cloud-based monitoring platform. This predictive maintenance feature adds 3-5% to initial cost but reduces lifecycle cost by 22% on average. For engineers working on critical infrastructure (hospitals, data centers), we recommend our “Smart RMU” with real-time analytics.

Finally, evaluate the manufacturer’s ability to provide training and remote support. Our factory includes 2 days of on-site commissioning training for every Ring Main Unit order, plus a 10-year warranty on the vacuum interrupter. When you calculate TCO, include decommissioning costs: SF6 units require certified gas recovery, whereas our eco-friendly dry air RMU can be scrapped without special procedures. By choosing Lugao Power Co.,Ltd., engineers align financial planning with operational excellence.

Summary: Integrating Technical Factors into a Coherent RMU Strategy

Selecting a Ring Main Unit is a multi-dimensional challenge that balances electrical robustness, environmental adaptability, safety, and economic foresight. Engineers must prioritize rated voltage and short-circuit capacity as the non-negotiable foundations. Then, the choice of insulation medium (SF6, dry air, or solid) shapes both regulatory compliance and physical footprint. Fault making/breaking capacities must be verified through type tests, not just datasheet claims. For harsh environments, demand proof of mechanical endurance and corrosion resistance. Lastly, a lifecycle cost analysis that includes modularity and predictive maintenance will prevent expensive retrofits. At Lugao, our factory engineers collaborate with clients to create customized RMU solutions that adhere to these principles. We invite you to leverage our 20 years of field data and our Ring Main Unit configurator to build a specification that delivers safety, reliability, and value for decades.

Frequently Asked Questions (FAQ) – Ring Main Unit Selection