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Ring main unit

A Ring Main Unit (RMU) is a factory-assembled, metal-enclosed switchgear device designed for use in medium-voltage (typically 7.2–36 kV) ring-type electrical distribution networks, where it serves as a compact interface for connecting loads, transformers, and feeders while enabling switching, isolation, and protection to maintain supply continuity.^[1]^[2]^[3] RMUs typically incorporate key components such as load break switches, circuit breakers, fused disconnectors, current transformers (CTs), potential transformers (PTs), and protective relays, often arranged in a modular, gas-insulated configuration (commonly using SF6) to ensure safe and reliable operation in a sealed environment.^[1]^[2] These units support three-position functionality—close, open, and earth—for each switch, allowing operators to segment the ring network, isolate faults, and perform maintenance without widespread interruptions.^[2] In secondary distribution substations, RMUs play a critical role by forming closed-loop configurations that enhance system reliability, minimize downtime during faults, and facilitate integration with smart grid technologies like SCADA for remote monitoring and control.^[1]^[2] Their compact design and low-maintenance requirements make them particularly suitable for urban and underground applications, where space is limited and continuous power supply is essential, supporting current ratings up to 630 A and contributing to efficient power distribution in modern electrical grids.^[1]^[2]

Overview

Definition and Purpose

A ring main unit (RMU) is a factory-assembled, metal-enclosed switchgear unit designed for use at load connection points in ring-type medium-voltage (MV) distribution networks, typically operating at voltages such as 11 kV, 24 kV, or 33 kV.^[1]^[2]^[4] The primary purpose of an RMU is to enable switching, sectionalizing, isolation, and protection functions within these networks, thereby minimizing power outages through the rerouting of supply in looped configurations.^[5]^[6] By incorporating elements such as switches and fuses, RMUs facilitate safe disconnection and fault management without interrupting the broader supply.^[7] Ring main distribution represents a closed-loop topology in electrical power systems, where feeders supply loads from two directions around the loop, in contrast to radial systems that rely on a single unidirectional feed from a substation.^[8] This arrangement enhances reliability by providing an alternative path for power flow if one segment fails, reducing downtime compared to radial setups.^[9] In a basic ring circuit schematic, RMUs are positioned at key load points—such as distribution transformers—around the closed loop formed by the main feeder cable, allowing sectional isolation while maintaining continuity through the opposite direction of the ring.^[1]^[2]

Historical Development

The origins of the ring main unit (RMU) trace back to the early 20th century in Europe, coinciding with the rapid urbanization and electrification of cities, where ring-type distribution networks were developed to provide reliable power supply in dense areas. These early systems, primarily air-insulated, evolved from basic switchgear setups used for medium-voltage (MV) tee-off points in transformer stations, addressing the need for sectionalizing and isolating faults in looped circuits. By the 1930s, European utilities, particularly in the UK, began implementing ring main configurations for urban grids to enhance supply continuity, marking the foundational step toward integrated RMU designs.^[10] Post-World War II reconstruction accelerated RMU development in the 1950s and 1960s, as demand for compact, reliable MV equipment grew in rebuilding urban infrastructures. Initial RMUs were predominantly air-insulated, but the introduction of sulfur hexafluoride (SF6) gas as an insulating medium in switchgear during the 1960s revolutionized the field, enabling smaller footprints and higher reliability. Companies like ABB and Schneider Electric pioneered SF6 applications in RMUs by the late 1970s and early 1980s, with the first SF6-insulated outdoor/indoor unit—Schneider Electric's Ringmaster—launched in the UK in 1987, significantly reducing size while improving arc-quenching capabilities for urban substations. This shift from air-insulated to gas-insulated units was primarily driven by space constraints in city environments, where traditional air designs proved too bulky for underground or compact installations.^[11]^[12] Key milestones in the 1970s included the push toward international standardization through the International Electrotechnical Commission (IEC), which established early norms for enclosed MV switchgear under precursors to modern standards like IEC 62271, promoting interoperability and safety across Europe. By the 2010s, environmental concerns over SF6's high global warming potential prompted a regulatory shift, with the European Union's F-Gas Regulation (EU) No 517/2014 restricting its use and accelerating the development of eco-friendly alternatives such as vacuum-interrupter and air-insulated or fluoronitrile-based designs. As of 2024, this evolution continued with launches of SF6-free RMUs, including Lucy Electric's EcoTec synthetic air unit and Schneider Electric's Ringmaster AirSet, advancing sustainable urban power distribution.^[13]^[14]^[15]^[16]

Design and Components

Key Components

A ring main unit (RMU) typically incorporates several core functional components to facilitate sectionalizing, isolation, and protection within medium-voltage distribution networks. The primary elements include load break switches, which are designed to interrupt load currents safely under normal operating conditions, rated up to 630 A and complying with standards such as IEC 62271-103 for class E3 performance in making and breaking capabilities.^[17] Ring switches, often integrated as part of the load break switch assembly, enable the connection and isolation of feeders to form the ring circuit, ensuring continuity in the distribution loop.^[1] For overcurrent protection, fuses are employed in switch-fuse combinations, typically using high-rupturing-capacity (HRC) fuse-links rated for transformer protection up to 1600 kVA, while circuit breakers, such as vacuum-interrupter types rated at 200 A or 630 A, provide advanced fault interruption with relay integration for selective protection.^[17] Voltage and current transformers, housed in dedicated metering modules, facilitate monitoring and billing by stepping down voltages to 100 V and currents to 5 A, often following DIN 42600 specifications.^[1] Common configurations of these components adapt the RMU to specific network needs, such as the 2-switch plus fuse setup, where two load break switches bookend a fuse-protected outgoing feeder for transformer connections, providing isolation on both sides while the fuse handles fault clearing.^[17] In contrast, the 3-switch configuration incorporates three load break switches for tee-off points, allowing radial or ring operation with sectionalizing between incoming and outgoing lines, and internal wiring typically routes busbars through insulated bushings to connect modules in a compact, gas-insulated tank.^[1] These arrangements support up to five functional modules per unit, with busbar connections enabling extensibility.^[17] Auxiliary elements enhance safety and usability, including cable terminations that use standardized interfaces like Interface C for 630 A screened separable connectors, ensuring secure MV cable entry with provisions for testing points.^[17] Earthing switches, rated for short-time withstand currents up to 25 kA, provide grounding for isolated sections during maintenance, interlocked with the main switches to prevent operation under load.^[1] Indicators for voltage presence, often capacitive-type per IEC 61243-5, and gas pressure monitoring via manometers alert operators to live conditions or low SF6 levels, with fault passage indicators optionally integrated for directional short-circuit detection.^[17] Material specifics contribute to the RMU's reliability and compactness, featuring copper or aluminum busbars with a cross-section of 240 mm² for 630 A ratings, fully insulated within the enclosure to minimize partial discharges.^[17] In air-insulated models, arc chutes direct and extinguish arcs during switching, while gas-insulated variants rely on SF6 at 1.4 bar for superior dielectric strength and arc quenching.^[1]

Insulation and Enclosure Types

Ring main units (RMUs) primarily employ three types of insulation for medium-voltage applications: sulfur hexafluoride (SF6) gas, vacuum, and solid dielectrics. SF6 gas insulation is the most prevalent in RMUs due to its superior dielectric properties, offering approximately 2.5 to 3 times the breakdown strength of air at atmospheric pressure, which enables compact designs by allowing smaller electrode gaps while maintaining high voltage withstand capabilities.^[18]^[19] This insulation fills sealed compartments to prevent arcing and faults, typically at pressures of 0.1 to 0.5 MPa, enhancing reliability in urban distribution networks.^[20] Vacuum insulation, often integrated with vacuum interrupters, provides an alternative for fault interruption and insulation in RMUs, particularly in configurations where arc quenching occurs within a high-vacuum environment (around 10^-5 to 10^-7 mbar), minimizing contact wear and enabling rapid dielectric recovery post-switching.^[21] Vacuum-insulated RMUs are suitable for load-breaking applications up to 24 kV and offer maintenance-free operation over extended periods, though they may require hybrid designs with additional air or solid insulation for busbars.^[22] Solid-dielectric insulation, such as epoxy resin, encapsulates live parts in RMUs to create fully insulated, gas-free units, providing environmental benefits and resistance to partial discharges.^[23] These eco-variants use materials like hydrophobic cycloaliphatic epoxy (HCEP) for complete sealing, achieving IP67 protection levels and eliminating the need for gas handling.^[24] Epoxy resin solid-insulated RMUs are increasingly adopted for 12-24 kV systems, offering compact footprints and high mechanical stability.^[25] Enclosures for RMUs are typically constructed from metal cabinets, such as stainless steel for corrosion resistance in outdoor installations, ensuring durability against environmental factors like humidity and pollutants.^[26] These enclosures achieve IP54 or higher ratings to protect against dust ingress and water jets, with fully sealed designs preventing moisture accumulation and maintaining internal insulation integrity.^[27] Models vary between compact versions for space-constrained urban sites and extended-range enclosures for enhanced accessibility during maintenance.^[28] Since the 2010s, transitions to SF6 alternatives have gained momentum to mitigate greenhouse gas emissions, with dry air and fluoronitrile mixtures (e.g., C4-fluoronitrile/CO2 blends) emerging as viable options for insulation in RMUs.^[29] These mixtures provide dielectric strengths approaching or exceeding SF6 at elevated pressures, while offering global warming potentials less than 1% of SF6, supporting sustainable power distribution.^[30] Dry-air insulated RMUs, often paired with vacuum technology, further reduce environmental impact without compromising safety.^[31] Recent developments include the launch of Lucy Electric's EcoTec synthetic air RMU in March 2024, ABB's dry air insulated model in April 2024, and Schneider Electric's Pure Air RMU trialed by SA Power Networks in November 2024.^[32]^[33]^[34]

Operation and Functionality

Integration in Ring Networks

Ring main units (RMUs) are integrated into ring main distribution networks by being placed at key substations or load connection points, forming a closed loop topology that is typically fed from dual transformer sources to ensure balanced power distribution. This configuration connects multiple RMUs via ring cables, starting and ending at the same transformer substation on different busbar sections, often using two parallel cables in double-spur setups where each carries up to 50% of the load.^[35] The setup enables open-ring operation, where a normally open point (NO) is maintained at one RMU to facilitate maintenance without interrupting the entire network, while allowing reconfiguration to closed-ring mode as needed.^[35]^[36] The redundancy mechanism of RMUs in ring networks relies on the NO at one unit, which divides the loop into two radial feeds from the dual sources, providing an immediate alternative path for power supply if one segment fails. In the event of a fault, operators can isolate the affected section by opening ring switches at adjacent RMUs and closing the NO, restoring supply to healthy sections without widespread outages and minimizing interruptions.^[35] This selective switching enhances overall network reliability and operational safety by ensuring more functional power components than strictly required, often supported by circuit-breakers and short-circuit indicators for precise fault detection.^[35] RMUs in these networks typically operate at voltage levels of 6-36 kV and handle continuous currents from 200-630 A, making them suitable for medium-voltage secondary distribution.^[35]^[36] For instance, in an urban grid, multiple RMUs can form a loop around a city block or dense infrastructure like high-rise buildings and data centers, where the ring topology provides resilient power to critical loads in 10-30 kV systems.^[35]

Switching and Protection Mechanisms

Ring main units (RMUs) facilitate switching operations primarily through load break switches (LBS) and circuit breakers, enabling the isolation and reconfiguration of network sections in ring topologies. These switches are typically rated for normal currents up to 630 A and short-circuit withstand capacities ranging from 20 to 25 kA for durations of 1 to 3 seconds, allowing safe interruption of load currents without full circuit breaker intervention.^[36]^[37] Manual operation uses lever mechanisms, while motorized variants employ spring or motor drives for remote control, with closing times around 50-90 ms and opening times of 40-80 ms in vacuum interrupter designs.^[36] To isolate a faulty section, the standard sequence involves opening the ring switch adjacent to the fault to de-energize the affected segment, followed by closing the alternate tie switch to restore supply from the opposite direction, minimizing outage duration.^[38] Protection mechanisms in RMUs center on fuse-link coordination and relay-based systems to safeguard transformers and feeders from overloads and short circuits. In switch-fuse configurations, high-rupturing capacity (HRC) fuses protect transformers up to 2100 kVA, with coordination achieved through time-current curves where the fuse melting characteristic follows an I²t = constant relationship, ensuring the upstream fuse operates slower than downstream devices for selectivity.^[36]^[37] Circuit breaker variants incorporate overcurrent relays, such as numerical multifunction types compliant with IEC 61850, which detect and trip on excessive currents while providing directional or non-directional protection for ring network faults.^[36] These relays coordinate with fault passage indicators that sense short-circuit or earth-fault currents starting from 100-400 A, triggering alarms or trips within 40-1600 ms.^[36] Fault handling in RMUs emphasizes rapid arc extinction and operational safety to prevent equipment damage and personnel hazards. In SF₆-insulated units, arc quenching occurs via the puffer mechanism, where SF₆ gas, which is compressed during operation, extinguishes the arc during switching, supplemented by pressure rise detection sensors that initiate emergency trips if internal arcs develop, limiting arc energy to below 5% of nominal.^[36]^[37] Internal arc classification (IAC) up to 25 kA for 1 second ensures containment, with pressure relief vents directing gases safely. Mechanical and electrical interlocks are integral, prohibiting operations like closing a load break switch if the earthing switch is engaged or accessing live compartments without isolation, thereby averting incorrect maneuvers.^[36]^[37] Automation basics in RMUs enable remote management through integration with supervisory control and data acquisition (SCADA) systems, supporting protocols like IEC 60870-5-101/104 and DNP3.0 for status monitoring and control commands.^[36] Motorized drives allow remote switching with cycle times under 3 minutes for close-open sequences, while relay response times remain below 100 ms to facilitate quick fault isolation and network restoration.^[36]^[37] This setup supports basic self-healing by automating the open-close sequence in ring configurations, reducing manual intervention.

Applications and Installation

Typical Uses in Power Distribution

Ring main units (RMUs) are extensively deployed in urban power distribution systems, where space constraints and high load densities necessitate compact solutions. In densely populated residential and commercial areas, RMUs are often installed in pole-mounted or pad-mounted enclosures to manage medium-voltage feeders, enabling efficient power allocation while minimizing outage durations through ring network configurations. These units support secondary distribution levels up to 24 kV and rated currents of 630 A, facilitating reliable supply to high-rise buildings and public facilities. For instance, in urban switching substations, RMUs integrate with customer transfer points to ensure continuity in high-demand environments like business centers.^[39]^[2]^[40] In industrial settings, RMUs play a vital role in safeguarding equipment such as motors and transformers within factories and manufacturing plants. They are configured in ring arrangements to provide uninterrupted power to critical loads, including hospitals and data centers, where even brief disruptions can have severe consequences. Typical applications include automobile factories, refineries, and water treatment facilities, where RMUs handle switching for overhead lines and cables with short-time withstand currents up to 25 kA for up to 3 seconds. This setup allows for fault isolation and rapid restoration, supporting continuous operations in environments requiring high reliability.^[39]^[1]^[40] At utility scale, RMUs are integrated into secondary substations within medium-voltage ring networks to distribute power from primary sources to end-users. These installations manage loads in public utility grids, often supporting transformer capacities up to 2 MVA per unit in configurations rated at 11-24 kV and 630 A, with overall ring systems handling aggregate demands approaching 25 MVA in balanced setups. Such deployments enhance grid stability in distribution networks serving broad urban and suburban areas.^[41]^[2]^[39] A notable case of RMU deployment involves renewable energy integration, particularly in solar farms where ring feeds connect distributed generation to the grid for enhanced stability. In these setups, RMUs manage fluctuating outputs from photovoltaic arrays, enabling bidirectional power flow and fault protection to maintain grid reliability during variable solar production. For example, in solar plants, RMUs rated up to 24 kV facilitate the aggregation of inverter outputs into medium-voltage rings, supporting seamless incorporation into utility networks.^[39]^[40]^[1]

Site and Maintenance Considerations

Ring main units (RMUs) require specific installation requirements to ensure safety and operational reliability, including proper grounding, cable entry configurations, and adequate spacing. Grounding must incorporate common earth connection points capable of handling fault currents, such as 25 kA rms for 1 second at 22 kV, using conductors sized at 4 x 70 mm² or 2 x 150 mm² copper lugs, with earth bars positioned inside front panels for easy access and clamp-on testing.^[42] Cable entry points are typically bottom-entry designs featuring split gland plates, rubber bushings, and supports, with minimum bushing center heights of 375 mm above the gland plate to facilitate secure connections for cables up to 400 mm² XLPE.^[42] Spacing adheres to electrical clearance standards, such as minimum safety distances of 300 mm on accessible sides per internal arc classification (IAC) requirements in IEC 62271-200, with overall unit dimensions limited to heights of 1,500-1,800 mm, depths of 800-950 mm, and widths up to 2,500 mm to fit constrained environments.^[43] Indoor variants are suited for concrete substations with metering and SCADA integration, while outdoor models mount on galvanized steel stands within IP44-rated ventilated kiosks for weather resistance.^[42] Maintenance protocols for RMUs emphasize regular inspections to preserve insulation integrity and mechanical function, particularly for SF6-insulated units. Periodic SF6 leak checks are conducted annually through gas pressure monitoring and analysis, targeting pressures of 3.5-5 bar at 20°C as per IEC 62271-203, to detect degradation early and comply with environmental regulations.^[44] Switch mechanism lubrication involves applying dielectric grease to sliding and rotating components every 3-5 years, or more frequently in harsh conditions, to mitigate mechanical failures that account for up to 60% of issues.^[44] Diagnostic tools, such as partial discharge (PD) monitoring systems, employ sensors to detect high-frequency emissions, with intervention required if PD levels exceed 10 pC within 12-18 months per IEC 62478 guidelines.^[44] Routine testing verifies the dielectric strength and operational limits of RMUs post-installation and periodically thereafter. Hipot tests apply 2x the rated voltage for one minute across insulation to confirm withstand capability, ensuring no breakdown occurs under stress conditions aligned with IEC 62271-1 routine dielectric requirements.^[45] End-of-life criteria focus on contact wear and insulation degradation, with replacement recommended after approximately 10,000 switching operations for load break switches or when insulation resistance falls below 500 MΩ during annual 5 kV megohmmeter tests, as higher contact resistance (>25 μΩ) can elevate thermal failure risks by 300% within two years.^[1]^[44] Urban deployment of RMUs often faces space limitations, necessitating compact and modular designs that allow extensibility without exceeding footprints of 2,500 mm width, enabling integration into dense substations while maintaining required clearances.^[42]

Advantages, Limitations, and Standards

Benefits and Drawbacks

Ring main units (RMUs) offer significant operational benefits in medium-voltage power distribution, primarily through enhanced reliability enabled by their integration into ring networks, which provide redundant power paths to minimize outages during faults. This redundancy allows for automatic switching or reclosing mechanisms that can restore supply in less than one minute for transient faults, substantially reducing downtime compared to radial systems.^[46] Additionally, RMUs are highly compact, with typical footprints under 2 m² for 630 A units, such as models measuring approximately 0.95 m × 0.85 m, making them ideal for space-constrained urban installations.^[47] Economically, RMUs contribute to cost savings over separate switchgear assemblies by simplifying installation and reducing civil works requirements, often lowering commissioning and setup expenses through their modular, all-in-one design. Lifecycle costs are further optimized with minimal maintenance needs, as sealed gas-insulated units require less frequent interventions than traditional air-insulated alternatives. Reliability metrics in ring configurations demonstrate clear improvements, with studies showing up to 43% reduction in annual outage duration (from 17.3 to 9.9 hours per customer), positively impacting System Average Interruption Duration Index (SAIDI) and System Average Interruption Frequency Index (SAIFI) values.^[48] Despite these advantages, RMUs have notable drawbacks, including high initial costs ranging from $2,200 to $6,200 (as of 2024) for basic 11-24 kV units, which can escalate to $10,000 or more for advanced configurations with circuit breakers.^[49] SF6 gas insulation, while effective, introduces handling complexities due to its potent greenhouse gas properties and risks of leakage or moisture ingress, necessitating specialized training and environmental compliance measures; however, a shift towards SF6-free alternatives like air- or vacuum-insulated designs is underway to mitigate these issues.^[50]^[51] Basic fuse-protected RMU models also exhibit limited fault current interruption capabilities, typically relying on load break switches that cannot handle high short-circuit currents like full vacuum circuit breakers, potentially requiring supplementary protection in demanding applications.^[3] Overall, while ring setups yield lower total ownership costs than radial systems through reduced outages and maintenance, the upfront investment and SF6 management can pose challenges for budget-sensitive deployments.^[1]

Relevant Standards and Safety

Ring main units (RMUs) are governed by international standards that ensure their design, performance, and safety in medium-voltage power distribution systems. The primary standard for medium-voltage metal-enclosed switchgear, including RMUs, is IEC 62271-200, which specifies requirements for prefabricated assemblies rated above 1 kV and up to 52 kV, covering aspects such as enclosure design, insulation coordination, and operational reliability.^[52] In North America, IEEE C37.20.3 provides guidelines for metal-enclosed interrupter switchgear, emphasizing enclosure integrity to prevent unauthorized access and protect against environmental hazards.^[53] These standards also incorporate protection ratings like IP54 for outdoor enclosures, which offers resistance to dust ingress and water splashes from any direction, ensuring suitability for exposed installations.^[54] Additionally, RMUs must demonstrate seismic withstand capability, typically up to intensity level VIII on the Modified Mercalli scale, to maintain structural integrity during earthquakes without compromising functionality.^[55] Safety features in RMUs prioritize operator protection against internal faults. Arc-flash resistance is a critical aspect, with many units classified under internal arc class AFLR per IEC 62271-200, indicating the ability to contain an internal arc fault for specified durations (e.g., 1 second at 20-25 kA) while directing hot gases away from accessible sides—front, lateral, and rear—to minimize injury risk.^[56] During operation and maintenance, personnel must adhere to personal protective equipment (PPE) requirements, including flame-resistant clothing, insulated gloves, safety glasses, and arc-rated hoods, as outlined in electrical safety standards to mitigate hazards from potential arc flashes or electrical contact.^[57] Compliance with these standards involves rigorous type testing to verify performance under extreme conditions. Short-circuit type tests assess the unit's ability to withstand currents up to 25 kA for 3 seconds without mechanical or thermal failure, ensuring the enclosure and components remain intact during fault events.^[58] Mechanical endurance tests require switching devices, such as load break switches and disconnectors, to complete at least 2,000 operating cycles under rated conditions, confirming long-term reliability without excessive wear.^[59] Regulatory frameworks have evolved to address environmental concerns related to RMU insulation gases. The European Union's F-gas Regulation (EU) 2024/573, which replaced the 2014 regulation and applies from March 2024, imposes stricter limits on sulfur hexafluoride (SF6) emissions—a potent greenhouse gas commonly used in gas-insulated RMUs—through leakage detection, repair mandates, enhanced phase-down quotas, and bans on SF6 in new medium-voltage switchgear starting in 2026, to reduce overall fluorinated gas usage by promoting alternatives and containment measures.^[60]

Modern Advancements

Smart RMU Technologies

Smart ring main units (RMUs) represent a significant evolution in power distribution technology, incorporating digital enhancements to enable grid modernization through enhanced monitoring, automation, and intelligence. These advancements allow RMUs to transition from passive switching devices to active components in smart grids, facilitating real-time data exchange, rapid fault response, and predictive capabilities that improve reliability and efficiency. By integrating Internet of Things (IoT) devices and advanced communication protocols, smart RMUs support seamless connectivity with broader utility systems, reducing outage times and optimizing resource allocation.^[61] IoT integration in smart RMUs involves embedding sensors for real-time monitoring of key parameters such as voltage, current, and temperature, which provide continuous data on equipment health and network conditions. These sensors enable plug-and-play connectivity with supervisory control and data acquisition (SCADA) or distribution management systems (DMS) via wireless channels like GPRS, 3G, or fiber optics. Communication protocols such as DNP3 and IEC 61850 are commonly employed to ensure standardized, secure data transmission between RMUs and central control centers, supporting interoperability in diverse grid environments.^[62]^[63] Automation features in smart RMUs include fault passage indicators (FPIs) that detect and locate faults with high accuracy across various earthing systems, such as isolated or solidly earthed networks, using algorithms integrated into intelligent electronic devices (IEDs). Auto-sectionalizers automatically isolate faulty sections and restore power to healthy ones, while remote control capabilities allow operators to manage load break switches, circuit breakers, and disconnectors via SCADA interfaces. These features achieve response times under 50 milliseconds through fast messaging in protocols like IEC 61850 GOOSE, enabling near-instantaneous fault isolation and minimizing disruptions.^[62]^[61] Advancements in smart RMUs incorporate AI-based predictive maintenance, particularly for anomaly detection in switchgear components like circuit breakers, where machine learning models such as support vector machines (SVMs) analyze sensor data to identify wear or malfunctions before failures occur. These systems process real-time electrical measurements to forecast equipment degradation, optimizing maintenance schedules and extending asset life. In the 2020s, companies like Siemens have deployed AI-driven solutions for predictive maintenance in distribution networks, including switchgear monitoring that integrates with their MindSphere platform for anomaly alerts, while GE Vernova applies similar AI/ML techniques to predict failures in grid assets, enhancing overall system resilience.^[64]^[65]^[66] Data analytics in smart RMUs facilitates integration with DMS for advanced grid functions, including load balancing, by leveraging real-time RMU data to analyze consumption trends and manage distributed energy resources (DERs). This enables utilities to perform peak load predictions, stabilize power flows, and execute demand response programs, with analytics software processing vast datasets from sensors and IEDs to support bidirectional power management. Such integration, as seen in GE Vernova's GridOS platform, allows for automated Volt/VAR optimization and emergency load shedding, directly contributing to efficient smart grid operations.^[66]^[67]^[68]

Environmental and Sustainability Aspects

Ring main units (RMUs) traditionally rely on sulfur hexafluoride (SF6) as an insulating gas due to its excellent dielectric properties, but this has significant environmental drawbacks. SF6 possesses a global warming potential (GWP) of 24,300 times that of carbon dioxide over a 100-year period, making it one of the most potent greenhouse gases regulated under international agreements.^[69] Despite design standards limiting annual leak rates to less than 0.5% as per International Electrotechnical Commission (IEC) guidelines for gas-insulated switchgear, even minimal leaks contribute to cumulative atmospheric emissions over the equipment's lifecycle.^[70] In response, the European Union has mandated a phase-out of SF6 in new medium-voltage switchgear, including RMUs up to 24 kV, effective from January 1, 2026, under Regulation (EU) 2024/573, to curb fluorinated gas emissions and align with broader climate goals.^[71] To address these concerns, manufacturers have developed sustainable alternatives that drastically reduce greenhouse gas emissions while maintaining performance. GE's g3 gas, a mixture of fluoroketones such as 3M Novec 4710 with CO2 and O2, achieves a GWP of approximately 467—representing over 98% reduction compared to SF6—and has been validated for use in gas-insulated RMUs.^[72] Vacuum technology, employed in SF6-free designs like Schneider Electric's RM AirSeT, uses pure air insulation and vacuum interrupters, eliminating SF6 entirely and thereby reducing related emissions by nearly 100%.^[73] Additionally, modern RMU enclosures incorporate recyclable materials, such as modular aluminum and steel components, facilitating easier disassembly and material recovery at end-of-life to minimize waste.^[74] Lifecycle assessments highlight further sustainability gains from these innovations. SF6-free RMUs, particularly those integrated with smart monitoring, can achieve 25% reductions in energy losses through optimized operation and lower no-load consumption, translating to decreased overall carbon footprints during use.^[74] End-of-life recyclability rates for medium-voltage switchgear, including RMUs, exceed 70% for the equipment, with key components like copper and aluminum supporting high recovery efficiency through circular design principles that reduce the need for virgin materials.^[75] Industry efforts to promote low-GWP technologies have accelerated since 2015, including partnerships under the United Nations Environment Programme's (UNEP) Technology and Economic Assessment Panel (TEAP), which advises on fluorinated gas alternatives for electrical equipment through the Montreal Protocol framework.^[76] These collaborations with manufacturers like ABB and Siemens have driven the adoption of eco-efficient switchgear, contributing to global reductions in F-gas emissions from power distribution systems. As of 2025, partnerships such as Schneider Electric and E.ON continue to deploy SF6-free solutions, enhancing compliance with the impending EU phase-out.^[77]^[78]

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Internal part of SRMU is completely insulated with HCEP(Hydrophobic Cycloaliphatic Epoxy), for clean and green application, and providing safety to operators.Missing: resin | Show results with:resin
[22]
SVI-630/12 Series Epoxy Resin Solid Insulated Switchgear RMU
Solid Dielectric Ring Main Unit adopted Epoxy Resin as the insulating material, is a new developed environmental friendly and high-tech product experiencing two ...
[23]
Technical Specification of RMU- Ring Mains Units for Distribution ...
Apr 21, 2021 · Enclosure with IP 54 standard protection for OUTDOOR ... All live parts should be inside a stainless steel enclosure for outdoor type RMU ...
[24]
Outdoor Sf6 Gas Insulated Ring Main Unit Rmu Box Rmu
Rating 5.0 · Review by MIC_BUYER... enclosure (typically stainless steel or powder-coated carbon steel) with a high IP rating (e.g., IP54 or higher). This protects the internal components from ...
[25]
RMU3NIBIXX2A1PZ04 - Ring Main Unit, RM AirSeT, 12kV, non ...
The pure air, main contacts, vacuum interrupters and busbar are preserved from ambient environment in a IP67 stainless steel tank for the service life.
[26]
[PDF] Technical report on alternative to SF6 gas in medium voltage & high ...
Mar 5, 2020 · 2 - SCOPE. The purpose of this report is to provide a mapping of the status of alternatives to SF6 gas filled switchgear by the end of 2019.
[27]
(PDF) Fluoronitriles/CO2 gas mixture as promising substitute to SF6 ...
Aug 9, 2025 · Fluoronitriles chemical gas compound based on 3M NOVEC 4710 have a high dielectric strength, more than 2 times that of SF6 and a low Global ...
[28]
Comparing SF6 Alternatives: Why DryAir Stands Out - RapidoxSF6
This article provides a comprehensive SF6 alternative comparison, focusing on leading substitutes and positioning DryAir as a standout option.
[29]
[PDF] Planning of Electric Power Distribution - Digital Asset Management
... network, all substations downward of the fault location up to the normally open switch will fail. In case of low-voltage-side meshing of the ring-main units ...
[30]
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Summary of each segment:
[31]
[PDF] Switchgear Type 8DJH for Secondary Distribution Systems up to 24 ...
Switching capacity and classification of switching devices. Page 11. 11 ... protection systems, e.g. protection system 7SJ45 or make Woodward/SEG type ...
[32]
Ring main distribution - witnessing the evolutionary process
Insufficient relevant content. The provided URL (https://ieeexplore.ieee.org/document/5187353) links to an IEEE Xplore page titled "Ring main distribution - witnessing the evolutionary process | IET Journals & Magazine," but the content is not accessible or detailed enough to extract specific information about ring main units, switching, protection mechanisms, or fault isolation in distribution networks based on the given input.
[33]
[PDF] Planning of electric power distribution | Medium voltage
mostly used in public and industrial energy systems of the secondary distribution level, e.g. in. • Local ring-main units, customer transfer substations, and ...
[34]
Understand the Importance of Smart Ring Main Unit (RMU)
Apr 13, 2023 · A typical ring main unit, known as an RMU for short, is a medium voltage switchgear that is used in electrical power distribution systems.<|separator|>
[35]
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Summary of each segment:
[36]
[PDF] Specification – SF6 Ring Main Units (RMU) - Horizon Power
Jan 20, 2017 · Part 0 : Introduction and list of methods. AS 2024. High voltage ... volume of SF6 gas present in the RMU at a specified date. The Vendor ...
[37]
[PDF] SafeRing/SafePlus 12-24kV Gas-insulated ring main unit ... - ABB
IEC 60787 and IEC 62271-105. The following trans- former and switchgear work ... Distance between units when side extension is used. 14 mm. 1) Manometer ...
[38]
[PDF] Key Maintenance Practices for Extending the Lifespan of Your Ring ...
Partial discharge monitoring provides early warning of insulation degradation in Ring Main Units. Advanced sensors detect the high-frequency emissions ...
[39]
[PDF] IS/IEC 62271-1 (2007): High-Voltage Switchgear and Controlgear ...
shall be tested by a significant routine dielectric test across the open contacts. The test voltage shall be stated by the manufacturer. The dielectric test ...
[40]
Auto-reclose scheme in a ring substation configuration - IEEE Xplore
... close operations, which means that in an automatic reclose of a circuit breaker the time spent is less than a minute. In consequence, the reliability starts ...Missing: unit outage
[41]
RN6D-T1C3-21NZAU - RMU, Ringmaster, RMU 630A 13.8kV, 21kA ...
Unit Type of Package 1, PCE ; Number of Units in Package 1, 1 ; Package 1 Height, 198.0 cm ; Package 1 Width, 125.0 cm ; Package 1 Length, 125.0 cm.Missing: compact | Show results with:compact
[42]
Alteration from radial to ring power distribution to improve system ...
Aug 7, 2025 · The ring application offers 43.07 percent annual outage reduction from 17.322 to 9.861 hours annually. ... The international indices such as SAIFI ...
[43]
11kv Ring Main Unit Price - High Quality IP4X 630A - Alibaba.com
4.3 332 RMU Ring Main Unit Electrical Panel 24kV 11kV High and Medium Voltage Gas Insulated MV&HV Ring Network SF6 Free Switchgear. $2,200-6,200. Min. order: 1 set.
[44]
Comparing RMU and VCB Panels Which Is Right for Your Needs
Sep 15, 2025 · RMU panels often use SF6 gas, which is a strong greenhouse gas. VCB panels use vacuum technology and do not release harmful gases.Missing: drawbacks | Show results with:drawbacks
[45]
What is a Ring Main Unit? — The Brains of the Power Grid
Sep 9, 2025 · International Standard: The IEC 62271-200 standard for high-voltage switchgear and controlgear defines the performance requirements and ...
[46]
IEC 62271-200:2021
May 27, 2021 · IEC 62271-200:2021 is applicable to prefabricated metal-enclosed switchgear and controlgear assemblies designed for alternating current, rated voltages above 1 ...
[47]
IEEE Standard for Metal-Enclosed Interrupter Switchgear
This standard covers metal-enclosed interrupter switchgear assemblies containing but not limited to such devices as interrupter/switches, selector switches, ...Missing: integrity | Show results with:integrity
[48]
Ring Main Unit (RMU) - VPCPL Energy
Degree of protection, For switchgear enclosure [Indoor]: IP3X For switchgear enclosure [Outdoor]: IP54 For gas filled vessel: IP67. Internal arc classification ...
[49]
XGW- SF6 Insulated RMU - SOJO
Seismic capacity: VIII (seismic intensity) SF6 relative pressure: 0.03 MPa (≤20℃) SF6 yearly leakage percent: ≤0.3‰. Arc ignition test
[50]
What does A-F, A-FL and A-FLR mean? | Schneider Electric UK
Nov 29, 2010 · AF, A-FL and A-FLR are defined in IEC 62217-200 and help to define internal arc performance of medium voltage or high voltage switchgear.
[51]
How to Safely Operate Medium Voltage Switchgear: MV ... - Liyond
Jun 23, 2025 · Pre-Operation Safety Preparation and Protection Personal Protective Equipment (PPE): Wear standard-compliant PPE, including insulated gloves, ...
[52]
Ring Main Unit - NATUS [EN]
The new NATUS Type NES-F-RK ring cable switchgear system has been developed using airtight technology for operating voltages of up to 12 kV.
[53]
[PDF] Technical Specification for - WBSEDCL
The temperature rise test shall be carried out on complete RMU unit and test ... Mechanical Endurance Capacity. 2000 Operation. 18. Thickness of the Steel Tank.
[54]
Regulation (EU) No 517/2014 of the European Parliament and of the ...
Regulation 517/2014 maintains measures from 842/2006, caps HFC sales, and bans F-gases with high Global Warming Potential (GWP).Missing: ring units
[55]
Smart Ring Main Unit Market Size, Growth Analysis 2025-2034
Smart RMUs enhance grid automation, fault detection, and remote control, making them essential for intelligent energy distribution. Their deployment supports ...
[56]
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### Summary of REC615 Features
[57]
[PDF] Between Global Data Policies and an Evolving iIoT Environment
The path-dependent inertia of IEEE 1815/DNP3, lack of awareness and perceived benefit of IEC 61850, and strategic. OEM preferences for DNP3 involve the kinds of ...Missing: main | Show results with:main
[58]
AI-Based Predictive Maintenance Models for Smart Grid Components
May 26, 2025 · This study explores advanced AI-based predictive maintenance models designed to anticipate failures and optimize maintenance schedules for key smart grid ...Missing: ring main units switchgear Siemens 2020s
[59]
AI-based predictive maintenance - Siemens Global
Artificial Intelligence is ideal for predictive maintenance because it can provide reliable maintenance recommendations for machines and plants.Missing: ring units switchgear GE deployments 2020s
[60]
What is The Role of Data Analytics in a Smart Grid? - GE Vernova
Oct 29, 2025 · Smart grid analytics software is one of the most important investments the modern utility can make. Implementing such software enables data to be leveraged.
[61]
[PDF] Smart Grid Readiness – Self Assessment Tool (SGR-SAT)
Feb 8, 2019 · With real time analysis of RMU data through SCADA, it is possible to detect sudden feeder voltages and current changes, any abnormal load ...
[62]
https://library.e.abb.com/public/dab802745eb649e59deca8a19dad35e6/REC615_product_presentation_2NGA002263_Ena.pdf
[63]
[PDF] IPCC Global Warming Potential Values - GHG Protocol
Aug 7, 2024 · Sulfur hexafluoride. SF6. 22,800. 23,500. 24,300. Hydrofluorocarbons (includes unsaturated hydrofluorocarbons)*. HFC-23. CHF3. 14,800. 12,400.
[64]
[PDF] SF6 Leak Rates from High Voltage Circuit Breakers
For the study dataset, the lower and upper bound weighted- average leak rate estimates of 0.2 and 2.5 percent, respectively, represent the best and worst case ...
[65]
SF6 Ban Effective 2026: Impact & Compliance Guide | REELOAD
Jun 29, 2025 · From 1 January 2026, SF₆ is banned in new switchgear up to 24 kV · From 1 January 2028, the ban extends to 52–145 kV switchgear · From 1 January ...
[66]
SF6 Alternative, G3, What's Going To Be The Next Step | TJ|H2b
Feb 9, 2022 · This specific gas mixture called “g3” has been proved to be the most technically and economically promising solution with the advantage of ...
[67]
SF6-free MV, DER & digital tools boost sustainable grids
Mar 23, 2023 · SF6-free MV switchgear, like RM AirSeT RMU, which uses pure air and vacuum technology for insulating and breaking, rather than the GHG SF6.
[68]
[PDF] A case study of design considerations for medium voltage switchgear
For Utilities, such switchgear is used extensively as Ring Main Units (RMU), and in this whitepaper we give insights about key circular choices an. RMU ...
[69]
Charted: The End-of-Life Recycling Rates of Select Metals
Apr 13, 2025 · Ranked: The End of Life Recycling Rates of Select Metals Gold has an 86% recycling rate according to the latest available data. Per the Boston ...
[70]
[PDF] Montreal Protocol on Substances that Deplete the Ozone Layer
May 4, 2021 · o the use of low GWP refrigerants in new equipment o leak reduction o retrofitting of some high GWP gases with lower GWP alternatives o end ...
[71]
Industry initiative to reduce F-gases - Siemens Energy
It would reduce F-gases by 90 percent by 2050, and ban using F-gases in switchgears with a GWP of more than 10 by 2026 to 2031 (high-voltage), depending on the ...