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SM AirSeT is a modular, SF6-free, air-insulated medium-voltage switchgear range from Schneider Electric, engineered for secondary distribution in commercial, industrial and utility networks up to 24 kV. This article explains the range’s core interruption technology, environmental and digital differentiators, selection criteria, and the standards framework buyers should verify before specifying it on a project. It is written for procurement engineers and EPC contractors sourcing medium-voltage switchgear across the Middle East and Gulf region (including Egypt and Saudi Arabia), North Africa, the CIS states, and Sub-Saharan Africa, where SF6-alternative and digitally connected switchgear is increasingly requested in tender specifications.

What Is SM AirSeT and How Does It Work?

SM AirSeT replaces sulfur hexafluoride (SF6) gas — traditionally used for dielectric insulation in compact MV switchgear — with a combination of pure air insulation and vacuum arc interruption. Schneider Electric brands this arrangement Shunt Vacuum Interruption (SVI)™, in which the load-break switch sits inside a sealed tank filled with air, and a vacuum bottle housed within the same assembly performs the actual current-breaking function. This design avoids the use of SF6 or any alternative insulating gas, and it produces no toxic by-products during breaking operations, since the switching medium is simply air that can be released to atmosphere at end of life rather than recovered or recycled.

The motivation for this shift is straightforward from an environmental standpoint: SF6 has a Global Warming Potential 24,300 times higher than CO2 , which is why regulators worldwide are progressively tightening restrictions on its use in electrical equipment. SM AirSeT is positioned as a drop-in-compatible alternative that preserves the compact footprint, the familiar 3-position switch operation, and switch-fuse-based transformer protection that installers already expect from earlier SF6 switchgear generations — meaning adoption does not require re-training or civil-works changes on retrofit projects.

Range Architecture and Functional Building Blocks

SM AirSeT is built as a modular functional-unit range, allowing panels to be configured for incoming, outgoing, ring, and transformer-protection duties within the same switchboard. Core functional characteristics include:

• 3-position switch (open / closed / earthed) for clear, visually verifiable switching states.
• Switch-fuse combination units for direct transformer protection without a separate circuit breaker panel.
• CompoDrive mechanism — a high-tech operating mechanism designed to extend mechanical endurance over the equipment’s service life.
• Plug-and-play motorization that can be added on-site without redesigning the panel.
• Footprint parity with prior-generation SF6 switchgear, so supplementary functional units can be added later without adaptation of existing civil works.

Because of these design choices, Schneider Electric states the range achieves an extended lifespan of 40 years , attributing this durability to the combination of SVI interruption technology and the CompoDrive mechanism.

Digital Connectivity: Three Tiers of Monitoring

A defining feature of SM AirSeT is that it is “digitally connected by default.” Every unit carries a QR code linking to a digital environment that includes a Digital Logbook for storing and sharing project data from design through end of life. Beyond this baseline, Schneider Electric offers the range in three connectivity tiers — SM AirSeT, SM AirSeT Active, and SM AirSeT Active Plus — which scale from basic edge/cloud connectivity up to optional partial discharge monitoring, arc fault detection, environmental monitoring, and remote terminal unit integration.

Key embedded sensor technologies referenced for these tiers include TH110 (thermal), CL110, LPVT and LPCT (low-power voltage/current transducers), and VIP (voltage indication/protection) sensors, alongside optical arc-detection sensors for fast internal arc clearance. These sensors support condition-based maintenance strategies, reducing the need for routine physical site visits, and feed into Schneider Electric’s EcoStruxure architecture for edge control, EcoStruxure Power SCADA, and EcoStruxure Asset Advisor services.

Nearby and Remote Operation

SM AirSeT supports operating and monitoring the switchgear from a smart device or local HMI, allowing personnel to carry out switching and status checks from a safer distance without direct physical interaction with the panel — a capability referenced against the NFPA70E safe-distance standard for electrical safety work practices.

Comparative Specification Table

No rated short-circuit breaking current, rated normal current, or frequency figures were disclosed in the reviewed catalog excerpt; buyers should request the full technical datasheet for these panel-specific ampacity and fault-current ratings before finalizing a specification.

Selection Criteria for Buyers

When evaluating SM AirSeT against conventional SF6 or SF6-alternative-gas switchgear, procurement teams should weigh:

1. Regulatory exposure to F-gas restrictions. Since SF6 carries a documented GWP many thousand times that of CO2, jurisdictions tightening F-gas rules make an air/vacuum solution a lower long-term compliance risk.
2. Retrofit compatibility. SM AirSeT is designed with the same footprint as earlier SF6 switchgear generations, which matters for brownfield substation upgrades where civil works cannot be modified.
3. Digital service tier required. Buyers should decide upfront whether baseline connectivity, Active-tier condition monitoring, or Active Plus (with partial discharge and full environmental monitoring) is needed, since this affects sensor bill-of-materials and lifecycle service contract options.
4. End-of-life handling. Because pure air requires no recovery or recycling process, decommissioning logistics and associated costs are simplified compared with gas-insulated equivalents.
5. Service and lifecycle contract fit. Schneider Electric’s EcoCare, EcoFit, and Life Extension programs (Essential/Advanced) offer different combinations of lifetime extension, waste reduction, and failure-rate improvement — worth comparing against total cost of ownership models used by the buyer’s engineering department.
6. Sustainability reporting needs. Projects targeting LEED™ credits (Building Product Disclosure and Optimization, Advanced Energy Metering) benefit directly from SM AirSeT’s environmental data transparency program.

Typical Applications

SM AirSeT is positioned for MV secondary distribution switchboards in:

• Commercial buildings and campuses requiring compact, indoor ring main unit-style functional units.
• Industrial facilities needing transformer protection via switch-fuse combination panels.
• Utility secondary substations where SF6 phase-out policies are already influencing procurement specifications.
• Retrofit projects replacing aging SF6 switchgear without redesigning the substation civil structure.
• Projects pursuing green-building certification where equipment-level environmental data and reduced end-of-life gas handling are valued.

Standards and Compliance Note

SM AirSeT is positioned within the metal-enclosed AC switchgear category typically governed by IEC 62271-200, the international standard applicable to AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV, which frames type-testing and rated-characteristic requirements for this class of MV equipment. Safe operating-distance practices referenced for remote/nearby control features are aligned with the NFPA70E standard for electrical safety in the workplace. Buyers should always request project-specific type-test certificates and the applicable national grid code annexes alongside the standard reference.

Related guides

See also our guides on medium voltage switchgear, metal-clad switchgear and MV protection relays.

F400 metal clad switchgear is Schneider Electric’s air-insulated, metal-enclosed medium-voltage switchgear range built around withdrawable (floor rolling) circuit breakers, designed to cover electrical power distribution from 1 kV up to 36/40.5 kV. This article explains what F400 is, its breaker/module variants, how to select the right configuration, its rated performance figures, applicable IEC/IEEE standards, and typical applications for utility, industrial, and infrastructure substations. The information here supports procurement and engineering teams sourcing MV switchgear for the Middle East and Gulf region including Egypt and Saudi Arabia, North Africa, CIS countries, and Sub-Saharan Africa, where reliable air-insulated switchgear is a core requirement for grid and industrial substations.

What Is F400 Metal Clad Switchgear

F400 is a metal-enclosed, air-insulated switchgear platform consisting of withdrawable functional units housed in individually earthed metallic compartments, intended for indoor installation in the MV section of HV/MV substations and high-power MV/MV substations. The design integrates a floor rolling trolley for the withdrawable circuit breaker to guarantee reliable connection and alignment to the busbars, eliminating the need for a separate external trolley during racking operations. More than 20,000 F400 cubicles have been installed worldwide, reflecting a long field track record across utility and industrial networks.

Circuit Breaker Technologies and Module Variants

F400 supports two interchangeable circuit breaker technologies — vacuum and SF6 — sharing the same operating mechanism and cubicle footprint, which allows full interchangeability between breaker types without redesigning the cubicle. The range is offered with three breaker variants — commonly identified in the catalog as SF1, SF2 and HVX — covering different rated voltage and breaking-current combinations described below.

Each functional unit includes a rotary-design voltage transformer for easy maintenance and replacement, and cable connection points positioned higher than 1 meter to simplify termination work. Up to four cable runs per phase are possible in the standard cubicle design. The busbar is located at the bottom of the cubicle, avoiding heavy overhead lifting during installation and maintenance.

Rated Performance: Circuit Breaker Specification Table

(Source: Schneider F400 Metal Clad Switchgear.pdf — breaker performance chart)

In addition to breaker breaking capacity, F400 enclosures are classified to withstand internal arcing at up to 31.5 kA for 1 second under the AFLR internal arc classification. An optional arc protection unit can detect an internal arc flash event and trip the feeding breaker automatically to limit fault energy.

Selection Criteria for F400 Cubicles

When specifying F400, buyers should evaluate:

• Rated voltage class — 36 kV or 40.5 kV depending on network requirements.
• Breaker technology — vacuum or SF6, selectable per project preference since both share the same cubicle and control interface.
• Internal arc protection level (LSC2B / AFLR) — F400 cubicles use earthed metallic partitions between MV compartments (LSC2B), isolating each compartment against electrical, mechanical and thermal fault effects.
• Seismic requirement — F400 has certified seismic withstand of 1.25 g, suitable for regions with seismicity up to magnitude 8 on the Richter scale.
• Footprint constraints — cubicle width from 900 mm, height from 2,255 mm (without VT) up to 2,335 mm (with VT), and depth of 2,724 mm (available with forced ventilation at 2500 A).
• Digital integration needs — compatibility with EcoStruxure Grid/Power architecture, PowerMeter, Circuit Monitor, GemControl, and Easergy series protection relays for remote monitoring and control.
• Maintenance interval expectations — F400 requires only routine operating checks, cleaning and greasing every 5 to 10 years, and is designed for a 30-year service life.

Safety and Operational Features

F400 emphasizes operator safety through several embedded design elements. All routine operations — opening, closing of circuit breaker and earthing switch — can be carried out remotely, keeping operators away from the cubicle front during switching. A single “anti-reflex” handle is used for all manual operations to reduce operator error. A Voltage Presence Indicator System (VPIS) on the front panel confirms the absence or presence of energy before the earthing switch is operated. Racking the withdrawable breaker in or out is only possible with the cubicle door closed, and mechanical/electrical interlocks are embedded throughout the range. The earthing switch is rated with making capacity, an important safety attribute for fault-closing scenarios.

Digital and Monitoring Ready Solutions

F400 is positioned as an EcoStruxure-ready switchgear platform, supporting connected sensors such as Easergy TH110 (thermal) and Easergy CL110 (environmental), arc sensors, and remote-controlled circuit breaker and earthing switch actuators, feeding into EcoStruxure Grid or EcoStruxure Power architectures for substation monitoring, asset advisory, and SCADA integration. Direct access to product documentation — catalogs, user guides, relay manuals — is available via QR codes printed on the front door of each cubicle.

Applications

F400 is applied across utility and industrial installations, including:

• Power supply companies: HV/MV substations, MV/MV substations, MV/LV substations, and power generation facilities.
• Industry: oil & gas, chemical, automotive, and mining/mineral/metal industries.
• Infrastructure: airports, ports, and water treatment plants.
• Seismic zones: certified for seismic environments up to magnitude 8 equivalent loading.

Standards and Compliance

F400 internal arc withstand performance is developed for all functional units to achieve the AFLR protection class in accordance with IEC/EN 62271-200. F400 is fully compliant with metal-enclosed switchgear IEC standards, and its seismic certification for 1.25 g references IEEE 693 (2018), IEC 60068-3-3, IEC 62271-210, and IEC 60068-2-6. Design, manufacturing and testing follow ISO 9001:2008 quality standard, with type testing performed for each performance level in the range.

Premset switchgear (marketed by Schneider Electric under the “PremSet” name within its Set Series family) is a compact, modular medium-voltage switchgear line built around a Shielded Solid Insulation System (SSIS), in which every live part of the main circuit is encapsulated in an earthed epoxy or EPDM shield so the assembly is classified as “accidentally touchable.” This article covers the product’s design philosophy, technical ratings, functional variants, selection criteria, applications, and governing standards for engineering and procurement teams. It is particularly relevant for buyers and EPC contractors sourcing MV secondary distribution equipment across the Middle East and Gulf region including Egypt and Saudi Arabia, North Africa, CIS countries, and Sub-Saharan Africa, where compact footprint, tropicalized ratings, and long service life are recurring procurement priorities.

What Is Premset Switchgear?

Premset is a 17.5 kV-class compact modular vacuum switchgear system positioned by Schneider Electric within its broader Set Series portfolio of MV and LV switchboards. The defining feature is the SSIS design: rather than relying solely on air or gas insulation with exposed conductors inside grounded enclosures, Premset embeds the switching, disconnecting, and earthing functions in solid, shielded insulation, reducing the probability of internal arc faults and enabling a smaller equipment footprint.

The system was previously known under the same “Premset” name and has been renamed “PremSet” as part of Schneider Electric’s Set Series naming consistency initiative, alongside other renamed products such as ComPact, MasterPact, TransferPact, and FuPact.

Core Technology: Shielded Solid Insulation System (SSIS)

The SSIS concept is the technical backbone of the Premset range. The entire main circuit — switch-disconnectors, earthing switches, current and voltage sensors, and metering units — is insulated with solid epoxy or EPDM material whose outer surface is maintained at earth potential throughout. Because the electric field does not propagate into the surrounding air, the switchgear body itself becomes safe to approach or touch under normal operating conditions.

This design is stated by the manufacturer to align with the PA (accidentally touchable) classification defined in IEC 62271-201, the standard covering AC metal-enclosed and solid-insulation switchgear assemblies for rated voltages above 1 kV up to and including 52 kV.

Key benefits attributed to SSIS include: – Reduced sensitivity to harsh environments such as humidity, dust, and pollution – Lower risk of phase-to-phase faults due to solid insulation between phases – Extended service life expectancy compared to conventional air-insulated designs

SSIS is applied not only to the main switching device but also to auxiliary functions, including the M06S compact metering unit, current transformers, and voltage transformers.

The “3-in-1” Integrated Core Unit

Rather than using separate devices for breaking, disconnecting, and earthing, Premset integrates all three functions into a single core unit with only three defined positions:

1. Closed — circuit connected
2. Open and disconnected — circuit isolated
3. Earthed — downstream conductors grounded

This single-line arrangement combines a switch-disconnector using vacuum interrupters with an earthing switch sealed in a tank at atmospheric air pressure, allowing MV cables to be earthed directly through the earthing switch without requiring any other device. The series arrangement of the two devices provides double isolation between busbars and cables.

Operators benefit from an intuitive mimic diagram with two clear position indicators, designed in accordance with IEC 62271-102, the standard governing high-voltage alternating-current disconnectors and earthing switches. All interlocks between the switching, disconnecting, and earthing functions are built-in as standard, positively driven, and keyless, applying equally to circuit breaker and load break switch variants.

Integrated Cable Test Feature

For maintenance teams, Premset includes a front-accessible cable test function implemented through dedicated earth rods, eliminating the need to enter the cable box, operate main switches, or dismantle cable terminations before testing. This cable test device is stated to meet the requirements of IEC 62271-200, the standard for AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV.

Technical Ratings and Specifications

Premset is offered across three voltage classes, with insulation levels and short-circuit performance summarized below.

Additional functional and environmental ratings:

For marine-rated protection and control chains, Premset Marine supports short-circuit performance up to 25 kA for 1 second.

Design and Connection Features

Premset’s flexibility is built around a common architecture used across every configuration, which simplifies customization and installation:

• Universal flat power connection — a patented single connection interface allowing cables to be routed from the front, rear, bottom, or top, while maintaining earth shield continuity.
• Embedded sensors — integrated current and voltage sensors built around the core function, avoiding the need for extra or larger cubicles.
• Standardized cabling height — all cable connections positioned at 700 mm to simplify installation across the range.
• Multiple cable box dimensions — a range of sizes to suit different substation rooms and cabling arrangements, with an embedded voltage option.

These design choices are intended to reduce both installation time and total lifecycle footprint, which is a common evaluation criterion for utilities and industrial buyers standardizing on a single switchgear platform across multiple substations.

Digital and Smart Grid Capabilities

Premset architecture supports distributed intelligence through intelligent electronic devices (IEDs) that communicate over a standard protocol with plug-and-play configuration. Supported functions include:

• Feeder automation with built-in local intelligence and communication
• Load management through integrated smart metering
• Asset management via advanced switchgear and transformer monitoring
• Automatic transfer system (ATS) functionality to reduce power supply interruption

Connected components in the ecosystem include fault passage indicators, protection relays, energy quality monitoring devices, switch controllers, and substation monitoring devices, linked via radio, 2G/3G, or Ethernet communication networks.

Applications

Premset switchgear is designed for use across a broad range of MV secondary distribution scenarios, including:

Buildings and industry – MV/MV and MV/LV consumer substations (direct, double feeder, loop, radial connections) – MV/LV consumer substations with MV backup – MV private networks

Distribution networks – MV/MV switching substations – MV/LV distribution substations and ring main units – MV distributed generation

Marine applications Premset holds DNV-GL type approval certification for marine use, offering an enhanced protective environment, improved reliability, and a smaller footprint compared to conventional shipboard switchgear. The marine variant retains the electrical and dimensional characteristics of the standard range, adapted with features such as PM partition-class compartmentalization, front access, IP41 or IP32 enclosure ratings (IP67 for HV parts), and optional thermal diagnostics. A dedicated tunnel above the cubicle is designed to absorb gases from internal arcing events specific to onboard installations.

Environmental and Long-Term Reliability Considerations

Premset is offered as an SF6-free design, positioning the range for markets and end users seeking to reduce dependency on SF6 gas insulation in MV switchgear. Combined with the SSIS insulation approach and reduced number of operating steps (a single motion each for opening/disconnecting and for earthing), the range is positioned to lower total cost of ownership through extended service life and simplified maintenance intervals.

Standards and Compliance

Premset switchgear is engineered against multiple IEC standards relevant to MV switchgear procurement:

• IEC 62271-200 — governing AC metal-enclosed switchgear and controlgear; referenced specifically for the integrated cable test device.
• IEC 62271-201 — governing AC metal-enclosed and solid-insulation switchgear assemblies; referenced for the PA (accidentally touchable) classification of the SSIS design.
• IEC 62271-102 — governing high-voltage AC disconnectors and earthing switches; referenced for the mimic diagram and position indication design.
• IEC 60068-2-6 — governing vibration testing procedures; referenced for the stated vibration withstand values.

Buyers specifying MV switchgear for regulated utility or industrial projects should request certification documentation confirming compliance with the applicable standard for the specific function (switch-disconnector, circuit breaker, or metering unit) being procured.

Selection Criteria for Buyers

When evaluating Premset switchgear against alternative MV switchgear platforms, procurement and engineering teams should assess:

1. Voltage class required — 7.2 kV, 12 kV, or 17.5 kV, matched to network nominal voltage and future expansion plans.
2. Short-circuit and current ratings — confirm the 25 kA/3 s short-time withstand and busbar/circuit-breaker current ratings meet system fault-level and load requirements.
3. Insulation philosophy — whether a solid, shielded insulation approach (as opposed to gas- or air-insulated designs) aligns with site environmental conditions (humidity, dust, pollution exposure).
4. Environmental rating needs — ambient temperature range and humidity tolerance, particularly relevant for installations in high-humidity or high-temperature GEO markets.
5. Digital integration requirements — whether feeder automation, remote monitoring, or automatic transfer system functionality is needed for the substation.
6. Application type — standard MV/LV distribution substation, ring main unit, distributed generation interface, or marine/offshore installation requiring DNV-GL type-approved equipment.
7. Cable connection layout — front, rear, bottom, or top cabling access based on civil works and substation room layout.

SM6-36 is a metal-enclosed, air insulated medium voltage switchgear range engineered for distribution networks up to 36 kV, combining SF6 or vacuum interrupting technology inside sealed-for-life switching poles with a modular cubicle architecture that allows incoming, outgoing, circuit-breaker, ring main unit, and metering functions to be assembled into a single harmonized line-up. This article covers the SM6-36 functional unit types, its key technical ratings, internal arc classification, selection criteria for utility and industrial substations, applicable IEC standards, and answers to the most common buyer questions. Power Solutions Hub supplies and supports SM6-36 switchgear across export markets including the Middle East and Gulf incl. Egypt and Saudi Arabia, North Africa, CIS, and Sub-Saharan Africa, where compact, low-maintenance MV switchgear is a standard requirement for grid and industrial substations.

What Is SM6-36 Air Insulated Medium Voltage Switchgear?

SM6 is a modular medium voltage cubicle range built on more than forty years of Schneider Electric experience in protection, monitoring, and control solutions for MV networks, with the switching poles based on more than thirty years of SF6 interruption technology. The SM6-36 variant extends the harmonized SM6 platform up to 36 kV, allowing operators to combine multiple functional cubicles — incoming/outgoing feeders, circuit-breaker units, ring main units, and metering panels — to build a complete distribution substation. Globally, more than 1,700,000 SM6 cubicles have been installed worldwide , following an earlier milestone of over 1,500,000 units by 2015 , reflecting broad reference deployment across utility, industrial, and infrastructure projects.

Each SM6-36 cubicle is designed with switching and earthing parts housed inside a sealed-for-life SF6-filled unit for the operational life of the equipment , and the overall product range is rated for a 30-year service life . All cubicles are 100% factory tested and do not require on-site testing before commissioning , which shortens installation time for export projects where local test facilities may be limited.

Functional Units and Variants Available in SM6-36

The SM6 platform is built around a common set of functional cubicle types, which can be combined to match the single-line diagram of the substation:

• Ring main unit (RMU) cubicles — load-break switch disconnector functions for ring or radial distribution feeders.
• Circuit-breaker cubicles — withdrawable or fixed circuit-breaker units for protected incoming/outgoing feeders and transformer protection.
• Metering cubicles — voltage/current transformer compartments for revenue or protection metering.
• Bus coupler and bus riser cubicles — for busbar sectionalizing and extension between switchboard sections.
• Fuse-combination cubicles — combined switch-fuse protection for transformer feeders where fuse-based protection is preferred over a circuit breaker.

At 36 kV, switching devices are based on the SF1 circuit-breaker range; the Evolis vacuum circuit breaker is not available at 36 kV and is used only on lower-voltage SM6 cubicles . Rated current for the switching devices is available at 630 A and 1250 A, with associated rated short-circuit breaking currents of 16 kA and 25 kA , giving buyers a choice of breaker duty depending on the transformer or feeder load to be protected.

Technical Specifications and IAC Comparison Table

The table below summarizes the key numeric ratings for SM6-36, alongside the SM6-24 variant from the same harmonized platform, to help specifiers compare internal arc classification (IAC) and voltage class options when selecting the correct cubicle family.

Note that internal arc gas evacuation options differ between the two voltage classes: SM6-24 offers both downward and upward gas evacuation, while SM6-36 uses downward gas evacuation as the standard arrangement .

Internal Arc Protection and Operator Safety

Internal arc withstand is a decisive selection criterion for utilities and industrial plants where operator safety and continuity of service are critical. SM6-36 offers 3-sided internal arc protection classified IAC: A-FL at 16 kA for 1 second , meaning the enclosure is validated to contain and vent arc fault energy away from the front, sides, and rear-adjacent accessible zones, protecting personnel standing in front of the switchgear during an internal fault event. Beyond the enclosure rating, SM6 cubicles incorporate a three-position switch structure to prevent incorrect switching sequences, a fully closed earthing switch, positive-indication position indicators, internal arc withstand in cable and connection compartments, clear mimic diagrams, an anti-reflex operating lever, and physically segregated compartments — all standard mechanical and procedural safeguards built into the SM6 design philosophy.

Selection Criteria for SM6-36 Switchgear

When specifying SM6-36 for a project, buyers and consulting engineers should evaluate:

• Rated voltage and insulation level — confirm the network’s nominal voltage falls within the SM6-36 envelope (up to 36 kV) rather than requiring the lower SM6-24 platform.
• Rated current and short-circuit duty — match the 630 A/1250 A current rating and 16 kA/25 kA short-circuit breaking current to the transformer and feeder fault-level study for the substation.
• Internal arc classification — verify whether 3-sided (A-FL) protection is sufficient, or whether the layout requires additional rear-side (A-FLR) considerations, and confirm the gas evacuation routing (downward) fits the substation civil design.
• Functional unit mix — determine which combination of ring main unit, circuit-breaker, metering, and bus coupler cubicles matches the required single-line diagram.
• Monitoring and connectivity needs — SM6 cubicles are compatible with PowerMeter metering units, Easergy P3 and Easergy Sepam multifunction protection relays, and self-powered VIP protection relays, plus connectable sensors (SC110, CL110, TH110) for continuous asset condition monitoring; the Easergy TH110 conductor temperature sensor is fitted as standard on cable terminations of relevant cubicles .
• Maintenance regime — factor in the reduced maintenance burden from sealed-for-life SF6 switching and earthing parts, versus operating mechanisms designed for lower maintenance frequency under normal service conditions.
• End-of-life SF6 handling — plan for certified SF6 recovery and disposal procedures in line with local environmental regulation before decommissioning.

Applications

SM6-36 switchgear is applied in primary and secondary MV distribution substations, industrial plant intake switchrooms, transformer feeder protection, ring main distribution networks, and utility distribution feeders where a 36 kV insulation class is required. Reference installations spanning airports, universities, hospitals, cement plants, data centers, and government facilities across multiple regions demonstrate the platform’s applicability to both utility-grade and industrial-grade MV distribution projects, including installations referenced in Egypt and Saudi Arabia among other Middle East and Gulf, North Africa, CIS, and Sub-Saharan Africa markets.

Standards and Compliance

SM6 switchgear is designed in accordance with the IEC standard framework applicable to metal-enclosed medium voltage switchgear . End-of-life SF6 gas recovery is compliant with IEC 62271-4:2013 , and disposal must follow local environmental regulation, including Regulation (EU) No 517/2014 for European countries, given SF6’s classification as a powerful greenhouse gas requiring certified recovery rather than atmospheric release .

SM6-24 air insulated medium voltage switchgear is a metal-enclosed, modular 24 kV cubicle range from Schneider Electric that combines air-insulated busbars with sealed-for-life SF6 switching and earthing units to deliver compact, low-maintenance distribution and protection functions for primary and secondary substations. This article covers the SM6-24 cubicle family — its construction, functional variants (DM1, CM, PM, QM, GBC and related types), circuit-breaker and switch-fuse ratings, internal arc (IAC) classification, applicable IEC standards, and the selection criteria buyers need before specifying equipment for a project. It is written for engineers, EPC contractors, and utility procurement teams sourcing MV switchgear for the Middle East and Gulf region including Egypt and Saudi Arabia, North Africa, CIS countries, and Sub-Saharan Africa, where SM6 cubicles have an extensive installed reference base.

What Is SM6-24 Air Insulated Switchgear?

SM6-24 belongs to Schneider Electric’s SM6 modular MV cubicle platform, which is manufactured with SF6 or vacuum switching technology and is designed for a 30-year service life. The switching and earthing parts sit inside a sealed-for-life SF6-filled unit, while the surrounding cubicle structure and busbar compartment use air as the primary insulation medium — hence the “air insulated” designation used in the manufacturer’s own catalogue naming for the SM6 range. This hybrid approach keeps the switching chamber maintenance-free while allowing straightforward busbar extension and connection work in the field.

Globally, Schneider Electric reports more than 1,700,000 SM6 cells installed worldwide, supported by over 2,500,000 SF6 medium voltage cells supplied by the company across its product history. The platform’s development timeline shows more than 300,000 SM6 cells in service by 1989 and over 1,500,000 by 2015, reflecting a mature, field-proven design rather than a recent introduction.

Cubicle Types and Functional Variants

The SM6-24 range is built from functional units that can be combined side by side to build a complete switchboard. Key cubicle designations referenced in the manufacturer’s operating and maintenance documentation include:

• DM1, DM1-W, DM1-Z — withdrawable circuit-breaker cubicles, where the circuit breaker can be racked out and racked in for isolation and maintenance.
• DM2, DMV, DMVL — additional circuit-breaker and disconnector cubicle variants used for incoming/outgoing feeder and coupling functions.
• CM, CM2, PM, QM, QMC, QMB, TM — fuse-combination and fused-switch cubicles for transformer protection, each with fuse-status indication and MV fuse replacement procedures defined in the manufacturer’s manual.
• GBC-A, GBC-B — cubicles with dedicated fuse replacement procedures, typically used in ring/feeder configurations.
• CVM — a metering/voltage variant included in the fuse-replacement instructions alongside CM/PM/QM types.

Each cubicle uses a three-position switch structure (open / closed / earthed) to prevent incorrect switching sequences, and the earthing switch has a full making capacity design, giving operators a mechanically interlocked, visually confirmed switching sequence.

High-Performance Breaking Devices

The SM6 range integrates SF1 and/or Evolis breaking devices, rated at 630 A and 1250 A, with breaking capacities of 16 kA and 25 kA. Note that the Evolis breaker option is not available at the 36 kV voltage level, meaning full breaker choice flexibility applies specifically to the SM6-24 platform.

Internal Arc (IAC) Classification

Internal arc withstand is a critical safety differentiator for MV switchgear specifiers. The SM6 range offers both 3-sided and 4-sided internal arc protection:

• IAC: A-FL (3-sided arc protection) — for SM6-24, rated at 12.5 kA 1s, 16 kA 1s, and 20 kA 1s; for SM6-36, rated at 16 kA 1s.
• IAC: A-FLR (4-sided arc protection) — for SM6-24, rated at 12.5 kA 1s, 16 kA 1s, and 20 kA 1s.

Gas evacuation can be routed downward for all units, or upward specifically for SM6-24 cubicles, giving substation designers flexibility for cable basements versus overhead exhaust ducting.

Selection Criteria for Buyers

When specifying SM6-24 air insulated medium voltage switchgear, evaluate the following:

1. Rated short-time withstand / IAC class needed — determine whether 12.5 kA, 16 kA, or 20 kA 1s arc withstand is required based on upstream fault level and personnel safety requirements (3-sided vs. 4-sided protection).
2. Breaking device type — confirm whether SF1 or Evolis vacuum circuit breakers suit the application, and required current rating (630 A or 1250 A) and breaking capacity (16 kA or 25 kA).
3. Functional unit mix — select DM1/DM2/DMV/DMVL for circuit-breaker feeders, CM/PM/QM/QMC/TM/GBC types for fused transformer protection.
4. Withdrawable vs. fixed design — DM1-W and DM1-Z variants allow the circuit breaker to be racked out for maintenance without de-energizing the entire busbar.
5. Monitoring and connectivity — compatibility with PowerMeter, Easergy P3/Sepam protection relays, VIP self-powered protection relays, and Easergy TH110 temperature sensors for condition-based maintenance.
6. Gas evacuation routing — downward or upward exhaust depending on switchroom layout.
7. End-of-life compliance — dismantling and SF6 gas recovery service aligned with IEC 62271-4:2013.

Spec Comparison Table

Applications

SM6-24 cubicles are used in primary and secondary distribution substations, industrial plants, data centers, airports, and utility ring main points where compact, low-maintenance MV switching combined with fused transformer protection is required. Reference installations documented by the manufacturer span airports, hospitals, universities, cement plants, and utility distribution companies across multiple regions, including projects in Egypt and Saudi Arabia. This global reference base supports specification confidence for new-build and retrofit projects in the Middle East and Gulf region including Egypt and Saudi Arabia, North Africa, CIS markets, and Sub-Saharan Africa.

Standards Compliance

SM6-24 cubicles are designed in accordance with IEC standards applicable to metal-enclosed MV switchgear with vacuum circuit breakers up to 24 kV. End-of-life dismantling and SF6 gas recovery follow IEC 62271-4:2013, alongside applicable local regulations such as EU Regulation No 517/2014 on fluorinated greenhouse gases. Because the switching units contain SF6 — a potent greenhouse gas — recovery, reclamation, or destruction of the gas is mandatory prior to disposal, and dismantling must only be performed by certified personnel.

Air-insulated switchgear (AIS) is medium-voltage switchgear in which the busbars and main current-carrying parts are insulated by air, rated for systems above 1 kV up to about 40.5 kV and built to IEC 62271-200. It is the most widely used MV switchgear type, spanning compact secondary-distribution cubicles up to large withdrawable primary panels, and — because it uses air — it is inherently SF6-free in its insulation.

This pillar guide explains what air insulation means, how AIS spans the network from secondary cubicles to primary metal-clad, how it compares with gas-insulated switchgear, the product families we supply, the key ratings, the SF6-free picture, and how to select — for MV distribution and industrial projects across the Middle East and Gulf (including Egypt and Saudi Arabia), as well as North Africa, CIS and Sub-Saharan Africa.

What “air-insulated” means

In an air-insulated panel the dielectric between live parts, and between live parts and earth, is air at atmospheric pressure. Because air has a lower dielectric strength than insulating gas, AIS needs larger phase-to-phase and phase-to-earth clearances — for a 36 kV system the minimum busbar air clearance is typically 35–36 cm — which makes an AIS panel physically larger than an equivalent gas-insulated one. In return, AIS is lower in capital cost, easy to inspect visually, straightforward to maintain, and needs no gas handling. The arc interruption itself is normally done by a vacuum interrupter inside the panel; it is the standing insulation that is air.

“Air-insulated” describes the insulation medium, not the construction. An AIS panel can be a simple secondary-distribution cubicle or a fully compartmented, withdrawable metal-clad primary panel — both are air-insulated. See the medium voltage switchgear pillar for the full insulation-vs-construction picture.

AIS across the network

Air insulation is used at every level of MV distribution, in two broad construction formats:

• Secondary-distribution cubicles. Compact, modular, often fixed-mounted air-insulated panels for the many small MV/LV substations — for example Schneider SM6 (12–36 kV modular cubicles) and ABB UniSec (up to 24 kV, 1250 A, 25 kA). These provide incomer, feeder, transformer-protection and bus-coupler functions in an air-insulated, serviceable format.
• Primary metal-clad panels. For high busbar current and fault level, the air-insulated panel takes the withdrawable, compartmented metal-clad form (ABB UniGear ZS1 up to 24 kV, 4000 A, 63 kA). The construction detail is covered in the metal-clad switchgear pillar.

So AIS and metal-clad are not alternatives on the same axis: metal-clad is one (primary, withdrawable) construction of air-insulated switchgear.

Functional units in an AIS line-up

An air-insulated switchboard is assembled from standard functional panels sharing a common busbar, so the single-line matches the installation. The usual functions are:

• Incomer panel — brings the supply into the switchboard, usually with a circuit breaker and protection.
• Feeder panels — the outgoing circuits to transformers, motors or sub-boards, each with its own breaker or switch and protection.
• Bus-coupler / bus-section panel — links two busbar sections, allowing the board to be split or joined for maintenance and redundancy.
• Transformer-protection panel — protects a distribution transformer, by a switch-fuse combination for smaller ratings or a circuit breaker with a relay for larger ones.
• Metering panel — houses the current and voltage transformers for measurement and revenue metering.

Because the panels are modular, a board can be extended later by adding units to the busbar — one of the practical advantages of air-insulated construction. This modularity, and the ability to see and inspect the equipment directly, is a large part of why AIS remains the most common MV switchgear worldwide.

Construction formats and solid insulation

Air-insulated switchgear comes in more than one construction:

• Fixed-mounted cubicles — the breaker or switch is built into the panel; simple and compact, typical of secondary distribution (e.g. Schneider SM6, ABB UniSec).
• Withdrawable metal-clad — the compartmented, draw-out primary format (see the metal-clad switchgear pillar).
• Shielded solid-insulation systems — some compact modular families, such as Schneider Premset (up to 17.5 kV), combine air insulation with shielded solid-insulated components and vacuum interruption, shrinking the footprint while keeping the serviceability of an air system.

The right format depends on the network level, the available space and the maintenance strategy — the same trade-offs that run through the whole medium voltage switchgear family.

AIS vs GIS — how to choose

The core selection question is insulation medium: air-insulated (AIS) or gas-insulated (GIS).

AIS is the default where space and a controlled indoor environment are available and where low capital cost and easy servicing matter. GIS wins where the site is space-limited, hot, dusty or humid.

Air-insulated product families

The AIS families we most often supply:

SF6-free and air insulation

Because AIS uses air as its insulation, its busbar and main insulation are inherently SF6-free. The remaining question is the interrupter: modern AIS uses vacuum interruption, so a fully SF6-free air-insulated panel is readily available. Families such as ABB UniSec Air are explicitly SF6-free, compliant with the EU F-gas Regulation (EU 2024/573), with a global warming potential of 0. Where a project mandates SF6-free equipment, air-insulated switchgear with vacuum interruption is a natural fit. See the gas-insulated switchgear pillar for the SF6-free gas alternatives.

Key ratings to specify

Confirm these against the manufacturer’s datasheet for your configuration:

How to choose

1. Space and environment. AIS suits installations with room for clearances and a clean, dry, controlled environment; for space-limited or harsh sites choose GIS.
2. Network level. Secondary distribution uses compact air-insulated cubicles (SM6, UniSec); primary distribution uses withdrawable metal-clad AIS.
3. Cost and maintenance. AIS has a lower capital cost and is easy to inspect and service.
4. SF6-free requirement. Air insulation with vacuum interruption is inherently SF6-free; confirm the family (e.g. UniSec Air) if the project mandates it.
5. Maintenance strategy. If a local team will inspect and service the equipment directly, the accessible, repairable air-insulated design suits that model; if minimal-maintenance, sealed operation is preferred, weigh a gas-insulated alternative.
6. Fault level and current. Match the short-circuit rating (up to 25 kA secondary, up to 63 kA primary) and the rated current to the network; high-current primary duty points to withdrawable metal-clad.

Standards

Air-insulated switchgear is governed by IEC 62271-200 (AC metal-enclosed switchgear 1 kV–52 kV), with the switching devices following the matching parts of the IEC 62271 series (-100 circuit-breakers, -102 disconnectors, -103 switches). Compact secondary substations may also be type-tested to IEC 62271-202.

Maintenance and inspection advantages

One of the enduring reasons AIS remains the most common MV switchgear is serviceability. Because the insulation is air, the live parts and connections are — with the panel safely isolated and earthed — directly accessible for visual inspection, cleaning, thermographic checks and component replacement. There is no sealed gas system to monitor or handle at end of life, and spare parts and repairs are straightforward. This makes AIS attractive where a local maintenance team looks after the installation and where the total cost of ownership favours a serviceable, repairable design over a sealed one. The trade-off is that this accessibility depends on keeping the switch room clean and dry, which in turn drives the environmental requirements below.

Air-insulated switchgear in hot and dusty climates

Because AIS relies on air clearances, the installation environment matters more than for a sealed panel. In Gulf, North-African and CIS projects, two points are decisive. First, ambient temperature: the rated current derates as the switch-room temperature rises, so the busbar and feeder ratings must be confirmed against the actual ambient — often meaning air-conditioning or a higher-rated busbar. Second, cleanliness and humidity: dust, salt and condensation can reduce the effective air insulation and cause tracking, so AIS switch rooms are kept filtered, dry and, where needed, climate-controlled. Where those conditions are difficult or expensive to maintain — a compact plot, a harsh outdoor-adjacent site, high pollution — a sealed gas-insulated panel is often the better long-term choice. Matching the switchgear type to the real site conditions is exactly the selection judgement this guide is built to support.

Applications

Air-insulated switchgear is used across MV distribution: secondary MV/LV substations, industrial plants, commercial buildings and infrastructure (compact cubicles), and the primary incoming substations of large facilities (metal-clad). Typical sectors include utilities and distribution networks, manufacturing and process industry, renewable energy plants (wind and solar collector substations), commercial and public buildings, water and infrastructure, and mining and oil & gas. It is the default MV switchgear where space and a controlled environment allow, and where low capital cost and easy maintenance are priorities — which is why it remains the most widely installed MV switchgear worldwide, complemented by gas-insulated panels where space or environment demand a sealed solution.

Metal-clad switchgear is a construction class of medium-voltage switchgear defined by IEC 62271-200, in which the main components — the withdrawable circuit breaker, the busbars and the cable connections — sit in separate compartments divided by earthed metal partitions, with the breaker mounted on a withdrawable (draw-out) truck. It is the standard format for primary distribution switchgear, and it is almost always air-insulated.

This pillar guide explains what “metal-clad” means, the withdrawable architecture and its service positions, the loss-of-service-continuity and partition classes, the main product families we supply, the key ratings, and how to select a panel — for MV distribution and industrial projects across the Middle East and Gulf (including Egypt and Saudi Arabia), as well as North Africa, CIS and Sub-Saharan Africa.

What “metal-clad” means

Metal-clad is a construction format, not an insulation medium — a distinction that matters (see the medium voltage switchgear pillar for the full insulation-vs-construction picture). A metal-clad panel is characterised by three things:

• Separate, earthed metal compartments for the busbar, the circuit breaker and the cables, so a fault or maintenance in one compartment is contained and does not expose the others.
• A withdrawable circuit breaker on a truck that can be racked in and out.
• Metal partitions (PM) between compartments that are earthed, giving a high level of operator safety.

This is distinct from a simpler metal-enclosed cubicle (where the parts are in one enclosure) and from a compact, sealed ring main unit. Metal-clad is the format chosen where high busbar current, high fault levels and maximum continuity of supply are required — typically the incoming substation of a large facility.

The withdrawable architecture

The defining feature of metal-clad switchgear is the withdrawable (draw-out) circuit breaker. The breaker sits on a truck that can be moved between three positions:

• Service — the breaker is fully engaged with the main and earthing contacts; the circuit is live and operational.
• Test — the breaker is disconnected from the main circuit but its control circuits are still connected, so it can be operated and tested safely without energising the primary circuit.
• Isolated / withdrawn — the breaker is fully disconnected and can be racked out and removed for maintenance or replacement, while the busbar and cable compartments remain in their state.

This is what gives metal-clad its continuity of supply advantage: a faulty breaker can be swapped for a spare in minutes without shutting down the whole board. Automatic shutters cover the live contacts when the breaker is withdrawn, maintaining the earthed-barrier safety.

Inside a metal-clad panel: the four compartments

The compartmented construction is what defines metal-clad, so it is worth seeing what each compartment does:

• Busbar compartment — carries the main three-phase busbars that run along the switchboard connecting all the panels. Isolated behind an earthed metal partition, it can stay energised while work is done on the breaker.
• Circuit-breaker compartment — houses the withdrawable breaker truck. This is the compartment that is opened for maintenance; the automatic shutters close over the live busbar and cable contacts as the truck is withdrawn.
• Cable compartment — where the MV power cables terminate, often with the current transformers and earthing switch. Separated from the breaker so cables can be worked on safely.
• Low-voltage (control) compartment — carries the protection relay, metering, control wiring and communications, physically separated from all the MV compartments.

Because these four zones are individually enclosed in earthed metal, a fault or an operator action in one compartment cannot propagate to the others — the safety principle at the heart of metal-clad design.

Vacuum circuit breakers and arc-resistant design

The switching device in a modern metal-clad panel is a withdrawable vacuum circuit breaker: the arc is interrupted in a sealed vacuum interrupter, which is compact, maintenance-light and has a long electrical life. The breaker truck’s racking mechanism, interlocks and shutters are all engineered so that the breaker can only be moved between service, test and isolated positions in a safe sequence — for example, it cannot be racked in or out while closed.

Metal-clad panels are also designed to be arc-resistant: if an internal arc fault occurs, the earthed compartments contain it and a pressure-relief path vents the hot gases safely away from the operator, as verified by the Internal Arc Classification (see below). This combination — compartmentalisation, withdrawable vacuum breaker and arc-resistant construction — is why metal-clad is specified where both high performance and operator safety are paramount.

Classification: LSC and partition class

Metal-clad panels are classified to IEC 62271-200 by how much stays in service during maintenance and by the type of partitions:

• Loss of Service Continuity — LSC2B: both the cable and busbar compartments can remain energised while working on the circuit-breaker compartment. This is the highest continuity class and the norm for metal-clad.
• Partition class — PM: the partitions between compartments are metal (earthed), as opposed to insulating partitions (PI). Metal partitions give the highest operator safety.
• Internal Arc Classification (IAC): metal-clad panels are internal-arc tested — for example ABB UniGear ZS1 and Schneider F400 carry IAC AFLR ratings, with F400 classified to withstand internal arcing as AFLR up to 31.5 kA for 1 s.

Metal-clad product families

The metal-clad families we most often supply:

Metal-clad vs metal-enclosed vs RMU

For the compact, sealed alternative used deeper in the network, see the ring main unit pillar; for the gas-insulated option, see gas-insulated switchgear.

Key ratings to specify

Confirm these against the manufacturer’s datasheet for your configuration:

How to choose

1. Network level and current. Metal-clad suits primary distribution with high busbar currents (up to ~4000 A) and high fault levels; for secondary distribution use an RMU.
2. Continuity of supply. If a faulty breaker must be replaced without shutting the board, the withdrawable LSC2B format is the answer.
3. Voltage and fault level. Match the rated voltage (up to 40.5 kV) and the short-circuit rating (up to 63 kA) to the network.
4. Internal-arc safety. Confirm the IAC class and accessibility (A/B, F/L/R) required by the installation.
5. Insulation. Metal-clad is air-insulated; if a sealed, compact, humidity-tolerant panel is needed, consider gas-insulated switchgear instead.

Standards

Metal-clad switchgear is governed by IEC 62271-200 (AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV up to 52 kV), which defines the metal-clad construction, the LSC categories, the partition classes (PM/PI) and the internal-arc classification (IAC). The withdrawable circuit breakers follow IEC 62271-100.

Digital and condition-monitored metal-clad

Modern metal-clad switchgear is increasingly a digital platform. The low-voltage compartment carries numerical protection relays with IEC 61850 communication, so the whole switchboard can be integrated into a substation-automation system with fast GOOSE interlocking, metering and event recording. Beyond protection, condition-monitoring options — temperature sensors on the busbar joints and cable connections, partial-discharge monitoring, and circuit-breaker operation counters — let operators move from fixed-interval maintenance to condition-based maintenance, catching a developing hot joint or insulation problem before it becomes a fault. For a critical primary substation, this monitoring capability is often specified alongside the raw electrical ratings.

Selecting metal-clad for hot and demanding climates

For Gulf, North-African and CIS projects, three points deserve attention when specifying metal-clad. First, temperature derating: the rated busbar current must be derated for high ambient temperatures, so a 4000 A busbar may be limited below its nameplate value in a hot switch room — confirm the derating before fixing the rating. Second, environment: because AIS relies on air clearances, the switch room must be kept clean, dry and dust-free (filtered ventilation or air-conditioning), otherwise pollution can degrade the insulation; where that environment is hard to guarantee, a sealed gas-insulated alternative may be better. Third, continuity: in facilities where an unplanned outage is very costly, the withdrawable LSC2B format pays for itself by allowing a breaker to be replaced without shutting the board.

Applications

Metal-clad switchgear is used at the primary distribution level: the incoming substations of industrial plants, power stations, large commercial and infrastructure projects, mining, and oil & gas facilities — anywhere high busbar current, high fault level and maximum continuity of supply are required. Its withdrawable design also simplifies maintenance over a long service life.

Extending and modernising a switchboard

Metal-clad switchboards are designed to be extended: because the panels share a common busbar behind earthed partitions, additional feeder panels can be added at the end of a line-up as a facility grows, subject to the busbar rating and the physical space. Existing boards can also be modernised — the withdrawable breaker trucks can be replaced with new vacuum breakers, and the low-voltage compartment’s electromechanical relays upgraded to numerical protection relays with IEC 61850 communication, extending the switchboard’s service life without replacing the whole installation. When specifying a new board, it is worth confirming the manufacturer’s extension and retrofit options up front, especially for a site that expects to grow.

A ring main unit (RMU) is a compact medium-voltage switchgear assembly used to switch and protect secondary distribution networks: it connects a substation into the MV ring, switches the incoming and outgoing ring cables, and protects the MV/LV distribution transformer — typically rated up to 24 kV (families to 40.5 kV).

This pillar guide covers what an RMU does and how it works in a ring network, the main types and brand families, how a unit is configured from functional blocks, the key ratings to specify, and how to select one — for MV distribution projects across the Middle East and Gulf (including Egypt and Saudi Arabia), as well as North Africa, CIS and Sub-Saharan Africa.

What a ring main unit does

An RMU sits at the secondary distribution level. In a distribution “ring”, each substation is fed from two directions so a fault or maintenance on one section does not cut supply; the RMU provides the two ring load-break switches (incoming and outgoing) plus a transformer feeder that protects the local MV/LV transformer. Combining these functions in one compact, usually sealed enclosure is what distinguishes an RMU from a full primary switchgear line-up.

It links up to the wider medium-voltage switchgear family: where primary switchgear feeds large loads, the RMU is the workhorse of compact secondary substations and kiosks.

How an RMU works in a ring network

Secondary distribution is usually built as an open ring. Substations are connected in a loop that runs from one primary source, through a string of distribution substations, back to another source — but the ring is deliberately left open at one point (a normally-open switch) so it operates radially. Each substation’s RMU carries two ring load-break switches, one for each direction of the ring cable.

The value of this arrangement is continuity of supply. If a cable section faults or needs maintenance, the operator opens the load-break switches on either side of that section to isolate it, then closes the normally-open point to feed the affected substations from the other direction. Supply is restored to almost all customers within minutes, without waiting for the cable repair. The RMU is the device that makes this switching possible at every node of the ring, which is why it is the standard building block of urban and industrial MV distribution.

The third function at each node is the transformer feeder: a tee-off from the ring that protects and supplies the local MV/LV distribution transformer, either through a switch-fuse combination or a circuit breaker with a protection relay (see below).

RMU types

RMUs are distinguished mainly by insulation medium and transformer-protection method:

• Gas-insulated RMUs: the live parts sit in a sealed SF6 (or SF6-free) tank — compact and maintenance-free for life. Examples: Schneider RM6, ABB SafeRing / SafePlus and Siemens 8DJH. See the gas-insulated switchgear pillar for the insulation detail.
• SF6-free RMUs: sealed units that use dry/clean air with vacuum interruption instead of SF6 — for example ABB SafeRing/SafePlus Air, a sealed dry-air ring main unit up to 24 kV with a global warming potential of 0 and a vacuum circuit-breaker with disconnector.
• Air-insulated / modular RMUs: modular cubicles such as Schneider SM6 provide the same ring-plus-transformer function in an air-insulated, serviceable format, up to 36 kV.
• Fixed vs extensible: an RMU can be a fixed, factory-set configuration or an extensible unit that can be enlarged on site as the network develops (for example SafeRing’s extendable version, or the RM6’s extensibility codes NE/RE/LE/DE).

How an RMU is configured — functional units

A modern RMU is specified as a string of functional units (modules), so the single-line matches the substation exactly. Using the Schneider RM6 nomenclature as an example, the core functions are:

• I — line (ring) switch: a switch-disconnector for an incoming or outgoing ring cable, rated 400 A or 630 A.
• Q — fuse-switch combination: transformer protection by a switch plus MV fuses, for smaller transformers (up to about 2 000 kVA).
• D / B — circuit-breaker function: transformer or feeder protection by a circuit breaker with a protection relay, for larger transformers (up to about 8 000 kVA).

A typical two-incomer, one-transformer substation is therefore an I-I-Q string (two ring switches plus a fuse-switch transformer feeder), while a substation whose transformer needs relay protection is an I-I-D (two ring switches plus a circuit-breaker feeder). The extensibility code chosen at order time decides whether more modules can be added later without replacing the unit — a double-extensible unit can grow on either side as the network develops. This “build what you need, extend later” approach is a large part of why RMUs dominate secondary distribution.

RMU brand families at a glance

The ring-main-unit families we most often supply:

RMU vs primary switchgear

For the full construction/insulation taxonomy, see the medium voltage switchgear pillar.

Transformer protection in an RMU

The transformer feeder is where the protection choice is made, and it is the point most worth getting right:

• Switch-fuse combination (fuse-switch). For smaller distribution transformers — up to about 2 000 kVA — a load-break switch in series with MV fuses is a compact, economical and fast solution. The fuses clear high fault currents very quickly, and the standard governing this device is IEC 62271-105 (AC switch-fuse combinations for above 1 kV up to 52 kV).
• Circuit breaker with a protection relay. For larger transformers — up to about 8 000 kVA — or where adjustable, selective protection and remote control are required, a circuit-breaker feeder with a numerical protection relay is used. This gives full overcurrent and earth-fault protection with settable curves and communications.

Choosing between them is mainly a question of transformer rating, the required selectivity with upstream and downstream devices, and whether remote control and metering are needed. Getting the fuse rating or the relay settings wrong is a common cause of nuisance tripping or, worse, unprotected faults — so the transformer feeder should always be coordinated with the rest of the protection scheme.

Key ratings to specify

Confirm these against the manufacturer’s datasheet for your configuration:

Ambient-temperature derating

Rated current derates with ambient temperature — a key point for hot-climate projects. For a 630 A gas-insulated busbar, usable current falls as the ambient rises: for example, from 630 A at 40 °C down to about 575 A at 45 °C, 515 A at 50 °C, 460 A at 55 °C and 425 A at 60 °C. In Gulf and North-African installations where indoor temperatures routinely exceed 45 °C, this derating should be applied before confirming the rated current.

Selection criteria

1. Insulation medium: gas-insulated (compact, sealed, harsh-environment), SF6-free sealed, or air-insulated modular (serviceable). See GIS vs AIS.
2. Transformer rating → protection type: switch-fuse for smaller transformers (up to about 2 000 kVA), circuit breaker with a relay for larger ratings.
3. Rated current: 400 A or 630 A busbars, then apply ambient-temperature derating for hot climates.
4. Short-circuit level: confirm the network’s prospective fault current against the RMU’s rated breaking and making capacity.
5. Extensibility: fixed or extensible, depending on whether the substation will be enlarged later.
6. Regulatory: where SF6-free equipment is mandated, choose a dry-air or air-insulated family with equivalent ratings.

Standards

Ring main units are metal-enclosed switchgear to IEC 62271-200 (above 1 kV up to 52 kV). The transformer-protection switch-fuse combination follows IEC 62271-105, disconnectors and switches follow IEC 62271-102 / -103, circuit-breakers follow IEC 62271-100, and a compact secondary substation enclosure may be type-tested to IEC 62271-202. Sealed gas-insulated RMUs are “sealed-for-life” (sealed pressure system) designs.

Applications

RMUs are used in compact secondary substations, kiosks and building distribution — utilities, industry, infrastructure and renewable connection points — anywhere a compact unit is needed to ring-connect and protect an MV/LV transformer. Because the enclosure is compact and (for gas types) sealed, RMUs are also the natural choice for packaged and prefabricated substations to IEC 62271-202. A concrete example is the Schneider RM6 ring main unit, whose functional-unit range shows how a substation is built as a string of ring switches and transformer feeders.

RMUs in compact and prefabricated substations

One of the most common ways an RMU is deployed is inside a compact (packaged) substation — a factory-built enclosure, kiosk or concrete housing that contains the RMU, the MV/LV distribution transformer and the low-voltage board in one prefabricated unit, type-tested to IEC 62271-202. The RMU is the MV heart of this package: it brings the ring cables in, switches them, and protects the transformer, while the whole assembly is delivered ready to connect on site. Because gas-insulated RMUs are compact and sealed, they let the entire substation shrink to a footprint that fits an urban plot, a basement, an industrial yard or a renewable-plant collection point. For utilities rolling out many identical substations across a network — a frequent pattern in Gulf, North-African and CIS distribution projects — this standardised RMU-plus-transformer package shortens delivery and simplifies maintenance across the fleet.

Gas-insulated switchgear (GIS) is medium-voltage switchgear in which the live parts — busbars, switching devices and connections — are enclosed in a sealed metal tank filled with an insulating gas (traditionally SF6, increasingly SF6-free), rated for systems above 1 kV up to about 40.5 kV.

This pillar guide explains what gas insulation is and how it works, the GIS types and product families we supply, the shift from SF6 to SF6-free technology, how gas insulation compares with air, the key ratings and standards to specify, and how to select the right panel — for MV distribution projects across the Middle East and Gulf (including Egypt and Saudi Arabia), as well as North Africa, CIS and Sub-Saharan Africa.

What “gas-insulated” means (and where GIS fits)

In a gas-insulated panel the primary conductors sit inside a hermetically sealed enclosure filled with insulating gas, so the dielectric strength no longer depends on room air, humidity or altitude. Because the gas is far more effective than air, the same voltage and current ratings fit into a much smaller footprint, and the sealed tank makes the switchgear maintenance-free for its service life — the “sealed-for-life” or “sealed pressure system” concept defined in IEC 62271-200.

GIS belongs to the wider medium-voltage switchgear family. Where large primary substations often use air-insulated metal-clad panels, GIS dominates compact secondary distribution — ring main units, compact substations and space-limited or harsh-environment sites.

How gas insulation works

The role of the insulating medium is to withstand the electric field between live parts, and between live parts and earth, without breaking down. Insulating gas has a much higher dielectric strength than air at the same pressure, so the phase-to-phase and phase-to-earth clearances can be far smaller for the same rated voltage. That single property is why a 24 kV gas-insulated panel can be a fraction of the volume of an equivalent air-insulated one.

Two design ideas follow from this:

• Sealed pressure system (“sealed-for-life”). The gas is enclosed in a welded steel tank that is filled and sealed at the factory and never opened in service. IEC 62271-200 recognises this as a sealed pressure system with an expected operating life during which no gas handling is required. Manufacturers describe the primary enclosure as hermetically sealed and safe-to-touch, independent of the external environment.
• Vacuum switching inside the gas tank. Modern MV gas-insulated panels do the actual current interruption with a vacuum interrupter (vacuum circuit-breaker or vacuum-tight switch), while the gas provides the standing insulation. Siemens 8DJH, for example, uses a vacuum circuit-breaker with a three-position disconnector inside its sealed vessels.

Because the tank is sealed for life, the internal parts are protected from dust, humidity, condensation and pollution — the reason GIS keeps its dielectric performance in conditions that would derate or foul an air-insulated panel.

GIS types and product families

GIS is supplied in several formats depending on the role in the network:

• Gas-insulated ring main units (RMUs): compact sealed units for secondary distribution — for example the Schneider RM6, ABB SafeRing / SafePlus and Siemens 8DJH. These combine ring load-break switches with transformer protection (fuse-switch or circuit-breaker) in one sealed tank. See the ring main unit pillar for the RMU function in detail.
• Extensible / block-type secondary GIS: panels that can be extended on site as the network grows — SafeRing/SafePlus offer an extendable version, and 8DJH is a modular block system for secondary distribution up to 24 kV.
• SF6-free gas-insulated switchgear: newer families that replace SF6 with dry/technical air or another low-GWP medium while keeping the sealed, compact design (covered in detail below).

GIS product families at a glance

The gas-insulated families we most often supply for MV secondary distribution:

For the construction and insulation split across the whole switchgear family, see the medium voltage switchgear pillar, which separates insulation medium (gas vs air) from construction format (metal-clad, ring main unit).

SF6 vs SF6-free gas-insulated switchgear

For decades SF6 was the standard MV insulating gas because of its excellent dielectric and arc-quenching properties. Its drawback is environmental: SF6 is a potent greenhouse gas with a global warming potential (GWP) of roughly 23,000 times that of CO₂ — Siemens switchgear datasheets cite a GWP of 22,800, and the IPCC (Sixth Assessment Report) value is 24,300 — which has made it the target of tightening regulation such as the EU F-gas Regulation (EU 2024/573).

SF6-free gas-insulated switchgear keeps the sealed, compact concept but replaces SF6 with a low-GWP medium, most commonly dry / clean air combined with vacuum interruption:

• Dry-air sealed RMUs — e.g. ABB SafeRing/SafePlus Air, a sealed ring main unit that uses dry air with a global warming potential of 0 and a vacuum circuit-breaker with disconnector, rated up to 24 kV with internal-arc accessibility AFLR 20 kA 1 s to IEC 62271-200.
• Pure-air + vacuum modular switchgear — e.g. Schneider SM AirSeT, an SF6-free range that combines pure air for insulation and vacuum for arc interruption (Shunt Vacuum Interruption, SVI), with the load-break switch mounted in a sealed tank filled with air containing a vacuum bottle.
• SF6-free air-insulated secondary switchgear — e.g. ABB UniSec Air, up to 24 kV, 1250 A and 25 kA, fully compliant with the EU F-gas regulation and using a dedicated vacuum interrupter (this is an air-insulated family; see the parent pillar for the insulation-vs-construction distinction).

The practical takeaway for a specifier: SF6-free MV switchgear now offers the same ratings and sealed-for-life convenience as SF6, with a clear regulatory and environmental advantage. Where a project or utility mandates SF6-free equipment, a like-for-like alternative is available across the RMU and secondary-distribution range.

GIS vs AIS — how to choose

The core selection question is insulation medium: gas-insulated (GIS) or air-insulated (AIS).

Beyond the headline comparison, the decision is really about total cost of ownership and site conditions. A GIS panel costs more to buy, but its sealed enclosure removes the need for a large, clean, climate-controlled switch room and cuts internal maintenance to near zero over its life. An AIS panel is cheaper and easier to inspect and service, but it needs the space and the controlled environment to keep its air clearances clean and dry. On a compact urban plot, an offshore or desert site, or a project with limited maintenance access, the sealed GIS solution frequently wins on lifetime cost even though its purchase price is higher.

Selecting GIS for hot and harsh climates

For Gulf and North-African projects — high ambient temperature, dust, humidity and coastal salt — the sealed GIS enclosure is often decisive, because its dielectric performance does not degrade with room conditions. Three engineering points matter when specifying for these markets:

1. Temperature derating. Rated normal current derates with ambient temperature. In installations where indoor temperatures routinely exceed 45 °C, apply the manufacturer’s derating before confirming the rated current — a 630 A busbar may be limited well below 630 A at 55–60 °C.
2. Sealed enclosure vs pollution. Because the live parts are hermetically sealed, dust, humidity and salt do not reach them, so GIS avoids the flashover and tracking risks that pollution creates on exposed air-insulated parts.
3. Compactness. Where floor area or substation footprint is constrained — kiosks, basements, packaged substations — the smaller GIS volume is a direct project benefit.

Key ratings to specify

When specifying a GIS panel, confirm these ratings against the manufacturer’s datasheet for your configuration:

The ABB SafeRing/SafePlus and Siemens 8DJH families are type-tested to the IEC 62271 series — including IEC 62271-100 (circuit-breakers), -102 (disconnectors), -103 (switches) and -105 (switch-fuse combinations) — with the complete panel to IEC 62271-200.

Internal-arc safety

Even a sealed panel must be safe if an internal fault occurs. Internal Arc Classification (IAC) to IEC 62271-200 states the accessibility that is protected (type A for authorised personnel, sides F/L/R) and the tested arc current and duration — for example AFLR 20 kA for 1 s. In a fault, a pressure-relief path directs the hot gases away from the operator, and the earthed metal enclosure contains the event. A gas-insulated RMU such as SafeRing Air is classified AFLR 20 kA 1 s, and the Siemens 8DJH panels are tested for resistance to internal faults up to 21 kA.

Standards

The governing standard for MV GIS is IEC 62271-200 (AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV). Gas-insulated panels are built as sealed pressure systems with a defined service life, and are classified by loss-of-service-continuity category (LSC), partition class (PM/PI) and internal-arc classification (IAC). The switching devices themselves follow the matching parts of the series — -100 (circuit-breakers), -102 (disconnectors), -103 (switches) and -105 (switch-fuse combinations) — and for a compact secondary substation the enclosure may also be type-tested to IEC 62271-202 (high-voltage/low-voltage prefabricated substations).

Applications

Gas-insulated switchgear is the workhorse of secondary distribution: compact substations and kiosks, utility ring networks, industrial and infrastructure distribution, renewable connection points and any space-limited or environmentally demanding installation. A gas-insulated ring main unit typically sits where an MV ring drops down to a distribution transformer, switching the incoming and outgoing cables and protecting the transformer — see the plug-in cable termination guide for how cables connect to a sealed GIS panel, and the ring main unit pillar for the secondary-distribution role in detail.

A plug-in cable termination — more precisely a separable connector — is a screened, touch-proof MV cable accessory that pushes onto the bushing of gas-insulated switchgear, so a cable can be connected or disconnected without cutting into the insulation.

This guide explains how a plug-in cable termination works on gas-insulated MV switchgear, the interface types you will meet, the four assembly steps, and how to select the right connector — with common mistakes to avoid.

Why use a plug-in cable termination on gas-insulated switchgear?

Gas-insulated switchgear (GIS) and ring main units are sealed for life, so their cable connection has to be made from outside the tank, onto standardised bushings. A plug-in cable termination provides exactly that: a screened (fully insulated, earthed outer layer) plug that mates with the bushing, is touch-proof, and can be removed for testing without disturbing the cable. This is what makes the compact, maintenance-free gas-insulated switchgear design practical in the field.

Interface types and current ratings

Separable connectors are standardised by a bushing interface defined in CENELEC EN 50180 / EN 50181, and each interface has a current class. The Nexans Euromold range covers cables from 12 up to 42 kV. The common interfaces are:

Interface A (250 A) is the workhorse for distribution transformers and small feeders; the 630 A and 1250 A interfaces are used where the feeder current or ring-through current is higher.

The bushing part accepts interface diameters between 40 and 60 mm, and the system is separable, allowing easy removal for testing. A typical screened elbow connector (e.g. 158LR) is built from a conductive EPDM insert and jacket, a conductor connector, a capacitive voltage test point, and — in the earthed (-G) version — an earthing lead.

Screened vs unscreened — why touch-proof matters

The single most important distinction is whether a connector is screened. A screened separable connector carries a semi-conductive outer layer that is bonded to earth, so the exposed surface stays at earth potential even when the cable is live. That makes the assembly touch-proof and lets it be operated safely inside a compact, sealed switchgear cabinet where clearances are small. Unscreened accessories have no earthed outer layer and are restricted to applications where touch-proofing is not required. On gas-insulated switchgear — where the whole point is a sealed, space-saving enclosure — screened, touch-proof connectors are effectively mandatory. The screened design is also what allows tee and cross connectors to be stacked on a single bushing to build ring and tap-off arrangements.

Function types — elbow, tee and cross

The elbow (angled) connector terminates a single cable onto a bushing; the tee connector provides a through-connection so a bushing can carry the ring plus a tap-off; and the cross connector extends this to multiple tap-offs. Unused positions are closed with dead-end plugs, and surge arresters (e.g. the 156SA) or stand-off / earthing plugs can be added onto the same interface. This building-block approach mirrors the switchgear itself and is why the accessory interface is standardised so strictly.

Fitting a plug-in cable termination: the four steps

1. Prepare the cable. Strip the cable to the connector’s dimensions and apply the stress-control geometry that controls the electric field at the screen cut-back.
2. Fit the connector body. Push the elbow or T-body over the equipment bushing until seated.
3. Screen and earth. Connect the semi-conductive outer screen to earth, making the assembly touch-proof.
4. Test point / plug. Fit the capacitive voltage test point and any insulating plug; unused positions of a tee are closed with a dead-end plug.

Selecting a plug-in cable termination

When you select a plug-in cable termination, five factors decide the right part:

• Interface & current: match Interface A (250 A), 630 A or 1250 A to the feeder current.
• Voltage class: e.g. 12/20 (24) kV — select the connector’s rated voltage above the network’s Ur.
• Cable type: confirm the connector suits the cable’s conductor size and screen type (e.g. copper-tape or wire screen).
• Function: elbow (single cable), tee (through-connection / ring), or cross (multiple tap-offs).
• Standards: the products meet CENELEC HD 629.1, EN 50180/50181, IEC 60502-4 and IEC 60137.

Common mistakes

• Wrong interface pairing — the connector interface must match the switchgear bushing interface (A/C); they are not interchangeable.
• Ignoring stress control — an incorrectly applied stress-control cone leads to partial discharge and early failure.
• Screen not earthed — leaves the assembly no longer touch-proof.
• Skipping the test point — the capacitive test point is needed to verify dead/live status safely before work.

Testing and commissioning

Before energising, the capacitive voltage test point on each connector is used to confirm the circuit is dead, and, after energising, to verify phase presence — without exposing any live metal. On commissioning, cable systems are typically subjected to an insulation and partial-discharge check; because the connector is separable, it can be taken off the bushing for these tests and refitted, which is a practical advantage over fixed heat-shrink terminations. Keeping the interface clean and correctly lubricated at assembly is what ensures a reliable, partial-discharge-free joint over the cable’s life.

A correctly specified plug-in termination is part of a coordinated MV system — the same fault levels that size the short-circuit rating also apply to the connector’s short-circuit withstand.