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Table of Contents
Introduction
Why the comparing AIS vs GIS vs SSIS vs RMU important? Modern power systems demand compact, reliable and sustainable medium voltage switchgear. With the multiple options available, Engineers are often asked a common question:
“What is the difference between AIS vs GIS vs SSIS vs RMU – and which switchgear is best for my project?”
This article offers a practical comparison of AIS vs GIS vs SSIS vs RMU for both primary and secondary distribution networks, focusing on insulation type, footprint, safety, environmental impact, and ideal applications.
Air-Insulated Switchgear (AIS)
AIS is the most traditional form of medium-voltage switchgear and uses air as its primary insulation medium.
Key Characteristics
- Insulation Medium: Ambient air
- Application: Mainly primary distribution switchgear
- Footprint: Large
- Typical ratings: High current, standard MV fault levels
Advantages of AIS
- Proven, robust technology
- Easy to understand and manufacture
- Easy access for operation, inspection and maintenance
- Lower upfront cost for sites where space is not a constraint
- No SF₆ gas – therefore, good environmental impact profile
Limitations
- Large room size and clearances required
- Performance is sensitive to dust, humidity and pollution as conductive parts are exposed to air.
- High maintenance cost
- Not ideal where real estate is expensive, like in dense urban substations
Best Suited For
- Utility substations and industrial plants
- Brownfield expansions where existing AIS is already installed
- Projects where cost is more important than compactness
Gas-Insulated Switchgear (GIS)
Gas-insulated switchgear (GIS) uses SF₆ gas to achieve very high dielectric strength in a compact housing. It has become the standard for many urban and critical substations.
Key Characteristics
- Insulation Medium: Pressurised SF₆ gas
- Footprint: Very compact – ideal where space is premium
- Application: Primary distribution networks at MV and HV levels
- Maintenance: Low, with sealed-for-life gas compartments
Advantages of GIS
- Extremely compact switchgear solution
- Excellent reliability in harsh environments (pollution, salinity, humidity)
- Highly suitable for metro, rail, tunnels, offshore platforms and city-centre substations
Limitations
- SF₆ is a potent greenhouse gas, so the environmental impact of SF₆ switchgear is significant
- Requires trained personnel and special gas handling equipment
- Higher initial investment compared to AIS
Best Suited For
- High-density urban substations
- Metro rail and transport hubs
- Oil & gas and process industries with harsh ambient conditions
SSIS – Shielded Solid Insulation System (Gas-Free Switchgear)
SSIS (Shielded Solid Insulation System) represents the next generation of SF₆-free, gas-free MV switchgear. Instead of gas, it uses solid epoxy or organic composite insulation, fully shielded and often combined with a modular, digital-ready design.
Key Characteristics
- Insulation: Solid dielectric with full metallic shielding
- Footprint: Compact – comparable to GIS
- Application: Primary distribution switchgear where safety and uptime are critical
- Environmental Impact: Excellent – no greenhouse gas
Advantages of SSIS
- SF₆-free switchgear supporting net-zero and sustainability goals
- High degree of personal safety: touch-proof, arc-flash-safe architecture
- Factory-assembled, modular design enabling fast installation
- Ideal foundation for digital switchgear technology, advanced protection and condition monitoring
Limitations
- Higher initial cost than AIS and many RMU-based solutions
- Requires adequate training to use advanced diagnostics and communication features
Best Suited For
- Data centres where high reliability and uptime is the key requirement
- Renewable energy plants (solar, wind, hybrid evacuation switchboards)
- Large commercial campuses, hospitals and IT parks aiming for green building certifications
- Users actively looking for an alternative to GIS to avoid SF₆ while retaining compactness, safety and reliability
Ring Main Unit (RMU)
A Ring Main Unit (RMU) is a compact device used mainly in secondary distribution networks, typically at 11 kV or 22 kV levels.
Key Characteristics
- Insulation: SF₆ / air / solid, depending on design
- Footprint: Very compact, wall- or floor-mounted
- Application: Secondary distribution, ring or radial networks
- Functionality: Switching and basic protection for transformers and outgoing feeders
Advantages of RMU
- Very small footprint – ideal for kiosks and compact substations
- Simple to operate, with low maintenance needs
- Cost-effective for utilities and building distribution networks
Limitations
- Not intended as a full-featured primary distribution switchgear
- Lower flexibility in protection and control compared to AIS/GIS/SSIS
- Fault current and rating options are more limited
Best Suited For
- Urban and rural distribution networks (utilities)
- Residential townships, commercial and mixed-use buildings
- Compact substations, package substations and distribution kiosks
Quick Comparison Table – AIS vs GIS vs SSIS vs RMU
Pictorial Comparision AIS vs GIS vs SSIS vs RMU
Which Medium Voltage Switchgear Technology Should You Choose?
- Choose AIS if:
Budget is tight, space is available, and you prefer a robust, familiar technology. - Choose GIS if:
You need a compact, high-reliability solution in space-constrained or harsh environments and are prepared to manage SF₆ responsibly. - Choose SSIS if:
You want gas-free, SF₆-free switchgear that is safe, compact and aligned with long-term sustainability and digitalisation goals—especially in data centres and renewable energy plants. - Choose RMU if:
You are designing secondary distribution networks or compact substations where a simple, cost-effective solution is sufficient.
Conclusion
From the above comparison of AIS vs GIS vs SSIS vs RMU, we now have clarity on which medium-voltage solution to use. In short, there is no single “best” technology; there is only the best-fit switchgear for your technical, environmental and commercial priorities.
The shift toward green, digital, and compact MV distribution solutions is accelerating. While AIS and GIS still dominate many applications, SSIS (gas-free primary distribution switchgear) is emerging as the most sustainable and technologically advanced solution for the next decade.
External References
https://new.abb.com/medium-voltage/switchgear/gas-insulated?utm_source=chatgpt.com
https://www.se.com/in/en/product-range/61301-premset/?parent-subcategory-id=87903#products