Gas Insulated Switchgear (GIS) vs Air Insulated Switchgear (AIS): Complete Comparison
When selecting medium voltage switchgear for a power distribution project, one of the most fundamental decisions is choosing between Gas Insulated Switchgear (GIS) and Air Insulated Switchgear (AIS). Both technologies serve the same primary function — switching, controlling, and protecting electrical circuits — but they differ significantly in design, footprint, cost, and application suitability.
This comprehensive comparison will help you understand the key differences and make the right choice for your specific project requirements.
What is Gas Insulated Switchgear (GIS)?
Gas Insulated Switchgear (GIS) uses sulfur hexafluoride (SF6) gas or environmentally friendly alternatives as the insulating medium. All live components — including busbars, circuit breakers, disconnectors, and earthing switches — are enclosed within a sealed metal housing filled with SF6 gas at low pressure (typically 0.15–0.4 bar).
The high dielectric strength of SF6 gas (approximately 2-3 times that of air at equivalent pressure) allows for extremely compact designs, reducing the physical footprint by up to 60% compared to conventional AIS. GOHO’s GHRM-12 series SF6 fully insulated compact switchgear is a typical example of GIS technology applied to ring main units (RMU).
Key specifications of modern GIS (12kV class):
- Compact footprint: 600-800mm width per panel (vs 800-1200mm for AIS)
- Gas leakage rate: ≤ 0.1% per year (sealed for life)
- Enclosure protection: IP67 (tank) / IP3X-IP4X (cabinet)
- Maintenance interval: Sealed-for-life — no scheduled maintenance required
- Typical applications: Urban substations, underground vaults, offshore platforms
What is Air Insulated Switchgear (AIS)?
Air Insulated Switchgear (AIS) uses ambient air as the primary insulating medium. Live conductors are separated by air gaps, with clearances determined by the voltage level and applicable standards (IEC 62271-200 for MV switchgear). AIS is the conventional, more widely deployed technology with decades of proven performance across all voltage levels.
GOHO’s KYN28-12 metal-clad withdrawable switchgear and KYN61-40.5 armored switchgear are examples of advanced AIS technology, offering metal-enclosed compartmentalized construction with air-insulated busbars and vacuum circuit breakers.
Key specifications of modern AIS (12kV class):
- Panel width: 800-1200mm per panel (depending on configuration)
- Insulation medium: Ambient air (no gas handling required)
- Enclosure protection: IP4X (enclosure) / IP2X (partitions)
- Maintenance: Periodic inspection and cleaning required
- Typical applications: Indoor substations, industrial facilities, power plants
GIS vs AIS: Head-to-Head Comparison
| Aspect | GIS (Gas Insulated) | AIS (Air Insulated) |
| Space Requirement | Up to 60% smaller — 800x1600mm per panel (12kV) | Larger — 1000x2800mm per panel (12kV) |
| Insulation Medium | SF6 gas or eco-friendly alternatives | Ambient air (free, unlimited) |
| Environmental Sensitivity | Immune — sealed design unaffected by humidity, pollution, salt spray | Sensitive — requires clean, dry environment; de-rating at high altitude |
| Maintenance | Sealed-for-life — no routine maintenance of primary components | Periodic cleaning, inspection, and component servicing required |
| Installation Time | Fast — pre-assembled modules, plug-in connections | Moderate — site assembly of busbars and components |
| Initial Cost | Higher (20-40% premium over AIS) | Lower — most economical option |
| Lifecycle Cost | Lower maintenance cost offsets higher initial investment | Higher maintenance costs over 20+ year service life |
| Altitude Performance | No de-rating required up to 5000m | De-rating required above 1000m (air density reduction) |
| Submersible Capability | Yes (IP67 tank can withstand temporary submersion) | No — water ingress causes flashover |
| End-of-Life SF6 Handling | SF6 recovery and recycling required | No gas handling — standard metal recycling |
When to Choose GIS
GIS is the preferred choice in the following scenarios:
- Space-constrained urban substations: Where real estate costs are high or available space is limited — GIS reduces footprint by up to 60%
- Underground installations: Vaults, basements, and tunnels where space is at a premium and environmental contamination is likely
- Coastal and industrial environments: Where salt spray, humidity, or chemical pollution would accelerate degradation of air-insulated components
- Flood-prone areas: The sealed IP67 tank can withstand temporary submersion without damage
- High-altitude installations: GIS performance is unaffected by reduced air density at altitudes above 1000m
- Offshore platforms and marine applications: Compact size and corrosion resistance are critical advantages
When to Choose AIS
AIS remains the better choice in these scenarios:
- Budget-constrained projects: Initial capital cost is 20-40% lower than GIS for equivalent ratings
- Indoor substations with adequate space: If space is not a constraint, the simpler AIS design offers reliable performance at lower cost
- Industrial facilities: Standard indoor industrial environments are well-suited to AIS, with maintenance personnel familiar with the technology
- Projects requiring frequent reconfiguration: AIS is generally easier to modify or expand
- Regions with environmental restrictions on SF6: Some jurisdictions are implementing SF6 phase-down regulations
Environmental Considerations: SF6 and Alternatives
SF6 gas has a global warming potential (GWP) of approximately 23,500 times that of CO₂, making it one of the most potent greenhouse gases. However, modern GIS equipment is designed with sealed-for-life enclosures that limit leakage to ≤ 0.1% per year, and proper end-of-life gas recovery is standard industry practice.
Eco-friendly alternatives to SF6 are emerging, including:
- Clean air / dry air: Compressed dry air insulation for medium voltage switchgear
- Fluoroketone (C5-FK) mixtures: GWP of less than 1
- Fluoronitrile (C4-FN) mixtures: GWP of less than 1
- Solid insulation: Epoxy resin encapsulated systems for specific applications
Frequently Asked Questions
Which technology lasts longer — GIS or AIS?
Both technologies have a typical service life of 25-35 years. GIS offers lower maintenance requirements throughout its life, while AIS may require more frequent component replacement (especially in harsh environments). The sealed design of GIS protects components from environmental degradation.
Is GIS cheaper than AIS in the long run?
Although GIS typically costs 20-40% more upfront, total lifecycle cost analysis often favors GIS in space-constrained or harsh environments due to reduced civil works (smaller building), lower maintenance costs, and higher reliability. For standard indoor installations with adequate space, AIS remains the more economical choice.
Does GOHO manufacture both GIS and AIS switchgear?
Yes — GOHO manufactures both technologies. Our GHRM series represents SF6 gas insulated compact RMU technology, while our KYN28-12, KYN61-40.5, and XGN series represent air insulated technology. This allows us to recommend the optimal solution for each project without technology bias.