When you specify medium voltage switchgear, the first big decision is the insulation platform: air-insulated (AIS) or gas-insulated (GIS). The two do the same job and carry the same ratings, but they differ in size, cost, maintenance, and environmental footprint, and the right choice depends on your site. STELLAR builds both, so this comparison is even-handed: the aim is to show where each one wins, not to push one over the other.
AIS vs GIS at a Glance
Here is the whole comparison in one view, before the detail.
| Factor | Air-insulated (AIS) | Gas-insulated (GIS) |
| Insulation | Ambient air | Sealed SF6 gas |
| Footprint | Large; needs clearances | 10% to 30% of AIS |
| Initial cost | Lower | Higher |
| Lifecycle cost | Higher land and maintenance | Lower land and maintenance |
| Maintenance | Frequent; exposed parts | Low; sealed, gas work for repair |
| Environment | No SF6 | Uses SF6, a greenhouse gas |
| Reliability | Contamination-sensitive | High; sealed from environment |
| Typical service life | About 20 to 30 years | 40 years and beyond |
| Expansion | Easy to extend | Harder; needs gas handling |
| Best for | Open, clean, low-cost land | Tight, urban, harsh sites |
The Core Difference: Air vs SF6 Insulation
Everything else follows from one choice: what keeps the live parts apart. AIS uses the air in the room, so the busbars and switches need generous clearance to hold off the voltage. GIS seals those same parts in SF6 gas, which has about three times the dielectric strength of air, so the clearances shrink and the whole assembly becomes far smaller and fully enclosed. Air is open, simple, and free; SF6 is sealed, compact, and a managed gas. From that single difference come the trade-offs in space, cost, maintenance, and environment below. For the products, see air insulated switchgear and gas insulated switchgear.
Footprint: The Biggest Advantage of GIS
Space is where GIS wins decisively. Because SF6 allows much smaller clearances, a GIS lineup typically needs only 10% to 30% of the floor area an AIS lineup uses at the same voltage, and for medium voltage the saving is often around two thirds. That smaller footprint also means a smaller building, shallower foundations, and shorter cable trenches. On a cramped or expensive site, that space saving is frequently the deciding factor, because it lowers the civil and land cost enough to outweigh the higher equipment price.
Cost: Capital vs Total Cost of Ownership
Cost is where the comparison gets misread, because the answer flips depending on how you measure it.
Initial cost
On purchase price alone, AIS is clearly cheaper. Its construction is simpler, with open copper busbars and air, and there are no precision gas tanks, seals, or gas-handling equipment to pay for. For a buyer comparing equipment quotes side by side, AIS wins on the sticker.
Total cost of ownership
Over the life of the installation the picture can reverse. GIS uses far less land and needs less routine maintenance, so on a high-value urban site the saved real estate and lower upkeep can make GIS cheaper overall. On a rural site with cheap land and easy access, the land saving means little and AIS stays the lower-cost choice. The honest way to decide is to compare total cost of ownership, equipment plus land plus maintenance over the service life, not the purchase price by itself.
Maintenance and Reliability
Because AIS parts sit in the open, they collect dust, moisture, salt, and the occasional rodent, so they need regular cleaning, inspection, and testing, and contamination is the main cause of AIS faults. The upside is that the open design makes those parts easy to reach, inspect, and repair on site. GIS seals the live parts away from the environment, so it runs with little routine maintenance and high reliability, and it shrugs off pollution and coastal salt. The trade-off is that any internal work on GIS means recovering and refilling the gas, which calls for trained staff and equipment. In short, AIS asks for more frequent but simpler maintenance; GIS asks for far less, but what it does need is specialised.
Environment and the SF6 Question
This is the one area where AIS has the clear edge. AIS uses only air, so it has no greenhouse-gas impact. GIS relies on SF6, which is a potent greenhouse gas, with thousands of times the global-warming potential of carbon dioxide, and it is now regulated in many markets, with leak monitoring and reporting required. Two things soften this for modern GIS: the gas is sealed for life with very low leak rates, and the switching is usually done by a vacuum interrupter, so SF6 is used only for insulation. The industry is also moving toward SF6-free designs using clean air or gas mixtures, mainly at the lower medium voltage classes. If avoiding SF6 is a hard requirement, AIS or a solid-insulated design is the straightforward route.
Insulation vs Interruption: A Common Confusion
A point that trips up a lot of specifications: the insulation medium and the interrupting medium are two different things. Air versus SF6 describes how the panel is insulated. It does not describe how the circuit breaker actually breaks the current. In modern medium voltage gear, both AIS and GIS usually break the current with a vacuum circuit breaker; in GIS the SF6 is there for insulation, not switching. So an AIS panel and a GIS panel can use the same vacuum breaker inside. When you read a specification, separate the two questions: how is it insulated, air or gas, and how does it interrupt, vacuum or SF6.
When to Choose AIS vs GIS
There is no single winner. Match the platform to the site:
Choose AIS when
• Land is available and not expensive, and the room is clean and dry.
• First cost is the binding constraint.
• You want to inspect and maintain the gear yourself without gas equipment.
• Avoiding SF6 is a requirement and a solid-insulated option is not needed.
• The layout will be extended often.
Choose GIS when
• Floor space is tight or expensive, such as urban, indoor, basement, or rooftop sites.
• The environment is harsh, coastal, dusty, humid, or high-altitude.
• Reliability and low maintenance matter more than first cost.
• The total cost of ownership, including land, favours the compact option.
Many projects use both, GIS where space is tight and AIS elsewhere, and a hybrid layout that mixes gas-insulated switching modules with air-insulated connections is a middle path for extending or upgrading an existing air-insulated substation. For the complete sealed substation, see the gas insulated substation page; for cable-ring distribution, a gas or solid ring main unit is the compact secondary option.
Get the Right Switchgear for Your Site
STELLAR builds both air-insulated and gas-insulated medium voltage switchgear to IEC 62271 and the equivalent GB standards, configured from your single-line diagram. Because we make both, we can advise on the platform that fits your space, budget, environment, and SF6 policy without a bias toward one technology. Send us the voltage, the fault level, the footprint you have, and the site conditions, and we will recommend AIS or GIS and quote your specification.
Frequently Asked Questions
What is the difference between AIS and GIS switchgear?
AIS uses ambient air to insulate the live parts, so it is simpler and cheaper but larger and exposed to the environment. GIS seals the live parts in SF6 gas, so it is far more compact and protected, at a higher cost and with SF6 to manage. They carry the same ratings; the difference is the insulation medium and what it costs in space, money, and maintenance.
Is GIS more expensive than AIS?
On purchase price, yes, GIS costs more because of its precision gas tanks, seals, and gas handling. Over the whole life it can be cheaper, because it uses far less land and needs less maintenance. On expensive urban sites GIS often wins on total cost of ownership; on cheap rural land AIS usually stays cheaper overall.
How much smaller is GIS than AIS?
A GIS lineup typically occupies only 10% to 30% of the floor area an AIS lineup needs at the same voltage, and for medium voltage the saving is often around two thirds. That is why GIS is chosen for tight urban, indoor, and underground sites where AIS would not fit.
Why is SF6 used in GIS, and is it bad for the environment?
SF6 is used because it has about three times the dielectric strength of air and quenches arcs well, which lets GIS be compact. It is also a potent greenhouse gas, so it is sealed for life with leak monitoring and is regulated in many markets. Modern GIS uses a vacuum breaker for switching, so SF6 is only the insulation, and SF6-free options are emerging at lower voltages.
Do AIS and GIS both use vacuum circuit breakers?
Often, yes. In modern medium voltage gear both AIS and GIS usually break the current with a vacuum circuit breaker. The air-versus-gas label refers to how the panel is insulated, not how it interrupts. In GIS, the SF6 is there for insulation while a vacuum interrupter does the switching.
When should I choose AIS over GIS?
Choose AIS when land is available and not costly, the room is clean and dry, first cost is the priority, you want simple in-house maintenance, or you need to avoid SF6. Choose GIS when space is tight or expensive, the environment is harsh, or reliability and low maintenance outweigh the higher purchase price.
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STELLAR
Switchgear & Power Transformer ManufacturerSTELLAR designs and manufactures switchgear, transformers, and complete power distribution equipment from our China facility, serving utilities, contractors, and distributors in 50+ countries. Every technical article on this site is reviewed by our in-house engineers to ensure accuracy against IEC standards and real project practice.
