GIS vs AIS: Gas Insulated vs Air Insulated Switchgear — Complete Comparison

There are two ways to insulate medium voltage switchgear, and the choice between them is one of the more consequential decisions a substation project makes. One method shrinks the equipment dramatically and seals it from the environment. The other keeps things simple and cheap but takes up more room and needs more looking after.

Both have been around for decades. Both are still being installed in volume. The reason they coexist is that they are not competing answers to the same question — they are answers to different questions.

GIS: everything in a sealed box

Gas insulated switchgear puts every live component — busbars, breakers, disconnectors, earthing switches — inside a sealed metal housing filled with SF6 gas at a modest pressure, typically 0.15 to 0.4 bar. SF6 has roughly 2-3 times the dielectric strength of air, which is what lets the whole assembly shrink so dramatically. A GIS panel at 12kV can be 600-800mm wide; an air-insulated equivalent needs 800-1200mm.

Because everything is sealed, the environment simply does not get in. Humidity, salt spray, dust, chemical pollution — none of it reaches the live parts. The tank is rated IP67, which means it can even sit through temporary submersion without damage. And there is essentially nothing to maintain on the primary side: no cleaning, no inspection of live conductors, because you cannot get to them and you do not need to.

That is the pitch in a nutshell: compact, immune to the environment, maintenance-free. GOHO’s GHRM ring main unit is this technology applied to secondary distribution.

AIS: air does the insulating

Air insulated switchgear does what the name says — ambient air is the insulator. Conductors are separated by air gaps, sized to the voltage and the requirements of IEC 62271-200. It is the conventional technology, the one that has been proven for decades, and the one most maintenance crews grew up with.

GOHO’s KYN28-12 and KYN61-40.5 are AIS products — metal-clad, compartmentalized, with air-insulated busbars and vacuum breakers. AIS gives you visible, accessible components. You can see the busbar, you can reach the connections, and you can service things without special gas handling.

The trade-offs are the flip side of GIS’s advantages: bigger footprint, sensitivity to the environment, and periodic maintenance. Air is free and unlimited, but it does not insulate nearly as well, so everything has to be bigger.

Side by side

When GIS earns its premium

GIS is the answer when space or environment makes AIS impractical:

• Urban substations where land costs more than the switchgear. A 60% smaller footprint is real money in a city center.
• Underground vaults and basements, where you cannot get the dimensions of an AIS room, and where contamination is a given.
• Coastal and industrial sites, where salt spray or chemical pollution would slowly eat an air-insulated assembly.
• Flood-prone locations. A sealed GIS tank shrugs off water that would flash over an AIS board.
• High-altitude projects. Above 1000m, thin air needs de-rating on AIS; GIS does not care.
• Offshore platforms, where both compactness and corrosion resistance are non-negotiable.

When AIS is the smarter buy

AIS remains the right call in the majority of standard installations:

• Budget-constrained projects. The 20-40% saving is hard to argue with when space is not a problem.
• Indoor substations with room. If the building already exists and fits, simpler is better.
• Ordinary industrial environments. Maintenance crews know AIS, and standard indoor conditions suit it.
• Frequent reconfiguration. AIS is easier to modify and expand than a sealed gas system.
• Regions tightening SF6 rules. Some jurisdictions are phasing down SF6, which shifts the calculus.

The SF6 question

SF6 deserves an honest mention. Its global warming potential is roughly 23,500 times that of CO2 — it is one of the most potent greenhouse gases around. Modern sealed-for-life GIS limits leakage to under 0.1% per year, and end-of-life gas recovery is standard practice. But the pressure to move away from SF6 is real, and alternatives are arriving: compressed dry air, fluoroketone and fluoronitrile mixtures with GWP below 1, and solid-insulated systems. For new medium voltage projects, these are increasingly worth asking about.

The straight answers

Which lasts longer? Both are 25-35 year equipment. GIS needs less maintenance over that life; AIS may need more component replacement in harsh environments.

Is GIS cheaper long-term? Often, in the right setting. The higher first cost can be offset by a smaller building, lower maintenance, and higher reliability. In a standard indoor room with space to spare, AIS stays the economical pick.

Does GOHO make both? Yes. GHRM is our GIS line; KYN28, KYN61, and the XGN series are AIS. We do not have a technology bias to protect, which makes it easier to recommend the right one for your site.

Contact GOHO for switchgear selection advice