Should a project specify gas-insulated or air-insulated switchgear?
Medium-voltage distribution has two established ways to keep energized parts from flashing over: separate them with air, or seal them inside an enclosure filled with an insulating gas. Air-insulated switchgear (AIS) relies on clearance and solid barriers in ambient air. Gas-insulated switchgear (GIS) shrinks those clearances by surrounding the conductors and switching devices with a dielectric gas inside a sealed metal enclosure. Both switch, isolate, and protect the same circuits; they differ in size, maintenance, environmental exposure, and — increasingly — in how regulators treat the gas itself.
The choice is rarely about performance alone. It is a site decision that weighs available floor area, ambient conditions, voltage class, service philosophy, and a shifting compliance picture around SF6. Getting it wrong means either paying for a building larger than it needs to be, or installing sealed equipment that a jurisdiction is about to restrict.
How the two technologies differ
The insulating medium sets everything else
In AIS, dielectric strength comes from air gaps, so bus bars, breakers, and terminations need physical separation and are typically reachable for inspection. That makes the lineup larger but keeps live parts visible and serviceable. GIS replaces most of those air gaps with pressurized gas inside a grounded shell, letting conductors sit far closer together.
The practical trade-off shows up in maintenance. As one manufacturer’s application guidance notes, in GIS “primary circuits and components are contained within a sealed tank or covered with solid encapsulated insulation,” so they “are not subject to destructive effects of the surrounding environment, thus requiring minimal or no maintenance for long periods.” AIS, by contrast, can see its “performance and dielectric integrity … negatively affected by environmental contaminants and humidity,” so “more elaborate and frequent maintenance may be required.” Sealing is an advantage in harsh or dirty environments and a limitation where crews expect to open and inspect primary compartments directly. Both approaches are available as metal-clad switchgear with compartmentalized, drawout breakers, or as simpler metal-enclosed switchgear for fixed-mounted devices.
Footprint and where it matters
Space is where GIS earns its premium — but only above a voltage threshold. According to the same engineering guidance, “at 5 and 15 kV, lineups of metal-clad switchgear and GIS have similar footprints,” while “at 27 and 38 kV, GIS offers a much smaller footprint” and “can reduce the footprint by up to 60 per cent, saving building costs and freeing up space for other use.” At 15 kV and below, the compactness case for gas-insulated switchgear is weak; the argument strengthens sharply at 27 kV and 38 kV, and in dense urban rooms, rooftop electrical spaces, and retrofit vaults where every square metre is expensive.
What SF6 rules are changing
The gas most GIS has historically used is sulfur hexafluoride (SF6) — an excellent insulator and arc-quenching medium, and a potent greenhouse gas. The U.S. EPA classifies SF6 among “high-GWP gases” whose global warming potentials “can be in the thousands or tens of thousands,” and which “persist in the atmosphere for hundreds or thousands of years.”
That profile is driving a wave of state action. California’s Air Resources Board finalized amendments in 2022 to “phase out use of SF6 in gas-insulated equipment (GIE) starting in 2025.” New York “implements a voltage-based phaseout of SF6-gas insulated equipment (GIE) beginning in 2027.” Massachusetts caps GIS at “a maximum annual SF6 leak rate of 1 percent for GIS purchases after 2015.” The direction is one-way: SF6-filled equipment now carries inventory, leak-tracking, and end-of-life obligations, and in some states a shrinking window to buy it at all. SF6-free GIS — using vacuum interruption with alternative gases or dry air — is maturing to fill the gap, and any GIS specification written today should state which insulating medium the design uses and how the owner will meet local reporting rules.
Where each approach makes sense
GIS fits space-constrained, higher-MV sites: substations feeding data centers, urban distribution, indoor rooms at 27–38 kV, and coastal or industrial environments where sealed primary parts resist contamination. AIS fits sites with room to build, lower voltage classes, owners who value direct access to primary compartments, and projects where SF6 policy or corporate sustainability targets discourage gas-filled equipment. Many campuses mix both — AIS at 5–15 kV lineups and GIS where footprint drives the design — often alongside low-voltage switchgear and MCCs downstream.
What a buyer should specify
Start with the governing standards as context. Medium-voltage switchgear is specified against the IEC 62271 series and the IEEE C37.20 family, with gas-insulated substations above 52 kV governed by IEEE C37.122 and IEC 62271-203; installation follows the NEC and Canadian CEC. From there, pin down rated voltage and short-circuit withstand; arc-resistance class if required; the insulating medium (SF6 or SF6-free) and the leak-rate and reporting obligations that come with it; the maintenance model the crews can actually support; and the true available floor area, since that is what decides whether the GIS premium pays back.
Entogo designs and builds both AIS and GIS lineups — from metal-clad and gas-insulated switchgear to pad-mounted RMUs and packaged units for substations and power distribution and utility projects — designed and built to the applicable IEEE and IEC standards; UL (cULus)/CSA certifiable on request. Because the switchgear, transformers, and enclosures come from one vertically integrated factory, a lineup can be matched to a site’s footprint, voltage class, and gas policy without the multi-year waits that dominate today’s market.
