The three main types of electrical switchgear are classified by how live parts are insulated and how the arc is interrupted: air-insulated switchgear (AIS), which uses air as insulation and is the lowest-cost, easiest to maintain option; gas-insulated switchgear (GIS), which seals live parts in SF6 or an alternative gas to save up to 70–90% of floor space; and vacuum switchgear, which interrupts current inside sealed vacuum bottles and is the standard choice for medium voltage up to about 40.5 kV.
Choosing the wrong type of switchgear for a project does not always reveal itself immediately. The equipment gets installed, it operates, and somewhere between year three and year seven the problems start — maintenance costs climb, outages become difficult to manage, or the installation footprint creates problems that were not anticipated during design. Most of those situations trace back to a specification decision made early in the project when the differences between air-insulated, gas-insulated, and vacuum switchgear were not fully understood.
This article explains how each technology works, where each one belongs, and how to make the right selection for your specific project conditions — whether you are building an outdoor transmission substation, an urban indoor distribution network, an industrial facility, or anything in between.
Types of Electrical Switchgear: Why the Type Matters
Switchgear is the combination of electrical disconnect switches, fuses, circuit breakers, and associated control equipment used to control, protect, and isolate electrical equipment in a power system. It appears at every voltage level — from the high-voltage transmission network down to the low-voltage distribution boards inside a building.
The type of switchgear you use determines how the electrical arc is interrupted during switching and fault conditions, what insulating medium separates live conductors from earthed metalwork and from each other, how much physical space the installation requires, how the equipment is maintained over its operating life, and how it performs under challenging environmental conditions.
Getting this selection right matters because switchgear is a long-life asset. Most installations are expected to operate reliably for 25 to 40 years. A technology choice that creates maintenance difficulties, space constraints, or environmental compliance issues five years into that operating life is an expensive problem to correct.
Air-Insulated Switchgear (AIS): The Traditional Standard
How AIS Works
Air-insulated switchgear uses atmospheric air as the primary insulating medium between live conductors and between live conductors and earthed metalwork. The live components — busbars, circuit breakers, disconnectors, earthing switches, current transformers, and voltage transformers — are mounted in open air within a metal-clad or metal-enclosed structure, physically separated by the clearance distances required at the operating voltage.
At medium voltages from 6kV to 36kV, AIS is typically supplied as indoor metal-clad switchgear. The KYN28-12 panels manufactured at Danli are a clear example — the vacuum circuit breaker, busbars, and instrument transformers are all installed inside a metal-clad cabinet with air as the insulating medium between conductors and compartments. At higher transmission voltages from 72.5kV upward, AIS is usually an outdoor structure where equipment is mounted on steel frameworks or concrete plinths with large air clearances between phases and to earth.
Where AIS Performs Best
Air-insulated switchgear is usually the most flexible and economical choice when land is available. For outdoor substations in rural or semi-urban environments, transmission switching yards, utility distribution networks, and industrial facilities with adequate substation room dimensions, AIS delivers reliable performance at a lower initial equipment cost than gas-insulated alternatives.
AIS is also the preferred technology when the maintenance team needs straightforward visual access to equipment. Because the live components are in open air and physically accessible, routine inspection, cleaning, and contact checking can be done without specialist gas handling equipment or sealed compartment procedures. Engineers and technicians who have worked with AIS for years find it familiar and maintainable with standard electrical safety procedures.
AIS Limitations
The main constraint of AIS is its physical footprint. The air clearances required at higher voltage levels take up significant space. A 132kV AIS substation requires a large outdoor area with substantial civil works — cable trenches, earthing grids, equipment plinths, perimeter fencing, and access roads. In urban environments where land costs are high and space is genuinely limited, that physical requirement becomes a serious problem.
AIS is also more exposed to environmental conditions. In coastal areas with salt-laden air, heavily industrialised zones with chemical pollution, and high-humidity tropical environments, the exposed insulator surfaces are subject to contamination flashover if cleaning and maintenance schedules are not rigorously followed.
Gas-Insulated Switchgear (GIS): Compact, Reliable, Higher Cost
How GIS Works
Gas-insulated switchgear encloses all live components — circuit breakers, disconnectors, busbars, earthing switches, and instrument transformers — inside sealed metal enclosures filled with sulphur hexafluoride (SF6) gas or, in more recent designs, alternative gas mixtures such as dry air, nitrogen, or g3 (a fluoronitrile-based gas mixture developed to address SF6 environmental concerns).
SF6 gas is used as both the insulating medium and the arc-quenching medium. Major components such as circuit breakers, disconnectors, busbars, and earthing switches are enclosed in a sealed metal enclosure. Because SF6 has a dielectric strength approximately three times that of air at atmospheric pressure, the required clearances between conductors inside the sealed enclosure are dramatically smaller than in AIS. This is what makes GIS so compact.
Where GIS Is the Right Choice
GIS is commonly used in urban substations, industrial plants, metro rail systems, offshore platforms, and indoor substations where space and reliability are important.
When an electricity utility needs to install a 33kV or 132kV substation inside an urban building, on the roof of a commercial tower, in an underground metro system, or on an offshore oil and gas platform, GIS is not just preferred — it is often the only technically viable option. The footprint reduction compared to AIS at the same voltage rating can be as high as 70 percent, which in constrained urban locations translates directly into project viability.
GIS also performs significantly better in polluted and harsh environments. Because all live components are sealed inside pressurised enclosures, the insulation performance is completely independent of external air quality. A GIS installation in a coastal industrial zone performs identically to one installed in a clean mountain environment.
Reliability is another GIS strength. The insulating life in gas-insulated systems is generally very long, often exceeding 40 years, provided the equipment is kept clean and free from environmental contamination. Because the internal components are sealed and protected from moisture, dust, and corrosive atmospheres, the degradation mechanisms that affect AIS insulators simply do not apply.
GIS Limitations
The primary limitation of GIS is cost. GIS is usually preferred where space, pollution resistance, or an enclosed installation is more important than initial equipment cost. The sealed enclosures, precision gas handling systems, specialised sealing technology, and factory assembly requirements mean that GIS carries a significantly higher price per bay than equivalent AIS equipment.
SF6 is also a potent greenhouse gas with a global warming potential approximately 23,500 times that of carbon dioxide over a 100-year period. Regulatory pressure on SF6 use is increasing across the European Union and other jurisdictions, which is driving manufacturers toward alternative insulating gas mixtures. Projects specifying SF6-based GIS today need to consider the long-term regulatory environment and whether SF6 recovery and handling obligations will affect operating costs over the equipment’s life.
Maintenance of GIS also requires specialist skills and equipment. Accessing internal components for inspection or replacement requires trained personnel with gas handling certification, specialised vacuum and filling equipment, and adherence to manufacturer procedures that are significantly more complex than AIS maintenance.
Vacuum Switchgear: The Medium Voltage Standard
How Vacuum Switchgear Works
Vacuum switchgear is a category defined by its arc interruption technology rather than its insulating medium. In vacuum switchgear, the circuit breaker uses a sealed vacuum interrupter — a bottle from which virtually all gas has been removed — as the arc extinction device. When the contacts inside the vacuum interrupter separate under current, an arc forms briefly in the metallic vapour from the contact surfaces. With nothing to sustain the arc after current passes through zero, it extinguishes cleanly and completely within one current half-cycle.
The insulating medium surrounding the vacuum interrupter and the busbar connections can be air, solid resin, or SF6 gas depending on the switchgear design. Vacuum circuit breakers installed in air-insulated medium voltage switchgear like the KYN28-12 use air as the external insulating medium. Vacuum interrupters installed in ring main units or compact secondary distribution switchgear are often embedded in solid resin or gas-filled compartments.
Why Vacuum Technology Dominates Medium Voltage
Vacuum interrupters have become the dominant arc interruption technology for medium voltage switchgear — from 1kV to 52kV — because they combine high reliability with very low maintenance requirements. The sealed vacuum bottle has no gas to leak, no contacts exposed to atmospheric contamination, and no arc products to clean up after fault interruption. A correctly specified vacuum interrupter is rated for 10,000 mechanical operations and typically 50 to 100 fault-current interruption operations before the contacts require attention.
This maintenance profile is significantly better than the oil circuit breakers and air blast circuit breakers that vacuum technology replaced from the 1980s onward. At Danli, our KYN28-12 switchgear uses VS1-12 and VH1-12 vacuum circuit breakers as standard, with compatibility for ABB VD4 and other internationally recognised vacuum breaker types depending on project requirements.
Vacuum technology also offers excellent performance across a wide range of medium voltage applications — motor switching, capacitor bank control, transformer protection, and general feeder protection — without the need for SF6 gas handling infrastructure that ring main units and GIS-based secondary distribution equipment require.
Where Vacuum Switchgear Is Used
Vacuum switchgear in metal-clad AIS construction is the standard for industrial medium voltage distribution across manufacturing, water treatment, oil and gas, mining, metallurgy, and power generation facilities worldwide. Secondary distribution substations that step down from 11kV or 33kV to 400V use vacuum circuit breakers in the incoming and outgoing feeder panels. Large motors above 100kW are typically switched and protected by vacuum contactors or vacuum circuit breakers in dedicated motor feeder panels.
Read More: High Voltage vs Low Voltage Switchgear
AIS vs GIS vs Vacuum: Side-by-Side Comparison
Understanding how the three technologies compare across the factors that matter most in project selection helps make the decision clearer.
Footprint and Space Requirements
AIS requires the most physical space at any given voltage level. GIS offers footprint reductions of 50 to 70 percent compared to AIS at the same voltage. Vacuum switchgear in medium voltage metal-clad construction is compact by design — a standard KYN28-12 panel at 12kV occupies a floor area of roughly 1.2 to 1.5 square metres per panel including access space.
Initial Cost
AIS usually has the lowest initial equipment cost. GIS usually has the highest initial equipment cost. Vacuum switchgear in medium voltage AIS construction falls between the two in absolute cost terms, though it is the standard choice at its voltage level rather than a premium option.
Maintenance Requirements
AIS requires regular visual inspection of insulators, cleaning in polluted environments, and periodic contact checking. GIS requires specialist gas handling skills and equipment but has fewer routine inspection needs. Vacuum switchgear in metal-clad construction requires minimal routine maintenance — primarily mechanical checks of the operating mechanism and periodic secondary wiring inspection.
Environmental Suitability
AIS performs well in clean, dry environments with adequate ventilation. It requires more attention in coastal, industrial, or high-humidity locations. GIS is largely immune to external environmental conditions once installed. Vacuum switchgear performance is unaffected by external environment because the arc interruption occurs inside the sealed vacuum bottle.
Operational Life
Switchgear is installed and expected to work for 25 years or longer. All three technologies are capable of achieving this operating life when properly specified, installed, and maintained. GIS installations in clean, sealed environments may exceed 40 years without significant component replacement. AIS in challenging environments may require more frequent insulator cleaning and contact maintenance to achieve the same service life.
The table summarises how the three compare:
| Factor | AIS | GIS | Vacuum (MV) |
|---|---|---|---|
| Insulating medium | Air | SF6 or alternative gas | Air insulation with vacuum interrupters |
| Footprint | Largest | Smallest | Compact |
| Initial cost | Lowest | Highest | Low to moderate |
| Maintenance | Easy access, more frequent cleaning | Low, but needs gas handling skills | Low; interrupters are sealed |
| Harsh environments | Sensitive to dust, salt and humidity | Excellent (sealed) | Good with suitable enclosure |
| Typical use | Outdoor substations, industrial plants with space | City substations, offshore, high altitude | Industrial and utility MV distribution |
Which Type Should You Specify?
Choose AIS When
You have adequate substation land or indoor room dimensions. Your project is in a location with manageable environmental conditions — dry, low pollution, not coastal. Your budget is constrained and the lower initial equipment cost of AIS makes the project viable. Your maintenance team has standard AIS experience and does not have SF6 gas handling certification.
Choose GIS When
Your substation must fit within a constrained urban footprint — an indoor substation, a rooftop installation, an underground facility, or an offshore platform. Your installation environment is harsh — coastal, chemically polluted, high humidity, or subject to significant dust and contamination. Long-term reliability and minimal maintenance are priorities over initial equipment cost.
Choose Vacuum Switchgear When
Your application is at medium voltage from 1kV to 52kV and you need reliable, low-maintenance arc interruption technology for motor switching, feeder protection, capacitor bank control, or transformer protection. Vacuum technology at this voltage range is not really an alternative to AIS or GIS — it is the arc interruption technology used inside both types of medium voltage switchgear. The question is simply whether the surrounding insulation structure is air-insulated, gas-insulated, or solid insulation.
A Note on Hybrid Switchgear
A fourth option worth mentioning is hybrid gas-insulated switchgear, sometimes called HGIS. Hybrid switchgear sits between AIS and GIS — it packages the main bay functions in a compact gas-insulated module while retaining air-insulated busbars or external connections. This makes it a practical choice for brownfield substation extensions where connecting to an existing AIS busbar arrangement but achieving a smaller bay footprint is the priority.
HGIS is particularly relevant in projects upgrading older AIS substations in locations where land for a full AIS extension is not available and a full GIS replacement is not justified by the project scope.
Summary
Choosing between the types of electrical switchgear comes down to four questions: how much space you have, how harsh the environment is, what the budget allows, and what skills your maintenance team has. For most industrial medium voltage projects, air-insulated metal-clad panels with vacuum breakers give the best balance; GIS earns its higher price where space, altitude or pollution make AIS impractical.
Frequently Asked Questions
By insulation: air-insulated (AIS), gas-insulated (GIS) and solid- or hybrid-insulated designs. By interruption medium: air, oil (legacy), SF6 and vacuum breakers. By voltage: low, medium and high voltage switchgear.
For medium voltage (about 3.3–40.5 kV), air-insulated metal-clad cabinets with vacuum circuit breakers are the most common choice in factories, mines, water plants and commercial buildings.
SF6 is a very strong greenhouse gas. Regulations, especially in the EU, are pushing new switchgear towards SF6-free alternatives such as vacuum interruption with clean-air or dry-air insulation.
GIS is worth it where space is very expensive or limited, at high altitude, in polluted or coastal air, or offshore. Where space is available, AIS or vacuum switchgear is usually more economical.
