Air Insulated Switchgear Guide: Construction, Benefits, and Applications

Air insulated switchgear (AIS) is the backbone of many medium-voltage distribution systems because it does not rely on special gases or complicated enclosures to work. Atmospheric air provides the electrical clearance between live parts, while conventional circuit breakers, disconnectors, and protection relays handle switching and fault clearing. For engineers and procurement teams, AIS remains attractive because of its transparent design, shorter delivery times, and familiar maintenance procedures. When you open an AIS panel, you can usually see the busbars, insulators, and moving contacts, which makes troubleshooting easier and helps build confidence with on-site staff.

What Is Air Insulated Switchgear?

Air insulated switchgear is an assembly of switching devices, busbars, instrument transformers, and protection relays arranged so that air is the main insulating medium between conductors. It is built for indoor and outdoor service across the medium-voltage range, with 12 kV, 24 kV, and 40.5 kV installations being the most common in industrial and utility networks. In indoor AIS, metal-enclosed panels provide segregation and operator protection. In outdoor AIS, open busbars and apparatus are mounted on support structures with clearance distances calculated from standard insulation levels.

For a broader look at switching equipment categories, you can refer to our comprehensive high-voltage switchgear guide.

How Air Insulated Switchgear Is Constructed

Clearances and Insulation

The spacing between phase conductors and from live parts to earth is fundamental to AIS design. It is determined by the power-frequency withstand voltage and lightning impulse withstand voltage expected for the system. The distances are then adjusted for altitude, pollution level, and the type of insulator used. In coastal or dusty areas, creepage distance is increased and the enclosure is designed to reduce contamination on live surfaces.

Switching and Protection

Inside a typical medium-voltage AIS panel, the switching function is performed by a vacuum circuit breaker, which offers high interruption capability and long electrical life. Disconnectors provide visible isolation, and earthing switches allow safe access to the cable compartment. Current transformers, voltage transformers, and a protection relay detect overload, short circuit, earth fault, and undervoltage conditions. Modern panels also include surge arresters, cable connections, and mechanical interlocks to enforce the correct operating sequence.

AIS vs GIS: What Is the Real Difference?

GIS is often mentioned as a space-saving alternative to AIS. In GIS, the conductors are enclosed in earthed metal housings filled with SF6 or another insulating gas, which sharply reduces the footprint. In AIS, air is the dielectric, so the same voltage level requires larger clearances and more physical space. The choice is frequently made on space, project budget, and maintenance capability rather than on electrical performance alone.

Many project teams still select AIS because the long-term familiarity and lower maintenance threshold fit local operating capabilities. GIS becomes attractive where land is expensive or the station must be installed in a building, underground, or offshore environment. For the medium-voltage range, both technologies are available from established manufacturers, but the rest of this article focuses on AIS selection and operation.

Advantages and Limitations of AIS

Advantages

• Lower upfront cost for most medium-voltage installations.
• Components are visible, which reduces the time needed to locate faults.
• Maintenance does not require gas handling or special leak-detection equipment.
• Replacement parts are commonly available from multiple suppliers.
• The technology is well understood by design institutes, contractors, and electricians.

Limitations

• Requires more space than GIS because of air clearances.
• Performance can be affected by dust, salt, humidity, altitude, and condensation.
• Open designs require stricter safety discipline during operation and maintenance.
• Switching actions can be noisier and more exposed than in a sealed gas enclosure.

These limitations do not make AIS outdated. They simply mean the site conditions and operating procedures need to be reviewed before selecting a particular panel family.

How to Choose Air Insulated Switchgear

Selecting AIS involves more than checking whether the panel fits on the floor. The buyer should confirm the following parameters against the site conditions:

1. Rated voltage: 12 kV, 24 kV, or 40.5 kV class.
2. Rated continuous current of the busbar and feeder.
3. Short-time withstand current and duration, for example 25 kA for 4 seconds.
4. Internal arc classification according to IEC 62271-200.
5. Environmental conditions: altitude, temperature, humidity, pollution, and seismic level.
6. Protection and control functions required by the network operator.

After these parameters are fixed, choose an equipment family that matches the installation. For a 24 kV manufacturing plant, Detong's 24kV armored removable metal-enclosed switchgear offers withdrawable vacuum circuit breakers, clear busbar compartment separation, and a compact indoor footprint.

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Common Applications of Air Insulated Switchgear

AIS is applied wherever space, climate, and safety regulations can be managed. Common settings include:

• Utility distribution substations and switching stations.
• Industrial plant incomers and distribution centers.
• Commercial buildings, hospitals, airports, and data centers.
• Wind and solar power plant medium-voltage collector circuits.
• Mining and infrastructure power distribution.

At 40.5 kV, utilities and industrial users often need a robust incoming arrangement. Detong's 40.5kV armored removable AC metal-enclosed switchgear is designed for these duties, combining single or double busbar arrangements with vacuum circuit breakers and enclosed cable connections.

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Distribution utilities also use ring main units to feed secondary networks. In these applications, a simple but maintainable design reduces supply interruption risk. Detong's AC metal-enclosed ring network switchgear supports loop feeding, transformer protection, and fault isolation without taking the entire feeder out of service.

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Maintenance and Long-Term Operation

Maintenance discipline keeps AIS reliable over a typical 25-30 year service life. The operations team should check:

• Torque on busbar and cable connections after major short-circuit events.
• Contact resistance across breaker poles during scheduled outages.
• Partial discharge activity with handheld detectors or temporary sensors.
• Thermal imaging of joints and cable terminations under load.
• Visual inspection for contamination, condensation, and animal intrusion.

Because AIS has no sealed gas compartments, staff can perform many checks with the system off or on, as permitted by local safety rules. Keep records of breaker operations, spring-charging time, and motor current. These records help predict wear before a fault occurs.

Bottom Line on Air Insulated Switchgear

Air insulated switchgear will continue to be a practical choice for medium-voltage distribution because it balances cost, safety, maintainability, and supplier choice. It is not the most space-efficient option, but it delivers what most substation owners need: clear construction, established operating methods, and lower dependency on specialized gas equipment. If you are planning a 12 kV to 40.5 kV project, start with the electrical parameters, compare AIS and GIS objectively, and select a product family that matches your maintenance capability.