GIS vs AIS RMU: How to Choose the Right Medium Voltage Switchgear for Your Project

One of the most common technical queries we receive from EPC contractors and consulting engineers during the secondary distribution design phase is: "Should we specify a Gas Insulated Ring Main Unit (GIS) or an Air Insulated Switchgear (AIS)?"

Both comply with IEC 62271-200 standards and perform the exact same fundamental tasks: isolating faults, protecting transformers, and ensuring network continuity. However, their structural design, capital expenditure (CAPEX), and operational expenditure (OPEX) differ significantly.

Here is a straightforward engineering breakdown to help you select the right Ring Main Unit (RMU)for your specific project conditions.

1. The Core Difference: Insulation Medium

The primary distinction lies in what separates the live parts (busbars, switches, breakers) from each other and the grounded metal enclosure.

• GIS RMU (Gas Insulated): All high-voltage components are hermetically sealed inside a laser-welded stainless steel tank filled with SF6 gas (or eco-friendly gas alternatives). Gas has superior dielectric strength, meaning live parts can be placed much closer together.

• AIS RMU (Air Insulated): Relies on ambient dry air for insulation. Because air has lower dielectric strength than gas, the physical distance (clearance) between live parts must be significantly larger to prevent arcing.

2. When to Choose a GIS RMU

GIS is the industry heavyweight for demanding applications. You should specify GIS when:

• Space is highly restricted: GIS is extremely compact—often up to 50% smaller than an equivalent AIS. It is the go-to choice for wind turbine towers, compact urban kiosks, and underground basements.

• The environment is harsh: Because the gas tank is typically rated IP67, GIS is completely immune to high humidity, salt spray, heavy dust, or even temporary flooding.

• Maintenance must be minimized: The sealed high-voltage compartment requires zero routine maintenance over its 25 to 30-year lifespan, drastically lowering your OPEX.

3. When to Choose an AIS RMU

AIS remains a highly relevant and practical choice for many global projects. You should specify AIS when:

• Budget is the primary constraint: AIS has a noticeably lower initial CAPEX compared to GIS. If you have ample installation space, AIS provides excellent value.

• Environmental regulations are strict: AIS is 100% SF6-free. If your project is in a region with strict greenhouse gas regulations (like parts of Europe) and you want to avoid the end-of-life disposal costs of SF6, AIS is a future-proof, eco-friendly option.

• Visible isolation is preferred: Many operators prefer AIS because it often features inspection windows. Maintenance crews can physically see the open/closed status of the isolating switches, providing an extra layer of psychological safety.

Quick Comparison Matrix.

Conclusion

There is no "one size fits all" switchgear. If your project faces severe weather conditions, high land costs, or requires a compact footprint, GIS RMU is the clear winner. However, if you are building an indoor substation with plenty of space, a tight budget, and strict green mandates, AIS RMU is the smarter financial and environmental choice.

(Insert a link here pointing to your GIS product page) -> Explore our fully sealed GIS RMU solutions here.

(Insert a link here pointing to your AIS product page) -> Discover our cost-effective, SF6-free AIS RMU solutions here.