Medium Voltage Switchgear Selection: Air-Insulated (AIS) vs Gas-Insulated (GIS)
Technical comparison covering IEC 62271-200 LSC2B-PM metal-clad AIS, SF6 and Clean-Air GIS, up to 70% footprint reduction, Internal Arc Classification (IAC AFLR
1. Medium Voltage Switchgear Architecture
Selecting medium-voltage switchgear (12 kV to 36 kV) requires evaluating civil room space costs, ambient harshness (humidity, dust, chemical pollution), and maintenance access.
2. Comparative Engineering Evaluation Matrix
| Engineering Parameter | Air-Insulated Metal-Clad AIS (IEC 62271-200) | Gas-Insulated Switchgear GIS (SF6 / Clean Air) |
|---|---|---|
| Insulation Medium | Ambient Air (Phase clearance: 120-220 mm) | SF6 / Synthetic Air (1.3 - 1.5 bar abs) |
| Footprint per Feeder (36 kV) | 1000 mm width × 2250 mm depth | 600 mm width × 1100 mm depth (-65% to -70%) |
| Loss of Service Continuity | LSC2B-PM (Partition Class Metallic) | LSC2A / LSC2B sealed tank |
| Internal Arc Withstand | IAC AFLR 25 kA / 31.5 kA 1.0 s | IAC AFLR 25 kA / 31.5 kA 1.0 s |
| Environmental Immunity | Sensitive to condensation, dust, marine salt | 100% Hermetically Sealed (IP65 stainless steel tank) |
| Cable Termination System | Air bushings with heat-shrink boots | Outer cone / Inner cone plug-in bushings (EN 50181) |
| Maintenance Interval | 3-5 years visual inspection & busbar torquing | Maintenance-free primary circuit for 30+ years |
3. Internal Arc Classification (IAC AFLR)
Per IEC 62271-200, switchgear must pass internal arc testing to protect personnel:
- A: Authorized personnel access only.
- F, L, R: Front, Lateral, and Rear protection.
- 25 kA / 31.5 kA 1.0 s: Fault current magnitude and duration.
4. Eco-Friendly Clean-Air GIS Trends
Modern regulations phase out greenhouse gas SF6 in favor of Clean-Air (80% N2 + 20% O2) with vacuum interrupters, achieving Global Warming Potential (GWP) < 1.
*Reference: IEC 62271-200:2021; IEEE Std C37.20.2-2022; CIGRE TB 740.*