Quick Answer: What Is GIS?
Gas-Insulated Switchgear (GIS) is a type of electrical switchgear in which the bus conductors, circuit breakers, disconnectors, and other components are housed inside sealed, grounded metal enclosures filled with an insulating gas. The gas provides superior electrical insulation in a fraction of the space required by conventional air-insulated equipment. GIS is the preferred solution when space is limited, the environment is harsh, or maximum reliability is required.
How Gas-Insulated Switchgear Works
The operating principle of GIS relies on the exceptional dielectric strength of the insulating gas. SF6, the most commonly used gas, has approximately 2.5 times the dielectric strength of atmospheric air at the same pressure. This means the clearance distances between live conductors and the grounded enclosure can be reduced by 60-80% compared to air-insulated designs.
Inside a GIS assembly, the key components are:
- Gas-tight enclosure: Typically welded or bolted stainless steel or aluminum, filled with insulating gas at 0.03-0.15 MPa above atmospheric pressure
- Bus conductors: Supported by epoxy-resin insulators (spacers) inside the gas-filled enclosure
- Circuit breaker: Usually a vacuum interrupter or SF6 puffer breaker, enclosed within the gas compartment
- Disconnectors and earthing switches: Integrated into the gas compartment for compact operation
- Cable terminations: Sealed bushings connecting external power cables to the internal gas-insulated bus
- Gas monitoring: Pressure sensors and density relays that alarm if gas leakage occurs
AIS vs GIS: Side-by-Side Comparison
| Parameter | AIS (Air-Insulated) | GIS (Gas-Insulated) |
|---|---|---|
| Insulating Medium | Atmospheric air | SF6, nitrogen, or fluoronitrile gas |
| Footprint | Large (100%) | Compact (20-50% of AIS) |
| Environmental Immunity | Sensitive to dust, humidity, salt, pollution | Completely sealed — immune to environment |
| Maintenance | Regular cleaning, insulator inspection | Minimal — sealed compartment, 15-20 year intervals |
| Reliability | Good | Excellent (failure rates 5-10x lower than AIS) |
| Installation | Outdoor or large indoor rooms | Compact indoor rooms, underground, offshore |
| Capital Cost | Lower equipment cost | Higher equipment cost, but lower total installed cost in space-constrained sites |
| Lifecycle Cost | Higher (more maintenance) | Lower (less maintenance, longer life) |
| Environmental Impact | No greenhouse gas risk | SF6 variants have GWP concern (SF6-free options emerging) |
| Extension | Easy — add new bays | Requires gas handling for bus extensions |
Types of Gas-Insulated Switchgear
Full GIS (Transmission Level)
Used at 66 kV to 800 kV for transmission substations. All components share a common gas bus. Extremely compact but requires specialized gas-handling equipment for installation and maintenance.
C-GIS / Compact GIS (Distribution Level)
Used at 12 kV to 40.5 kV for medium-voltage distribution. Each functional unit has its own sealed gas compartment, making the equipment modular and factory-testable. This is the most relevant type for industrial, commercial, and urban distribution applications.
NAIJI Electric offers two C-GIS models:
- GSN3-12 — 12 kV, 3150 A, 40 kA. Uses nitrogen (N2) insulation instead of SF6, making it a truly eco-friendly option. Stainless steel welded enclosure, 500 x 1025 x 2250 mm.
- GSN1-40.5L — 40.5 kV, 2500 A, 31.5 kA. Dual gas chamber design with SF6. Annual leakage rate below 0.05%. Stainless steel laser-welded, 600 x 1700 x 2400 mm.
The Shift to SF6-Free GIS
The global warming potential of SF6 (23,500 times CO2) has driven a major industry push toward SF6-free alternatives. The EU F-Gas Regulation is progressively restricting SF6 in new electrical equipment, and many utilities worldwide now specify SF6-free switchgear for new installations.
Current SF6-free approaches include:
- Nitrogen (N2): Used at medium voltage (up to 24 kV). Non-toxic, zero GWP, abundant. NAIJI Electric's GSN3-12 uses this approach.
- Dry air / technical air: Clean, dry air at moderate pressure. GWP of zero. Suitable for medium voltage.
- Fluoronitrile mixtures (C4F7N / C5F10O): Synthetic gases with GWP less than 1, used with CO2 or N2 as carrier gas. Being adopted at transmission voltage levels.
- Vacuum + solid insulation: Eliminates the need for any insulating gas. Emerging at medium voltage.
For a deeper look at this topic, see our article: SF6-Free Switchgear: The Future of Medium Voltage Distribution.
Where Is GIS Used?
- Urban substations: Where real estate is expensive and space is extremely limited
- Underground substations: In city centers, beneath buildings, or in metro systems
- Offshore platforms and ships: Where salt spray and vibration would destroy AIS equipment
- Industrial plants: Petrochemical, mining, and steel plants with high dust or corrosive atmospheres
- Data centers: Where reliability and compact footprint are critical
- High-altitude installations: Where air density is low, reducing the insulating capability of AIS
- Rail transit: Traction substations in metro and railway systems
How to Select GIS for Your Project
- Voltage class: Match the GIS rated voltage to your system (12 kV, 24 kV, 40.5 kV, or higher)
- Rated current: Size the bus bar and circuit breaker for your maximum load current plus future expansion
- Fault level: Ensure the GIS short-circuit rating exceeds the prospective fault current at the installation point
- Environmental conditions: Consider temperature range, humidity, altitude, corrosive atmosphere, and seismic requirements
- SF6-free requirement: Check if local regulations or corporate policies mandate SF6-free equipment
- Space constraints: Compare GIS dimensions to available room size and access routes
Need help selecting the right gas-insulated switchgear? Contact the NAIJI Electric engineering team for a project-specific recommendation.