Switchgear Classification: AIS vs GIS vs Hybrid by Insulation & Construction (2026)

Why Switchgear Classification Matters for Engineers and Buyers

Medium voltage switchgear sits at the heart of every power distribution system — from utility substations and industrial plants to data centers and renewable energy farms. Choosing the wrong type leads to oversized installations, unnecessary cost, or inadequate protection. Yet the classification landscape is genuinely confusing: IEC and ANSI use different terminology, manufacturers mix marketing names with standard categories, and the same physical product may be called "metal-clad," "metal-enclosed," or "C-GIS" depending on who is speaking.

This guide provides a single, definitive classification framework. We organize all medium voltage switchgear along three independent axes — insulation medium, construction type, and switching function — so that any switchgear on the market can be described precisely by its coordinates on these three axes. Every classification references the governing IEC or ANSI standard clause.

Classification by Insulation Medium

The insulation medium determines how the switchgear maintains electrical clearance between live parts and between live parts and earth. It is the single most important classification axis because it dictates the physical size, environmental sealing, maintenance regime, and cost of the switchgear.

Air-Insulated Switchgear (AIS)

Air-insulated switchgear uses atmospheric air as both the primary and supplementary insulation medium. The live bus bars, cable terminations, and instrument transformers are separated by air gaps sized according to the rated lightning impulse withstand voltage (BIL). At 12 kV, the minimum phase-to-phase air clearance per IEC 62271-200 is 125 mm (BIL 75 kV) or 210 mm (BIL 95 kV). At 24 kV, clearances increase to 270 mm (BIL 125 kV).

AIS is the most mature and cost-effective switchgear technology. It requires no special gas handling equipment, no sealed enclosures, and can be maintained by standard-qualified electricians. The trade-off is physical size: a 12-bay AIS switchgear lineup at 12 kV typically occupies 8-12 meters of wall length and 2.5-3 meters of depth, versus 5-7 meters for equivalent GIS.

Typical AIS products include the ASN3-12 metal-clad switchgear (12 kV, draw-out VCB, rated up to 3,150 A and 40 kA) and the ASN2-24 metal-enclosed switchgear (24 kV).

Standards: IEC 62271-200 (AC metal-enclosed switchgear), ANSI C37.20.2 (metal-clad), ANSI C37.20.3 (metal-enclosed interrupter).

Gas-Insulated Switchgear (GIS)

Gas-insulated switchgear encloses all live components in a sealed metal enclosure filled with an insulating gas — historically SF6, but increasingly nitrogen, dry air, or fluoronitrile (C4-FN) blends. The high dielectric strength of SF6 (approximately 2.5x that of air at atmospheric pressure) allows the same insulation performance in a much smaller volume. At 12 kV, a GIS cubicle is typically 400-600 mm wide versus 800-1,000 mm for AIS — a 40-50% reduction in footprint.

The primary advantage of GIS is environmental sealing. The stainless-steel welded enclosure (annual leak rate <0.1% per IEC 62271-203) protects internal components from moisture, dust, salt spray, and pollution. This makes GIS the preferred choice for: coastal and offshore installations, underground substations, industrial environments with conductive dust, and high-altitude installations (>1,000 m) where air insulation degrades.

NAIJI Electric's GSN3-12 is a 12 kV eco-friendly GIS using nitrogen instead of SF6, while the GSN1-40.5L serves 40.5 kV applications with dual-chamber SF6 design and annual leak rate below 0.05%.

Standards: IEC 62271-203 (high-voltage GIS), IEC 62271-200 (metal-enclosed, which includes C-GIS at MV), IEEE C37.122 (gas-insulated substations).

Solid-Insulated Switchgear (SIS)

Solid-insulated switchgear replaces both gas and air gaps with cast epoxy resin or silicone rubber encapsulation of live parts. The vacuum interrupter, bus bars, and cable connectors are all embedded in solid insulation, eliminating the need for any gas and reducing the switchgear to its most compact possible form. A typical 12 kV solid-insulated ring main unit is 350-450 mm wide per functional unit.

The key advantage is truly zero-maintenance insulation — there is no gas to leak, no pressure to monitor, and no air gap to degrade with pollution. The disadvantage is that solid-insulated components are generally not field-repairable; if the epoxy cracks or a vacuum interrupter fails, the entire module must be replaced. This makes SIS best suited for applications where replacement is acceptable but routine maintenance is difficult or impossible: underground cable networks, remote renewable energy plants, and developing markets with limited maintenance infrastructure.

Standards: IEC 62271-200 with LSC (Loss of Service Continuity) class PM or PI, IEC 62271-202 (prefabricated substations that often use SIS).

Hybrid Insulation

Many modern switchgear designs use a combination of insulation technologies. A common hybrid approach uses vacuum interrupters (no arc-extinguishing gas), solid epoxy insulation around the interrupter poles, and air insulation for the bus bar compartment. This "vacuum + air" or "vacuum + solid" approach captures the size benefits of solid/gas insulation around the switching element while keeping the bus compartment simple and accessible.

Comparison table — insulation media:

Classification by Construction Type

Construction type defines the physical arrangement of compartments, the degree of internal separation, and whether the circuit breaker is fixed or removable. This classification has the most direct impact on maintenance strategy and operational safety.

Metal-Clad Switchgear

Metal-clad switchgear is the premium construction type, defined by ANSI C37.20.2 and IEC 62271-200 (with specific compartmentalization requirements). The defining features are:

• Draw-out circuit breaker: The circuit breaker can be physically removed from the cubicle for testing, maintenance, or replacement without de-energizing the main bus. This is the single most important feature — it enables breaker rotation programs and minimizes outage time.
• Metal barriers between compartments: Grounded metal partitions separate the circuit breaker compartment, bus compartment, cable compartment, and instrument compartment. If an arc fault occurs in one compartment, the metal barriers limit propagation.
• Automatic shutters: When the circuit breaker is withdrawn, mechanical shutters cover the stationary primary contacts to prevent accidental contact with energized bus bars.
• Separate instrument compartment: CTs, VTs, and protection relays are in a dedicated compartment, accessible without exposing live parts.

Metal-clad switchgear is specified for all critical applications: utility primary distribution substations, large industrial plants, power generation stations, and hospital/data center main switchboards. The cost premium over metal-enclosed switchgear is 20-40%, justified by the safety and maintenance advantages.

NAIJI Electric's ASN3-12 is a 12 kV metal-clad switchgear with draw-out VCB, rated up to 3,150 A continuous and 40 kA short-circuit. The ASN3i-12 adds intelligent monitoring with IEC 61850 communication.

Metal-Enclosed Switchgear

Metal-enclosed switchgear (ANSI C37.20.3 / IEC 62271-200 without full compartmentalization) uses a metal enclosure but does not require the draw-out circuit breaker, automatic shutters, or full metal compartment barriers of metal-clad. The circuit breaker may be fixed-mounted, and barriers between compartments may be non-metallic or partially omitted.

Metal-enclosed switchgear is more economical and physically compact than metal-clad. It is used in secondary distribution, industrial substations, commercial buildings, and applications where the reduced maintenance flexibility is acceptable. Many gas-insulated switchgear (C-GIS) products are classified as metal-enclosed because the sealed gas enclosure inherently provides compartment separation.

Pad-Mounted Switchgear

Pad-mounted switchgear is designed for outdoor ground-level installation on a concrete pad, typically serving underground distribution networks. The enclosure is weather-sealed (typically NEMA 3R or IP54+) with tamper-resistant lockable doors. Pad-mounted switchgear commonly integrates load-break switches, fuses, and sometimes vacuum fault interrupters in a compact outdoor enclosure.

Pole-Mounted Equipment

While technically not "switchgear" in the IEC sense, pole-mounted vacuum circuit breakers and reclosers are an essential part of the medium voltage classification. These are individual switching devices mounted on utility poles for overhead distribution line protection. NAIJI Electric's ZW32-12 (12 kV) and ZW32-24 (24 kV) are pole-mounted VCBs with spring or magnetic actuator mechanisms.

Classification by Switching Function

Each cubicle in a switchgear lineup performs a specific function. Understanding these functional types is essential for specifying a lineup configuration.

Circuit Breaker Panel (Feeder or Incomer)

Contains a circuit breaker (vacuum or SF6), current transformers (CTs), voltage transformers (VTs), and a protection relay. Used as: incomer panel (receiving power from a transformer or upstream bus), feeder panel (distributing power to downstream loads), or tie panel (connecting two bus sections). This is the most common panel type — a typical 12-bay lineup has 8-10 circuit breaker panels.

Load-Break Switch Panel

Contains a load-break switch (SF6 or vacuum type) capable of making and breaking load current but not fault current. Used in ring main units and secondary distribution where fault protection is provided by upstream devices. Load-break switches are rated per IEC 62271-103 with typical ratings of 630 A continuous and 20 kA short-time withstand.

Bus Section / Bus Coupler Panel

Contains a circuit breaker or isolating switch connecting two bus sections. A bus section panel with a circuit breaker enables split-bus operation — essential for maintaining supply to half the lineup during maintenance or when one transformer is out of service. IEEE 141 (Red Book) recommends split-bus configurations for all installations above 2 MVA.

Metering Panel

Contains voltage transformers and potentially revenue-grade current transformers for energy metering. The metering cubicle is typically located at the incoming feeder position. Accuracy class CT and VT (0.2S or 0.5S per IEC 61869) are required for billing applications.

Capacitor Switching Panel

Designed specifically for switching power factor correction capacitor banks. Capacitor switching imposes unique demands on the circuit breaker due to high inrush currents (up to 100x rated current) and restrike risk. IEC 62271-100 Class C2 breakers are specified for capacitor switching duty. NAIJI's VCBs are available with capacitor switching capability for power factor correction applications.

IEC vs ANSI Voltage Class Mapping

One of the most common sources of confusion in switchgear specification is the mismatch between IEC and ANSI voltage classes. The table below maps equivalent ratings:

Key voltage note for Chinese equipment: China uses 10 kV (rated voltage 12 kV) and 35 kV (rated voltage 40.5 kV) as its primary MV distribution classes. NAIJI Electric's product range covers both — from 12 kV VCBs and switchgear up to 40.5 kV indoor VCBs and switchgear. For ANSI-market exports, our switchgear is tested to equivalent ANSI ratings with appropriate BIL levels.

Selection Decision Tree

Use this decision framework to narrow down the right type of switchgear for your application:

Step 1 — Determine system voltage. This fixes the voltage class (12 kV, 24 kV, 36 kV, or 40.5 kV) and associated BIL requirement.

Step 2 — Determine installation environment. Indoor, clean environment → AIS is cost-optimal. Outdoor, coastal, dusty, underground, or high-altitude → GIS or SIS is recommended. Space-constrained indoor → GIS offers 40-50% footprint reduction.

Step 3 — Determine fault level. System short-circuit current defines the required breaking capacity. Common distribution fault levels: 20 kA (light industrial), 25 kA (urban distribution), 31.5-40 kA (heavy industrial, utility primary). Higher fault levels generally require metal-clad construction.

Step 4 — Determine maintenance strategy. If you need to test or replace breakers without outage → metal-clad with draw-out VCB. If sealed maintenance-free is acceptable → GIS or SIS. If lowest cost with periodic maintenance is acceptable → metal-enclosed AIS.

Step 5 — Determine environmental policy. SF6-free mandate → vacuum AIS, vacuum + solid insulation, or nitrogen GIS. SF6 permitted → full range of options including SF6 GIS for maximum compactness.

NAIJI Electric Switchgear Product Matrix

NAIJI Electric manufactures switchgear across the full classification spectrum:

Whether you need a standard 12 kV metal-clad lineup for a utility substation or a custom 40.5 kV GIS solution for a constrained industrial site, NAIJI Electric has the product range and engineering expertise to deliver. Contact our technical team for a switchgear selection consultation based on your specific application requirements.