![Medium Voltage Switchgear Specification: Voltage, BIL & kA Rating Guide [2026]](/images/blog/mv-switchgear-guide.jpg)
What Is Medium Voltage Switchgear?
Medium voltage (MV) switchgear is the critical link between high voltage transmission networks and low voltage distribution systems. It controls, protects, and isolates electrical equipment operating between 1kV and 40.5kV — the voltage range that powers factories, commercial buildings, hospitals, data centers, and utility distribution networks.
A typical MV switchgear lineup consists of multiple panels, each containing a circuit breaker, current transformers, voltage transformers, protection relays, and bus bars. The lineup might include an incoming panel receiving power from a transformer, a bus section panel for tie connections, and multiple outgoing feeder panels distributing power to loads.
Types of Medium Voltage Switchgear
Air-Insulated Switchgear (AIS)
AIS is the workhorse of medium voltage distribution. Air provides the primary insulation between live parts, with phase-to-phase and phase-to-ground clearances defined by IEC 62271-200. Common AIS types include:
- Metal-clad switchgear — Each component (breaker, bus, cable) is in its own grounded metal compartment. The highest level of safety and the standard for utility and industrial applications. NAIJI's KYN28-12 is a metal-clad design rated 12kV, 31.5kA.
- Metal-enclosed switchgear — Components share compartments with insulated barriers. More compact and cost-effective for less critical applications.
- Fixed-mounted switchgear — The circuit breaker is permanently installed. Lower cost but requires de-energizing the entire switchgear for breaker maintenance. NAIJI's XGN-12 is a fixed-mounted ring main unit design.
Gas-Insulated Switchgear (GIS)
GIS uses pressurized gas (traditionally SF6, increasingly clean air or fluoronitrile mixtures) for both insulation and arc extinguishing. The key advantage is a dramatically smaller footprint — a GIS switchgear room can be 50-70% smaller than an equivalent AIS room. This makes GIS the preferred choice for urban substations, offshore platforms, and underground installations where space costs are high.
Hybrid Switchgear
Hybrid designs combine GIS compactness for the bus section with AIS for circuit breakers, offering a middle ground between footprint and cost.
Voltage Classes and Ratings
| Voltage Class | Rated Voltage | Common Applications | Typical Short-Circuit Rating |
|---|---|---|---|
| 12 kV class | 3.6 / 7.2 / 12 kV | Industrial plants, commercial buildings, secondary distribution | 20-31.5 kA |
| 24 kV class | 17.5 / 24 kV | Primary distribution, wind farms, large industrial | 25-31.5 kA |
| 36 kV class | 36 / 40.5 kV | Sub-transmission, utility substations, large solar farms | 25-40 kA |
In North America, the common voltage classes differ from IEC standards: 4.16kV, 13.8kV, and 34.5kV are the standard distribution voltages. ANSI-rated switchgear must meet C37.20 requirements, which define different insulation levels and testing procedures than IEC.
Key Specification Parameters
When specifying MV switchgear for a project, engineers must define these critical parameters:
- Rated voltage (Ur) — Must equal or exceed the system nominal voltage
- Rated current (Ir) — Continuous current carrying capacity of bus bars and feeders (typically 630A to 4000A)
- Short-circuit rating (Ik) — Maximum fault current the switchgear can withstand and interrupt (20kA to 50kA)
- Insulation level (BIL) — Lightning impulse withstand voltage (75kV to 200kV depending on voltage class)
- Arc resistance (IAC) — Internal arc classification per IEC 62271-200 Annex A or ANSI C37.20.7
- IP rating — Degree of protection (typically IP4X for indoor, IP65 for outdoor)
How to Choose the Right Type
The selection depends on three primary factors: available space, budget, and safety requirements.
For standard industrial and utility applications with adequate room, metal-clad AIS switchgear is the default choice — it offers the best combination of safety, maintainability, and cost. When floor space is severely limited, GIS switchgear is worth the premium. For distribution networks with multiple cable connections, ring main units (RMUs) provide a compact and reliable solution.
NAIJI Electric manufactures a complete range of MV switchgear from 3.6kV to 40.5kV. View our product catalog or request a quote for your project specifications.