MV switchgear is the electrical equipment that controls, protects, and isolates circuits operating at medium voltage, typically between 1 kV and 35 kV under international (IEC) standards or 601 V to 69 kV under North American (ANSI/NEC) standards. It sits at the critical junction between power generation and the lower-voltage systems that feed buildings, factories, and homes. If you’ve ever seen a large metal cabinet humming inside a substation or industrial facility, you were likely looking at medium voltage switchgear.
Where MV Fits in the Voltage Spectrum
The term “medium voltage” covers a broad middle range in the electrical grid. Under IEC standards used internationally, medium voltage spans 1 kV to 35 kV, sometimes extended to 52 kV. In North America, the ANSI/NEC definition is wider: 601 V to 69 kV. The difference matters if you’re comparing equipment specs across regions, but the core function is the same. MV switchgear handles voltages too high for the panels in your home or office but lower than what you’d find on long-distance transmission lines.
What’s Inside the Cabinet
MV switchgear is essentially a collection of protective and switching devices housed in a single enclosure. Each component has a specific job, and they work together to keep power flowing safely.
Circuit breakers are the primary switching devices. In medium voltage applications, these are typically vacuum or gas-insulated types. They can interrupt fault currents automatically when a protective relay detects an abnormal condition, or they can be operated manually to take a section of the network offline for maintenance.
Busbars are the heavy copper or aluminum conductors that carry current between the incoming supply and the outgoing circuits. They act as the electrical backbone inside the switchgear, connecting all the individual circuit breaker compartments to the main power feed.
Disconnectors provide a visible, physical gap in the circuit so maintenance workers can confirm that a section is truly de-energized. In many MV designs, secondary disconnects also interface with the circuit breaker’s control wiring, providing position indication and auxiliary power when the breaker is racked into its operating position.
Instrument transformers scale down the high voltages and large currents to levels that protective relays and meters can safely read. Current transformers (CTs) connect in series with the main circuit and produce a proportional secondary current, usually 1 or 5 amps. Voltage transformers (VTs) do the same for voltage, stepping it down to 120 V for relay and metering use. Without these, the protection system would have no way to “see” what’s happening on the power circuit.
Air-Insulated vs. Gas-Insulated Designs
MV switchgear comes in two main configurations based on how internal components are insulated from each other and from the enclosure.
Air-insulated switchgear (AIS), often called metal-clad switchgear, uses air as the primary insulating medium between live parts. It’s the traditional design, and its main advantage is ease of access. You can open compartments, inspect components, and replace parts relatively straightforwardly. The trade-off is that performance degrades in dirty, humid, or corrosive environments, which can mean more frequent cleaning and maintenance.
Gas-insulated switchgear (GIS) seals the primary circuits inside a tank filled with insulating gas or encapsulates them in solid insulation. Because contaminants, moisture, and even rodents can’t reach the live components, GIS requires minimal or no maintenance for long periods and is considered more reliable overall. It’s also inherently arc-resistant since there’s no pathway for environmental intrusion to trigger an internal fault.
At lower medium voltages (5 kV and 15 kV), the physical size of AIS and GIS lineups is similar, so the choice comes down to environment and maintenance preferences. At 27 kV and 38 kV, GIS offers a dramatically smaller footprint, reducing space requirements by up to 60 percent compared to equivalent metal-clad equipment. That space savings is the primary reason GIS dominates at those higher voltage ratings, especially in urban substations or facilities where floor space is expensive.
How Protection Works
The protection system inside MV switchgear uses relays connected to instrument transformers to detect faults and automatically trip the circuit breaker. Several types of protection cover different failure scenarios.
Overcurrent protection handles the most common faults: short circuits and overloads. Instantaneous overcurrent relays trip immediately when current spikes far above normal, while inverse-time relays allow brief overloads but trip faster as the current gets higher. Using both together lets the system distinguish between a severe fault (trip now) and a moderate overload (wait briefly to see if it clears on its own).
Ground fault protection detects current leaking to earth, which can indicate damaged insulation or a downed conductor. Non-directional ground overcurrent relays work well in simpler networks, while directional versions are needed in more complex systems where fault current could flow in either direction. The directional type requires voltage transformers to establish a reference point for determining which way the fault current is moving.
Busbar protection uses differential relays that compare the current entering and leaving a bus section. If more current flows in than out, some of it is leaking through a fault, and the relay trips all breakers connected to that bus. This is one of the fastest-acting protection schemes because busbar faults are particularly dangerous and need to be cleared in milliseconds.
Arc Safety Ratings
An internal arc, where a fault creates an explosive electrical discharge inside the switchgear enclosure, is one of the most dangerous events in electrical infrastructure. Arc-resistant switchgear is designed to channel that energy safely away from anyone standing nearby.
Equipment is tested and classified using the Internal Arc Classification (IAC) system. The classification tells you which sides of the enclosure are safe for an operator to stand near during an arc event. An IAC AFLR rating, for example, means the front, lateral (side), and rear panels are all arc-safe. During testing, the enclosure must prevent dangerous projectiles from escaping, keep doors from blowing open, avoid forming holes below operator height, and maintain its grounding connection intact. The goal is protecting people even though the equipment inside may be destroyed.
GIS designs have an inherent advantage here. Because the live components are sealed away from the environment, there’s no possibility of contamination-triggered arcing inside the gas-filled compartments. Arc-resistant GIS tested to IEC standards achieves high safety ratings without a significant increase in equipment size.
Common Applications
MV switchgear appears wherever medium voltage power needs to be distributed, controlled, or protected. Utility companies use it in distribution and transmission substations to isolate faults and maintain grid stability. When a tree falls on a line feeding a neighborhood, it’s MV switchgear at the substation that detects the fault and disconnects that circuit before damage spreads.
Data centers rely on MV switchgear for power reliability and backup system coordination. A single unexpected outage in a data center can cost hundreds of thousands of dollars, so the switchgear’s protection and fast switching capabilities are critical. Renewable energy installations, particularly solar and wind farms, use MV switchgear to connect variable power output to the grid in a stable, controlled way. Industrial facilities like manufacturing plants, mining operations, and water treatment plants also depend on it to feed large motors, furnaces, and process equipment that run at medium voltage.
The Shift Away From SF6 Gas
For decades, sulfur hexafluoride (SF6) was the go-to insulating gas in GIS because of its excellent electrical properties. The problem: SF6 is one of the most potent greenhouse gases known, with a global warming potential thousands of times greater than carbon dioxide. The industry is now actively replacing it.
The two main alternatives are vacuum technology and alternative fluorinated gas mixtures. Vacuum circuit breakers paired with dry air (a mix of nitrogen and oxygen) eliminate fluorinated gases entirely. Major manufacturers including Siemens Energy, Mitsubishi Electric, and Meiden now offer fluorinated-gas-free vacuum circuit breakers for voltages up to 72.5 kV and beyond. For higher voltages, some manufacturers use alternative gas mixtures (such as C4-FN blends) that have a fraction of SF6’s environmental impact. The EPA maintains a directory of SF6-alternative circuit breakers, and the list grows steadily as the technology matures.
Key Standards Governing MV Switchgear
Two primary standards define how MV switchgear must be designed, tested, and rated. IEC 62271-200, most recently updated in 2021, is the international standard covering metal-enclosed switchgear for voltages above 1 kV. It specifies requirements for insulation, temperature rise, short-circuit withstand, and internal arc testing. In North America, IEEE C37.20.2 covers metal-clad switchgear and serves a similar role. Equipment sold globally is often tested and certified to both standards, since utilities and industrial buyers in different regions may require compliance with one or the other.