Medium voltage switchgear can be classified in several different ways.
Terms such as AIS, GIS, SIS, metal-clad, metal-enclosed,
withdrawable switchgear and RMU do not all describe the same characteristic.
AIS, GIS and SIS mainly describe the insulation system.
Metal-clad and metal-enclosed describe the construction of the assembly.
Fixed and withdrawable describe how the main switching device is installed,
while an RMU describes a compact switchgear arrangement commonly used in
secondary distribution networks.
When classified by insulation technology, the three main modern types of
medium voltage switchgear are air-insulated switchgear (AIS),
gas-insulated switchgear (GIS), and solid-insulated switchgear (SIS).
Metal-clad, metal-enclosed, fixed, withdrawable and RMU describe different
construction, mounting or application characteristics and should not be
treated as additional insulation types.
What Is Medium Voltage Switchgear?
Medium voltage switchgear is an assembly of switching, protection, measurement
and control equipment used to distribute electrical power and isolate faults
within a medium voltage network.
A typical MV switchgear lineup may include:
- Vacuum circuit breakers
- Load break switches
- Disconnectors and earthing switches
- Current transformers (CTs)
- Voltage transformers (VTs)
- Protection relays
- Busbars
- Cable termination compartments
- Metering and control equipment
protection relays monitor electrical quantities such as current and voltage.
When the protection criteria are met, the relay sends a trip command to the
circuit breaker. The circuit breaker then interrupts the fault current and
isolates the affected circuit.
What Voltage Range Is Considered Medium Voltage?
The exact boundaries of medium voltage vary between standards, countries and
industry practices. For this reason, engineers should specify the actual rated
voltage and insulation requirements of the equipment rather than rely only on
the general term “medium voltage.”
For metal-enclosed switchgear, IEC 62271-200 covers AC metal-enclosed switchgear
and controlgear with rated voltages above 1 kV and up to and including 52 kV.
This is the scope of the equipment standard and should not be interpreted as
a universal rule that every system below 52 kV is always classified as medium voltage.
Common distribution systems include voltage classes such as 3.3 kV, 6 kV,
10/11 kV, 12 kV, 20/24 kV, 33/36 kV and, depending on the market and project,
40.5 kV equipment.
Main Types of Medium Voltage Switchgear by Insulation
The clearest starting point is the insulation system. Modern medium voltage
switchgear is primarily divided into AIS, GIS and SIS.
Air-Insulated Switchgear (AIS)
Air-insulated switchgear uses air as the principal insulation medium
between energized conductors and earthed components.
AIS is widely used in utility substations, industrial plants,
commercial infrastructure and renewable energy projects.
Typical Advantages
- Straightforward construction
- Convenient component access
- Flexible configuration
- Broad equipment availability
- Suitable for many primary distribution systems
Key Considerations
- Requires sufficient electrical clearances
- Typically requires more space than GIS
- Environmental conditions should be evaluated
- Switchroom dimensions affect configuration
AIS is often a practical option when adequate switchroom space is available
and convenient inspection, maintenance and component access are important.
Gas-Insulated Switchgear (GIS)
Gas-insulated switchgear places primary conductive components within
sealed compartments using a gaseous insulation system.
Traditional GIS designs have widely used SF6 because of its dielectric
performance. Modern medium voltage systems are also increasingly available
with SF6-free insulation technologies.
Typical Advantages
- Compact equipment footprint
- Sealed primary insulation system
- Reduced exposure to dust and moisture
- Suitable for constrained electrical rooms
- Useful in demanding environmental conditions
Key Considerations
- Greater equipment complexity
- Insulation technology should be evaluated
- Gas management may be required for SF6 equipment
- Life-cycle requirements should be considered
GIS is commonly considered for urban substations, infrastructure,
offshore applications, industrial facilities and other projects where
installation space or environmental isolation is a major consideration.
Solid-Insulated Switchgear (SIS)
Solid-insulated switchgear uses solid dielectric materials around or
between energized conductors. Depending on the design, epoxy-based
insulation may be used to encapsulate major primary circuit components.
Solid insulation can provide a compact solution while reducing dependence
on conventional insulating gases.
Typical Advantages
- Compact construction
- Reduced exposure of energized components
- No conventional insulating-gas handling for solid-insulated sections
- Suitable for compact secondary distribution
Key Considerations
- Insulation material quality is critical
- Manufacturing processes affect long-term performance
- Electric field control must be carefully engineered
- The complete tested assembly should be evaluated
What About Oil-Insulated Switchgear?
Oil was historically used as both an insulating and arc-interruption medium
in electrical switching equipment.
Oil-insulated equipment is no longer one of the principal technologies
specified for most modern medium voltage switchgear projects. Vacuum interruption
combined with air, gas or solid insulation is now much more common.
Oil-insulated switching equipment is therefore better regarded as a
legacy or specialized technology rather than an equal modern
alternative to AIS, GIS and SIS.
AIS vs GIS vs SIS: Key Differences
There is no single insulation technology that is automatically suitable for
every medium voltage project. The correct choice depends on electrical ratings,
available space, environment, maintenance strategy and project requirements.
| Factor | AIS | GIS | SIS |
|---|---|---|---|
| Main Insulation | Air | Insulating gas | Solid dielectric material |
| Typical Footprint | Larger | Compact | Compact |
| Environmental Isolation | Depends on enclosure and installation | High for sealed primary systems | High around encapsulated primary components |
| Component Accessibility | Generally easier | More dependent on sealed construction | Design-dependent |
| Equipment Complexity | Low to moderate | Higher | Moderate to high |
| Typical Applications | Industrial plants, substations and infrastructure | Space-constrained, urban and harsh environments | Compact and secondary distribution systems |
| Key Selection Factor | Space, accessibility and environment | Footprint, sealing and insulation technology | Insulation quality and electric field control |
Medium Voltage Switchgear Types by Construction
Insulation medium is only one classification method. Switchgear can also be
categorized according to the physical construction and compartment arrangement
of the assembly.
Metal-Enclosed Switchgear
Metal-enclosed switchgear contains switching, protection and associated
equipment within grounded metal enclosures.
In IEC-based projects, the construction may be further described using
characteristics such as Loss of Service Continuity category, partition class,
Internal Arc Classification and compartment accessibility.
Metal-Clad Switchgear
The term metal-clad switchgear is particularly common in
ANSI/IEEE specifications.
Metal-clad construction typically uses grounded metal barriers and
compartmentalization between major primary circuit elements. Withdrawable
circuit breakers are also commonly used in these systems.
metal-clad and metal-enclosed should not simply be treated as two quality
levels. Their exact meaning depends on the applicable IEC or ANSI/IEEE
construction requirements.
Fixed vs Withdrawable Medium Voltage Switchgear
Fixed Switchgear
In a fixed configuration, the main switching device remains mechanically
installed in its operating position.
Fixed designs may offer:
- Simpler mechanical construction
- Compact equipment dimensions
- Fewer moving primary connection mechanisms
Fixed configurations are widely used in compact metal-enclosed switchgear
and secondary distribution systems.
Withdrawable Switchgear
Withdrawable switchgear allows the circuit breaker to move between defined
operating positions such as service, test and isolated positions, depending
on the equipment design.
This arrangement can provide greater flexibility for circuit breaker testing,
isolation and maintenance and is commonly used in primary distribution switchgear.
Types by Switching and Protection Function
Circuit Breaker Switchgear
Circuit breaker panels use a circuit breaker to make and interrupt current.
Vacuum circuit breakers are widely used in modern medium voltage systems.
Typical applications include:
- Incoming feeders
- Outgoing distribution feeders
- Transformer feeders
- Large motor feeders
- Bus couplers
- Generator connections
Protection relays detect abnormal electrical conditions and command the
circuit breaker to trip when the programmed protection criteria are met.
Load Break Switchgear
A load break switch is designed to make and break specified load currents.
It should not be assumed to provide the same fault-interruption capability
as a circuit breaker unless the complete equipment has been designed and
rated for that duty.
Load break switches are commonly used in secondary distribution networks
and ring main units.
Switch-Fuse Combination
A switch-fuse combination uses a switching device together with high-voltage
fuses and is frequently applied to distribution transformer protection in
secondary distribution systems.
Metering and Auxiliary Cubicles
Not every cubicle in a switchgear lineup is designed to interrupt current.
A complete assembly may also contain:
- Metering panels
- CT and VT compartments
- Bus riser panels
- Protection and control panels
- Cable connection panels
- Bus sectionalizer or bus coupler panels
Where Does a Ring Main Unit Fit?
A Ring Main Unit (RMU) should not be treated as another
insulation technology alongside AIS, GIS and SIS.
An RMU is a compact medium voltage switchgear arrangement designed primarily
for secondary distribution and ring network applications.
Depending on the equipment design, an RMU may use:
- Gas insulation
- Solid insulation
- Air insulation
- Alternative SF6-free insulation technologies
Typical RMU configurations contain ring switches for incoming and outgoing
feeders together with transformer protection using either a circuit breaker
or switch-fuse combination.
RMUs are widely used in secondary substations, urban distribution networks,
commercial developments, industrial facilities, renewable energy systems and
compact substations.
View Zoliov’s
Ring Main Unit series
for available compact secondary distribution solutions.
Important IEC Classifications for MV Switchgear
When comparing IEC metal-enclosed switchgear, engineers should look beyond
voltage and current ratings. Several classifications help describe compartment
accessibility, service continuity and personnel protection.
Loss of Service Continuity — LSC
Loss of Service Continuity describes how opening an accessible high-voltage
compartment affects the ability of other functional units or compartments
to remain energized.
Common categories include:
- LSC1
- LSC2
- LSC2A
- LSC2B
It should also not be interpreted as a simple numerical quality score where
a larger number automatically means better equipment.
Partition Class — PM and PI
Partition class describes the nature of partitions or shutters between
accessible compartments and live high-voltage components.
- PM: arrangements using earthed metallic partitions or
shutters for the applicable separation. - PI: arrangements where one or more applicable partitions
or shutters use insulating material.
Internal Arc Classification — IAC
Internal Arc Classification indicates that the equipment has been evaluated
under specified internal arc test conditions.
An IAC designation may specify:
- Accessibility type
- Accessible sides
- Internal arc fault current
- Arc duration
Designations such as AFL or AFLR should therefore not be presented as an
automatic characteristic of every medium voltage switchgear assembly.
The exact classification depends on the tested equipment configuration.
How to Select the Right Medium Voltage Switchgear
Switchgear should not be selected simply from an application name such as
“solar switchgear,” “wind switchgear” or “data center switchgear.”
Projects within the same industry can require very different equipment.
Selection should start with the actual electrical system and site conditions.
1. Rated Voltage and Insulation Level
Confirm the required:
- Nominal system voltage
- Rated voltage of the switchgear
- Power-frequency withstand voltage
- Lightning impulse withstand voltage
2. Rated Current
Determine the required continuous current for incoming feeders,
outgoing feeders, busbars, transformer circuits and other major loads.
Future expansion should also be considered when it forms part of the
project design.
3. Short-Circuit Capability
The network study should establish values such as:
- Prospective short-circuit current
- Rated short-time withstand current
- Rated peak withstand current
- Circuit breaker short-circuit breaking current
- Short-circuit making current
4. Installation Environment
Site conditions may include:
- Indoor or outdoor installation
- Ambient temperature
- Altitude
- Humidity and condensation
- Dust and pollution
- Salt or corrosive atmosphere
- Seismic requirements
A sealed GIS or solid-insulated arrangement may provide advantages in
some demanding environments, while AIS can remain an effective solution
in controlled electrical rooms.
5. Available Installation Space
Equipment width alone should not determine the final choice.
Engineers should also consider panel depth, cable bending radius,
front and rear access, aisle clearance and pressure-relief arrangements.
6. Service Continuity and Maintenance
Critical facilities may require part of the electrical system to remain
in service while another compartment or functional unit is isolated.
In IEC projects, LSC category, compartment arrangement and network
redundancy should therefore be evaluated together.
7. Internal Arc Requirements
If Internal Arc Classification is required, specify:
- Required accessibility
- Accessible sides
- Arc fault current
- Arc duration
- Equipment installation arrangement
- Pressure-relief or exhaust strategy
8. Fixed or Withdrawable Circuit Breaker
Withdrawable circuit breakers may be preferred where testing,
maintenance and isolation flexibility are priorities.
Fixed configurations can be suitable where mechanical simplicity and
compact dimensions are more important.
9. Insulation Technology
For GIS and compact distribution projects, determine whether conventional
SF6 technology is permitted or whether an SF6-free solution is required.
The evaluation should include installation requirements, maintenance,
leakage management where applicable, recycling and end-of-life handling.
Zoliov Medium Voltage Switchgear Solutions
Primary and Secondary Distribution Equipment
Zoliov manufactures medium voltage switchgear and ring main units for
different primary and secondary distribution requirements.
Equipment can be configured according to the project single-line diagram,
rated voltage, rated current, short-circuit level, protection philosophy,
installation environment and applicable standards.
Medium Voltage Switchgear
Ring Main Unit
- KYN28A-12 Withdrawable Switchgear
- KYN61-40.5 Withdrawable Switchgear
- XGN Series Fixed Metal-Enclosed Switchgear
- GTXGN-12 Solid-Insulated RMU
- SRM16-12 Gas-Insulated RMU
Frequently Asked Questions
What are the main types of medium voltage switchgear?
When classified by insulation technology, the main modern categories
are air-insulated switchgear (AIS), gas-insulated switchgear (GIS),
and solid-insulated switchgear (SIS).
Metal-clad, metal-enclosed, fixed, withdrawable, and RMU describe
other characteristics rather than additional insulation types.
What is the difference between AIS and GIS?
AIS mainly uses air as the insulation medium and therefore requires
sufficient electrical clearance between energized components.
GIS uses a sealed gaseous insulation system and can achieve a more
compact equipment arrangement.
The appropriate choice depends on installation space, environmental
conditions, project standards, maintenance strategy, and life-cycle
requirements.
Is all GIS switchgear insulated with SF6?
No. SF6 has traditionally been widely used in gas-insulated switchgear,
but modern medium voltage GIS is also available with SF6-free
insulation technologies.
Is an RMU the same as GIS?
No. RMU describes a compact secondary distribution switchgear
arrangement, while GIS describes an insulation technology.
An RMU can be gas-insulated, solid-insulated, air-insulated, or use
another suitable insulation technology. Some RMUs are therefore GIS,
but not every RMU is gas-insulated.
What is the difference between metal-clad and metal-enclosed switchgear?
Metal-enclosed is a broad construction description for equipment
installed within grounded metal enclosures.
Metal-clad is a more specific construction classification commonly
associated with ANSI/IEEE systems and includes requirements concerning
compartmentalization, barriers and equipment arrangement.
Is fixed or withdrawable switchgear better?
Neither configuration is universally better. Withdrawable switchgear
can provide greater flexibility for circuit breaker isolation,
testing and maintenance.
Fixed switchgear can provide a simpler and more compact arrangement.
The choice should be based on service continuity, maintenance
philosophy, available space and project requirements.
What information is needed for an MV switchgear quotation?
For an accurate proposal, provide as much project information as possible, including:
- Single-line diagram
- System and rated voltage
- Frequency
- Rated current
- Short-circuit level
- Incoming and outgoing feeder quantities
- Transformer or motor ratings
- CT and VT requirements
- Protection relay requirements
- Indoor or outdoor installation
- Ambient temperature and altitude
- Internal arc requirements
- Applicable IEC, ANSI/IEEE, or local standards
Conclusion
Medium voltage switchgear does not have one simple classification system.
Understanding the different classification dimensions prevents terms such
as AIS, GIS, metal-clad, withdrawable and RMU from being incorrectly treated
as equivalent types.
AIS, GIS and SIS describe insulation technology.
Metal-enclosed and metal-clad describe construction.
Fixed and withdrawable describe the mounting arrangement of the main switching
device. Circuit breaker panels, load break switches and switch-fuse combinations
describe switching functions, while an RMU describes a compact secondary
distribution arrangement.
For an actual project, the final selection should be based on electrical
ratings, short-circuit requirements, service continuity, site environment,
available space, maintenance strategy and applicable standards.
Need Help Selecting Medium Voltage Switchgear?
Send Zoliov your single-line diagram, system voltage, rated current,
short-circuit level and project conditions. Our engineering team can
recommend an appropriate switchgear configuration based on your
electrical system and installation requirements.
