Indoor Switchgear: Types, Components, Applications & Selection Guide

Indoor switchgear is electrical switchgear designed for installation inside a building, electrical room, substation, plant, or other protected enclosure. It is used to control, protect, isolate, and distribute electrical power while operating within the environmental conditions specified for the equipment.

Indoor switchgear is widely used in medium-voltage distribution systems, industrial facilities, commercial buildings, substations, renewable-energy projects, and infrastructure installations. Depending on the application, it may incorporate vacuum circuit breakers, load switches, disconnectors, earthing switches, busbars, instrument transformers, protection relays, and communication systems.

Unlike outdoor switchgear, indoor equipment benefits from a controlled installation environment. However, indoor installation does not eliminate engineering considerations such as temperature, humidity, condensation, ventilation, cable routing, maintenance access, fire protection, and electrical clearances.

This guide explains the types, components, applications, advantages, and selection criteria for indoor switchgear.

What Is Indoor Switchgear?

Indoor switchgear is an assembly of electrical switching and control equipment intended to operate inside a suitable building or enclosed electrical installation.

Its primary functions include:

• Switching electrical circuits
• Interrupting fault currents
• Protecting electrical equipment
• Isolating circuits for maintenance
• Earthing selected circuits
• Distributing power
• Measuring electrical parameters
• Controlling electrical equipment
• Communicating with automation and monitoring systems

The exact construction varies according to voltage level, fault level, insulation technology, protection requirements, and installation environment.

For medium-voltage applications, indoor switchgear commonly uses vacuum circuit breakers (VCBs), although other switching and insulation technologies are also available.

How Does Indoor Switchgear Work?

Indoor switchgear controls electrical power through a combination of switching devices, protection systems, measurement equipment, and mechanical interlocks.

Under normal operating conditions, electrical current flows through the incoming feeder, busbar, and outgoing feeder or transformer connection.

If a fault occurs, sensors or instrument transformers provide information to the protection system. The protection relay evaluates the abnormal condition and, when the configured protection criteria are met, sends a trip signal to the circuit breaker.

The basic operating sequence is:

1. Power enters the switchgear through an incoming feeder.
2. Current passes through switching equipment and onto the busbar.
3. Outgoing feeders distribute power to transformers, motors, distribution networks, or other loads.
4. Protection equipment monitors the system for specified abnormal conditions.
5. The circuit breaker opens when a trip command is issued.
6. Isolation and earthing functions can be used according to the equipment design and operating procedure.

Interlocking systems can help ensure that switching, isolation, and earthing operations occur in an appropriate sequence.

Main Components of Indoor Switchgear

Indoor switchgear can contain numerous components depending on its design.

1. Vacuum Circuit Breaker

A vacuum circuit breaker interrupts current using vacuum interrupters.

When the breaker contacts separate, an electrical arc forms and is extinguished within the vacuum interrupter according to the interrupter’s operating principle.

VCBs are commonly used in medium-voltage switchgear because they can provide circuit interruption within a compact switching assembly.

For more information, see this guide to vacuum circuit breaker designations.

2. Busbars

Busbars provide the electrical connection between incoming and outgoing circuits.

Indoor switchgear may use different busbar arrangements depending on the required system architecture.

Possible configurations include:

• Single busbar
• Sectionalized busbar
• Double-bus arrangements
• Other project-specific configurations

The selected arrangement affects operational flexibility, maintenance procedures, protection design, and equipment configuration.

3. Disconnectors

Disconnectors provide electrical isolation under their specified operating conditions.

They serve a different function from circuit breakers. A circuit breaker is designed to interrupt specified currents, including fault currents within its rating, while a disconnector is primarily associated with isolation.

4. Earthing Switch

An earthing switch connects an isolated circuit or equipment section to earth when the operating conditions permit.

Earthing switches are commonly integrated with mechanical and/or electrical interlocking arrangements.

5. Current Transformers

Current transformers, or CTs, can provide current signals for protection and measurement.

Typical applications include:

• Overcurrent protection
• Earth-fault protection
• Metering
• Monitoring
• Control

The CT specification needs to match the protection and measurement requirements of the system.

6. Voltage Transformers and Voltage Sensors

Voltage transformers or other voltage-sensing arrangements provide voltage information for measurement, protection, and control.

Depending on the switchgear design, voltage detection may be integrated into the assembly.

7. Protection Relay

Modern indoor switchgear frequently incorporates numerical protection relays.

Depending on the system, protection functions can include:

• Overcurrent protection
• Earth-fault protection
• Overvoltage protection
• Undervoltage protection
• Other application-specific protection functions

The protection philosophy should be coordinated with upstream and downstream equipment.

8. Control and Communication Equipment

Indoor switchgear can incorporate local control panels and communication interfaces for integration with plant automation or SCADA systems.

Depending on the project, these interfaces may support:

• Remote status monitoring
• Remote operation
• Event recording
• Fault indication
• Measurement
• Communications

Types of Indoor Switchgear

There is no single type of indoor switchgear. The appropriate configuration depends on the electrical system and project requirements.

Indoor Air-Insulated Switchgear

Air-insulated switchgear, or AIS, uses air as the insulation medium for applicable internal clearances.

Indoor AIS can be arranged in metal-enclosed compartments containing breakers, busbars, cable connections, protection equipment, and auxiliary systems.

One benefit of an indoor installation is that the equipment can operate within a building environment designed to control exposure to weather.

Indoor Gas-Insulated Switchgear

Gas-insulated switchgear, or GIS, encloses relevant energized components within an insulated environment.

GIS can provide a compact equipment arrangement and may be considered where space is constrained.

The insulation medium and environmental implications depend on the particular technology. Where gas-insulated equipment is considered, the project should evaluate the specific gas technology, maintenance requirements, environmental considerations, and applicable regulations.

Learn more about GIS SF6 gas insulation switchgear.

Solid-Insulated Switchgear

Solid-insulated switchgear uses solid dielectric materials for insulation around selected primary components.

This technology can be used to develop compact medium-voltage switchgear arrangements with enclosed primary components.

See the guide to solid insulation type switchgear.

Indoor RMU

A ring main unit, or RMU, is a compact medium-voltage distribution assembly designed for specific network arrangements.

RMUs can combine switching, isolation, and protection functions within a compact enclosure and may be used in distribution substations and other medium-voltage installations.

Withdrawable Indoor Switchgear

Withdrawable switchgear uses a removable or withdrawable switching device, commonly a vacuum circuit breaker.

The breaker can have defined service, test, and disconnected positions depending on the equipment design.

This arrangement can facilitate maintenance and replacement procedures, but the switchgear room must provide sufficient space for the required withdrawal path.

More information is available in this guide to withdrawable chassis for vacuum circuit breakers.

Indoor MV Switchgear

Indoor medium-voltage switchgear is one of the most common applications of indoor switchgear technology.

Depending on the system and equipment, medium-voltage installations can use ratings such as 11 kV, 12 kV, 20 kV, 22 kV, 24 kV, or 33 kV.

The correct rating depends on the actual system voltage, maximum operating voltage, utility requirements, equipment specification, and applicable standards.

An indoor MV switchgear lineup may contain:

• Incoming circuit breaker
• Busbar
• Outgoing feeders
• Vacuum circuit breakers
• Disconnectors
• Earthing switches
• CTs
• VTs or voltage sensors
• Protection relays
• Cable compartments
• Auxiliary circuits
• Control and communication systems

Short-circuit capability is a critical part of the specification. The switchgear must be appropriately rated for the prospective fault conditions at its installation location.

Applications of Indoor Switchgear

Electrical Substations

Indoor switchgear can be installed in substations where electrical equipment is housed inside a dedicated building.

The equipment can control incoming and outgoing feeders, transformer connections, and distribution circuits.

Industrial Facilities

Manufacturing plants and processing facilities often have dedicated electrical rooms containing medium-voltage and low-voltage distribution equipment.

Indoor switchgear can provide protection and switching for:

• Transformers
• Motors
• Distribution feeders
• Industrial loads
• Plant utility systems

Commercial Buildings

Large commercial facilities can require dedicated electrical distribution equipment.

Indoor switchgear can be incorporated into electrical rooms serving buildings, campuses, infrastructure facilities, and other large installations.

Solar Power Plants

Solar installations can use indoor switchgear in inverter stations, substations, control buildings, and other electrical facilities.

A simplified energy path is:

PV modules → inverter → transformer → MV collection system → substation → grid

Indoor switchgear may be used on the medium-voltage side for feeder protection, switching, isolation, and collection-system control.

Wind Farms

Wind projects may use indoor switchgear in turbine substations, collector substations, and central electrical facilities.

A simplified arrangement is:

Wind turbine → turbine transformer → MV collector system → wind farm substation → grid

The switchgear rating and protection arrangement must be matched to the collector network and grid connection design.

Battery Energy Storage Systems

Battery energy storage facilities can include transformers and medium-voltage collection or grid-interface equipment.

Indoor switchgear can be used where the project architecture places electrical equipment inside dedicated buildings or enclosures.

Indoor vs Outdoor Switchgear

The fundamental electrical functions are similar, but the installation environments differ.

Indoor installation should not be interpreted as automatically eliminating environmental risks.

Electrical rooms can experience elevated temperatures, high humidity, condensation, dust, corrosive atmospheres, or inadequate ventilation. These conditions should be considered during equipment selection and building design.

Advantages of Indoor Switchgear

Environmental Protection

A suitable building can protect electrical equipment from direct exposure to rain, solar radiation, wind, and airborne contamination.

Controlled Installation Environment

The indoor environment can potentially be managed through ventilation, heating, cooling, humidity control, and other building systems.

Organized Electrical Layout

Indoor switchgear can be integrated with protection panels, control equipment, transformers, and other electrical systems in a structured electrical room.

Reduced Outdoor Footprint

Moving equipment into a building can simplify the external substation arrangement, although the building itself requires space and appropriate construction.

Easier Integration

Indoor switchgear can be conveniently integrated with control, automation, communications, and other electrical-room equipment.

Limitations and Design Considerations

Indoor switchgear also introduces requirements that should not be overlooked.

Heat Dissipation

Switchgear produces heat during operation. The electrical room must provide appropriate ventilation or cooling according to the equipment losses and environmental conditions.

Condensation

Temperature changes can produce condensation, which can affect electrical insulation and equipment reliability.

Humidity control and appropriate anti-condensation measures may therefore be important.

Room Dimensions

The room must accommodate more than the switchgear’s cabinet dimensions.

Designers should consider:

• Front access
• Rear access where applicable
• Side clearances
• Cable access
• Breaker withdrawal path
• Maintenance space
• Lifting and handling
• Emergency access
• Ventilation

Cable Routing

Cable trenches, ducts, floor openings, and termination compartments need to be coordinated with the switchgear layout.

Fire and Building Requirements

The electrical room may need to satisfy applicable fire protection, building, ventilation, and emergency-access requirements.

These requirements vary according to the project location, building type, equipment, and local regulations.

How to Select Indoor Switchgear

A structured selection process helps avoid costly changes during installation.

Step 1: Identify the System Voltage

Determine the nominal voltage and maximum operating voltage of the electrical system.

Step 2: Calculate the Required Current

Review the continuous load current and expected operating conditions.

Step 3: Determine the Fault Level

Evaluate the prospective short-circuit current at the switchgear installation point.

Review:

• Rated short-circuit breaking capability
• Short-time withstand current
• Peak withstand current
• Protection coordination
• System grounding

Step 4: Choose the Switchgear Technology

Compare the suitability of:

• Air-insulated switchgear
• Gas-insulated switchgear
• Solid-insulated switchgear
• RMUs
• Withdrawable configurations
• Fixed circuit-breaker configurations

Step 5: Define Protection Requirements

Determine which protection functions are required and how the switchgear will coordinate with upstream and downstream protection.

Step 6: Review the Electrical Room

Verify that the room accommodates:

• Switchgear
• Cable termination
• Maintenance access
• Breaker withdrawal
• Ventilation
• Control panels
• Auxiliary equipment
• Personnel access

Step 7: Consider Future Expansion

If additional feeders may be required later, consider whether the switchgear lineup and electrical room can accommodate expansion.

Step 8: Confirm Standards and Project Requirements

The applicable standards depend on the equipment type, voltage level, and application.

The IEC 62271 series is relevant to many medium- and high-voltage switchgear and controlgear applications, while IEC 61439 applies to many low-voltage switchgear and controlgear assemblies.

The exact standard requirements should always be confirmed for the particular equipment and project.

Common Mistakes When Selecting Indoor Switchgear

Choosing Equipment Based Only on Rated Current

Rated current is only one parameter.

Voltage, fault level, insulation requirements, switching duty, environmental conditions, and protection functions also matter.

Ignoring Electrical Room Ventilation

Even an indoor installation can experience significant heat accumulation.

Ventilation and cooling should be considered using the actual equipment losses and room conditions.

Not Allowing Enough Maintenance Space

The switchgear may fit physically while still being difficult or unsafe to maintain.

Always review manufacturer installation drawings and required clearances.

Forgetting Breaker Withdrawal Space

For withdrawable equipment, the room layout must accommodate the complete withdrawal and handling process.

Treating Indoor Conditions as Automatically Ideal

Humidity, condensation, dust, temperature, and corrosive atmospheres can still occur indoors.

Ignoring Future Expansion

A tightly packed initial installation may become difficult to expand later.

Indoor Switchgear Safety

Safety should be incorporated into both the switchgear design and the electrical room.

Important considerations include:

• Appropriate voltage and current ratings
• Adequate short-circuit ratings
• Proper protection coordination
• Earthing
• Mechanical and electrical interlocking
• Isolation procedures
• Access control
• Arc-fault considerations
• Equipment labeling
• Maintenance procedures
• Manufacturer operating instructions
• Applicable electrical installation requirements

Internal-arc performance, where required, should be specified and verified according to the applicable equipment standard and project requirements.

Frequently Asked Questions About Indoor Switchgear

What is indoor switchgear?

Indoor switchgear is electrical switching and protection equipment designed for installation inside a building, electrical room, substation, or other suitable enclosure.

What is indoor switchgear used for?

It is used for electrical switching, protection, isolation, distribution, measurement, control, and sometimes automation of electrical circuits.

What type of circuit breaker is commonly used in indoor MV switchgear?

Vacuum circuit breakers are commonly used for medium-voltage indoor switchgear. The appropriate breaker depends on the system voltage, current, fault level, switching duty, and project requirements.

Is indoor switchgear better than outdoor switchgear?

Indoor and outdoor switchgear are designed for different installation environments. The appropriate choice depends on the project requirements, available space, environmental conditions, maintenance strategy, and equipment configuration.

Can indoor switchgear be used for solar projects?

Yes. Indoor switchgear can be used in solar substations, inverter stations, control buildings, and medium-voltage collection systems when appropriately rated.

Can indoor switchgear be used in wind farms?

Yes. It can be installed in wind-farm substations, collector substations, and other electrical buildings where the equipment meets the required electrical and environmental specifications.

What is the difference between indoor AIS and GIS?

Indoor AIS generally uses air for applicable insulation clearances, while GIS encloses relevant energized components within an insulated environment. GIS can offer a compact arrangement, but the appropriate technology depends on the project.

Does indoor switchgear need ventilation?

Potentially, yes. Switchgear generates heat, and the electrical room should maintain environmental conditions within the equipment’s specified operating range.

Does indoor switchgear need air conditioning?

Not necessarily. Whether heating, ventilation, air conditioning, or another environmental-control system is required depends on equipment losses, ambient conditions, building design, and specified operating limits.

What voltage levels are used with indoor switchgear?

Indoor switchgear is available across different voltage classes. Medium-voltage applications commonly include systems such as 11 kV, 12 kV, 20 kV, 22 kV, 24 kV, and 33 kV, depending on the project and equipment.

What should I consider when buying indoor switchgear?

Consider system voltage, rated current, short-circuit level, insulation requirements, switching technology, protection, environmental conditions, cable arrangement, room dimensions, maintenance access, expansion requirements, and applicable standards.

What is withdrawable indoor switchgear?

Withdrawable indoor switchgear uses a removable switching device, commonly a vacuum circuit breaker, that can move between defined operating positions. The configuration can support maintenance and operational procedures according to the equipment design.

Indoor Switchgear Selection Checklist

Before ordering or specifying indoor switchgear, verify:

• Nominal system voltage

• Maximum system voltage

• Rated current

• Frequency

• Short-circuit breaking capability

• Short-time withstand current

• Peak withstand requirements

• Insulation requirements

• Indoor ambient temperature

• Humidity and condensation conditions

• Altitude

• Pollution level

• Circuit-breaker technology

• Busbar arrangement

• CT/VT requirements

• Protection functions

• Earthing arrangement

• Cable-entry configuration

• Maintenance clearances

• Breaker withdrawal space

• Ventilation/cooling requirements

• Communication interfaces

• Future expansion

• Applicable standards

• Manufacturer drawings and technical documentation

Conclusion

Indoor switchgear is an important part of modern electrical distribution systems, providing switching, protection, isolation, measurement, and control within a protected installation environment.

Medium-voltage indoor switchgear can use technologies including vacuum circuit breakers, air insulation, gas insulation, solid insulation, RMUs, and withdrawable configurations. The appropriate solution depends on the electrical system and the physical and environmental requirements of the installation.

For engineers and procurement teams, the key is to evaluate the complete system rather than selecting equipment based solely on cabinet dimensions or rated current. Voltage, fault level, protection, insulation, cable access, room ventilation, maintenance clearances, interlocking, and applicable standards should all be included in the specification.

A well-defined indoor switchgear specification helps ensure that the equipment fits not only the electrical system, but also the building, operating procedures, maintenance strategy, and long-term project requirements.