MV Distribution System: Components, Types, Design & Selection Guide

An MV distribution system is the part of an electrical power network that distributes medium-voltage electricity from substations or other upstream sources to transformers, facilities, industrial loads, commercial buildings, infrastructure, and downstream distribution networks.

A properly designed MV distribution system must do more than move electrical power. It must provide appropriate switching, protection, isolation, fault management, metering, and operational flexibility while satisfying the project’s voltage, current, short-circuit, environmental, and utility requirements.

This guide explains what an MV distribution system is, how it works, the main equipment involved, common network configurations, and the factors engineers and procurement teams should consider when selecting MV distribution equipment.

What Is an MV Distribution System?

An MV distribution system, or medium-voltage distribution system, is an electrical network used to distribute power at voltage levels above low voltage and below the high-voltage transmission range.

The exact voltage classification varies by standard, utility, and region. Common MV distribution levels include examples such as:

• 6.6 kV
• 10 kV
• 11 kV
• 12 kV
• 20 kV
• 22 kV
• 24 kV
• 33 kV

These values are examples rather than universal requirements. The appropriate system voltage depends on the utility network, project requirements, equipment ratings, load characteristics, and applicable standards.

A typical MV distribution system may include:

• MV switchgear
• Circuit breakers
• Load switches
• Disconnectors
• Earthing switches
• Ring main units
• Busbars
• Current transformers or current sensors
• Voltage transformers or voltage sensors
• Protection relays
• MV cables
• Distribution transformers
• Metering equipment
• Automation and communication equipment

The basic objective is to distribute electrical energy safely and reliably while providing sufficient control and protection against abnormal operating conditions.

How Does an MV Distribution System Work?

A simplified power path can be represented as:

Grid or generation source → primary substation → MV busbar → MV feeders → distribution switchgear → transformer → LV distribution → loads

In a utility network, a substation may receive power at a higher voltage and transform it to an MV level suitable for local distribution.

MV feeders then carry power to different locations. At downstream substations, a transformer can reduce the voltage to LV for final utilization.

For industrial facilities, the arrangement may instead look like:

Utility supply → incoming MV switchgear → MV busbar → outgoing feeders → transformers → LV switchboards → equipment

The exact topology depends on the facility and its operational requirements.

An MV distribution system also needs to respond to abnormal conditions. When a fault occurs, protection equipment detects the relevant electrical condition and can initiate circuit-breaker operation to isolate the affected section.

This selective isolation helps prevent a fault from unnecessarily interrupting unaffected parts of the network.

Typical MV Distribution System Components

1. MV Switchgear

MV switchgear is one of the central elements of an MV distribution system.

It provides functions such as:

• Switching
• Circuit protection
• Isolation
• Earthing
• Busbar connection
• Metering
• Protection and control
• Local or remote operation

Depending on the application, MV switchgear can use technologies such as air-insulated construction, gas-insulated construction, solid insulation, vacuum circuit breakers, load switches, or combinations of these technologies.

The specific configuration should be selected according to voltage, current, fault level, environmental conditions, installation arrangement, and applicable standards.

2. Circuit Breakers

Circuit breakers provide controlled switching and fault interruption within their rated capabilities.

In many MV applications, vacuum circuit breakers (VCBs) are used for feeder and transformer protection.

A circuit-breaker selection should consider parameters including:

• Rated voltage
• Rated normal current
• Short-circuit breaking capacity
• Short-time withstand current
• Making current
• Operating sequence
• Mechanical and electrical endurance
• Protection and control requirements

For detailed information about VCB terminology, see this guide to vacuum circuit breaker letter designations.

3. Load Switches

Load switches are designed primarily for switching normal load currents within their specified ratings.

They are commonly found in MV distribution arrangements where routine switching and network sectionalizing are required.

A load switch should not automatically be treated as equivalent to a circuit breaker. Their switching and fault-interruption capabilities depend on the specific device and rating.

4. Disconnectors and Earthing Switches

Disconnectors provide isolation where required by the system design and operating procedure.

Earthing switches provide a means of connecting an isolated circuit or section to earth under specified conditions.

Interlocking arrangements are important because switching, isolation, and earthing operations must follow the required operating sequence.

5. Ring Main Units

A ring main unit (RMU) is a compact MV distribution assembly commonly used in distribution networks and transformer substations.

An RMU may incorporate combinations of:

• Load switches
• Circuit breakers
• Disconnectors
• Earthing switches
• Fuses
• Protection devices
• Measurement equipment

The exact configuration depends on the application.

RMUs can be useful where space efficiency, network sectionalizing, and transformer connection are important considerations.

6. MV Busbars

Busbars provide common electrical connection points between incoming and outgoing circuits.

An MV switchboard may use a single busbar, sectionalized busbar, double-bus arrangement, or another configuration depending on operational requirements.

Busbar design must account for:

• Continuous current
• Short-circuit withstand
• Thermal performance
• Dielectric requirements
• Physical arrangement
• Clearances and insulation
• Connections and terminations

7. Current and Voltage Measurement

Current transformers, voltage transformers, sensors, or alternative measurement technologies can provide signals for:

• Protection
• Metering
• Monitoring
• Automation
• Power-quality functions

The measurement architecture should be compatible with the protection and control system.

8. Protection Relays

Protection relays monitor electrical conditions and determine when protective action is required.

Depending on the network, protection functions may include:

• Overcurrent protection
• Earth-fault protection
• Overvoltage or undervoltage protection
• Directional protection
• Transformer protection
• Feeder protection
• Other project-specific functions

Protection settings should be coordinated with the upstream and downstream network rather than selected independently.

Types of MV Distribution Systems

The physical equipment can vary significantly depending on network topology.

Radial MV Distribution System

A radial system supplies loads primarily from one direction.

A simplified arrangement is:

Substation → feeder → transformer → load

Advantages include relatively straightforward design and protection coordination.

The trade-off is that a fault or outage affecting the feeder can interrupt downstream customers or equipment.

Ring MV Distribution System

A ring configuration provides a network path that can allow sections to be supplied from different directions, depending on the network design and switching arrangement.

RMUs are frequently associated with ring distribution networks.

The actual level of redundancy depends on the network topology, normally-open points, protection scheme, and operating procedures.

Networked MV Distribution System

More complex networks can use multiple interconnected feeders and substations.

These arrangements can provide greater operational flexibility but also require more sophisticated:

• Protection coordination
• Switching procedures
• Automation
• Fault management
• Network studies

The complexity should match the operational requirements of the facility or utility.

MV Distribution System vs LV Distribution System

MV and LV distribution systems perform related functions but operate at different voltage levels and typically require different equipment and protection arrangements.

The exact voltage boundary depends on the applicable standards and local electrical system.

MV Switchgear in the Distribution System

MV switchgear is often the operational center of an MV distribution network.

A typical switchgear lineup may contain:

1. Incoming feeder
2. Busbar
3. Outgoing feeders
4. Circuit breakers or load switches
5. Protection relays
6. Measurement equipment
7. Earthing switches
8. Cable compartments
9. Control and indication equipment

For more product-related information, see the switchgear products page.

Air-Insulated MV Switchgear

Air-insulated switchgear, or AIS, uses air as the primary insulation medium around relevant conductors and components.

It can provide a practical solution for many indoor and outdoor distribution applications, subject to the specific equipment design and environmental requirements.

Gas-Insulated MV Switchgear

Gas-insulated switchgear uses an insulating gas or gas mixture within its relevant insulated compartments.

GIS can provide a compact arrangement, which can be valuable where installation space is limited.

However, the overall installation must still account for cable connections, operating access, maintenance, control equipment, and other system requirements.

Solid-Insulated MV Switchgear

Solid-insulated designs use solid dielectric materials for insulation around selected primary components.

They can be considered where the project has particular requirements related to environmental conditions, compactness, insulation technology, or equipment architecture.

More information is available in this guide to solid-insulation switchgear.

Applications of MV Distribution Systems

MV distribution systems are used across a wide range of electrical infrastructure.

Utility Distribution

Utilities use MV networks to distribute electricity from substations toward local distribution areas.

Feeder arrangements may incorporate sectionalizing equipment, RMUs, reclosers, circuit breakers, and automated control systems depending on the utility’s architecture.

Industrial Facilities

Factories, processing plants, mines, and other industrial sites may receive electrical power at MV.

The MV system can distribute electricity to multiple transformers or large MV loads.

Industrial applications require careful consideration of:

• Motor starting
• Short-circuit levels
• Load diversity
• Protection coordination
• Harmonics
• Process continuity
• Future expansion

Commercial Buildings

Large commercial facilities may use MV distribution where the site’s electrical demand or utility supply arrangement justifies an MV service.

The system can feed multiple transformers serving different building areas.

Renewable Energy Projects

Solar farms and wind farms can use MV collection systems to gather power from multiple generation units before sending it toward a project substation.

A solar architecture may include:

PV arrays → inverters → step-up transformers → MV collector system → project substation → grid

Wind projects may use a similar concept:

Wind turbines → turbine transformers → MV collector network → wind farm substation → grid

The actual architecture depends on project size, turbine or inverter configuration, grid requirements, and utility interconnection conditions.

Key MV Distribution System Design Considerations

Selecting an MV distribution system should begin with the electrical and operational requirements rather than the equipment name alone.

1. Rated Voltage

The equipment must have a suitable voltage rating for the network.

The system voltage should be determined from the utility or project design, while equipment ratings must satisfy the applicable requirements.

2. Continuous Current

The switchgear, busbars, cables, transformers, and other components must be capable of carrying the expected continuous load current.

Future load growth should also be considered where applicable.

3. Short-Circuit Level

Short-circuit calculations are essential when selecting MV equipment.

The switchgear and associated components need appropriate short-circuit withstand and interruption capabilities.

Using only the normal operating current is not sufficient for equipment selection.

4. Protection Coordination

Protection devices should operate selectively where the system design permits.

A coordination study can evaluate the relationship between:

• Upstream protection
• Feeder protection
• Transformer protection
• Downstream protection

The objective is to isolate faults appropriately while minimizing unnecessary interruption.

5. Network Topology

Determine whether the project requires:

• Radial distribution
• Ring distribution
• Dual supply
• Sectionalized busbars
• Networked operation

The topology affects equipment configuration, protection, switching procedures, and operational flexibility.

6. Indoor or Outdoor Installation

Environmental conditions can have a significant effect on equipment selection.

Outdoor installations may need to account for:

• Rain
• Humidity
• Condensation
• Dust
• Pollution
• Solar radiation
• Temperature
• Altitude
• Corrosion
• Salt exposure

Indoor installations may provide a more controlled environment but still require appropriate ventilation, temperature management, humidity control, cable access, and maintenance space.

7. Cable Termination

MV cable termination is an important interface between switchgear and the distribution network.

Engineers should confirm:

• Cable type
• Conductor size
• Termination technology
• Cable entry direction
• Phase arrangement
• Screen/earth connection
• Available cable compartment space
• Bending radius requirements

8. Automation and Communication

Modern MV distribution systems may incorporate monitoring, remote control, and digital protection.

Possible functions include:

• Remote switching
• Fault indication
• Energy measurement
• Event recording
• Protection monitoring
• SCADA integration
• Equipment status monitoring

Connectivity should be considered alongside cybersecurity and operational requirements.

Safety Considerations for MV Distribution Systems

MV electrical equipment presents significant electrical hazards, so safety must be incorporated into system design and operating procedures.

Important considerations include:

• Appropriate equipment ratings
• Isolation procedures
• Earthing
• Mechanical and electrical interlocking
• Access control
• Protection against energized parts
• Correct cable testing procedures
• Maintenance procedures
• Switching procedures
• Arc-flash assessment where applicable
• Clearly defined operating responsibilities

Equipment should always be operated and maintained according to the manufacturer’s instructions and the applicable electrical safety requirements.

Common MV Distribution System Design Mistakes

Several issues can create problems during installation or operation.

Choosing Equipment Only by Voltage

A suitable voltage rating does not automatically mean the equipment is appropriate.

Current rating, fault level, insulation requirements, environmental conditions, protection, and installation configuration must also be checked.

Ignoring Future Expansion

A distribution system designed only for today’s load can become difficult to expand.

Where future growth is expected, engineers should consider spare feeder positions, busbar capacity, transformer capacity, physical space, and cable routing.

Underestimating Cable Space

Compact equipment can reduce the main enclosure footprint while still requiring substantial space for cable bending, termination, testing, and maintenance.

The complete installation footprint matters.

Treating an RMU as the Entire Distribution System

An RMU is one type of MV distribution equipment. It does not replace the need to evaluate the broader network architecture, transformer, cables, protection, earthing, and upstream/downstream coordination.

Overlooking Environmental Conditions

Outdoor and harsh industrial installations can expose equipment to moisture, dust, pollution, corrosion, temperature variation, and other stresses.

The environmental rating of the specific equipment should be verified against the actual installation conditions.

MV Distribution System Selection Checklist

Before purchasing MV distribution equipment, confirm:

• System voltage

• Rated current

• Short-circuit level

• Short-time withstand requirement

• Network topology

• Number of incoming and outgoing feeders

• Transformer requirements

• Protection philosophy

• Metering requirements

• Cable type and termination

• Indoor or outdoor installation

• Ambient conditions

• Altitude

• Pollution and corrosion exposure

• Earthing arrangement

• Interlocking requirements

• Local/remote control requirements

• Communication and SCADA requirements

• Maintenance access

• Future expansion requirements

• Applicable standards and utility specifications

Frequently Asked Questions About MV Distribution Systems

1. What is an MV distribution system?

An MV distribution system is an electrical network that distributes power at medium-voltage levels between substations, transformers, facilities, and downstream distribution networks.

2. What voltage is used in an MV distribution system?

There is no single universal MV voltage. Examples include 6.6 kV, 10 kV, 11 kV, 12 kV, 20 kV, 22 kV, 24 kV, and 33 kV. The applicable voltage depends on the local network and project requirements.

3. What is the difference between MV and LV distribution?

MV distribution operates at medium-voltage levels and commonly uses equipment such as MV switchgear, VCBs, RMUs, and MV cables. LV distribution operates at lower voltage and commonly uses equipment such as LV switchboards, ACBs, MCCBs, and MCBs.

4. What equipment is used in an MV distribution system?

Typical equipment includes MV switchgear, circuit breakers, load switches, disconnectors, earthing switches, busbars, RMUs, protection relays, measurement devices, cables, and transformers.

5. What is an RMU in an MV distribution system?

An RMU, or ring main unit, is a compact MV distribution assembly used for functions such as network connection, switching, isolation, and transformer feeder protection depending on its configuration.

6. Are VCBs used in MV distribution systems?

Yes. Vacuum circuit breakers are commonly used for MV feeder, transformer, and other switching/protection applications where their ratings and characteristics are suitable.

7. Is an MV distribution system always a ring network?

No. MV systems can use radial, ring, dual-supply, sectionalized, or more complex network arrangements.

8. Why is protection coordination important?

Protection coordination helps ensure that the appropriate protective device responds to a fault while reducing unnecessary interruption of healthy parts of the network.

9. Can MV distribution systems be automated?

Yes. Depending on the project, MV distribution equipment can incorporate remote monitoring, remote switching, digital protection, event recording, SCADA communication, and other automation functions.

10. What factors affect MV switchgear selection?

Important factors include voltage, continuous current, short-circuit level, insulation requirements, network topology, protection, environmental conditions, cable configuration, installation space, maintenance requirements, and applicable standards.

11. Can MV switchgear be installed outdoors?

Yes, equipment specifically designed and rated for outdoor installation can be used outdoors. Environmental conditions and the manufacturer’s installation requirements must be evaluated carefully.

12. What is the role of a transformer in an MV distribution system?

A distribution transformer commonly reduces MV electricity to a lower voltage suitable for downstream LV distribution and end-use loads.

13. What standards apply to MV distribution switchgear?

The applicable standards depend on the equipment and application. IEC 62271 is an important standards series for many high-voltage switchgear and controlgear applications, while other standards may apply to cables, transformers, protection, installation, and system-level requirements.

Always verify the exact standards specified for the project and jurisdiction.

Conclusion

An MV distribution system provides the infrastructure needed to distribute electrical power between substations, transformers, industrial facilities, commercial sites, infrastructure, and other electrical loads.

A complete system can include MV switchgear, circuit breakers, RMUs, load switches, busbars, protection relays, measurement equipment, cables, transformers, and communication systems.

Successful design depends on more than selecting equipment with the correct voltage. Engineers and procurement teams should evaluate current capacity, short-circuit levels, protection coordination, network topology, cable interfaces, environmental conditions, safety, automation, maintenance access, and future expansion.

For project-specific equipment selection, the MV network should be evaluated as an integrated electrical system rather than as a collection of individual components.

Quick MV Distribution System Checklist

Network: What is the source, topology, voltage, and required redundancy?

Equipment: What switchgear, breakers, RMUs, transformers, cables, and measurement devices are required?

Protection: What are the fault levels and protection coordination requirements?

Environment: Is the equipment installed indoors or outdoors, and what environmental stresses apply?

Operation: Is local operation sufficient, or are remote monitoring and control required?

Maintenance: Is there enough space for inspection, testing, cable work, and equipment maintenance?

Future: Can the system accommodate expected load growth or additional feeders?

Answering these questions early can help create a more technically appropriate and maintainable MV distribution system.