Ring Main Unit (RMU) Internal Structure Explained: Components, Functions and Working Principles

In modern medium-voltage power distribution systems, the Ring Main Unit (RMU) has become one of the most widely used switching devices due to its compact design, high reliability, and excellent power supply continuity. It is extensively applied in urban distribution networks, industrial parks, commercial buildings, renewable energy projects, and compact substations.

Compared with traditional metal-clad switchgear, RMUs occupy significantly less installation space while maintaining essential switching, protection, and isolation functions. This makes them an ideal solution for locations where space and investment costs are critical considerations.

This article provides a detailed overview of the internal structure, key components, and operating principles of Ring Main Units.

What Is a Ring Main Unit (RMU)?

A Ring Main Unit is a factory-assembled medium-voltage switchgear designed for ring-type distribution networks. Its primary function is to distribute electrical power, isolate faults, and maintain power supply continuity by allowing electricity to flow from multiple directions within a ring network.

RMUs are commonly available for voltage ratings of:

• 12kV

• 24kV

• 36kV

The most common rated current is 630A, making RMUs particularly suitable for secondary distribution networks and customer substations.

Main Internal Components of an RMU

Although RMUs are available in various configurations, most units contain the following core components.

1. Load Break Switch (LBS)

The Load Break Switch is one of the most important switching devices inside an RMU.

Functions:

• Switching normal load current

• Isolating feeders during maintenance

• Sectionalizing distribution networks

• Providing visible switching status

Modern RMUs commonly utilize SF6 gas-insulated or vacuum-interruption load switches for reliable arc extinguishing and long service life.

2. Circuit Breaker Unit

In transformer protection applications, the RMU may incorporate a Vacuum Circuit Breaker (VCB).

Functions:

• Interrupting fault current

• Protecting transformers and feeders

• Cooperating with protection relays

Compared with load switches, circuit breakers provide higher breaking capacity and enhanced protection performance.

3. Earthing Switch

Safety is a critical concern during maintenance operations.

The earthing switch provides:

• Safe grounding of isolated circuits

• Protection against induced voltages

• Personnel safety during maintenance

Mechanical interlocking systems prevent incorrect operation sequences and improve operational safety.

4. Busbar System

The busbar serves as the electrical backbone of the RMU.

Functions:

• Connecting incoming and outgoing feeders

• Distributing electrical power among functional units

• Ensuring low-resistance current paths

Busbars are generally made of high-conductivity copper and designed to withstand both continuous current and short-circuit conditions.

5. Cable Compartment

The cable compartment is used for connecting incoming and outgoing medium-voltage cables.

Features include:

• Front or rear cable access

• Touch-proof cable terminations

• Cable testing facilities

• Safety interlocking mechanisms

Proper cable termination is essential because poor connections can generate excessive heat and potentially lead to insulation failure.

6. Gas Tank or Insulation Chamber

For gas-insulated RMUs, the switching devices are installed inside a sealed stainless-steel gas tank.

The insulation medium may be:

SF6 Gas

• Eco-friendly gas mixtures

• Clean Air technology

The gas chamber provides:

• Electrical insulation

• Arc extinguishing capability

• Protection from environmental contamination

Because of the excellent dielectric strength of SF6 gas, gas-insulated RMUs can be significantly smaller than traditional air-insulated switchgear.

7. Protection Relay System

Modern intelligent RMUs are often equipped with microprocessor-based protection relays.

Functions include:

• Overcurrent protection

• Earth fault protection

• Short-circuit protection

• Event recording

• Remote monitoring

Protection relays continuously monitor system conditions and issue trip commands when abnormal conditions occur.

8. Monitoring and Automation Devices

As distribution networks become increasingly intelligent, RMUs are evolving into Smart RMUs.

Typical monitoring devices include:

• Current sensors

• Voltage sensors

• Temperature monitoring

• Gas pressure monitoring

• Fault indicators

These devices support SCADA integration and remote operation, significantly improving grid reliability and maintenance efficiency.

Air-Insulated vs Gas-Insulated RMUs

Air-Insulated RMU (AIS)

Advantages:

• Lower equipment cost

• Easier maintenance

• No SF6 gas handling requirements

Disadvantages:

• Larger dimensions

• Higher installation space requirements

Gas-Insulated RMU (GIS)

Advantages:

• Extremely compact footprint

• High reliability

• Better environmental resistance

• Reduced maintenance requirements

Disadvantages:

• Higher initial investment

• Specialized maintenance procedures

Gas-insulated RMUs are currently the preferred solution for urban distribution systems where installation space is limited.

Common RMU Configurations

Typical configurations include:

2-Way RMU

• Two load break switches

• Suitable for ring network connections

3-Way RMU

• Two ring switches

• One transformer feeder switch

Most commonly used in compact substations.

4-Way and 5-Way RMU

• Multiple outgoing feeders

• Suitable for industrial parks and commercial developments

Modular RMUs can be expanded according to future load growth requirements.

Advantages of Ring Main Units

The growing popularity of RMUs is largely attributed to the following benefits:

Compact Design

Compared with conventional switchgear, RMUs require significantly less installation space.

High Reliability

Ring network structures provide alternative power paths during faults, ensuring continuous power supply.

Enhanced Safety

Fully enclosed structures minimize the risk of accidental contact with live parts.

Reduced Maintenance

Sealed-for-life designs greatly reduce routine maintenance requirements.

Fast Fault Isolation

Faulted sections can be isolated quickly without interrupting power supply to the entire network.

The Ring Main Unit has become a key component of modern medium-voltage distribution systems. Its compact structure integrates load switches, circuit breakers, earthing switches, busbars, protection devices, and intelligent monitoring systems into a single enclosure.

Whether installed in urban power grids, industrial facilities, renewable energy projects, or compact substations, RMUs provide an ideal balance of safety, reliability, and cost-effectiveness. As smart grid technology continues to evolve, intelligent RMUs equipped with remote monitoring and automation functions will play an increasingly important role in future power distribution networks.

At BeiKe Electric, we provide customized Ring Main Units, medium-voltage switchgear, compact substations, and integrated power distribution solutions designed to meet the requirements of utilities, industrial users, and EPC contractors worldwide.