Ring Main Unit (RMU) - Components, Functions, Types and Applications

Ring Main Unit (RMU) is a switchgear device used in secondary distribution systems, i.e., between the distribution substation and the end consumer to ensure continuous power supply and isolate the faulty section from the network.

The main purpose of using a ring main unit is to provide an uninterrupted power supply to consumers even in fault conditions. It is basically a small sized switchgear unit enclosed completely within a cabinet and consists of disconnectors, fuses, and circuit breakers.

Read this article to learn the structure, types, advantages, and applications of a ring main unit (RMU).

What is a Ring Main Unit?

A Ring Main Unit (RMU) is a small switchgear unit which consists of circuit breakers, fuses, and disconnector switches for providing uninterrupted power supply. RMUs are mainly used in secondary or medium-voltage power distribution systems.

The main purpose of using a ring main unit is to enhance the reliability and reduce the power interruption time. It is primarily used in urban and industrial areas where continuous power supply is critical.

Ring main unit is used for isolating a part of the distribution network for maintenance and supply the load from an alternative feeder for that duration.

Components of Ring Main Unit

A typical ring main unit consists of the following key components to perform switching function −

• Circuit Breaker − To provide switching during maintenance and protection during fault. It protects the electrical distribution network from faults by interrupting the flow of current.

• Load Break Switch − It is used to perform switching function during normal operating conditions of the distribution network.

• Fuses − These are also provided for protection purposes. Fuses protect the distribution network from overcurrent.

• Earth Switch − It is provided to ensure safety of working personnel during maintenance.

• Busbar − These are conductors used for providing connection between outgoing circuits and the RMUs internal circuit.

• Metering Unit − Different types of meters are provided in a ring main unit to measure voltage, current, and energy in the distribution network.

Functions of Ring Main Unit

Being a switchgear device, a ring main unit performs the following crucial functions in an electrical distribution network −

• Circuit Control − RMU is used for switching the circuits from one feeder to another in a loop network.

• Isolation − It is another key function that an RMU performs. This function is essential for separating a fault part from rest of the circuit.

• Protection − As RMU consists of several devices like circuit breakers, fuses, earth switch, etc. Hence, it provides protection against overload, short-circuit, or earth-fault currents.

A ring main unit is considered a versatile and important device in a secondary distribution network, mainly in urban and industrial areas.

Types of Ring Main Units

Depending on insulation medium used, the ring main units can be classified into the following three main types −

• Air Insulated Ring Main Unit

• Oil Insulated Ring Main Unit

• Gas Insulated Ring Main Unit

Let’s discuss about each type of ring main unit in detail along their features.

Air Insulated Ring Main Unit

An air insulated ring main unit is simply a switchgear enclosed within a metal cabinet and it is primarily used in distribution automation systems. It consists of an air insulated vacuum load break switch as its core component. Therefore, the air insulated ring main unit has a simple construction and easy maintenance.

This type of ring main unit is mainly used in distribution networks in residential areas, small factories and commercial buildings. Air insulated ring main units are known for their small structure, easy installation, cost effectiveness, and flexible operation.

Oil Insulated Ring Main Unit

Oil insulated ring main unit is another type of RMU specially used to install in harsh and corrosive environmental conditions. In these ring main units, modular ring switches and fuses circuit breaker are used as the core components. This type of ring main unit is known for its compact design and fully system extendable feature.

These ring main units can withstand the harsh environmental conditions, typically ranging from 10°C to 55°C with a humidity level up to 100%. The oil insulated ring main units are commonly seen in mining and oil field applications.

Gas Insulated Ring Main Unit

Gas insulated ring main units use $\mathrm{SF_6}$ gas as the insulating medium and are used in low to medium voltage distribution networks to provide uninterrupted power supply and protect the electrical equipment. These types of ring main units are suitable to use in both indoor and outdoor installations because of their compact design and efficient operation.

The gas insulated ring main units have less maintenance cost, modular design, pollution resistant, etc. Thes can also be expanded easily. Therefore, the gas insulated ring main units are widely used in transformer substations, industrial areas, wind power plants, solar power plants, etc.

Advantages of Ring Main Unit

Ring main units are crucial components in secondary distribution systems that allow to easily manage the issues associated with electrical power distribution. These units minimize the power outage time and improve the distribution network reliability. The use of a ring main unit in an electrical distribution network offers the following main advantages −

• RMUs require less maintenance cost and hence reduce the down time.

• RMUs are easy to install that reduce the installation and commissioning time and cost.

• RMUs have a compact design, hence they are easy to install and require less space.

• The use of RMUs in a distribution network allows to supply the load from an alternative. This improves the reliability of the power system.

• An RMU consists of several protecting devices like circuit breaker, fuses, earth switch, etc. Hence, this enhances the safety of the power distribution network.

• RMUs have modular design. Hence, these can be easily expanded if needed.

Disadvantages of Ring Main Unit

Apart from the advantages, a ring main unit can also have certain disadvantages which are listed below −

• RMUs have high upfront cost.

• RMUs have relatively smaller lifespan as compared to other types of switchgears.

• These are mainly suitable for small substations and are not for large substations.

Applications of Ring Main Unit

Ring main units (RMUs) are used in medium voltage secondary distribution networks for isolation, protection, and control purposes.

Some of the common applications of ring main units are listed below −

• Small distribution substations

• Small industrial facilities

• Wind and solar power plants

• Residential and commercial distribution networks

• Underground cable distribution networks, etc.

Conclusion

A Ring Main Unit (RMU) is a small switchgear device used in medium voltage power distribution networks to enhance their reliability. It is a device used for supplying load from an alternate source when the main supply network is out of service due to fault or maintenance reasons.