(RMU) Ring Main Unit: Definition, Construction, and Application

What is a Ring Main Unit?

In an electrical power distribution system, a ring main unit (RMU) is a factory assembled, metal enclosed set of switchgear at the load connection points of a ring-type distribution network. It contains different types of switches for different purposes for example some switches connect with load, some feed a distribution transformer etc.

CEED High-quality Multiple Power Ring Main Unit RMU Equipment

The metal enclosed unit connects to the transformer either through a bus throat of standardized dimensions or through cables and is usually installed outdoors. Ring main cables enter and leave the cabinet. This type of switchgear is used for medium-voltage power distribution.

RMU is an extensible and non-extensible ring main unit for the secondary distribution network. RMU can be supplied in various configurations suitable for most switching applications in 24 kV distribution networks. RMU is a completely sealed system with a stainless-steel tank, gas tight metal enclosure, containing all the live parts, switching-disconnector, earth switch, fuse switch, the circuit breaker.

Structure of a Ring Main Unit (RMU)

The RMU cabinet is connected by output and input cable compartments by isolating breakers and cables out to transformers using load breakers and protective fuses.

In general, the switch element (including load breaker, cutter machine, and ground knife, busbar, and terminals) is placed in an SF6 gas tank made of a substance that is particularly resistant to the environment, such as dirt, oil grease, not rust.

High Voltage Electrical Rmu Enclosure Distribution Panel Sf6 Ring Main Unit

RMU cabinets are divided and connected by separate compartments, with RMU standards with two compartments, four compartments, six compartments, depending on customer requirements with functions of them.

In the past, most protection schemes for medium / low voltage transformers in distribution grids used fuse combined with a cutter knife commonly. It’s a simple and cost-effective solution. On the other hand, circuit breakers in the past were often expensive and used in high/medium voltage (HV / MV) stations instead of medium / low voltage (MV / LV) stations.

Recently, mid-voltage cabinet manufacturers such as CEED Electric design a compact RMU cabinet breaker that incorporates SF6 gas insulators, self-powered relays, and optimized current transformers at a total cost of ownership the equivalent of a mid-voltage fuse-cutter solution.

Types of Ring Main Unit

There are three main types of ring main units that you need to be aware of. These include gas-insulated, air-insulated, and solid-insulated. Each is necessary for its own type of specific application. This is why it’s so imperative that you understand the differences between them so that you can choose the right smart RMU to meet your needs.


Gas-insulated RMUs are a great choice for indoor and outdoor secondary distribution networks. RMUs like CEED Electric provide a wide selection of voltage ratings based on customer requirements and work to safeguard your electrical equipment at all times. They’re ideal for power plants, industrial zones, transformer substations, etc. Gas-insulated are the most widely utilized RMUs on the market today.

Gas Insulated Switchgear


Air-insulated RMUs are airtight cabinets whose primary switch is an air-insulated vacuum load switch. These metal enclosures feature very sturdy construction and are made to perform under difficult conditions. They’re great for factories, businesses, high-rise buildings, and residential blocks.

Air Insulated Switchgear


Solid-insulated RMUs are typically constructed with oil as the insulating material. They are specifically designed to withstand corrosive and hard environments. It’s ideal for installations in oil fields and mining environments.

What Is the Ring Main Unit Function?

The fundamental roles of the electrical switchgear are:

Circuit control: It regulates the circuit to transfer operations.

Isolation: It isolates the malfunctioning device from the remainder of the circuit.

Protection: It safeguards the circuit against the ground-fault current, short-circuits current fault, and overload.

Compared with medium voltage explosives, the circuit breaker paired with relays will strengthen the capacity to protect selectively; Avoid inappropriate functioning of the transformer’s surge current; Detect phase-to-phase fault currents and modest earth intensity. RMU switchgear electrical cabinets are employed with many various purposes in the secondary circuit distribution system, with the role of input switching for transformer stations, integrated transformer stations, etc., in many sectors of business.

The ring main unit is modest in size and can fit many sorts of structures. It may be utilized for long-term reliable power supply and rarely high voltage switching operation. So ring the main unit is currently commonly employed in residential buildings, public areas, industrial buildings, and other buildings, primarily in medium and small size compact units.

The ring main unit’s purpose is to increase the power supply’s reliability. It is mostly used in power distribution systems in industries including mining and construction. It is used in residential, port, and high-rise structures as well. The high-voltage circuit is frequently regulated by a load switch or a vacuum contactor due to its low load capacity.

Genuine distributors of RMU 

Currently, The CEED Electric RMU cabinets are trusted by customers to use more popularly because of its advantages.

Customers who have demand for RMU cabinet products, please contact hotline +86 18109601818 or email: sales@ceedelectric.com for advice and a free quotation.

CEED Electric is committed to providing genuine products with the most preferential prices and the best customer care services.


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