What Is RMU Ring Main Unit?

Ring Main Unit
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RMU ring main unit combines electrical machinery to interconnect, test, and incorporate fixed type breaker with transformer protection function. RMU cabinets are modest in size, extremely dependable, secure, very easy to keep, quick to replace, and extend. Depending on the requirements, the RMU switchgear is made in a variety of voltages which is suitable for both indoor and outdoor applications. 

One of the best RMU manufacturers produces a broad range of RMU testing equipment for OEMs for assuring the integrity of the item and for end consumers to achieve optimal operation of their resources.

What Is RMU Ring Main Unit?

ring main unit RMU-switchgear
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Ring Main Unit (RMU) is medium voltage switchgear that is employed in secondary distribution networks. They are utilized in-ring main electrical power distribution systems to provide a continuous power supply to the customer by shifting sources in event of any emergency or repair on the primary source. In many circumstances, the RMU switchgear may be holding idle for a period of years without activating. But, when a problem arises and a feeder goes out of operation, they need to function rapidly to move the supply to an alternate feeder so that the supply to the user is not disrupted and at the same time the distribution transformer is safeguarded. 

Ring Main Unit requires frequent examination and upkeep to guarantee that they perform consistently when necessary. SF6 RMUs are primarily manually operated. A unique approach for data gathering is employed during assessment due to the manual nature of RMUs. In instances of motorized or coil-based ring main units, the motor or coil is driven via the network, and the current onset in the motor or coil is used as an initiation for data capture. Contact timings, contact bounce, non-simultaneity of contacts, contact travel and speed measurements, spring charging time, coil current and voltage characteristics, motor current characteristics, contact resistance, insulation testing, endurance tests, etc. are among the few tests done on rmu switchgear.

If we talk about what is RMU in an electrical system, the ring main unit (RMU) is an electrical device attached to the power supply system of the secondary distribution network. The function of the ring main unit (RMU) is to link the loop network. This helps to increase the stability of the power supply i.e. the loop function of the loop system or the load cut of the ring network.

Load break switches and fuses are used in the core. Each ring main unit is a flexible connection mode made up of 3 to 5 total switch boxes that may be used to satisfy the power supply needs of various power distribution network nodes. The switch uses a three-phase linkage three-station SF6 gas load break switch or circuit breaker. The high-performance spring mechanism can notice fast switching operation. DCDC

Since the power switching device and the hard bus are bundled in the same sealed metal casing, it is commonly used as insulation and arc extinguishing medium. Different modules and busbar units, such as the incoming unit, the measuring unit, and the TV unit, make up a single ring main unit, and many combinations can be arbitrarily assembled according to actual needs. When ring main units are used, there is no need to be concerned if the first source of power supply fails because power supply recovery can be accomplished rapidly with the support of the secondary source.

What Is the Ring Main Unit Function?

Ring Main Unit
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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.

What is the Types of Ring Main Units

Types of RMU
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There are generally 3 categories for the classification of RMU switchgear. Ring Main Units (RMU) are basic in design and compact in volume and need minimal application space. The ring main unit (RMU) has several pros of less price, good operation reliability, low maintenance cost, good cost performance, and can increase the power supply characteristics. A convertible switch or a circuit breaker with vacuum or gas-insulated interrupters can be employed to isolate the transformer. Protective relays may be included in the equipment to activate the circuit breaker in the event of a malfunction. Here are the three fundamental classifications of Ring Main Unit (RMU):

There are generally 3 categories for the classification of RMU switchgear. Ring Main Units (RMU) are basic in design and compact in volume and need minimal application space. The ring main unit (RMU) has several pros of less price, good operation reliability, low maintenance cost, good cost performance, and can increase the power supply characteristics. A convertible switch or a circuit breaker with vacuum or gas-insulated interrupters can be employed to isolate the transformer. Protective relays may be included in the equipment to activate the circuit breaker in the event of a malfunction. Here are the three fundamental classifications of Ring Main Unit (RMU):

i) On the basis of Structure

  • Extensible Ring Main Unit
  • Compact Ring Main Unit

ii) On the basis of insulation

  • Electrical insulation cabinet in an air environment
  • Electrical insulation cabinet in a vacuum environment
  • Electrical insulation cabinet in SF6 gas environment

iii) On the basis of voltage level

  • 24kV cabinet
  • 36kV cabinet

What Are Ring Main Unit Components

Ring Main Unit
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The output and input cable compartments of the RMU cabinet are connected by isolating breakers and cables to transformers through load breakers and protection fuses. Typically, the switch element consists of the load breaker, cutter machine, and ground knife, and the busbar and terminals are housed in an SF6 gas tank. This is composed of a material that is resistant to dirt, oil grease, and corrosion. Different sections split and connect RMU cabinets, with ring mian units specifications including two compartments, four compartments, and six compartments, based on client needs and purposes. 

In the past, most medium or low voltage transformer protection solutions in distribution grids relied on a fuse and a cutter knife. It’s a straightforward and cost-effective option. Circuit breakers, on the other hand, were typically expensive in the past and were utilized in high or medium voltage (HV / MV) stations rather than medium/low voltage (MV / LV) stations. Liyond , a mid power distribution cabinet maker, lately developed a small RMU cabinet breaker that includes SF6 gas insulators, self-powered relays, and improved current transformers at the same total cost of maintenance as a mid-voltage fuse-cutter solution. Improved service quality while lowering investment costs is one of the causes electrical providers and private consumers prefer the circuit breaker solution. 

The circuit breaker with self-powered relays is simpler to set up than a medium voltage fuse. It can also protect both upstream and downstream supply. The most essential ring main unit components are the load break switch and fuse, which may offer fault safety for the loop network and increase the functional security of electricity supply lines.

  • Switches
  • Conductors
  • Fusible Switch or Circuit Breaker
  • Distribution Transformer

Advantages of Ring Main Units

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SF6 ring main unit is an inventive technology that makes it easier to handle the numerous issues of power distribution. The RMU unit is an all-in-one system since it is a secure switchgear. It is quick to install, and simple to maintain, which also allows utilities to enhance network reliability and availability. This helps decrease operational expenses substantially. 

Reduce maintenance and spare parts: Today’s relays need essentially no maintenance since they integrate self-test functions. Moreover, the protection circuit may be simply verified upon installation using inexpensive test equipment. 

Better endurance in hard environments: The circuit breaker in the RMU unit is situated inside the chamber filled with SF6 gas, thus it is not influenced by environmental conditions. It gives increased safety against magnetizing overloading current flow, phase-to-phase fault currents, and ground contact current with low-intensity RMU substation assist in better coordinating security with other medium voltage and low voltage protective devices. 

Basic compact design: RMU transformer is SF6 insulated compact switch fitted with SF6 RMU and vacuum circuit breaker. Its tiny form does not need too much area to set up and run. 

Time and Cost-effective: The RMU switchgear is entirely easy to install, saves installation and operating time. Regardless of the temperature and endure the climatic conditions, these units’ administration and upkeep expenses are very minimal. 

Safety and Performance: RMU substation’s state-of-the-art compact design and newest technology offer perfect safety, connection, dependability, and efficiency.  It gives stable functioning in any harsh environmental situations.Intelligence capabilities: It is loaded with smart electronics and communication capabilities, RMU unit readily interfaces with the Distribution Management Network and produces Smart Grid solutions. The lifespan of the gadget is over 20 years.

Disadvantages of Ring Main Units

Ring main unit switchgear
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A few of the disadvantages of a ring main unit (RMU) are as follows:

  • They might be inexpensive but sometimes compromises on certain features. 
  • They are suitable for compact small substations but aren’t purposeful for bigger facilities.

Application of Ring Main Uint

RMU switchgear
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The Ring Main Units are utilized in underground cable transmission networks at the load interface and connection sites of a ring-type distribution network. Buyers choose the rmu switchgear because of its two primary applications. The ring main unit’s primary function is to keep the voltage supply from decreasing at different load substations. The interruption of supply is instantly replaced in a conventional urban ring main system by manual switching. 

Second, it improves the medium-voltage supply’s stability in the event that one of its sections fails. There is always a solid supply or an additional route accessible for each substation. Both the ends of the ring, irrespective of being at the equal metering point, are able to supply power to every substation within the ring. Ring main units offer a similar degree of stability as parallel distribution feeder systems, but they can handle a bigger load substation and are connected via various paths. The RMU transformer is also preferred due to its space limitations and installation costs. 


Ring Main Unit is also appropriate for usage in the following applications:

  • Small secondary substations 
  • Wind power plants 
  • Solar power plants 
  • Smaller Industries 
  • Resorts, office towers, residential housing projects, shopping complexes, business centers, hospitals, airports, etc. also have good usage of RMU substation.

What Is the Ring Main Unit Working Principle

working of switchgear
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Ring Main Unit consists of one incomer feeder and one or more outgoing feeders. The incomer feeder is either Vacuum Circuit Breaker or SF6 Circuit Breaker with related Disconnect Switches and Earthing switches. This income feeder is made with the advantage of bushing projecting from the shell of RMU switchgear for transmitting cable attachment. The outgoing feeder includes Load Break Switches with related Disconnect Switch and Earth Switch. Provision for a wired connection is given in the outgoing feeder. 

Describing the ring main unit working principle further, the Load Break Switch is nothing but an isolator with the potential of breaking the circuit while in a load situation. The incomer, as well as outgoing feeders, are contained in an SF6 environment to make the design small. Adjustable apparatus for examining SF6 gas pressure is put on the RMU unit to evaluate the gas pressure. The scale of this instrument is separated into three or more subscales for varied temperatures. Moreover, each sub-scale is having a green and red-colored zone.

 For a certain temperature, one should consult the proper sub-scale and the gas pressure should be within the green-colored zone. Mechanical instructions are given on the panel for Circuit Breaker as well as switches. The current Transformer of sufficient ratio is fitted in the incomer feeder. The secondary of this CT is coupled to a self-powered relay for essential safety. A self-powered protective mechanism is also fitted on the RMU transformer. This relay is a microprocessor-based Numerical Relay with a user interface. 

This relay provides limited protection such as Over Current and Earth Fault prevention. Though various manufacturers offer a different set of protection or manufacturer may supply protection dependent on the user’s request. In the distribution system, SF6 ring main unit is frequently utilized as it is a full product and just needs installation and cable connection. Everything else is inside the bundle. This drastically decreases the commissioning period.

How to Test Ring Main Unit

ring main unit RMU-switchgear
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In an RMU distribution system, the feeder ring spans the entire supply region and eventually ends at the substation from which it began. The unit is shaped like a ring and creates a closed loop. Because each portion of the ring transmits less current than the radial system, SF6 RMU has the advantage of requiring less conductor material. Another advantage is that voltage fluctuations are reduced. In comparison to the design of a radial system, it is more complicated to design. Go through the RMU testing process and find the best switchgear service for your facility.

  • Examine the bus bar’s tension. 
  • Make sure the earthing busbar is secure. 
  • Measuring and recording the contact resistance should be done. 
  • Check the busbar or breaker contact resistance, as well as the breaker timing test. 
  • Attach the auxiliary power supply to each cubicle. 
  • The primary injection can be used to check the current transformer’s ratio or circuit. 
  • If necessary, check the secondary injection for relays. Check out the RMU functioning test. 
  • Examine the remote signals. Perform the switchboard’s high voltage circuit breaker assessment. 
  • CT and switchgear insulation resistance tests should be conducted then. 
  • Carry out testing and commissioning after the installation is complete.

Evaluation of Contact Resistance at Busbars

At the connection point, inject a DC current of 100A for each cycle and record the low resistance measurement, which should be in Micro ohms. Alternatively, utilize a device that allows for direct reading, such as a micro ohmmeter with a 100A DC current injection. inject current to both sides of the test object from a 100A DC output.  The voltage drop at the test object was recorded using input DC, the current was measured from within, and the software calculated the test item’s resistance using these numbers.


Insulation Resistance Evaluation

  • On secondary terminals, disconnect the Ct earth connection. 
  • To determine the insulating resistance between the core and ground equipment to be used, do the following tests: Insulation testers with a voltage of 5 kV. 
  • CT primary insulation resistance was measured using a 5KV DC voltage. 
  • Other phases will be held in mind when measuring each phase. 
  • By applying 500V DC to each core concerning earth and core to core, the secondary circuit insulation resistance can be determined. 
  • Between the primary and secondary windings, there is insulation.

Circuit Breaker Examination

  • Insulation Resistance Measurement: Evaluate the insulation resistance stage to earth with 5.0 kV DC whenever the breaker is in a locked state. Then turn on the breaker and check the insulating resistance between poles. All of the aforementioned shall be done in all phases.

  • Contact Resistance Exam: Before beginning the test, the breaker must be in a closed position. Inject a DC of 100 A into each side top and bottom, and use the multimeter to detect the millivolt fall. Alternatively, attach the contact resistance kit and take direct measurements in micro ohms.


  • High Voltage Output Clearances: The unit’s high voltage discharge comes from a screened cable with an unscreened finish, and proper clearances between the following sections and any other conducting item must be maintained, HV Connection to the output, section of the output cable that isn’t screened, components of the test object that are not grounded.

Difference Between RMU and VCB(Vacuum Circuit Breaker)

Vacuum Circuit breaker
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Here are a few differences between an RMU and VCB based on its,

  • Functions: A ring main unit (RMU) is a factory-assembled, metal-enclosed set of switchgear. It is employed at the main intersection points of a ring-type supply chain in an electrical power transmission system. This RMU switchgear is utilized for medium-voltage power distribution, with voltages ranging from 7200 to 36000 volts.


    Whereas, a vacuum circuit breaker is a type of circuit breaker that uses a vacuum medium to quench the arc. In a vacuum chamber in the breaker known as a vacuum interrupter, the action of turning on and    shutting power conducting contacts as well as interrelated arc interruption takes place.

  • Applications: Ring Main Units (RMU) can be used efficiently in any compact substation or facility. Vacuum Circuit Breaker (VCB) technology is best suited for applications involving medium voltage. Vacuum technology for greater voltages has been explored. Metal-clad switchgear and porcelain encased circuit breakers both used vacuum circuit breakers.

  • Purpose: In a secondary distribution system, the ring main unit is employed. It is mostly utilized to provide a continuous power source. It also safeguards your secondary side transformer from transient currents on occurrence. The vacuum is attached as the arc cooling medium in vacuum circuit breakers. The vacuum has the best insulating properties. As an outcome, it outperforms all other mediums in terms of arc cooling. If you find the Vacuum circuit breaker purposefully, you can contact the leading VCB manufacturer for further inquiry.

Where to Find Reliable RMU Manufacturers

ring main unit
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One of the best switchgear companies Liyond is committed to creating and producing circuit breakers, switchgears, and switchgear components of the highest quality and reliability. Countable circuit breaker producers with strategic access to supplies and equipment for efficient manufacturing and prompt delivery has played an important role in the success of a number of well-known companies. 

All the products are built to GB and IEC norms, with a durable part interlocking mechanism that ensures secure functioning and physical security. The products provide steady execution of all functions thanks to specific surface treatment and long-lasting lubricating grease, extending the lifespan and lowering maintenance expenses.


Electrical switchgear is an important component for your substation which safeguards your electrical system from any faults. After going through the blog you now know everything about Ring Main Unit (RMU) switchgear. You can find the absolute switchgear solutions for you and can also get custom switchgear for your facilities.

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