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Home / Technical Articles / Ring Main Unit (RMU) as an important part of secondary distribution substations

RMU

A Ring Main Unit (RMU) is a totally sealed, gas-insulated compact switchgear unit. The primary switching devices can be either switch disconnectors or fused switch disconnectors or circuit breakers.

Ring Main Unit as an important part of Secondary Distribution Substations
Ring Main Unit as an important part of Secondary Distribution Substations (on photo: ABB’a 24kV RMU type ‘SafeRing’)

Different combinations of these primary switching devices within the unit are commonly used.

An example of distribution network with Ring Main Units
An example of distribution network with Ring Main Units (combinations of RMU units by Schneider Electric)

In case a circuit breaker is the switching device, it is also equipped with protective relaying, either with a very basic self-powered type or a more advanced one with communication capabilities.

The rated voltage and current ranges for RMUs typically reach up to 24 kV and 630 A respectively. With many of the manufacturers of RMUs, the basic construction of the unit remains the same for the whole of the voltage range.

The increase in rated voltage is handled by an increase in the insulating gas pressure.

Single-line representation of a typical RMU configuration
Figure 1 – Single-line representation of a typical RMU configuration

The figure above shows a typical RMU configuration where load disconnectors are the switching devices for the incoming cable feeders and circuit breaker works as the switching device for distribution transformer feeder.


Three-position design // Closing, Opening and Earthing

All of the switching devices are of three-position design, having the possibility to close or open or earth the feeder in question.

All switches can be operated with the included operating handle
All switches can be operated with the included operating handle

Closing

Closing the moving contact assembly is manipulated by means of a fast-acting operating mechanism. Outside these manipulations, no energy is stored. For the circuit breaker and the fuse-switch combination, the opening mechanism is charged in the same movement as the closing of the contacts.

Turn the operating handle clockwise to charge the close/open spring. Then push the green button. (A)
Turn the operating handle clockwise to charge the close/open spring. Then push the green button. (A)

Opening

Opening of the switch is carried out using the same fast-acting mechanism, manipulated in the opposite direction. For the circuit breaker and fuse-switch combination, opening is actuated by:

  • A pushbutton
  • A fault.
Push the red button (B) to open fuse switch disconnector
Push the red button (B) to open fuse switch disconnector

Earthing

A specific operating shaft closes and opens the earthing contacts. The hole providing access to the shaft is blocked by a cover which can be opened if the switch or circuit breaker is open, and remains locked when it is closed.

Close earthing switch by turning operating handle clockwise
Close earthing switch by turning operating handle clockwise

The figure below shows typical outlook of a three-feeder RMU. In the figure, the combination consists of load disconnectors for the incoming two feeders and a fused load disconnector for the distribution transformer feeder. The incoming and outgoing medium-voltage cables are attached using elbow-type plug-in cable ends.

Outlook of a typical three-feeder 24 kV RMU unit
Figure 2 – Outlook of a typical three-feeder 24 kV RMU unit (ABB’s SafeRing RMU)

Whereas the RMU type of units represents the very compact gas-insulated design for a dedicated purpose, the secondary medium-voltage switchgears represent an air-insulated, quite freely extendable and configurable solution.

References:

  • Distribution Automation Handbook // Elements of power distribution systems – ABB
  • RM6 Ring main Unit catalogue – Schneider Electric
  • MV RMU SafeRing catalogue – ABB

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author-pic

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

59 Comments


  1. byju
    Feb 15, 2020

    Is there any difference between an RMU and a compact switchgear unit ?


  2. dyondzo moonlight
    Oct 24, 2019

    i just want to know how much to refill a SF6 gas on the RMU.


  3. Okufuwa Luke Temitope
    Oct 16, 2019

    I always enjoy articles from EEP. please, I wish to get daily updates from this portal. Many thanks.


  4. Olajide Sunkanmi
    Jun 27, 2019

    Good day,

    Please is it possible to interface an RMU with SCADA?
    if yes is it in operatin anywhere


    • Bhushan Aher
      Jun 09, 2020

      Yes, we can Interface an RMU with SCADA by using any Make of FRTU.Also we can operate from Any where by Connecting 2/3/4G Modem.


  5. hoang sky
    May 11, 2019

    i want buy RMU , who can help em . thank you and best regards.


  6. Mohd Hasnizam Bin Che Hamid
    Jan 25, 2019

    Hello there,

    I am looking forward to do a revamp for various of older version of RMUs. Really seek an expertise from you that can do a modification on the RMUs from mechanical to a scada ready that can be monitored the status, alarm and measurement to send to RTU and Scada.
    Some of the RMU brands are from:
    – ABB
    -Lucy
    -Merlin Gerin
    -Etc
    Highly appreciate your response. Kindly please advise.
    For further clarification, please do feel free to contact me.
    Thank you.
    Looking forward.

    Best regards,
    Hasnizam
    Sales Engineer
    Progressive Impact Technology Sdn. Bhd. (Malaysia)


  7. Halid Mwasumile
    Dec 15, 2018

    The concept of the article about RMU is perfect since it gives clear understanding on how you can benefit with RMU in ring distribution network


  8. Raj
    Dec 10, 2018

    I didn’t like the way the article is presented.No clear data on advantages,applications and differentiation w.r.t other insulation equipment’s like VCB,SF6 etc


  9. Santosh Salunke.
    Dec 08, 2018

    I am really interested in total RMU unit Schematic line diagram to understand , as well as RMU v/S AMU I am interested in understanding all different relay protection s for RMU. It’s operation & site testing & Commissioning of unit etc.


  10. Navin Kumar Agarwal
    Dec 08, 2018

    Ring main Unit is very much in use in middle east; in fact for U/ground distribution network it is safe and effeicent electrical equipment and catagorized of its’ number of Incomer switches and Tee Off; like : 2+1, 2+2, 3+1, 3+2, etc.


  11. Miguel Angel Rendon
    Dec 08, 2018

    I recently quote a RM6. It is expensive. 40K! But in the other hand they are reliable and save tons of money in case of shor circuit failure..


  12. DEVI
    Nov 10, 2018

    In our cabling project , there are two switching stations proposed , my doubt is , instead of switching station ,any other alternate way is there to lay cable . whether Switching staion can be replaced by RMU.


  13. Zaid Siddique
    Oct 12, 2018

    Can an RMU be used on the incomer side of 11KV transformer instead of only circuit breaker?


    • Gurjit
      May 06, 2020

      Yes offcourse..But generally maximum current rating of R.M.U is upto 630 amps. Only where as panels available with higher rating also. Apart from this most of the incomer panel designed with line P.T…But RMU’s built with only live line indicators, for P.T it requires some sort of extension units..


  14. Biswanath Das
    Sep 18, 2018

    Can a RMU made of three LBS ? If so what are the advantages?


  15. David Sackey
    Jul 03, 2018

    Please I need help. Can anyone email me a PDF of a primary and secondary sub
    stations that explains the it’s constituents .thank You

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