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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.

64 Comments


  1. VENKATESH N Gothe
    Sep 06, 2023

    Sir,
    In our factory colony 6 tranformers of 11/0.433kv are installed.11Kv is from state board.How to use RMU in our colony for un iterruped supply?Pl.guide.


  2. OYALOWO IDRIS ADEBAYO
    Dec 22, 2022

    Sir I have an ABB (RMU) 11kv and is on service but for some days now if we on it is no supplying voltage out meaning the output is not transmitting out at closure


  3. ochelle alison ida
    May 20, 2021

    is it possible to merge two RMUs to synchronize between the power source and generator ?


  4. Eliphew Temitope olajide
    Nov 27, 2020

    Hello, Please i want to make enquiry for the following items
    1. 11kv Siemens Extensible Panel comprising 1no incomer 1no BUS and 2no Outgoing Feeders
    2. 11kv Sf6/Vacuum Sabre 200A RMU Circuit breaker extension
    3. 11kv Sf6/Vacuum Sabre 630A RMU Circuit breaker Isolator.

    I’ll appreciate your swift response.

    Regards


  5. Edvard
    Nov 17, 2020

    This article is available to everyone without any charge.


  6. ANUSHAMOORTHY
    Jul 08, 2020

    SIR I HAVE ONE RMU UNIT SCHNEIDER RM6, WITH 3WAY, WE USED ONE INCOMER 630 A AND AN O/G FOR TRANSFORMER 2000KVA. NOW WE HAVE PLAN TO INSTALL ANOTHER TRANSFORMER NEAR TO THIS RMU. CAN WE USE THIS UNUSED 3RD FEEDER AS INCOMER FOR NEW RMU/TRANSFORMER UNIT. THE MAIN CABLE IS ENOUGH TO SUPPORT TWO 2000KVA TRANSFORMERS (3CX120SQMM UNARMORED CABLE, 13.8KV)


    • Palanimurugan p
      May 06, 2022

      3way RMU /CCT2

      C – EB Power supply main Incoming cable connected to your RMU OD-1
      C – OD-2 this device power supply out going to Another one RMU OD-1 or OD-2
      incomer.,

      T2- VL-1 device power supply cable connected your Transformer..

      OD-1&2 power supply only incomer & Outgoing to one RMU to Other RMU there is no Relay & CT Protection…

      VL-1 – Relay & CT protection Required for transformer only available for this devices.

      OD-1 & 2 this device is CT & Relay Protection not available,
      so this device power supply don’t given directly to Transformer..


    • Durgesh Singh
      Sep 20, 2022

      120 sqmm size is low for 2X2000 KVA transformer.


  7. datngotien
    Jun 26, 2020

    Can anyone explain why it’s necessary to earth the load disconnector, please? As far as I know it’s for safety. Thạnk you very much.


  8. Oluwole Olubode
    Mar 01, 2020

    Is R.M.U a switch device or a control device?


    • MOHAMMAD IZHAR AHMAD
      Feb 22, 2023

      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.


  9. byju
    Feb 15, 2020

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


  10. dyondzo moonlight
    Oct 24, 2019

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


  11. Okufuwa Luke Temitope
    Oct 16, 2019

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


  12. 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.


  13. hoang sky
    May 11, 2019

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


  14. 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)


  15. 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


  16. 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


  17. 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.


  18. 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.


  19. 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..


  20. 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.


  21. 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..


  22. Biswanath Das
    Sep 18, 2018

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


  23. 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


  24. Jackson Kilonzo
    May 21, 2018

    I was having a problem with RMUs but now everything is crystal clear. Thanks a lot. Keep up with the good work


  25. Sriyan Lasantha
    May 20, 2018

    Can we use RMU without insulated gas for a short period? If so, what is the risk?
    And what is the maximum temperature RMU can withstand?


    • Edvard
      May 20, 2018

      No. Gas pressure relay will trip and won’t let you close CB.


  26. Nitesh Naik
    Apr 29, 2018

    can anyone tell about the designing of RMU


  27. rodolfo patawaran
    Feb 25, 2018

    Hi Edward, could you differentiate the use of 3 way and 4-way RMU?


  28. ISABIRYE David
    Feb 14, 2018

    i am particularly grateful to those who have contributed the educative information on various electrical engineering topics. How I wish that more stuff relating to 3, 4 or 5 feeder RMUs could be address on the portal.


  29. ROSE ANN
    Jan 31, 2018

    How can RMU Loop operate ?


  30. ahmed
    Dec 28, 2017

    i want to ask about position off , there is protection prevent operator to connect cable from off position to earth when cable is energized ?


  31. Manuel Levin Gomez
    Nov 25, 2017

    Hi there,

    I would like to be a member in this portal.
    I want to know how to install AMU. Please inform me, I am in trouble.

    Yours sincerely

    Manuel Levin Gomez


  32. siboleke makaula
    Oct 30, 2017

    Hi i would like to know about “tiger ring main unit”. i want know just a brief background literature, its primary operation, how is it different from the normal RMUs, and when do you use them? and are the limits for choosing tiger ring main units


  33. Habeeb Ali
    Mar 21, 2017

    HI I AM very happy to read the article and more interested and benefits so l want more information about RMU
    Thanks


  34. SYED IBRAHIM
    Nov 18, 2016

    Hi,
    Good, very useful, why use shield gas than others


  35. Amgad Shafik
    Nov 14, 2016

    Thanks for these interesting information, I do have a situation here and i need an advice, Our project is supplied from a substation to our main ring unit then to a switchgear…..We had a problem in our ring main unit and will be replaced, but I had an advice to replace the existant FUSED ring main unit to non-Fused ring main unit. Is this safe and logic or it is preferable to have the FUSED ring main unit as it is?????


    • Amir Nazih
      Feb 16, 2024

      A fused disconnect switch is essential for the RMU transformer feeder short circuit protection however for the RMU incomers, a disconnect switch is sufficient without a fuse


  36. nunu
    Sep 19, 2016

    why we select RUM rating current 630A?


    • Tarun
      Dec 02, 2016

      Since it is a secondary side switchgear maximum current can’t go above 630A in any case. So 630A is suitable.


  37. Vu
    Sep 02, 2016

    Hi Edvard,
    The RMU offers 2 options compact and modular.
    Could you tell us the pros and cons of these 2 options?
    Thank you,


  38. ing kej cikaj
    Sep 02, 2016

    Hi everyone…Can anyone help me….i need to know the difference between RMU vs Switchgear…Thank you for helping…good day everyone


    • sathish
      Oct 27, 2018

      RMU is a Ring unit , if any one incoming source get affect , the power can feed from other source because of ring circuit. MV switch gear has one incomer .

      RMU can use up to 24 KV. MV Switch gear can be use 33 KV , 66 Kv.


  39. mku
    May 03, 2016

    as per standard can we connect 3 transformer to one ring main unit


  40. aswajith asokan
    Jan 20, 2016

    Can we operate the OLU of an RMU to earth position without earthing the incoming side and other outgoing side…?????


  41. zftsa zau
    Jan 19, 2016

    I still can’t wrap my head around the “ring” terminology here. As per the diagram (combinations of RMU units by Schneider Electric) there are 2 different sources, hence its no ring.


  42. ABHISHEK RAY
    Nov 06, 2015

    IT’S not a proper supportive!!!!


  43. Gómez
    Nov 03, 2015

    We are specialist to protect RMUs. Relays designed to protect the Secondary Transformers and Distribution Substations of Electrical Networks. The Relays are self powered by the operating current. No batteries, chargers or any other external power sources are required. Do you know how work them?
    http://www.fanox.com/products/primary-secondary-distribution-protection-power-td/selfpowered-sia/


    • ALDRIN PETER
      May 05, 2016

      dear,

      Normally the relay used in RMU are earthfult and overcurrent relay.This relay is worked by the CT that connected in the out going cable.these relays are normally micro processor relays.Relay coil is energised by the secondary of CT.A small batteries is there in relay it just for display.if battery is not working it not affect the working of relay.


  44. Mohib
    Jul 31, 2015

    What is the difference between a normal RMU and metered RMU?


    • ALDRIN PETER
      May 05, 2016

      Dear,
      Normal rmu deosnot metered the outgoing current and voltage,There is no PT ,ammeter and voltmeter. mtered rmu have all these items,.


  45. Naing Win
    Jun 24, 2015

    more detail, it will be better.


  46. KINCHIT PATEL
    Jun 06, 2015

    HOW CAN I WILL BE MEMBER OF THIS SITE OR PORTAL????


  47. yuvaraj
    May 05, 2015

    Good


  48. Arshed sufyan
    May 05, 2015

    I am interested in this topic


  49. Vipul Gupta
    Apr 07, 2015

    Very good article and very informative site i must say


  50. ramadan
    Mar 06, 2015

    Good information


  51. Faluse Oluwamayowa
    Feb 18, 2015

    thank you…very useful info


  52. FELIX Jr.
    Feb 15, 2015

    THIS RMU ARTICLE IS GOOD INDEED FOR IT’S USE IN UNDERGROUND POWER DISTRIBUTION SYSTEM AS IT’S POPULAR IN THE MIDDLE EAST COUNTRIES. BUT LONG AGO BEFORE WHEN SF6 GAS IS NOT YET WIDELY POPULAR DUE TO IT’S COST, THE INSULATING MEDIUM USED WERE OIL AND VACUUM FOR IT’S CIRCUIT BREAKER OR SWITCHES. THE RMU RING MAIN LOOP IS NOT ACTUALLY CLOSED LOOP SINCE THE UPSTREAM INCOMERS NEED NOT BE PARALLELED OR SYNCHRONIZED, SO THAT ONE CKT. BKR. IN THE CLOSED LOOP IS OPEN, IN FACT.


  53. KISHOR DATTATRAYA PATIL
    Feb 12, 2015

    EEP website helps a lot to every stage of electrical engineering professional.

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