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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’)

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.

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

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

23 Comments


  1. SYED IBRAHIM
    Nov 18, 2016

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


  2. 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?????


  3. 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.


  4. 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,


  5. 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


  6. mku
    May 03, 2016

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


  7. 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…?????


  8. 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.


  9. ABHISHEK RAY
    Nov 06, 2015

    IT’S not a proper supportive!!!!


  10. 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.


  11. 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,.


  12. Naing Win
    Jun 24, 2015

    more detail, it will be better.


  13. KINCHIT PATEL
    Jun 06, 2015

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


  14. yuvaraj
    May 05, 2015

    Good


  15. Arshed sufyan
    May 05, 2015

    I am interested in this topic


  16. Vipul Gupta
    Apr 07, 2015

    Very good article and very informative site i must say


  17. ramadan
    Mar 06, 2015

    Good information


  18. Faluse Oluwamayowa
    Feb 18, 2015

    thank you…very useful info


  19. 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.


  20. KISHOR DATTATRAYA PATIL
    Feb 12, 2015

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

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