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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. ochelle alison ida
    May 20, 2021

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


  2. 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


  3. Andy Carson
    Nov 17, 2020

    How do you justify charging to access this information when it is all ABB Vendor information which is freely available on the web free of charge???


    • Edvard
      Nov 17, 2020

      This article is available to everyone without any charge.


  4. 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)


  5. 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.


  6. Oluwole Olubode
    Mar 01, 2020

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

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