Technical Specification of RMU- Ring Mains Units for Distribution Switchgears

Ring Main Unit

The RMU or Ring Main Unit is a factory assembled, metal enclosed set of switchgear used at the load connection points of a ring-type distribution network  in an electrical power distribution system. 

RMU is functions to install on medium voltage distribution network and mainly used for protection of transformers in compact substations. It is used for medium voltage distribution in compact substations, small buildings, residential housing complex, large shopping malls, airports, wind power, etc.

The Scope of RMU installation related works


The Installation of 11KV Outdoor SF6 Insulated RMU covering erection, testing and commissioning with associated equipment including civil work, supply & laying of 11kv cable, cable jointing kit etc. of RMU.

Generally the RMUS should be Modular, extensible type on both sides with provision of attaching/connecting with SNAP FIT arrangement without External Busbars, additional load break switches and circuit breakers in future whenever required. 

However, RMU shall be extensible on both side however one side is occupied by metering panel and other side is free for extension on vice versa is also possible depending on site condition. 

Alternatively Extension shall be possible by adding trunking chambers and required accessories or by plug-in bushing type arrangement.  

Configaration of a 11kV RMU:

Ring Main Unit configaration may various way like- 2 way, 3 way and more; see below a 11kV RMU configaration.

a) 2-Way - 11KV Gas (SF6) Insulated RMU with One 630A load break switches and One SF6 Insulated VCB of suitable rating. 

b)  3-way - 11KV ,Gas (SF6) Insulated RMU with 2 Nos. 630A load break switch and 1Nos. SF6 insulated VCB of suitable rating or  3 Nos 630A load  break switch
c)  4-way- 11KV Gas (SF6) Insulated RMU with 2Nos 630A Load break switches  and 2Nos. SF6 Insulated VCB of suitable rating– or 3 Nos 630A Load break switches and 1Nos , SF6 Insulated VCB of suitable rating. 

d)  5-way -11KV Gas (SF6) Insulated RMU with 2Nos 630A load break switch and 3No. SF6 insulated VCB of suitable rating - 
e)  6-way -11KV Gas (SF6) Insulated RMU with 3Nos 630A Load break switches.

Technical Requirement of a 11kV  RMU -Ring Main Unit

The Ring Main Unit (RMU) shall be installed at 11 KV junction points to have continuous  supply by isolating faulty sections. The RMU shall be extensible on both sides and consists of the following combinations of load break switches and Circuit breakers for a nominal voltage of 12 KV using SF6 gas as insulating and Vacuum as arc quenching medium. The basic technical requirement of a 11kV RMU is considered as below:

  1. Fixed type SF-6 gas insulated / Vacuum circuit breakers. It should be maintenance  free, having stainless steel robotically/ TIG / MIG welded enclosure for IN DOOR /OUTDOOR RMU. However, offer with high quality of the welding which has necessary extensive leakage test with leak rate of 0.075% per annum can be accepted. The RMUs to be used are only outdoor type. 
  2.  Low gas pressure devices- 1.4 Bar pressure. 1.4 bar pressure of SF6 gas in chamber of RMU is required.
  3.  Live cable indicators- High operator safety.
  4.  Fully rated integral earthing switch on each device.
  5. Back up relay with auxiliary supply (24V DC) shall be provided.
  6. For indoor Cable boxes should be front access and interlocked with earth switch. No rear /side access required. For outdoor RMUs cable boxes shall be on front.
  7. Cable testing possible without disconnection of cables.
  8. Compact in dimension.
  9. Circuit Breaker with back up relay with auxiliary supply (24V DC) shall be provided.
  10.  Low pressure, sealed for life equipment, can operate at “0” bar pressure.
  11. Cable earthing switch on all switching device-standard, for operator safety.
  12. Enclosure with IP 54 standard protection for OUTDOOR RMUs and IP2X for INDOOR RMUs
  13. All live parts should be inside a stainless steel enclosure for out door type RMU  & minimum 2 mm thickness of stainless steel robotically/TIG/MIG welded enclosure for Outdoor / indoor RMU. 
For better and safe performance RMU shall be enclosed in a single compact metal clad, outdoor enclosure suitable for all weather conditions. The switchgear/steel gas tank shall be filled with SF6 as per IEC/IS Standards relative pressure to ensure the insulation and breaking functions. The steel gas tank must be sealed for life and shall meet the “sealed pressure system” criteria in accordance with the IEC 298 standard. The RMU must be a system for which no handling of gas is required throughout the desired very long service life. 

Common Routine Tests for RMU

Any electrical equipment like RMU must needs some routine test to ensure the optimized performance. Likeway the common routine test for RMU from a recognized testing lab is as below:

  1. Micro-ohm test (Contact Resistance test) for the assembly inside the tank. 
  2. Circuit breaker analyzer test so as to ensure the simultaneous closing of all poles for VCB. 
  3. SF6 gas leak test. 
  4. Partial Discharge test on the complete gas tank so as to be assure of the proper insulation level and high product life. 
  5. High voltages withstand. 
  6. Secondary test to ensure the proper functioning of the live line indicators, fault passage indicators and relays. 
  7. As per IEC/IS standards Mechanical operation of RMU switch Must be carried out.

RMU Design Features

RMU design featuresof the proposed RMU, as described in the  reference design and materials. The key design features include those that relate to: 

  1. Maintainability, expandability, and life span
  2. Ability to operate in severe outdoor environmental condition
  3. Immunity to electrical stress and disturbance.
  4. Acceptable insulation properties. 
RMU may be indoor type or outdoor type, so design also be considered  the installing place.

Indoor RMU:
  • Modular design, Panel type with frnt cable access.
  • RMU must be made of robotically/TIG/MIG welded stainless steel.
  • Offered RMU must be extensible. 
Outdoor RMU:
  • Stainless steel enclosure for OUT DOOR RMU application. The manufacturers shall conform the normal current ratings mentioned in GTP at 50 deg. Ambient without derating or as per IEC Standard. 
  • Enclosure with I.P.54 standard protection.
  • Offered RMU must be extensible.
  • Cable boxes shall be on Front sides.
The RMU outdoor metal clad, Switch Gear, Load break isolators, Vacuum circuit  breakers shall be equipped with an earth bus securely fixed along the base of the RMU.  

The size of the earth bus shall be made of IEC/IS standards with tinned copper flat for RMU and M.S.Flat for Distribution Transformer, earth spike and neutral earthing. Necessary terminal clamps and connectors shall be included in the scope of supply.  

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