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.
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.
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.
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.
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.
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.
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.
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
Related electrical guides & articles
Edvard Csanyi
Hi, I'm an electrical engineer, programmer and founder of EEP - Electrical Engineering Portal. I worked twelve years at Schneider Electric in the position of technical support for low- and medium-voltage projects and the design of busbar trunking systems.I'm highly specialized in the design of LV/MV switchgear and low-voltage, high-power busbar trunking (<6300A) in substations, commercial buildings and industry facilities. I'm also a professional in AutoCAD programming.
Profile: Edvard Csanyi
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.
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
is it possible to merge two RMUs to synchronize between the power source and generator ?
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
This article is available to everyone without any charge.
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)
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..
120 sqmm size is low for 2X2000 KVA transformer.
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.
Is R.M.U a switch device or a control device?
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.
Is there any difference between an RMU and a compact switchgear unit ?
i just want to know how much to refill a SF6 gas on the RMU.
I always enjoy articles from EEP. please, I wish to get daily updates from this portal. Many thanks.
Good day,
Please is it possible to interface an RMU with SCADA?
if yes is it in operatin anywhere
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.
i want buy RMU , who can help em . thank you and best regards.
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)
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
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
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.
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.
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..
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.
Can an RMU be used on the incomer side of 11KV transformer instead of only circuit breaker?
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..
Can a RMU made of three LBS ? If so what are the advantages?
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
I was having a problem with RMUs but now everything is crystal clear. Thanks a lot. Keep up with the good work
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?
No. Gas pressure relay will trip and won’t let you close CB.
can anyone tell about the designing of RMU
Hi Edward, could you differentiate the use of 3 way and 4-way RMU?
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.
How can RMU Loop operate ?
i want to ask about position off , there is protection prevent operator to connect cable from off position to earth when cable is energized ?
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
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
HI I AM very happy to read the article and more interested and benefits so l want more information about RMU
Thanks
Hi,
Good, very useful, why use shield gas than others
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?????
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
why we select RUM rating current 630A?
Since it is a secondary side switchgear maximum current can’t go above 630A in any case. So 630A is suitable.
Hi Edvard,
The RMU offers 2 options compact and modular.
Could you tell us the pros and cons of these 2 options?
Thank you,
Hi everyone…Can anyone help me….i need to know the difference between RMU vs Switchgear…Thank you for helping…good day everyone
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.
as per standard can we connect 3 transformer to one ring main unit
Can we operate the OLU of an RMU to earth position without earthing the incoming side and other outgoing side…?????
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.
IT’S not a proper supportive!!!!
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/
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.
What is the difference between a normal RMU and metered RMU?
Dear,
Normal rmu deosnot metered the outgoing current and voltage,There is no PT ,ammeter and voltmeter. mtered rmu have all these items,.
more detail, it will be better.
HOW CAN I WILL BE MEMBER OF THIS SITE OR PORTAL????
Good
I am interested in this topic
Very good article and very informative site i must say
Good information
thank you…very useful info
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.
EEP website helps a lot to every stage of electrical engineering professional.