What is RMU (Ring Main Units) Design in Electrical Panels?

RING MAN UNITS

The criteria for the connection of large electrical loads are detailed in Imperial

College Technical Policy Statement E4 and for the separation requirements of

LV switchboards in Technical Policy Statement E5. This Technical Policy

Statement sets out the technical requirements for the College's main electrical

distribution equipment and includes interface and protection arrangements

(see Appendix 1).

2.0 HV Switchgear

Overview

Multi-panel switchboards or ring main units (RMUs) control the College's

6.6kV and 11kV networks. Both networks are connected as ring circuits but

operate as radial feeds. Each ring has a third feeder coupled, where

possible, at a node representing one half of the total ring current. The

distribution feeders are protected against over-current and earth faults at the

main switchboards and do not rely on intermediate downstream protection.

Therefore, incoming circuits on intermediate switchgear panels are not

equipped with protection relays and are used for manual sectionalising only.

This replicates the function of the ring switches on RMUs.

Multi-panel switchboards are used in substations where a third feeder

interconnection is made and/or other switched HV functions are required

e.g. PFC Capacitors and remotely switched transformer feeders.

RMUs are used in substations utilizing plain transformer feeders and offer

cost and possible space savings over multi-panel switchboards. Further

savings are made if the RMUs can be close coupled to the transformers and

some substations within Imperial College are equipped with up to 3 x

1600kVA transformers connected in this way.

2.1 HV Multi-panel Switchboards

These shall be procured from Imperial College Framework Agreement

Manufacturers and incorporate the following:

• 12kV minimum rms working voltage

• 630A minimum circuit breaker and bus-bar rating

• 25kA – 3s symmetrical fault rating

• Micom P121 or Sepam1000 protection device on transformer

feeders only. Device equipped with auxiliary relays to receive LV

intertrip and lockout signals.

• Vacuum breaking medium

• Bus-section circuit breaker

• All circuit breakers to be independent manual closing control and

fitted with 30V DC trip coil for local manual and protection trips

• All circuit breakers to have lockable electrical trip control switches

• One set of auxiliary contacts shall be wired out on the transformer

panels to provide an inter-trip signal to the transformer LV circuit

breaker, which also acts as an interlock to prevent closure of the LV

circuit breaker until the HV circuit breaker is closed.

2.2 HV Ring Main Units (RMUs)

These shall be procured from Imperial College Framework Agreement

Manufacturers and incorporate the following:

• Through symmetrical fault rating 25kA 3s

• Independent manual ring switch operation with minimum 630A

rating for load switching and through fault making capacity.

• 200A rated vacuum or SF6 circuit breaker for controlling outgoing

transformer feeder.

• Circuit breaker symmetrical breaking capacity of 21kA 1s

• Non-TLF protection e.g. Schneider VIP 300 unit or discrete relay as

in 2.1

• 30V DC shunt trip coil

• One set of auxiliary contacts shall be wired out on the transformer

panels to provide an inter-trip signal to the transformer LV circuit

breaker, which also acts as an interlock to prevent closure of the LV

circuit breaker until the HV circuit breaker is closed

• Suitable for close coupling to the transformer. Close coupled RMUs to

have ground braced framework and not rely on the transformer LV

flange for sole support.

3.0 Transformers

These shall be procured from Imperial College Framework Agreement

Manufacturers and be as follows:

• KNAN Midel liquid cooled type

• Nominally 11/0.433kV but if operated from the Imperial College

6.6kV network 11/6.6/0.433kV dual ratio.

• Vector Group Dyn11

• Suitable for mounting close coupled RMU

The following shall be fitted:

• Liquid Temperature Gauge with maximum temperature indicator

and alarm and trip contacts

• Pressure relief device with trip contacts

• Marshalling cubicle for accessory small wiring

• Off-circuit HV tappings at ± 2.5% and ± 5% with lockable switch

• Transformers in excess of 1600kVA, LV bus-bar flanges for

coupling bus-bar ducting

In exceptional circumstances it may be desirable to install cast resin

transformers with the following fittings:

• Nominally 11/0.433kV but if operated from the Imperial College

6.6kV network 11/6.6/0.433kV dual ratio.

• Vector Group Dyn11

• Thermocouple winding temperature system with 2

thermocouples/winding and monitoring module providing

temperature indication, alarm and trip signals

• IP23 enclosure. The IP index may be reduced if AN cooling

cannot be achieved but clearances to live conductors must be such

to comply with the IP standard "finger" test. Forced cooling of the

enclosures shall be avoided

• Access to core and windings shall be limited by either a) interlocked

doors with keys released by both HV and LV circuit breakers or b)

removable panels with fastenings operated with special tools

• Off-circuit HV tappings at ± 2.5% and ± 5% by means of bolted

links

• Transformers in excess of 1600kVA, LV bus-bar flanges for

coupling bus-bar ducting

4.0 LV Switchgear

These shall be procured from Imperial College Framework Agreement

Manufacturers. Switchboards shall be of Form 4 Type 7 construction in

accordance with Imperial College Technical Policy Statement E5 and

incorporate the following:

• Air Circuit Breakers (ACBs) on all incoming and bus-section

switches. The incoming ACBs to be equipped with Alstom P121

relays and, if transformer incomers, they shall have facility for

restricted earth fault (REF), trip lockout with contacts to intertrip HV

circuit breaker and auxiliary relays to accept trip and alarm signals

from the transformer liquid temperature device and trip signal from

the over-pressure device and, separately, trip receive from HV

circuit breaker.

• Incoming panels and outgoing circuits rated 50kW or greater, including

spares, shall have Socomec meters equipped with RS 485 Comms

output modules only. Socomec A40 meters shall be used for the

incoming circuits and Socomec A20 meters used for the output circuits.

The RS 485 outputs shall be "daisy chained" using screened twisted

pair & drain wire cable (Beldan or equivalent) and terminated in a

separate marshalling cubicle. The number of output networks to be

used, compatible with the College Trend System will be advised by the

College Engineering Team but no more than 31 instruments shall be

linked together

• Meters on sub-distribution panels supplied from switchboards

described above shall be equipped similarly, except the incoming

meter shall be omitted.

• No protection or metering required on bus-section breakers

• A separate cubicle shall be provided for the termination of all

metering outputs and external control circuits. Safe access shall be

possible with the switchboard live

• At the South Kensington Campus incoming and bus-section circuit

breakers shall not be interlocked. At remote campuses LV interlocking

is at the discretion of the HV network owner.

• Outgoing switches up to and including 630A shall be fuse-switch

type

• 30V DC auxiliary supply (shared with HV equipment)

• Outgoing switches equal to or greater 800A shall preferably be ACB

type However, switches in the range 800A – 1250A can be MCCB type

if it can be shown that discrimination can be achieved with downstream

devices

• Preferred switches are of Schneider manufacture

5.0 Cable Systems

HV Cable

• XLPE, Cu, SWA with LS0H oversheath, sized 240mm² for all main

feeders and 95mm² for all transformer feeders, except when close

RMU coupled.

• Cleated to ladder rack and not tie wrapped.

LV Cable & Conductor Systems

• XLPE, Cu, SWA with LS0H oversheath, multicore

• XLPE, Cu, AWA with LS0H oversheath, 1c for transformer incomers

equal to or less than1600kVA

• Transformer incomers > 1600kVA to be connected by bus-bar

trunking

• Cables to be cleated to ladder rack or tray. Tie-wraps may used on

earth conductors or cables, 50 mm² or smaller only

6.0 Auxiliary Equipment

Battery & Charger

• 30V DC with charger failure alarm for remote signalling. (BMS)

Substation Cooling

• Preferably is air natural. If forced air is required is naturally vented

input and forced output.

• Temperature alarm required for remote signalling. (BMS)

7.0 Earthing

This section shall be read in conjunction with Technical Policy Statement E7 –

Earthing Requirements.

Wall mounted main earth bar to be provided. If immediate ground access is

available two earth electrodes are to be provided with target earth resistance

of 10 ohms each. The following bonding shall be made at all South

Kensington substations:

• Panel HV switchgear, one bond at each end

• RMU, one bond. This may be to transformer if close coupled

• Transformers 2. One on each lug or boss

• LV Switchgear, one bond at each bus-bar section

• LV Neutral Earth within switchboard to main earth bar via

removable links. One link for each transformer incomer.

• Incoming HV cable armour. One bond per cable

• All outgoing cable earth conductors bonded to main switchboard

earth bar

For other Imperial College campuses the requirements of the local electricity

operator shall be adhered to. Where LV earths are the responsibility of

Imperial College, then earth electrodes less than or equal to1 ohm shall be

provided and permission to bond to the HV earth sought from the electricity

operator.

RELATED DOCUMENTS :

E4 – Connection of large electrical loads

E5 – Separation form for LV switchgear

E7 – Earthing

Growing in the electrical feld is shocking , by chuck

Hi Forum,

I was about to post the same question on RMU. Chunk, your explanation seems to be some extract of a code or bok. Request you to make a simpler explanation as to wht is the need of RMU & how it defere from normal HT Breaker panel. Is it so that if RMU is prposed there is'nt any need of HT breaker panel before Transformer or vice versa. Pls. clarrify.

Since it is not posible to draw the schematic, I am trying to gove you the required detail by giving you an expample:-

1 Suppose you want to feed 11 kv power to number of CSS/ OR Transfomers, for example 4 nos of 1250 KVA CSS 11kv/415v.,

2. Than you can do this by selecting 2 Nos of out going feedeers from 11 kv distribution breaker panel with 2 Nos of required size of 3 c 11 kv cable.

3. Th efirst feeder cable to terminate RMU -1 (RMU one incoming breaker / isolator / Load break switch as per application of load) wioth two numbers of Out going breakers/ isolators/ load break switch as per the application of load.

4. Out of two out going of RMU one is connected to CSS-1

5. Other Out going of RMU to incoming of RMU -2

6. Same way Out going of RMU-1 to CSS -2 and Other out going feeder to RMU-3

7. From RMU -3 out going 1 to CSS-3 and outher out going feeder to RMU-4 out going

8. RMU - 4 will receive incoming from 2nd feeder of 11 KV out going distribution panel.

9. RMU -4 out going one will recive cable from RMU -3 and RMU -4 other out going feeder to CSS-4

Hence with 2 nos of Dirstibruin panel feeder aqnd 2 nos of cable 4 nos CSS receives the power with reliablity for any of cable fails the system will recive power to all 4 CSS

Hope this will give you idea about RMU And it's applcations.

nascon

Nascom, u r explanation is clear. thanks. But i have seen in few designs where RMU is fed directly from HT metering panel with out HT panel. is that ok?

Dear

This all depend on the requirement of the power distribution

nascon

nascon u make me confused by stating that outgoing of one RMU is going to connect incomming to another RMU, i have opened the RMU and found that it have a two separate compartment for incommer and outgoing feeder, so one IC is from main feeder and other IC will connect to another IC of 2nd RMU and OG terminal is only for TRX connection not to another IC of other RMU, and this form a ring so its name is ring main unit.

GOOD....

THANK YOU...

EASY TO UNDERSTAND.

HOW MANY OUTGOING PROVISION FOR EACH RMU?

Can it be possible to have those incoming feeders from two different sub stations?

Is there any isolating device between RMU-3 and RMU-4?

How the automatic action takes place in restoring supply when a cable fault occurs?

With out more exact clairifacation of your statement , if what I quoated is not a complete answer to your guestion . Maybe your question is why there is a R M U spec, to start with. When dealing with mass of public of sheared public services civial fire police , insurence . some standared of ORDER in very acceptable . Not grasping the shearing of services is not getting it at all . A monarche or a dictsator type of background will hanper your thoughts in this area. We do want to look farther in life as to the people that may use this building.

It is all about WHAT IF .

yeha RMU does not need HT breaker panel, it is nothing but package of switchgeare. what we have done in one of our project is HT cable comming from 132/11 kv ss directly terminated to RMU and from RMU to TRX and from TRX to LV panel, RMU we have used are 3 function and 4 function. all the RMU have 2 I/C and not necessarily main feeder it could be ring feeder also. through this vartually we ignore the parallel connection of TRX.

Let me inform you that the number of readers to a thread is always greater than the number of writers to such a thread.

Hint: English is not my native language like many other readers in this forum.

So,

Let me recommend all writers to this thread: Please, always use spelling check feature, it is available on the writing menu bar, and it is fast enough to be used.

Thank you for these important new information for me.

Regards... Samak

I always get distracted from spelling when somebody is checking shorts.

RMU as an important part of secondary distribution substations. Ring main unit is a totallty sealed, gas-insulated compact switchgear unit. Switching devices is either disconnectors or circuit breaker.