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Topic Title: RMUs vs panels
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Created On: 30 November 2012 04:26 PM
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 30 November 2012 04:26 PM
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brianjones25

Posts: 11
Joined: 05 March 2003

We are having (another) cost squeeze in our company, and the idea of using RMUs on rings been raised, instead of using panels and radial feeds (on new projects not retrofit). Process is continuous with campaigns of many years, installed load c.12MVA, MV distribution on site usually 10-20kV, current practice is 1 MV panel (e.g. BVP17/ YSF6/ genie evo) with radial feeds out to 6 or 7 duplex subs. Indications are that RMUs are significantly cheaper than panels, so intuition says there has to be a downside. Anyone else considered RMUs in industrial environment or have any experiences they could share? TIA, Brian
 30 November 2012 07:12 PM
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MickeyB

Posts: 181
Joined: 18 January 2003

I have specified and operated RMU 'switchboards' for simple secondary distribution, 1MVA, 1.6MVA transformers with no operational problems. The largest being a 7 way line up. 5 transformers with 2 ring switches. I think the main drawbacks to the RMU solution are:

1. Relay protection and metering options are limited, (due to self powered relay options), Schneider offer one of the Areva P1xx relays self powered. You could add a battery charger and see what relays can be installed in some 'box' somewhere....

2. Fault ratings can be limited, although for most situations adequate.

3. Current rating options are limited when compared with line up switchgear, however, for most situations adequate.

4. Motor/ auto operation is limited, although you can get motorisation of the sf6 breaker and ring switches.

Physically and quality wise you do get what you pay for with the older RMU's..... Traditional RMU's can be a pain in the a*se to operate, very fiddly on some models.....Schneider is one of the better traditional RMU's. You can get an RMU with vacuum from Schneider, ABB, Lucy and Eaton that do look a little more robust and more like line up switchgear..... but smaller. The new 'compact' switchgear may be just what you're looking for.

Isolation and earthing is the same. If you operate it very rarely and have relatively small and simply distribution requirements then an RMU switchboard can work. I would say for 99% of small installations an RMU switchboard would work fine.
 01 December 2012 01:28 PM
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ArthurHall

Posts: 737
Joined: 25 July 2008

I largly agree with Mickey. However his point 1 about only being able to use self powered relays is not strictly true as normaly you can specify a normal trip coil and use any relay you like, but you will also have to supply a battery.

If you have a fault on a ring cable then you lose that whole leg until the faulty section has been isolated, this can take an hour or so. Where on radial systems you only use one transformer but it is off for the repair time. You need to take a good look at your system and decide which system or combination is best for you.

Another consideration is the standard of opperators and control systems. Radials are safer as the interlocks tend to work with you, where on rings it is possable to close an earth switch onto a live cable. This is one of the main reasons why industrial sites prefer radials.
 02 December 2012 01:05 AM
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alancapon

Posts: 5831
Joined: 27 December 2005

Originally posted by: ArthurHall
. . . Radials are safer as the interlocks tend to work with you, where on rings it is possable to close an earth switch onto a live cable. . .

I would suggest that the the earth switch should have a fault-making rating to allow you to do so. Closing an earth onto a live cable is one of those things that can happen.

Regards,

Alan.
 10 December 2012 11:07 AM
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rosemarie

Posts: 5
Joined: 10 December 2012

It is obvious that you don't understand electrics as a VCB panel can be used with a ring mains.

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 10 December 2012 11:09 AM
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Avatar for timothyboler                                      .
timothyboler

Posts: 230
Joined: 25 July 2008

Originally posted by: alancapon

Originally posted by: ArthurHall

. . . Radials are safer as the interlocks tend to work with you, where on rings it is possable to close an earth switch onto a live cable. . .


I would suggest that the the earth switch should have a fault-making rating to allow you to do so. Closing an earth onto a live cable is one of those things that can happen.



Regards,



Alan.


...and possibly get the earth switch interlocked with a capacitative voltage presence indicator like on the Siemens units i've seen. No substitute for proper procedures and training though.

I assume the RMUs will be ring-main switches and a SF6 or vacuum CB for the tee-off, and the ring will be operated in open mode? The main disadvantage of the RMU is that maintenance is more difficult; you must isolate the whole unit.

Regards, Tim

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