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Topic Title: ring main
Topic Summary: sub mains
Created On: 03 October 2013 09:48 PM
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 03 October 2013 09:48 PM
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SKElectrical

Posts: 892
Joined: 01 February 2009

What are the thoughts of wiring a sub mains as a ring?
Specifically: existing 35mm supplying one DB.... and then wiring in another 35mm to an adjacent DB... and then connecting the two together.
 03 October 2013 10:31 PM
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peteTLM

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why connect them? Does the load require parallel conductors?

They would have to share a single protective device, 2x 35mm conductors into a single device, and a single point of failure.

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 04 October 2013 10:15 AM
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broadgage

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Cant see the point, in most circumstances.
Adds complexity and is potentialy confusingly non standard.

The incoming terminals of a typical DB wont accept two full size conductors.

I did do this or something rather similar once.
A factory had two similar fuse boards next to each other each supplying machines, litghting and small power.
One was new and one decades old, both suffered from excessive voltage drop during motor starts and consequent lamp flicker.

Paraleling the two submains greatly reduced the problem, but was rather a bodge and only a short term fix.
 04 October 2013 04:25 PM
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OMS

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Originally posted by: peteTLM

why connect them? Does the load require parallel conductors?

They would have to share a single protective device, 2x 35mm conductors into a single device, and a single point of failure.


They wouldn't need to share a single protctive device - in most cases you would actually avoid that and connect them to two seperate devices to provide resilience.

Ring main distribution (as opposed to ring final circuits) is incredibly common for distribution systems - virtually every hospital has them, as an example.

Regards

OMS

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 04 October 2013 04:38 PM
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daveparry1

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That doesn't sound right OMS? it would be like putting the two legs of a ring on separate breakers surely? (I've seen that a few times though!)

Dave.
 04 October 2013 04:59 PM
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Parsley

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I could be wrong I think OMS is referring to a main switch panel that possible has non essential and essential compartments supplied from separate tranformers and a NO link between the two sides of the panel, if the supply is lost from one of the transformers the link can be closed to feed the other side of the board.

Or maybe an open ring main where there are several TX on the site but that would be HV distribution.

Regards

Edited: 04 October 2013 at 05:16 PM by Parsley
 04 October 2013 07:34 PM
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OMS

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Originally posted by: daveparry1

That doesn't sound right OMS? it would be like putting the two legs of a ring on separate breakers surely? (I've seen that a few times though!)

Dave.


That's it Dave - exactly that - either end of the ring is fed from a sepeate breaker or fuse - at each load point you have an "in" and "out" isolator and depending on the supply arrangements you run ring open or ring closed

Regards

OMS

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 04 October 2013 07:41 PM
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OMS

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Originally posted by: Parsley

I could be wrong I think OMS is referring to a main switch panel that possible has non essential and essential compartments supplied from separate tranformers and a NO link between the two sides of the panel, if the supply is lost from one of the transformers the link can be closed to feed the other side of the board.

Not exactly - they could both be essential feeders to the switchboard - there may or may not be a closed bus coupler - and the rings may or may not be operated normally as closed rings.

What is useful is that you use all of the ring mains closed to import power to one side of the board when it's relevant feeder may have failed - once you realise you have paralleled the transformers and designed accordingly, the worlds your oyster in terms of configurations



Or maybe an open ring main where there are several TX on the site but that would be HV distribution.

Again, not really - it could be LV and it could easily be ring closed - either LV and HV you may well have a closed ring, but you'd probably have unit protection at that point looking for where (or in what section) the fault has occured and then switching the fault out of circuit and resupplying as two radials to the relevant loads.

Regards


Do a google for HTM 06-01 for example - there'll be plenty of examples in there

Regards

OMS

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 04 October 2013 07:45 PM
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SKElectrical

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I know Suppliers use rings as they supply us with power.

The setup I am discussing is at a factory where the Main Section Board has only one way to spare. So the thinking is to wire a 35mm ring supplying 2 x dist boards and slap the lot on a 160A (or 200A I cant remember)
 04 October 2013 07:52 PM
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OMS

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Originally posted by: SKElectrical

I know Suppliers use rings as they supply us with power.



The setup I am discussing is at a factory where the Main Section Board has only one way to spare. So the thinking is to wire a 35mm ring supplying 2 x dist boards and slap the lot on a 160A (or 200A I cant remember)


What sort of demand have you got ?

I'd suggest a bit of design before you proceed - one option might be a new 200A supply to an isolator with a busbar chamber and 2 x 100A supplies - one to the old board - one to the new

Keep in mind that most Type B Dist Boards are limited to 125A - so you shouldn't have protection in circuit that exceeds that !

Slap the lot on an unknown fuse size doesn't bode well

Regards

OMS

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 04 October 2013 08:12 PM
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SKElectrical

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I'm just subbing for the elec contractor for this work, so I won't be having an input on design.
The switches of the boards will be changed to 250A switch diconnectors.
The loads are high and are simultaneous.
I don't think its great practice but if the factory won't pay for another / bigger Section Board then I suppose it's not that bad a solution.
 04 October 2013 08:43 PM
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OMS

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They don't need another section board - whatever you do will still be limited to 200A max - as for "not that bad a solution" - no comment.

By the time you swap out the board incommers to 250A (and note that doesn't mean you have 250A pan assemblies with 250A copper work) and run another 35mm2 back to the section board, it has to be a little bit more sensible don't you think to offer a slightly more engineered design solution as I suggested above ?

Alternatively why don't you make those swich disconnectors auto MCCB's and supply from a BBC and a 200A supply ?

That said, if it's not your design and you have no responsibility then don't worry about it - don't sign anything would be my advice and don't tell too many people - that kind of lash up gets remembered

Regards

OMS

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 06 October 2013 11:25 AM
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peteTLM

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Originally posted by: OMS

Originally posted by: peteTLM



why connect them? Does the load require parallel conductors?



They would have to share a single protective device, 2x 35mm conductors into a single device, and a single point of failure.




They wouldn't need to share a single protctive device - in most cases you would actually avoid that and connect them to two seperate devices to provide resilience.



Ring main distribution (as opposed to ring final circuits) is incredibly common for distribution systems - virtually every hospital has them, as an example.



Regards



OMS



Indeed, but i had guessed that was being suggested wasnt heading for quite as elegant as such a system


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Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 07 October 2013 10:59 AM
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AJJewsbury

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That doesn't sound right OMS? it would be like putting the two legs of a ring on separate breakers surely? (I've seen that a few times though!)

One of my earliest posts was an idea to feed a ring final circuit from a pair of 20A MCBs (e.g. one DP one with both poles used as L) - each leg being separately terminated - that way you had proper overload protection for each leg and the possibility of safely supplying 40A rather than 32A total for the same amount of copper. Even if you had two separate MCBs, as long as as the worst case disconnection time was no more than half the requirement (e.g. 0.4s/2 = 0.2s for TN) so even if the 2nd MCB didn't start to trip until the other had opened, the overall disconnection time would still be acceptable. It's just isolation that would require a bit of care.

- Andy.
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