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Topic Title: Split Con Sub Main
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Created On: 19 March 2013 08:31 PM
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 20 March 2013 01:13 PM
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alancapon

Posts: 5759
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In my opinion, thee covering on the neutrals would probably melt - it is not the same thickness of insulation you would expect to see on the phase! The DNO issue is probably slightly different, as it is usually the entire cable that is chopped. To be honest, I prefer the version with the aluminium phase conductor, as this tends to "burn back" on a cable strike.

Regards,

Alan.
 20 March 2013 05:54 PM
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statter

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ESCQR would seem to prevent SCNE being used underground now as there is no complete 'screen', I seem to recall something in 7671 about round cables having to have an outer layer of earthed conductors but I have to say I cant find it.

I like SCNE cable esp for overhead supplies to outbuildings etc using concentric 'dead ends' to support it.
 20 March 2013 05:55 PM
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statter

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sorry ESQCR (finger trouble)
 20 March 2013 06:05 PM
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peteTLM

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Originally posted by: MrP

As Andy says because it's not in the book doesn't mean it doesn't comply the humble Grommet and 3871 mcb's are not in the book

Why would anyone use swa for a surface domestic submain you would have to be crackers

Split con every time



MrP


It doesnt comply as the outer screen/ armour is used partly for a live conductor. Ok up to june 2008 but not after. ok for DNO stuff where the meter is in the flat but not when the meter is downstairs in a cupboard and 3871 applies to the riser/ submain.
In the OP situation, id leave it be but code it-

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

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 20 March 2013 06:30 PM
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bajb

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Have you got a reference for that?
 20 March 2013 06:38 PM
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prophet

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Originally posted by: peteTLM

Originally posted by: MrP



As Andy says because it's not in the book doesn't mean it doesn't comply the humble Grommet and 3871 mcb's are not in the book



Why would anyone use swa for a surface domestic submain you would have to be crackers



Split con every time







MrP[IMG][/IMG]




It doesnt comply as the outer screen/ armour is used partly for a live conductor. Ok up to june 2008 but not after. ok for DNO stuff where the meter is in the flat but not when the meter is downstairs in a cupboard and 3871 applies to the riser/ submain.

In the OP situation, id leave it be but code it-


Reg no please.

The cores round the Split con cable are not armouring, so like was mentioned above, i would treat as T+E. If installed correctly there should be no problem.
 20 March 2013 07:26 PM
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alancapon

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Originally posted by: statter
ESCQR would seem to prevent SCNE being used underground now as there is no complete 'screen' . . .

Not quite correct I am afraid. On the DNO's side of the meter, which the ESQCR regulations refer to, it is still allowed.

Regards,

Alan.
 20 March 2013 10:06 PM
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leckie

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If you used a split concentric cable as a sub-main, it would need rcd protection if less than 50mm from the surface or out of a wiring zone if the cable was buried. That's if it is an installation that falls with BS7671
 21 March 2013 01:19 AM
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whjohnson

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Surely it wouldn't need RCD protection if the part of the run which is 'buried' (or covered over with plasterboard) is enclosed in steel conduit?

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Political correctness is a doctrine, fostered by a delusional, illogical minority, and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.
 21 March 2013 05:48 AM
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MrP

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The application multiple apartment supplies, central metering position basement, ladder to and up the risers, supply into each apartment dressed it even looks like SWA
In the apartment wooden meter board flush up to the ceiling cable entres between wall and board
Entering the main switch room via ladder dressed, 12x12 PVC trunking the end cap is cut to match the cable profile looks like it grew there no RCD, no steel wire armour, no glands, its quick, its neat, its light and it complies.
Put 1000,s of metres of this stuff in

Unless someone can state other wise with appropriate regulation number

MrP
 21 March 2013 06:37 AM
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leckie

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No, it wouldn't need rcd protection if it is enclosed in steel conduit. But if you going to go to the trouble of fitting it in steel conduit it would be easier to just use a swa I would have thought.

I've never seen split concentric cable offered for sale or listed at any wholesalers. If used where would you get the cable rating, volt drop characteristics, etc. presumably the manufacturers as its not in the regs book.
 21 March 2013 06:45 AM
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leckie

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Might have answered my own question!

I found this, though its described as a service cable.
http://www.voltimum.co.uk/file...ingsPVCinsulation.pdf

So can you use this cable for sub-mains etc, within BS7671 or not?
I can see it useful, as Mr.P pointed out.
 21 March 2013 12:56 PM
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weirdbeard

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Originally posted by: AJJewsbury

I do have a worry about the fault protection of the N with split-con though - as each strand is separately insulated, it seems possible that under fault conditions (damaged cable) that not all the strands would share the fault current equally as we'd normally be able to assume for a stranded cable. If 16mm2 is split between 7 strands and only one of those strands is carrying the fault current then you've effectively got a single 2.29mm2 conductor which might be a challenge to adiabatic calculations,


Not used the type of cable in question myself, but I wouldn't have thought that it is at any more risk of suffering damage to the neutral conductor than any other type of cable?
 21 March 2013 01:15 PM
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AJJewsbury

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I wouldn't have thought that it is at any more risk of suffering damage to the neutral conductor than any other type of cable?

Agreed - it has not more (or less) risk of being damaged - just (I feel) less likely to survive (safety) if it is.
- Andy.
 22 March 2013 05:37 PM
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statter

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Originally posted by: alancapon

Originally posted by: statter

ESCQR would seem to prevent SCNE being used underground now as there is no complete 'screen' . . .


Not quite correct I am afraid. On the DNO's side of the meter, which the ESQCR regulations refer to, it is still allowed.



Regards,



Alan.


....... Yes sorry thought the meter was at the far end which it isn't. I still think that ESQCR might well prohibit use of SCNE by DNOs underground. The relevant regulation is 13.2...
(2) The protection referred to in paragraph (1) shall comprise -
(a)in respect of joints or terminations of a conductor in a low voltage system, some form of mechanical protection; and .
(b)in respect of any other part of any conductor, an electrically continuous metallic screen connected with earth, .
so placed as to ensure that, so far as is reasonably practicable, any tool or device likely to be used in the vicinity will make contact with that protection or screen before it can make contact with any conductors not connected with earth.

I know the neutral will normally be connected with earth but so is the live (through the transformer winding). Based on this it may well not comply.....? CNE would be fine I think because the outer conductors are dual purpose NE.

Probably an academic point.
 22 March 2013 06:35 PM
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weirdbeard

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Originally posted by: AJJewsbury

I wouldn't have thought that it is at any more risk of suffering damage to the neutral conductor than any other type of cable?


Agreed - it has not more (or less) risk of being damaged - just (I feel) less likely to survive (safety) if it is.

- Andy.


Really though Andy, we can't design to install cables to survive the loss of even 1 strand, let alone 5 more - a knick through the outer sheath of an SWA is enough to ensure that the cable is damaged past it's specifications, though the resulting corrosion might take a bit longer to cause noticable problems!
 22 March 2013 09:30 PM
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AJJewsbury

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Really though Andy, we can't design to install cables to survive the loss of even 1 strand, let alone 5 more - a knick through the outer sheath of an SWA is enough to ensure that the cable is damaged past it's specifications, though the resulting corrosion might take a bit longer to cause noticable problems!

Not quite sure I follow... we do though try and design so that cables will be safe during faults - including ones originating from damage to the cable itself. Think about the N in a split-con being 7 separate wires in parallel (they can't share fault current from the point of the fault like a normal stranded conductor as they're separated from each other by plastic) - and have a read of appendix 10.

- Andy.
 25 March 2013 01:38 PM
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weirdbeard

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Originally posted by: AJJewsbury

Think about the N in a split-con being 7 separate wires in parallel (they can't share fault current from the point of the fault like a normal stranded conductor as they're separated from each other by plastic) - and have a read of appendix 10.
.


Hi Andy, I think I was thinking along the lines of what ever type of cable is used, for example, if its a T+E and 6 strands of a conductor are cut through leaving only one remaining, then even though the rest of the strands are touching along the remaining portion there will still be a section with only one strand taking the current., so I can't see how having the conductor strands seperately insulated is any different?
 25 March 2013 01:58 PM
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OMS

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But isn't the point where you sever 6 out of seven strands also the point of fault - so all seven strands are still effective in the fault "loop"

Not so if you insulate each strand from the remaining strands - essentially you have conductors in prallel - how many in parallel is a subject for a detailed analysis of the fault

Regards

OMS

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 25 March 2013 02:01 PM
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AJJewsbury

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Hi Andy, I think I was thinking along the lines of what ever type of cable is used, for example, if its a T+E and 6 strands of a conductor are cut through leaving only one remaining, then even though the rest of the strands are touching along the remaining portion there will still be a section with only one strand taking the current., so I can't see how having the conductor strands seperately insulated is any different?

I guess the main difference is that the missing strands (or rather than one remaining strand) will only be a problem at one point - i.e. at the termination or point of failure (if they're cut by the damage that causes the short) - so the area of damage is limited and come repair time, it's obvious where the damage is and so what needs to be replaced.

With split-con, it's the entire length of the cable up to the fault that's been severely overheating (in one strand) - I would have thought that would mean a significantly in increased fire risk in all the areas it passes through, not just the location where the fault occurred, and (presuming it survived that) a long length of cable that's suffered damaged that might not be apparent.

- Andy.
IET » Wiring and the regulations » Split Con Sub Main

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