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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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 19 March 2013 08:31 PM
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primo

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Could anyone point me in the direction of a reg number that would prohibit the use of a split con cable for a sub main?

The set up is TP TNC-S supply in house converted to flats. All meters in one cupboard in hallway with a switch fuse to basement flat. The sub main from the switch fuse is a split concentric cable with what looks like 16mm line conductor. Earths are into a connector block then to a piece of 6mm that goes to main earthing terminal. Same terminations at the consumer unit in basement.

I think it needs replacing, or at least a separate 16mm earth run to the basement so I can install 10mm bonding.

But could do with a reg to back this up if poss.

Thanks.
 19 March 2013 09:22 PM
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leckie

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Is a split concentric cable referred to in any BS7671 table? What is its current rating,reference method, etc? I don't think it's in the book so I would think you cannot use it. Someone more knowledgeable will confirm no doubt.
 19 March 2013 09:28 PM
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primo

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Exactly, not referred to in the regs so need bait of lateral thinking to apply a reg and a code to it!
 19 March 2013 09:48 PM
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bajb

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In this country we normally work on the basis that if something is not forbidden then it is allowed, rather than if something is not allowed then it is forbidden. Manufacturer's data is available. For example:
http://www.batt.co.uk/upload/f...ebs7870_1355413546.pdf
 19 March 2013 09:59 PM
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alancapon

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I would expect a 16mm² to have 7 neutral cores adding up to 16mm² and 4 earth cores also adding up to 16mm². Where you do run into problems, is that it cannot be counted as SWA, as only a third of the protective armouring is earthed, the rest is connected to the system neutral, a live conductor.

Regards,

Alan.
 19 March 2013 10:27 PM
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bajb

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I'll be interested if there is a definitive answer to whether it can be used under 7671. I did use it to run a 25mm 3 phase sub main and a 16mm single phase sub main in my own place. It would have been much more difficult threading swa through a listed building.
 19 March 2013 11:52 PM
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peteTLM

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Originally posted by: alancapon

I would expect a 16mm² to have 7 neutral cores adding up to 16mm² and 4 earth cores also adding up to 16mm². Where you do run into problems, is that it cannot be counted as SWA, as only a third of the protective armouring is earthed, the rest is connected to the system neutral, a live conductor.



Regards,



Alan.



There was a reg introduced with the 17th that didnt allow the armouring of a cable to be used as a neutral as the neutral was now classed as a live conductor.
Number escapes me. Where's John or OMS when you need them!

So no you cant use split con after the meter. Before the meter the DNO can do whatever they like.

Before we get ahead of ourselves, how old is it? why do you think it should be replaced? what condition is it in?

Its not armoured as such, does it have RCD protection or is it in trunking or tray?

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 20 March 2013 12:57 AM
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whjohnson

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Is it split con or straight con?

Straight con is only used by the DNOs and is thus beyond 7671.

Split con has been used for years for sub mains and even electric shower feeds.

I ran 15 metre 25mm (integral 16mm multicore cpc around the outside) submain in my place when I resited the CU in the rear outbuilding ( it used to live on the lounge wall behind the telly!) I ran it through the back of the external meter box and up beyond ceiling height via a vertical run of galvenised steel conduit. After that, it was a doddle to thread through the pre-drilled joists to the back of the house.

It would have been a right btd of a job to do it in 3 core 25mm SWA I can tell you!

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 20 March 2013 04:48 AM
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MrP

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Split con is the sparkies friend
Used it on many apartment block sub main installations put 1,000 of meters of this stuff in
Easy to terminate, lighter, goes in faster and has none of that unnecessary steel wire armour
And it complies, If I were in the business of installation it would be my first choice

MrP
 20 March 2013 07:01 AM
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alancapon

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Originally posted by: peteTLM
There was a reg introduced with the 17th that didnt allow the armouring of a cable to be used as a neutral as the neutral was now classed as a live conductor.

Number escapes me. Where's John or OMS when you need them!

So no you cant use split con after the meter. Before the meter the DNO can do whatever they like. . .

I agree. It is ok regarding size of conductors, but fails on what two-thirds of the armour is used for. Apologies for not making that point in my earlier post. As such, it does not comply with BS7671

The rules for the DNOs as you say are a little different - the requirement is for all phase conductors to be enclosed by a metallic covering, at or about earth potential. For this application, use of a neutral is acceptable.

Regards,

Alan.
 20 March 2013 07:30 AM
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bajb

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I do not consider the neutral/earth conductors as 'armour'. I only laid it where armour was not required, for example visible on walls in a roof space or (well) under floorboards.
 20 March 2013 07:40 AM
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primo

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Originally posted by: peteTLM




Before we get ahead of ourselves, how old is it? why do you think it should be replaced? what condition is it in?



Its not armoured as such, does it have RCD protection or is it in trunking or tray?


Okay, maybe getting ahead of myself a bit there, not sure why I said I think it needs replacing, probably due to not knowing what size earth it provided hence wanting to run a separate 16mm to the CU.

Given Alan's comments I will check when returning to carry out works.

I would say its been there for at least 20 years, no RCD protection but not concealed and no requirement for it to be armoured. Its clipped direct, straight through the floor from intake cupboard to CU position.
 20 March 2013 07:41 AM
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primo

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Originally posted by: whjohnson

Is it split con or straight con?






Definitely split.
 20 March 2013 07:43 AM
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primo

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Originally posted by: alancapon


I agree. It is ok regarding size of conductors, but fails on what two-thirds of the armour is used for. Apologies for not making that point in my earlier post. As such, it does not comply with BS7671






Regards,



Alan.


So if no need for it to be armoured then you think okay?
 20 March 2013 07:44 AM
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primo

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Originally posted by: bajb

I do not consider the neutral/earth conductors as 'armour'. I only laid it where armour was not required, for example visible on walls in a roof space or (well) under floorboards.


As above!
 20 March 2013 09:38 AM
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Parsley

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See GN1 table F4 applications of cables for fixed wiring.

Split concentric thermoplastic insulated and sheathed.
Uses= general
Comments 1. Additional protection may be neccessary where exposed to mechanical stresses. 2. Protection from direct sunlight maybe neccessary, black sheath colour is better for cables in sunlight.

Regards
 20 March 2013 09:58 AM
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primo

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Thanks Parsley, just had a look at that, that's useful.
 20 March 2013 10:16 AM
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AJJewsbury

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There was a reg introduced with the 17th that didnt allow the armouring of a cable to be used as a neutral as the neutral was now classed as a live conductor.
Number escapes me. Where's John or OMS when you need them!

As I recall, the change under the 17th was to remove split-con (BS 7870?) cables from the list of cables that provide an earthed metallic covering in regs 522.6.100(ii), 522.6.101(i) etc. - i.e. where the cable is concealed in a wall or <50mm from the top/bottom of a joist and doesn't have RCD protection and located in 'safe' zones.

It's not mentioned in appendix 1 simply because it's not referred to in BS 7671 - it's a list of standards referred to, not a list of acceptable standards. For example BS 6004 (T&E) only gets a mention courtesy of section 717 and a few appendices.

It's absence from appendix 4 is irrelevant, as that appendix is informative, not normative.

So as far as I can tell, it's just as acceptable as T&E.

- Andy.
 20 March 2013 12:28 PM
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MrP

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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
 20 March 2013 01:02 PM
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AJJewsbury

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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 to mention loop impedance (even though it'll be fed from both ends).
- Andy.
IET » Wiring and the regulations » Split Con Sub Main

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