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Topic Title: Exporting TN-C-S to outbuilding/garage
Topic Summary: What is the validity of TTing in sceanrio described.
Created On: 21 April 2013 04:21 PM
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 21 April 2013 04:21 PM
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Thripster

Posts: 648
Joined: 22 August 2006

Would welcome your views please. Please note that I have posted exactly the same question on another forum as I am keen to get as many views on this as possible - it seems to be a contentious issue!

Domestic property, TN-C-S described as PME feeding 10mm² to outside wall of house where it changes (inside IP rated box) to 3 core 6mm² SWA to outbuilding 25m away. SWA terminated in plastic box, no banjo fitted. Three cores carry on to plastic DB where only two cores are used as Line and Neutral. Outbuilding TT'd. MET of outbuilding DB connected to rod and to incoming copper water pipe. Other end of copper water pipe bonded to MET of house DB. In effect therefore, the outbuilding DB is connected to the house MET by the copper pipe and would suffer the same consequences as had the SWA and unused third core of the SWA been used to earth the outbuilding DB. How, under the regulations, should this situation be dealt with? The 10mm² T/E is protected by a 40A MCB at the house DB and is not RCD protected. It does however, run under floor boards to an outside IP rated box. Noting that the combination of one, unused 6mm² SWA core used as earth and SWA armour might just be enough (depending upon who you talk to) meet thermal constraints and bonding requirements - why not just PME the outbuilding?

Thanks
 21 April 2013 04:53 PM
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Thripster

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Problem I've got here is that the 10mm² CPC is only 4mm² - I've yet to do the sums but I doubt whether it will meet thermal constraints and certainly will not meet 10mm² bonding requirement - so may have to stick with TT - unless I can include the low resistance of the copper pipe in parallel? A risky proposition I feel as that pipe may be removed/altered at any time. The only other concer I have is that the 10mm² cable is not RCD protected between house DB and outside wall - however, it is run under the floor boards and, as such, may not need RCD protection.
 21 April 2013 05:32 PM
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daveparry1

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I think it best that you stay with TT for the outbuilding but you'll need to do something about that copper pipe as at the moment it's giving earth continuity between the two buildings so therefore negating the TT earthing system. I'd think about incorporating a length of plastic pipe in the system, I wouldn't just use a plastic coupler somewhere because that could mean that someone could be in contact with both sections of copper! The cable under the house floor doesn't need rcd protection, you do of course need rcd at the outbuilding though!

Dave.
 21 April 2013 05:49 PM
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Thripster

Posts: 648
Joined: 22 August 2006

Thanks Dave. I'll book a plumbing course straight away:-) I think there is an opportunity to put a plastic section in before the pipe reaches the outbuilding. Would be interesting to see whether this situation has been encountered by others?

Regards
 21 April 2013 06:03 PM
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daveparry1

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It is quite unusual due to having the water supply, if it wasn't for that or if it were in plastic exporting the pme earth wouldn't be an issue to most of us on here although I know some say it shouldn't be done,

Dave.
 21 April 2013 11:18 PM
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leckie

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Isn't there a minimum length of plastic insert? Not by regs, but by design consideration. I know water isn't a great conductor but if you fit a 50mm plastic insert, that might be too short. I think it is something like 200mm minimum, but my memory may be playing tricks with me! Will be due to resitivity of water by volume.
 22 April 2013 06:55 AM
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Thripster

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Good point leckie - will have a look.

Regards
 22 April 2013 10:38 AM
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OMS

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Joined: 23 March 2004

What's the cross sectional area of copper of a 15mm dia copper pipe. What does 543.2.1 (vii) say.

What does 543.2.6 say - is a water pipe prohibited by 543.2.3

Is the cross sectional area of a water pipe plus your 4mm CPC in parallel equivalent to the requirements of Table 54.8.

Would you be better advised to reconnect all the earthing rather than attempting (like the original installer) an non TT, TT conversion

Regards

OMS

-------------------------
Failure is always an option
 22 April 2013 01:57 PM
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Thripster

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Thanks OMS, have considered it but cannot rely on future occupants leaving unsightly pipe in place, even with safety notices so think, on balance, that a TT installation is the better option.

Regards
 22 April 2013 02:07 PM
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ebee

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copper pipes (OK then tube) 15mm = 31.46 sq mm & 22mm = 66 sq mm - Does that sound right? I`ve used 22/7 as an approximation of Pi on a non scientific calculator

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 22 April 2013 02:17 PM
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sparkingchip

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"Is the cross sectional area of a water pipe plus your 4mm CPC in parallel equivalent to the requirements of Table 54.8. "

Yes, but one frosty night leading to a burst pipe with a bit of plastic used for the repair and this design consideration is straight out the window, so perhaps it is better to put a plastic insert into the pipe by design. But then someone could alter and repair the pipe with copper and brass after a burst thus removing the plastic insert, so there isn't a clear argument there either way

Andy
 22 April 2013 02:18 PM
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OMS

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Originally posted by: Thripster

Thanks OMS, have considered it but cannot rely on future occupants leaving unsightly pipe in place, even with safety notices so think, on balance, that a TT installation is the better option.

Regards


If they remove the pipe, they remove the extraneous conductive part and thus the need for bonding ?

It's going to be "PME" in the shed whatever you do - unless you do fit an insulating section in the MWS from below ground, right through into the shed. Then you could TT it

Regards

OMS

-------------------------
Failure is always an option
 22 April 2013 02:21 PM
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OMS

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Originally posted by: ebee

copper pipes (OK then tube) 15mm = 31.46 sq mm & 22mm = 66 sq mm - Does that sound right? I`ve used 22/7 as an approximation of Pi on a non scientific calculator


22/7 - Old Skool, Ebee -

Yes, allowing for a wall thickness of about 0.7 on 15mm and 1.0mm on 22mm copper tube I guess we could conclude that there's loads of copper

Regards

OMS

-------------------------
Failure is always an option
 22 April 2013 02:23 PM
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OMS

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Originally posted by: sparkingchip

"Is the cross sectional area of a water pipe plus your 4mm CPC in parallel equivalent to the requirements of Table 54.8. "



Yes, but one frosty night leading to a burst pipe with a bit of plastic used for the repair and this design consideration is straight out the window, so perhaps it is better to put a plastic insert into the pipe by design. But then someone could alter and repair the pipe with copper and brass after a burst thus removing the plastic insert, so there isn't a clear argument there either way

Andy


Ahhh - I've see you've met that fine old fella, Mr Arbut Whatif

Regards

OMS

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Failure is always an option
 22 April 2013 02:30 PM
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Thripster

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The pipe runs from the cellar in the house where it is bonded to the house MET, under the floors to the back and out along the terraced wall at about three feet high and on into the outbuilding. It is electrically continuous. With a break in the SWA at the outbuilding and a plastic section in the 15mm/1/2" before the outbuilding, I can make it true TT.

Regards
 22 April 2013 02:40 PM
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OMS

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Originally posted by: Thripster

The pipe runs from the cellar in the house where it is bonded to the house MET, under the floors to the back and out along the terraced wall at about three feet high and on into the outbuilding. It is electrically continuous. With a break in the SWA at the outbuilding and a plastic section in the 15mm/1/2" before the outbuilding, I can make it true TT.

Regards


You can - what purpose does it serve though ? - ie what risk, to who, from what fault scenario, are you trying to address

In terms of "shock" risk, you probably are more at risk touching the pipe and standing in the garden than you are in the shed.

regards

OMS

-------------------------
Failure is always an option
 22 April 2013 04:52 PM
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Thripster

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Looking at the various thoughts I think the solution would be to get a tardis, go back in time and proceed to not make the OP.

My main concern is to maximise safety whilst complying with the regulations. As you can see, the subject generates a difference of opinion. There has been much talk in the past about the dreaded lost neutral - in my scenario, as it stands, and whether TT or PME, touching the pipe in the garden could result in a dangerous touch voltage between hand (s) and feet. So I have considered removing the bond at the house end assuming that I can establish that there is no path to earth elsewhere along it's length. The driver for my decision to make it TT is that the 'reliable' CPC in the 10mm is only 4mm - it needs to be 10mm. I do not think that relying on the copper pipe is long term safe as the person who removes it in the future is unlikely to understand the electrical consequences. Upon reflection, it may be better to put a plastic section in where the pipe exits the house (or before if possible) so that danger is minimised. The conclusion that we (perhaps) draw is that it is not a simple situation.

Regards
 22 April 2013 05:28 PM
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OMS

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Originally posted by: Thripster

Looking at the various thoughts I think the solution would be to get a tardis, go back in time and proceed to not make the OP.

LoL - hind sight is a wonderful thing.


My main concern is to maximise safety whilst complying with the regulations.

OK - that's what we all try to do.

As you can see, the subject generates a difference of opinion.

Maybe

There has been much talk in the past about the dreaded lost neutral

so much so that BS 7671 doesn't even require you to think about it !!

- in my scenario, as it stands, and whether TT or PME, touching the pipe in the garden could result in a dangerous touch voltage between hand (s) and feet.

Not in a TT system it won't - the risk is the pipe connected to the system neutral in a PME system

So I have considered removing the bond at the house end

and leave the water pipe in the house unbonded ?

assuming that I can establish that there is no path to earth elsewhere along it's length.

is it likley to pick up an earth potential in the garden then ?

The driver for my decision to make it TT is that the 'reliable' CPC in the 10mm is only 4mm - it needs to be 10mm.

Agreed - you have 30mm of copper connecting the MET to the erth marshalling bar in the shed, plus whatever CPC you have.

I do not think that relying on the copper pipe is long term safe as the person who removes it in the future is unlikely to understand the electrical consequences.

They don't need to - if they remove it, you no longer have an extraneous conductive part in the shed and thus no need for bonding and thus no need to worry about just how much copper you have going to the shed.

Upon reflection, it may be better to put a plastic section in where the pipe exits the house (or before if possible) so that danger is minimised.

The conclusion that we (perhaps) draw is that it is not a simple situation.

Indeed - now you have two options - restore the system to a PME "export" to the shed or try and isolate the water connection syatem earth, drop in an electrode, add an RCD and make it TT.

Depends on how you view the risk of a lost PEN - and consider that there are whole rows of houses on metallic services - some have been TT'd - some not - but they all have a common water pipe and bonding.


Regards


As you said:

why not just PME the outbuilding?


Why not indeed

Good luck, whatever you decide any how

regards

OMS

-------------------------
Failure is always an option

Edited: 22 April 2013 at 05:42 PM by OMS
 22 April 2013 06:01 PM
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Thripster

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Think we are talking cross purposes - if TT'd at shed, bonding between MET at shed and incoming copper, bonding at house MET then lost neutral would cause pipe and TT installation in shed to attain a higher voltage wrt to soil - potentially dangerous in garden. If PME'd then same. As John said, plugging in a tool in the outbuilding for use outdoors could also intoduce the (vanishingly) rare situation whereby there is a pd between the case of the tool and soil under lost neutral conditions.

Don't consider it safe to rely on copper pipe as making up the 10mm requirement between shed and house ( although using the the third unused core and SWA would come close) as it is pig awful and, if it were mine, it would come straight out - if I feel that way then a future occupier may do the same. This is probably the only reason that I have not gone the way you advocate.


Thanks for your time and thoughts on this
 22 April 2013 06:04 PM
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Thripster

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Meant to say that the limiting factor is the 4mm cpc in the 10mm T/E - so that, along with unreliability of copper pipe duration is why I'm headinv towards the Isle of Man.
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