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Topic Title: which supply
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Created On: 22 April 2014 07:27 AM
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 22 April 2014 07:27 AM
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OldSparky

Posts: 592
Joined: 28 June 2011

Morning peeps,

yes 4 days off and now its hard to get back in to it..

I have doubt in my head and need a little reassurance.

A house with the main incomer across the yard, about 35 mts is fed with a 16mm t&e, this is TT'd , the supply in TNCS.

I have to install a second sub main for storage heaters, i would like to keep this supply TNCS and make the house the same rather than TT.

All bonding is in place, though the only services are oil from a plastic tank, pipe above the ground, and a well for the water.

The sub mains would not be rcd protected.

your advice would be appreciated.
 22 April 2014 06:31 PM
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weirdbeard

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Hi OS, not quite clear about what you are doubting.....is it just that the sub-mains won't be RCD protected?

I can't see thats going to be a problem if you ditch the TT assuming the Zs is good and they aren't concealed in a wall etc.
 22 April 2014 06:50 PM
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daveparry1

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What's the installation method of the t/e sub-main OS? 16mm sounds a bit small for a 35m run to feed a whole house doesn't it?
 22 April 2014 06:57 PM
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aligarjon

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The cpc on a 16mm T+E is not big enough for bonding on a pme system or is there a seperate one run in with it ?

Gary

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Specialised Subject. The Bleedin Obvious. John Cleese
 22 April 2014 07:03 PM
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weirdbeard

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Originally posted by: daveparry1

What's the installation method of the t/e sub-main OS? 16mm sounds a bit small for a 35m run to feed a whole house doesn't it?


Dave, changing the earthing arrangements won't affect the loading on the existing cable. (Just a guess, but if anything, the addition of a new dedicated heating sub will reduce the loading on the existing cable, by removing the need for plug-in type heaters? )
 22 April 2014 07:29 PM
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daveparry1

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Agreed WB but 16mm t/e on a 35m run still sounds small for a whole house supply.
 22 April 2014 07:32 PM
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OldSparky

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hi peeps

yep the 16mm only does lights and sockets, more than enough..

the new sub 25mm concentric is for the heating..

as for the earth being too small i could put that on a time delay rcd, i also doubt whether you could call it a PME system? there are no services in the house that are connected to outside utilities, all pipe work is plastic.

sub mains all overhead then clipped surface..
 22 April 2014 07:57 PM
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weirdbeard

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Originally posted by: OldSparky

as for the earth being too small i could put that on a time delay rcd,


This is getting complicated!....an RCD time delayed or otherwise, won't mitigate the earthing conductor size unless the existing CSA conductor is giving a too high Zs at the point of use in relation to the Ze????


i also doubt whether you could call it a PME system? there are no services in the house that are connected to outside utilities, all pipe work is plastic.


It was a long weekend, but surely not long enough to forget that PME has nothing to do with what's within the installation itself? (other than perhaps the appropriate sizing of MPB's if required) ?
 22 April 2014 08:02 PM
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weirdbeard

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Originally posted by: daveparry1

What's the installation method of the t/e sub-main OS? 16mm sounds a bit small for a 35m run to feed a whole house doesn't it?


If I was an actual proper electrician......I'd recommend at least 35mm SWA be trenched at least 6ft drawn through titanium hardend carbon fibre ducting
 22 April 2014 10:06 PM
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OldSparky

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Joined: 28 June 2011

Originally posted by: weirdbeard

Originally posted by: OldSparky



as for the earth being too small i could put that on a time delay rcd,




This is getting complicated!....an RCD time delayed or otherwise, won't mitigate the earthing conductor size unless the existing CSA conductor is giving a too high Zs at the point of use in relation to the Ze????





i also doubt whether you could call it a PME system? there are no services in the house that are connected to outside utilities, all pipe work is plastic.





It was a long weekend, but surely not long enough to forget that PME has nothing to do with what's within the installation itself? (other than perhaps the appropriate sizing of MPB's if required) ?


sorry explanation gone wrong

I have have seen many a job with the earth rod still connected when premises have been converted from TT, i have always disconnected these, what would your thoughts be on this?
 23 April 2014 11:46 AM
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AJJewsbury

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I have have seen many a job with the earth rod still connected when premises have been converted from TT, i have always disconnected these, what would your thoughts be on this?

A few years ago the IET actually suggested adding a local electrode to PME installations to reduce the dangers from a high impedance network CNE. The trouble is you need a very low rod resistance (or very low load) to make the precaution effective, but on the basis that every little helps, I'd be very tempted to leave any extra earthing connected. By the letter of the book, the rod would count as an extraneous-conductive-part and so it connection should meet PME bonding requirements (e.g. min 10mm2 Cu) - but in most circumstances it would be easy enough to show that the resistance to earth would limit any current to a low enough value that the original earthing conductor could be safely reused.

- Andy.
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