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Evening all,

I’m currently looking into running a submain approx 100m buried from a PME supply to a small outbuilding (timber frame, metal sheet clad) that is being used next to a glamping tent as a place to cook and shower etc. The sub main will be a 16mm SWA..I was initially going to run a 3 core SWA or potentially a 2 core SWA with a seperate 10mm CPC (slight cost reduction) with the later option. I’ve since thought that perhaps a TT install would suffice, and, as possibly in the future I might be asked to extend the power into the tent, which, would require a TT setup. Any thoughts of this from a safety point or preference, also, if deciding on a TT and running a 2 core SWA as a submain which would obviosuly be a cost saver compared to a 3 core, however, would it be best practice to still have a CPC available seeing as a trench is now open...future proofing....? Cheers.
 
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buzzlightyear

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have you tested the metal sheeting for extraneous and all that .any water pipes in the building and if so then TT it .have you read the guidance's note 8.
 
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  • #3
have you tested the metal sheeting for extraneous and all that .any water pipes in the building and if so then TT it .have you read the guidance's note 8.
Hi there, the pipes are all plastic, there isn’t an electrical installation yet in the outbuilding but I will be bonding an earth to the metal sheets.
 
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I was having a nose on the Western Power webpage, information for electrical installers. When providing a TN-C-S earthing arrangement, they ask that the installation meets BS7671 and Engineering Recommendation G12. Here it is:


Worth a read through, of particular interest will be 6.2.3.4 and 6.2.6.2
Thanks for that, some interesting points in there, just to add to the fun here the Ze is slightly high, ScottishPower haven’t been able to solve it yet, the ground is rocky as hell here!
 

suffolkspark

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Have you done any volt drop calcs? What size are you intending your overload protection to be for the submain. 100m is a fair run
 

davesparks

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If the protective conductor of the submain is suitable for use as a main bonding conductor then I see no reason not to use the TNCS earth.

Unless you put a lot of time and money into it a TT earth is never going to give you as good or reliable an earth connection as a TNCS supply.

If you are concerned about the potential difference between the TNCS earth and true earth at the load end of the submain then an additional local earth rod should solve this.
 
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  • #9
Have you done any volt drop calcs? What size are you intending your overload protection to be for the submain. 100m is a fair run
Hi there,

Yes I have, it wasn’t a post relating to the size of the cable it was relating to the earthing scenario which I’ve sorted now..cheers.
 
I would just TT it. As long as the requirements for the electrode resistance are met you will always meet the requirements for ADS.
 

davesparks

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I would just TT it. As long as the requirements for the electrode resistance are met you will always meet the requirements for ADS.
Why 'TT it' as you say? I cannot see a valid reason to not use a good solid TNCS earth connection in this scenario. The TNCS earth will meet the requirements for ADS whereas a TT earth will almost certainly require the use of RCDs for fault protection.

The only time it's normally necessary to create a seperate TT earthing system for an outbuilding is when it's impossible or impracticle to provide the necessary main bonding, in this case and as far as we know the proposed 10mm or 16mm CPC will satisfy the requirement for main bonding.
 
Yes without further information on the installation and it’s requirements for bonding I can’t say but because the OP stipulated the future requirement for a TT installation I based my comment on that.
 

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