WYLEX RCDO

... Roof is boarded and covered with two layers of felt, insulation inside and also plasterboarded. Its very snug and dry. ... The light are fluorescents that are all internal to the workshop, there is no external lighting on the workshop light circuit which is wired in 1.5mm twin and earth, all fittings are correctly earthed.
Fair enough, but the point being made was that if any of the wiring in the lighting circuit (including that to switches etc.) was 'buried' <50mm below the surface of the plasterboard, then the regulations would require the circuit to have RCD (or RCBO) protection (and also routed in the prescribed 'safe zones'). If the wiring were entirely on the surface, and visible, there would be no such requirements.

Kind Regards, John

Ah! Of course. No all the workshop wiring is surface mounted. At the back of the workbenches, and in some other areas, it is in plastic trunking.
 
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How is the SWA terminated on the house side, and is that circuit covered by an RCD (30mA) in the house consumer unit?
 
Ah! Of course. No all the workshop wiring is surface mounted. At the back of the workbenches, and in some other areas, it is in plastic trunking.
Fair enough - but, as I pointed out, in other circumstances garlicbread's comment may have been pertinent.

Kind Regards, John
 
How is the SWA terminated on the house side, and is that circuit covered by an RCD (30mA) in the house consumer unit?
I think most people would say that, given that there is RCD/RCBO protection in the outhouse, it would (assuming we're talking a TN installation) be better for there not to be any RCD protection at the house end of the SWA.

Kind Regards, John
 
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Trefor, I'm reminded that no-one has yet asked what earthing arrangements you're having for the outhouse and whether there are any extraneous-conductive parts (entering metal water pipes, structural metal etc.) in the outhouse.

Kind Regards, John
 
If the lighting cabling is concealed less then 50mm from the finished surface and does not have an earthed metallic covering (e.g. SWA or MICC) then in a domestic environment (your garden workshop is still classed as domestic!) then it'll have to be RCD protected to comply with the 17th edition. If you had wired it with the cabling surface in trunking or conduit then RCD protection would not be necessary
Like me, you need to go on a 'fast typing' course :)

Cheers, John
 
Trefor, I'm reminded that no-one has yet asked what earthing arrangements you're having for the outhouse and whether there are any extraneous-conductive parts (entering metal water pipes, structural metal etc.) in the outhouse.

Kind Regards, John

Hi John
No there are no pipes etc. The only 'service' is the SWA three core buried between 600mm and 450mm (beneath a path). Live, neutral and earth, with the earth conductor and the armouring all connected to the earth point at the house end.
 
No there are no pipes etc. The only 'service' is the SWA three core buried between 600mm and 450mm (beneath a path). Live, neutral and earth, with the earth conductor and the armouring all connected to the earth point at the house end.
Fair enough. Do I take it that you are going to use that earth as the earth for the outhouse? Do you know what earthing system the house has (TN-S, TN-C-S or TT)?

Kind Regards, John
 
If the lighting cabling is concealed less then 50mm from the finished surface and does not have an earthed metallic covering (e.g. SWA or MICC) then in a domestic environment (your garden workshop is still classed as domestic!) then it'll have to be RCD protected to comply with the 17th edition. If you had wired it with the cabling surface in trunking or conduit then RCD protection would not be necessary

It is surface wired. As it is a workshop I wanted to be able to easily move things around if that became necessary.
 
No there are no pipes etc. The only 'service' is the SWA three core buried between 600mm and 450mm (beneath a path). Live, neutral and earth, with the earth conductor and the armouring all connected to the earth point at the house end.
Fair enough. Do I take it that you are going to use that earth as the earth for the outhouse? Do you know what earthing system the house has (TN-S, TN-C-S or TT)?

Kind Regards, John

I thought that it was TNS but looking again I think that its TN-C-S.
 
I thought that it was TNS but looking again I think that its TN-C-S.
Fair enough. Some people don't like 'exporting' TN-C-S earths to outhouses, because of some theoretical (and very improbable) hazards but that becomes much less of an issue if, as in your case, there are no extraneous-conductive-parts in the outhouse that would need bonding (which then requires one to confirm that the earth connection between house and outhouse is adequate for bonding purposes). What is the distance involved (from house to outhouse)? Do I take it that the outhouse has some sort of 'proper floor' (not in contact with the ground)?

Kind Regards, John
 
I thought that it was TNS but looking again I think that its TN-C-S.
Fair enough. Some people don't like 'exporting' TN-C-S earths to outhouses, because of some theoretical (and very improbable) hazards but that becomes much less of an issue if, as in your case, there are no extraneous-conductive-parts in the outhouse that would need bonding (which then requires one to confirm that the earth connection between house and outhouse is adequate for bonding purposes). What is the distance involved (from house to outhouse)? Do I take it that the outhouse has some sort of 'proper floor' (not in contact with the ground)?

Kind Regards, John

The workshop is approx. 25mtr from the house. Its a solid concrete floor over a damp proof membrane.
 
The workshop is approx. 25mtr from the house. Its a solid concrete floor over a damp proof membrane.
OK. As you probably understand, the concern some have about 'exporting a TN-C-S' earth is that in the (very rare) event of a supply-side neutral fault, the potential of the TN-C-S-derived 'earth' can sometimes rise to considerably above true earth potential. If someone is able to touch that exported earth (e.g. exposed-conductive-parts of equipment, or maybe metalclad accessories) simultaneously with being in contact with 'true earth', they could be at risk. One would, where possible, seek to minimise such risks by bonding (to the earthing system) anything which might be at true earth potential, but that is not practical with a concrete floor (at least, not after it has been laid). However, if you're fairly confident (e.g. because of the DPM) that the floor is unlikely to develop a significant path to earth, then you might well conclude that the (extremely small) potential risk was not a concern.

As you are probably aware, the alternative would be to not use the TN-C-S at the outhouse, but to use a local earth rod there, and constitute the outhouse as a TT installation. The SWA armour would then be connected to the house earth at the house end (to protect the cable), but not to anything at the outhouse end (and you would not have/use an earth core in the CPC).

Kind REgards, John
 

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