DNO TN-C-S supply to an industrial unit building which contains ATEX zones

Patrick,

I don't have my copy of BS EN 60079-14 with me at the moment, but I can tell you that the standard does state that where a TN earthing system is used it shall be TN-S with separate protective conductor and neutral in the explosive atmosphere.

Hope this helps!

Hi Ross,

Thank you for your response.

So that's the DNO supply to the building must be TN-S?

Thanks again.

Patrick Benson said:

So that's the DNO supply to the building must be TN-S?

Or TT.

If the supply to the building is TN-C-S, it cannot become TN-S unless you involve another transformer and make a new earthing system. The reason is that these designations apply to the whole system.

In a building, typically you can't have more than one earthing system, because Regulation 411.3.1.1 requires that simultaneously-accessible exposed-conductive-parts to be connected to the same earthing system.

Hi,

Thank you for your comments, much appreciated.

So, a TN-C-S building supply earthing arrangement should not be used for the earthing arrangement in ATEX zones.

I'll have a good read of BS EN 60079-14 a.s.a.p.

Thanks again.

shall be TN-S with separate protective conductor and neutral in the explosive atmosphere.

While I agree that should be unambiguous (after all the TN-S designation is meant to describe the entire system, not just a portion of it) some standards (including some older versions of BS 7671) have described the usual form of TN-C-S as "the supply is TN-C and the arrangement in the installation is TN-S".

So it might be possible that that standard is trying to say the a TN-C-S supply is acceptable provided the installation "is TN-S" ... you could even read the existence of the "separate protective conductor and neutral in the explosive atmosphere" clause as accepting that combined conductors might exist somewhere in the system (hence TN-C in at least part).

Or it might indeed mean to say TN-C-S isn't acceptable at all ... as I gather is the case for petrol stations (the pumps area at least).

   - Andy.

shall be TN-S with separate protective conductor and neutral in the explosive atmosphere.

While I agree that should be unambiguous (after all the TN-S designation is meant to describe the entire system, not just a portion of it) some standards (including some older versions of BS 7671) have described the usual form of TN-C-S as "the supply is TN-C and the arrangement in the installation is TN-S".

So it might be possible that that standard is trying to say the a TN-C-S supply is acceptable provided the installation "is TN-S" ... you could even read the existence of the "separate protective conductor and neutral in the explosive atmosphere" clause as accepting that combined conductors might exist somewhere in the system (hence TN-C in at least part).

Or it might indeed mean to say TN-C-S isn't acceptable at all ... as I gather is the case for petrol stations (the pumps area at least).

   - Andy.

AJJewsbury said:

So it might be possible that that standard is trying to say the a TN-C-S supply is acceptable provided the installation "is TN-S" ... you could even read the existence of the "separate protective conductor and neutral in the explosive atmosphere" clause as accepting that combined conductors might exist somewhere in the system (hence TN-C in at least part).

I think the UK's view on this is probably illustrated in the APEA 'Blue Book' Guidance for Design, Construction, Maintenance and Decommissioning of  Filling Stations.

Electrical installations for hazardous areas is a specialist field, and if it's something one is new to, I would strongly recommend considering a course, such as CompEx, before carrying out work ... and also design (there are design courses) ... for such an area.

Thank you Andy.

That is an interesting interpretation.

In terms of any recommendation of not using TN-C-S, I assume the intent could be to mitigate the potential loss of the PEN conductor. That said, if there was a new on-site sub station supplying the building and the DNO supply was new, this may also potentially legitimize the use of TN-C-S as a PEN conductor fault may be considered to be very unlikely.

Thank you all for your responses, much appreciated.

Patrick Benson said:

In terms of any recommendation of not using TN-C-S, I assume the intent could be to mitigate the potential loss of the PEN conductor.

 Patrick Benson  It's not just Open-PEN that's the issue with using TN-C and TN-C-S in hazardous areas, but diverted neutral current which can and does occur even if the neutral is intact.

As I said in my last post, this is a specialist area.

Thank you for your comments, which are noted and appreciated.

Just to add that Earthing arrangements are remarkably varied around the world - in some parts (especially "multiply earthed neutral" (MEN) the norm) it's common to see TN-C not just to the installation, but within the installation too (often up to the final distribution board) - so international standards (and indeed national standards that are derived from them) the wording may well be trying to cover situations that the like of us wouldn't have even have thought of as a possibility.

I think the UK's view on this is probably illustrated in the APEA 'Blue Book' Guidance for Design, Construction, Maintenance and Decommissioning of  Filling Stations.

Thanks Graham - I'd remembered there was a "coloured book" for filling stations ... but my memory wasn't certain of the actual colour! I presume though that not all ATEX situations come under filling station rules though.

   - Andy.