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Transportable unit earthing system

Former Community Member
Former Community Member

I’ve been asked to test an industrial site spread across a couple of fields. There are several shipping containers used as offices, stores, and housing some control panels. The supply is tncs and likely pme.



 



The supply to most of the containers is protected by 30ma rcd and where it is not the final circuits within, on and adjacent to them are protected by an rcd in the dB within the container.



 



Presumably they count as mobile or transportable units, although there is no intention to ever move them about. I would have thought the risk really comes from them being made of steel rather than mobility.



 



717.411.4 says pme cannot be used except where under the control of skilled or instructed person and the suitability and effectiveness of the means of earthing has been confirmed.



 



How would you make a judgement as to the suitability and effectiveness of earthing in a case like this? I’d appreciate any suggestions thanks.

 



 


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    If you are not happy, the safest is to TT island the containers  and pretend they are caravans - the supply cable can be armoured and the armour connected to the PME earth, but  rod only for the CPC at the load end.  Equally, depending on how many of the risk markers above are on the low side you may decide that   to leave it PME is just fine



    Agreed ... but in forming the "TT island", you should consider the fact that separation is required from any buried metalwork connected to the PME earthing terminal. Conversely, this has the adverse effect that if you can touch any exposed-conductive-parts connected to the TT island, at the same time you are standing on ground affected by the rise of earth potential due to the broken PME CNE conductor, you haven't achieved anything, and you will simply have returned the touch voltage due to the PME broken neutral. BS 7430 provides suitable information to determine the required separation distances, and the effect of buried metalwork connected to the PME system on the surface potential. All of this is, of course, site-dependent.
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    If you are not happy, the safest is to TT island the containers  and pretend they are caravans - the supply cable can be armoured and the armour connected to the PME earth, but  rod only for the CPC at the load end.  Equally, depending on how many of the risk markers above are on the low side you may decide that   to leave it PME is just fine



    Agreed ... but in forming the "TT island", you should consider the fact that separation is required from any buried metalwork connected to the PME earthing terminal. Conversely, this has the adverse effect that if you can touch any exposed-conductive-parts connected to the TT island, at the same time you are standing on ground affected by the rise of earth potential due to the broken PME CNE conductor, you haven't achieved anything, and you will simply have returned the touch voltage due to the PME broken neutral. BS 7430 provides suitable information to determine the required separation distances, and the effect of buried metalwork connected to the PME system on the surface potential. All of this is, of course, site-dependent.
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