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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.

 



 


Parents
  • 0
    True, and be careful, a mixed system with simultaneous  access can become the worst of both worlds.

    The area of influence around an electrode, in which most of the voltage drop occurs, be the electrode a rod or a patch, like the base of a container, is related to its longest dimension, and will be a few of them.


    If you have many containers over a wide area, it is not wise to link the TT earths together, as although it does lower the overall Zs, this matters not much when there is an RCD, and it can be better during fault if each container is only locally earthed, so it can 'ride the waves ' of any local bounce in ground potential, much like a ship bobbing in the sea.

    In the end, you want the door handle of the cabin, and the ground around it,  where the person holding that handle will have their feet, to be within a few volts of each other. What that voltage  is relative to some other far-away earth potential is less relevant.
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  • 0
    True, and be careful, a mixed system with simultaneous  access can become the worst of both worlds.

    The area of influence around an electrode, in which most of the voltage drop occurs, be the electrode a rod or a patch, like the base of a container, is related to its longest dimension, and will be a few of them.


    If you have many containers over a wide area, it is not wise to link the TT earths together, as although it does lower the overall Zs, this matters not much when there is an RCD, and it can be better during fault if each container is only locally earthed, so it can 'ride the waves ' of any local bounce in ground potential, much like a ship bobbing in the sea.

    In the end, you want the door handle of the cabin, and the ground around it,  where the person holding that handle will have their feet, to be within a few volts of each other. What that voltage  is relative to some other far-away earth potential is less relevant.
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