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Can an extraneous conductive part be used as a legitimate earth fault path?

DG

Have been having a discussion with an equipment manufacturer who is supplying their OEM electrical-package pre-wired within a steel container. We are installing an incoming supply to marked terminals with agreed circuit protection. Their internal circuits are marshalled at an earthing terminal fixed to the steel chassis, and the steel chasis has clearly marked earthing points where we plan to bond an earthing conductor between these points and the site earthing system. 

I had thought this was to provide bonding of extraneous conductive parts (steel chasis) only, and that an main earth terminal would be present, marshalling their internal equipment cpc's and existing small earth terminal mentioned above, for us to bring a cpc to.

No MET is present/designated and when challenged, they have said that the steel container is to provide earth fault current path and that no main earth terminal is required as the earthing points on the steel work are the intended means of earthing. Is this correct? Shouldn't the internal earthing terminal should have a means of earthing via a cpc or similar and not the actual steel equipment frame? 

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  • 0

    Hi DG

    If I understand correctly, your supply circuit lacks an earth terminal connection and only includes line(s) and neutral terminal connections. The internal CPCs are connected to the frame, and there is an earth connection point on the frame. Is this accurate?

    -Andrew

  • 0 in reply to AMK

    That is not quite how I understand it. The OP suggests that the steel container will be connected to the supply earth - presumably TN-(C)-S. Now all you need for a CPC is a bit of green and yellow to the nearest convenient point.

    Whilst the laws of physics seem to allow this, good connections to the container are essential. Whilst preservation of the earthing conductor is essential to this, the same applies to any installation.The only difference is that the container could be quite well grounded to terra firma and if the supply earth is lost, you now have some form of TT earth, which could be at a different potential.

    As Mike says, proceed with caution!

  • 0 in reply to AMK

    Hey AMK, yep that's right, so then i wanted to confirm whether it was acceptable to rely on the steel frame where no main earth terminal provision is made to allow the incoming supply to also come with a cpc and bring all the internal equipment / circuit earths together . . . 

    it seems that   refererences regarding extraneous conductive parts does cover it then, as 543.2.6 does clearly lay out allowance for an extraneous conductive part to be used as a cpc provided that i - iv conditions are satisfied. 

  • 0 in reply to AMK

    Yes that's accurate 

  • 0 in reply to DG

    Just to clarify the clarification as it were, the steel container can (probably) be used as a c.p.c., but that's not the same as saying it can be used as a means of Earthing - you'd still need a c.p.c. from the supply (if TN) or local electrode (if TT) to the container.

      - Andy.

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  • 0 in reply to DG

    Just to clarify the clarification as it were, the steel container can (probably) be used as a c.p.c., but that's not the same as saying it can be used as a means of Earthing - you'd still need a c.p.c. from the supply (if TN) or local electrode (if TT) to the container.

      - Andy.

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