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

In BS 7671 terms it is permitted to use extraneous-conductive-parts as protective conductors (see 543.2.1 (vii) ... with a few conditions (see 543.2.6) which pretty much boil down to it being electrically adequate (impedance, fault withstand, etc.) and reliable (not likely to be removed by other trades etc. or nullified by corrosion) On the face of it shipping container could well be fine.

Depending on the nature of the contents on the container, it might not be a BS 7671 matter though - if the whole thing counts as a machine or other assembly that falls under a specific equipment standard, other rules might apply.

   - Andy.

It also depends where the container itself is to be dropped. If it is outdoors on its skids it will haf heartedly serve as its own electrode, but if being supplied TT, as a lot of outdoor kit like portakabins is, it will need its own electrode. (2 wires cheaper than 3, and L-N reversal is less dangerous if you are not using N to derive a local CPC/earth.) I'd expect provision such as large studs and nuts with anti-shake washers for an electrode to be connected as well as to terminate any CPC of the incomer.

If the container is to be inside a dry structure like a hanger with a concrete floor, then  to use the building supply CPC alone is more sense, but it needs adequate cross section for the maximum fault current.

If for example things like 13A sockets get their CPC by a fly lead to the body of the container, rather than dedicated wire, while OK, then it needs to be mechanically sound (welded nuts or similar, not friction fit ) -when done well it is possible to use the container as MET and also CPC for all internal circuits, however, personally I'd be inspecting pretty closely if that was the case, and looking for lock-washers and giving everything a jolly good tug before accepting that approach.

Equally, if the kit in the container has externally connected services, like water cooling systems, or for the maximum complexity, the container includes the option of a diesel genset as well, then there are multiple requirements to satisfy, such as those for main bonds as well as the actual CPC cross-section, and the cpc and MET may need a far larger cross section, than the normal load alone suggests.

So, in summary, it can be OK if properly done, but more info is needed,  especially if it is one of the harder corner cases.

Things like this https://marshallgroup.com/en/land-maritime/deployable-infrastructure are only containers too, but wiring a lot of those together gets very complex very quickly.

Mike.

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

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!

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 AJJewsbury  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. 

Yes that's accurate 

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.

Yes for sure, there will be a dedicated means of earthing (cpc) from main earthing system to the steel frame container connection .
thanks again ! 

In my experience, the absence of a dedicated terminal or bar is atypical in modern equipment. In contrast, older distribution boards, such as those manufactured by Otterman, typically featured a single stud for connecting CPCs. Upon reflection, a specialist fixed wiring company conducted an EICR and identified this configuration in the older distribution boards as a C2 classification. However, this is a slight digression from the main point.

You may have heard me reciteing my well worn saying at various presentaions I have done that says, " Earthing is not bonding and bodning is not earthing" followed up with "we dont earth gas pipes." There are 4 types of Protective Conductor.

In my view the steel container should be bonded to the MET with a main protective bonding conductor.

You can loop off one bonding conductor to another which I think without looking BS 7430 says has to be unbroken. I have certinly specified taking a bond off a welded stud on structural steel work on to a gas installtion pipe to avoid running a very long bond from the front of an indoor foorbal pitch to a gas pipe entering at the rear of a building.

Then we have Regulation 411.3.1.1 that a circuit protective condcutor to be run to each point in wiring and each accessry. Note is says a circuit protective condcutor not a bonding condcutor.

Any comptent designer, and every installtion tempoary or permanent has to be designed, will specify a CPC shall be run out with each circuit for me I would also say for singles in trunking the CPC shall me the same CSA as the live conductor.

Why not just design it correctly in the first place rather than get absolution using an individual interpretaion of a particular regulation.

JP