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Any similarity between an EVC point and a socket for a refrigerated cargo container?

AncientMariner
Unless things have changed since I retired in 2002, I am curious regarding the similarity between an EVC point and a socket for a refrigerated cargo container.


On board ship, refrigerated cargo containers were simply plugged in to the ships electrical system. 3-phase, 3-wire plus earth, so a 4-pole plug and socket. The acceptable voltage being 380 to 460v 60 or 50 Hz. Most ships being 60 Hz, but some I sailed on had been designed for possible MOD charter and were 50 Hz. (there were some dual voltage containers, ie for 3-phase 230 volt supplies which some ships had.)


The lowest power consumption was for frozen cargo, whereas cargo which was carried chilled or even warm, due to fresh air requirements rather than recirculation, resulted in higher power consumption.


Considering that the container was connected via 10 metre or so cable, this looks similar to an EVC connection?  In rough weather, I have experienced heavy seas over the deck causing cables to be ripped out at the container end and when the weather subsided, I found that the doors of a container full of French Fries were having some cooked on deck by a fizzing broken cable.


Circuit protection either three cartridge fuses or a MCB, never came across any RCDs. Some ships fed the sockets directly off the main 440v bus, so an earth fault on a container, usually the defrost heater, would show as an earth on the ship's main 440v bus, other ships had the luxury of a number of isolating transformers. A quick Google tells me that some ships can carry 500 refrigerated containers, some more. This explains why my last ship generated at 6.6 kV.


Containers held on the quay side were plugged into pillars and I guess the same for when containers were at their destination, or awaiting stuffing. I never saw one of these in those days https://catalog.eslpwr.com/wp-content/pdfs/s_3500-02.pdf but certainly looks serious.


Yet the requirements suggested in  http://digitalfizz.com/cargostore/wp-content/uploads/Reefer_Power.pdf of RCD protection and under volt release, seems less stringent to that for a EVC point?


Clive

  • 0
    Quite a lot of TNS private transformers do not have the NE link at the transformer, but rather at or very near the main LV switchgear, the star point is brought out via insualtor bushes and treated with the same care as the 3 LV phases.

    There are more cunning cases where more than 1 Xfrmer feed a common bus as well, and there may be more than 1 NE bond, or not, but I would still consider these to be an example of a PNB, personally, certainly not pure TNC-s unless there is an NE bond both at the Xformer and at the load end.

    The real defining questions are where do fault currents flow, and can you get neutral currents from the load circulating in the earthing as part of normal operation.


    I think rather like classifying sea creatures, we have created what seemed like enough categories, and then found out about extra variations that are not a neat fit to any of the rather arbitrary classifications.
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    ebee:

    Thanks fd from the for that Graham, I`m still digesting your comments.

    As an aside, a couple of years back I did an install on farm grounds fed from the farmhouse. The farm itself was fed from a pole pig in the next field say 20 metres away. It was the only customer on that Tx. Two wires (single phase) to the farmhouse and the N was split to N & PE in the service head and had an earth rod in the garden just where the supply entered the external wall then to the head. I decided it was TNC-S but perhaps it was TNS (PNB) ?




    Sounds like PNB but BS 7671 just calls it "TN-C-S (PME)" - some DNOs have (at least in the past) applied PME stickers to PNB supplies, but as G12/4 says, we should now consider them in the same way as far as the consumer's installation is concerned. It's definitely not TN-S as the protective and neutral functions are combined in the supply cable, especially if the customer has no control over the supply of other customers from that same transformer (and in this case single earth electrode).

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    Thanks Graham, I was thinking it was perhaps the PNB variant of TNS as only one consumer, only one earth point (in so far as I can see) therefore a transformer but with long leads rather than short ones in a substation. So from its one and only N it is earthed then sep from PE. If it was 3Ph we`d call it star point and it`s very near to consumer rather than at or very near to pole pig. I expect ask 10 contractors you`d get 11 different answers. LOL.
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    It's definitely not TN-S as the protective and neutral functions are combined in the supply cable, especially if the customer has no control over the supply of other customers from that same transformer (and in this case single earth electrode).



    Maybe I'm picturing this wrongly, but with a simple PNB arrangement (with both the only electrode and N-PE link at the single consumer's cut-out) how is the supply cable providing a protective earth function? The means of earthing is connected at the cut-out and the path from consumer's exposed-conductive-parts to that means of earthing doesn't pass along the supply cable. For sure earth fault current will flow back to the source along the supply cable's N - as it would flow back along the N conductor to the star point from the N-PE link on a pure TN-S system - apart from (possibly) larger lengths of conductors I'm not seeing a difference. For me a system is TN-C-S where the N-PE link is downstream of the (first) electrode connection (as then you have a combined conductor between the N-PE link and the electrode).


    I take the point that the DNO might change the arrangement at a later date - and add more electrodes to the N, but that's the case with most TN-S DNO supplies, but we don't say we shouldn't call them TN-S in the meantime. I'm not sure that adding more consumers without adding more electrodes would necessarily be a problem - indeed isn't there a problem with the ESQCR demand for two or more electrodes on a PME system if we start calling a single-electrode (PNB) system PME?


       - Andy.

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    AJJewsbury:


    Maybe I'm picturing this wrongly, but with a simple PNB arrangement (with both the only electrode and N-PE link at the single consumer's cut-out) how is the supply cable providing a protective earth function? The means of earthing is connected at the cut-out and the path from consumer's exposed-conductive-parts to that means of earthing doesn't pass along the supply cable. For sure earth fault current will flow back to the source along the supply cable's N - as it would flow back along the N conductor to the star point from the N-PE link on a pure TN-S system - apart from (possibly) larger lengths of conductors I'm not seeing a difference. For me a system is TN-C-S where the N-PE link is downstream of the (first) electrode connection (as then you have a combined conductor between the N-PE link and the electrode).


    I take the point that the DNO might change the arrangement at a later date - and add more electrodes to the N, but that's the case with most TN-S DNO supplies, but we don't say we shouldn't call them TN-S in the meantime. I'm not sure that adding more consumers without adding more electrodes would necessarily be a problem - indeed isn't there a problem with the ESQCR demand for two or more electrodes on a PME system if we start calling a single-electrode (PNB) system PME?


     


    First and foremost, it's not TN-S because this is a DNO arrangement, in accordance with the Distribution Code (which includes G12/4). It's not up to the installer or consumer to define what the earthing arrangement is ... where there is doubt, they must enquire from the supplier ... in this case, we are clearly dealing with PNB as stated in G12/4, and therefore it's the same rules as PME.


    From my side, however, the supply cable to the property has the Neutral split out at the service head (according to ebee's post)- there is no separate supply protective conductor - and therefore it's clearly TN-C-S. Other customers (if they are in the future served by the transformer) would also get two wires (or 4 if three-phase star), and have a protective conductor brought out of the Neutral at the Service Head, which then may be connected to an earth electrode by the DNO if they need it.


    I do accept, however, that this is little different to the arrangement of the mains switchboard of a private TN-S system that has a source of supply connected in the same way, the differentiator perhaps being there is no "service head".

  • Former Community Member
    0 Former Community Member

    gkenyon:



    It's definitely not TN-S as the protective and neutral functions are combined in the supply cable, especially if the customer has no control over the supply of other customers from that same transformer (and in this case single earth electrode).


     



    I'd say it definitely is TN-S if it's a single consumer - all that's happened is the neutral earth bond has been expressed out to the consumers installation (usually to keep the N-E Voltage offset low)  - there is no combined function of the neutral - it's the system neutral up to the consumer position and then it's earthed - it is not recombined after that point.


    We can call it different things depending on who owns that bit of the system - but the physics don't change at all.


    What rules a supplier wishes to impose on a consumer are a different argument, I'd say


    Regards


    OMS

     
     

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    OMS, I wouldn't disagree - but:


    (i) there is nothing stopping the supplier adding another customer; and

    (ii) who's duty to maintain the earth electrode?
  • 0
    Oh heck.

    What have I started.

    Graham, OMS, AJJ and MAPJ1.

    4 of the great and good on here not agreeing 100% gives mere mortals such as me no chance.

    Ebee - next time shut up!
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    ebee:

    Oh heck.

    What have I started.

    Graham, OMS, AJJ and MAPJ1.

    4 of the great and good on here not agreeing 100% gives mere mortals such as me no chance.

    Ebee - next time shut up!




    Part of the problem is that certain things used to be viewed in our standards (BS 7430 in particular) in a different way.


    Another consideration is that distributors don't always use the same standards ... and terminology.


    But I think we do agree in terms that a single customer supplied by a DNO from PNB differ only from the perspective that other customers may be supplied fron CNE cable, or wire overhead, whereas other installations (or buildings) supplied from the private transformer version would be supplied from the main switchboard in SNE cable as full TN-S; but that otherwise electrically, they are, from the purposes of most of the installation, little different to TN-S. Given the Distribution Code requirements, though, we are not permitted to treat the public supply (DNO) version of PME as TN-S.

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