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

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

Parents

  • ebee:

    That is a very interesting document.

    Also it seems to me that PNB is TNS as has been discussed a few times on here, yet PNB to mutiple users is treated as PME too. Again interesting.




    PNB earth terminals to a single customer must also be treated as PME. See Section 4.11 (pages 17 and 18) - the maximum number of consumers is 4 (and it's unstated that the minimum is, obviously, 1, although I guess a spare PNB could exist with no customers). The statement in the past para of 4.11 on page 18 holds true for 1 customer.


    So, I assume you are talking about what some term "PNB" in an installation with a private transformer or generator, where the connection of the system with Earth is made somewhere downstream of the transformer or generator?


    Couple of considerations:



    • In a TN-S system, the system must be earthed at one point only. If there are multiple sources, the connection with earth at, at least, one of the sources must necessarily be "remote" - does that make it anything other than a TN-S system at any point?

    • the term PNB is is not used at all in BS 7671. Figure 9A shows what is called a multiple source TN-C-S system which is perhaps quite close to what some term "PNB" - but it's still called "TN-C-S" in BS 7671. Similarly, Fig 44.9 - whilst in the example shown, Source 1 might be classed as "TN-S", what is the Source 2 classed as, say if Source 1 were disconnected (the PE is shown as provided, but of course the N-E bond is remote ...).

    • The current version of BS 7430 does not include the terms "PNB" or "Protective Network Bonding", it too only recognises TN-C-S and PME.

    My conclusion are:



    • Where this arrangement is used in public supplies, it's definitely PNB as defined in G12/4, and PME conditions apply to the consumer installations.

    • Where all sources of energy are "private", the installation may be considered TN-S from the main LV switchboard onwards (differentiator being that the installation operator is responsible for the earthing). The portions where earthing of the exposed-conductive-parts of the transformer or generator is carried out via a Neutral which is earthed remotely are perhaps more correctly classed as TN-C-S (but not PME). This type of system should really not be referred to as "PNB" and is very different from PNB supplied by a distributor, who is bound by ESQCR and the Distribution Code including G12/4.


    I wonder if the world might be a better place if TNC-S was not allowed. But there again we can never be sure that N & PE are never joined anywhere either by foolishness of others or accidently I suppose in the real world




    I'm sure a lot of people have shared this thought.

Reply

  • ebee:

    That is a very interesting document.

    Also it seems to me that PNB is TNS as has been discussed a few times on here, yet PNB to mutiple users is treated as PME too. Again interesting.




    PNB earth terminals to a single customer must also be treated as PME. See Section 4.11 (pages 17 and 18) - the maximum number of consumers is 4 (and it's unstated that the minimum is, obviously, 1, although I guess a spare PNB could exist with no customers). The statement in the past para of 4.11 on page 18 holds true for 1 customer.


    So, I assume you are talking about what some term "PNB" in an installation with a private transformer or generator, where the connection of the system with Earth is made somewhere downstream of the transformer or generator?


    Couple of considerations:



    • In a TN-S system, the system must be earthed at one point only. If there are multiple sources, the connection with earth at, at least, one of the sources must necessarily be "remote" - does that make it anything other than a TN-S system at any point?

    • the term PNB is is not used at all in BS 7671. Figure 9A shows what is called a multiple source TN-C-S system which is perhaps quite close to what some term "PNB" - but it's still called "TN-C-S" in BS 7671. Similarly, Fig 44.9 - whilst in the example shown, Source 1 might be classed as "TN-S", what is the Source 2 classed as, say if Source 1 were disconnected (the PE is shown as provided, but of course the N-E bond is remote ...).

    • The current version of BS 7430 does not include the terms "PNB" or "Protective Network Bonding", it too only recognises TN-C-S and PME.

    My conclusion are:



    • Where this arrangement is used in public supplies, it's definitely PNB as defined in G12/4, and PME conditions apply to the consumer installations.

    • Where all sources of energy are "private", the installation may be considered TN-S from the main LV switchboard onwards (differentiator being that the installation operator is responsible for the earthing). The portions where earthing of the exposed-conductive-parts of the transformer or generator is carried out via a Neutral which is earthed remotely are perhaps more correctly classed as TN-C-S (but not PME). This type of system should really not be referred to as "PNB" and is very different from PNB supplied by a distributor, who is bound by ESQCR and the Distribution Code including G12/4.


    I wonder if the world might be a better place if TNC-S was not allowed. But there again we can never be sure that N & PE are never joined anywhere either by foolishness of others or accidently I suppose in the real world




    I'm sure a lot of people have shared this thought.

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