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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

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    Aboard ship you have what perhaps an ideal environment as far as "earthing" is concerned - i.e. one big steel structure - so there's only one earthing system (the ship) and everything is practically always at the same potential, whether you're touching the container or the "ground" (deck) or an adjacent items of electrical equipment - no voltage gradients around electrodes and (presumably) none of the dreaded PME - so most of the complications with shoreside EV charge points aren't there. If you don't have unqualified people milling about you can perhaps dispense with RCDs (or EV style pilot wire checks) too. So I'd quite believe that all that's really needed is an MCB or fuses.

      - Andy.
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    Does every container arrive on board with an electrical test certificate?


    Andy Betteridge
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    I really liked that one Sparkingchip!


    Actually I was once asked to certfy a container thingy, It was to be hired out for use as an onsite works canteen, rest room & WC for say 6 people max.l did the viz, dead tests then hooked up to the nearest supply to live test inc RCDs

    Did the cert listing Dead tests only with a big note "Must be PIRed by competant blah blah blah before use" and similar labels on consumer unit and intake commando.


    I bet it justs gets plugged in and used each time with no I & T.


    I also advised the hirer that it must be certified prior each hire
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    Difficult to say whether they arrive on board with an electrical test certificate. All containers are supposed to have certificates and if they are reefer containers this will include the electrics, and should be subject to periodic inspection, but......


    When you are unloading/loading thousands of containers in a port (in the space of less than a day) who is checking the certification? This is probably eased with modern systems using blockchain tracking of each container but checking each one is still a challenge.


    With regard to the use of three wire/four wire distribution for the reefer sockets, some companies insist on it (e.g. Maersk - but their ships have a significant reefer capacity, numbered in the thousands, though they are cagey about just how many) while others don't. The basic deciding factor is how much does it cost to find an earth fault and fix it (during the life of the ship) compared to how much does it cost to install transformers. The fewer reefer sockets the less likely to have transformers.


    RCDs are very uncommon on ships since most have IT systems which means the RCD may not provide any protection. Reefer sockets fed via a transformer with an earthed neutral on the secondary side provide an exception, but as Clive says, the most common fault is through sea-water ingress (or more likely green sea damage) resulting in an earth fault. The plugging and unplugging are not carried out live so there should be no danger to personnel. It is further complicated by the fact that you would need the RCDs to be out on deck (or at least in deckhouses) and therefore need to monitor the supplies to make sure an RCD trip didn't result in the cargo being ruined. If you have one RCD per circuit then that is a lot of detection while if you have one RCD per block of sockets I would be surprised if you were able to get it to stay in. The current arrangement, with large MCCBs used to feed multiple sockets having trip detection means that there is some testing to find the faulty circuit on a trip, but it is probably a block of 12 or 24 reefers.


    On the whole I think that there is not a great deal of commonality between this and EVC points, since EVC points are accessible to the general public, they are plugged/unplugged with much greater frequency and generally by non-trained personnel (though on a ship the level of training of the personnel may be sometimes questionable), and the earthing is not the same since a container is in direct contact with the ship's metal structure while an EV is sitting on insulating rubber tyres.


    Alasdair
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    Well, yes and no.


    18th Edition currently does not permit connection of a Mobile & Transportable Unit (Section 717) to PME at all, but does permit connection of an EV on charge to PME under some circumstances.


    The BS 7671 requirements for other "shore" connections for containers, as they appear to be labelled in some places, would be use of H07RN-F in the connecting lead, and industrial outlet of the fixed installation protected by a 30 mA RCD (regardless of the current rating of the unit at present).
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    Certainly when a refrigerated container in onboard the ship, I would expect it to connected to the ships hull by the earth core in the cable and hopefully by contact through the container's mounting points. Although most of the ships that I sailed on had hatch lids. so say 9 containers stacked down guides below the hatch lid and perhaps 5 or 6 on top. The reason I said hopefully, is that the hatch lid sits on a rubber strip around the hold, so perhaps a single container on a lid could conceivably not have a connection via contact between the lid and hatch coaming. Thinking back, I have actually seen a single container on a hatch lid; this on a ship where the below deck containers were either general of if refrigerated, they used a blown air system from air chillers in each hatch. 


    Actually what I was trying to get discussion on was when the container is off the ship and sitting on the quayside. Many container yards where they would be plugged in are surfaced with brick paviors, just like my driveway! OK the container is metal to brick rather than rubber tyre to brick, so if we have two containers not touching but close enough for a person to place a hand on each, should due to rough handling the earth core have pulled out of one of the plug terminals and there is also an earth fault we would seem to be relying 100% on an RCD somewhere?  


    So going back to an EVC, if the supply to the house where the EVC point is mounted, is TN-S is anything more than an RCD required for shock protection? 

    Clive

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    AncientMariner:


    Actually what I was trying to get discussion on was when the container is off the ship and sitting on the quayside. Many container yards where they would be plugged in are surfaced with brick paviors, just like my driveway! OK the container is metal to brick rather than rubber tyre to brick, so if we have two containers not touching but close enough for a person to place a hand on each, should due to rough handling the earth core have pulled out of one of the plug terminals and there is also an earth fault we would seem to be relying 100% on an RCD somewhere?  




    Agreed - it's like that with mobile/transportable units.






    So going back to an EVC, if the supply to the house where the EVC point is mounted, is TN-S is anything more than an RCD required for shock protection? 

    Clive

     




    The simple answer is, no, nothing more than an RCD would be required, but for a house it's not that simple. If you're sure the supply is definitely TN-S, and is guaranteed to remain so, protection by RCD is definitely be OK, and none of the messing around with the requirements for PME supplies in 722.411.4.1 applies.


    However, is the supply really TN-S? determining whether the supply is really TN-S (and will remain so) is difficult these days. The problem is that DNOs carry out repairs on TN-S networks with CNE (combined neutral and earth) cable, or occasionally make a CNE joint with earth electrodes. This is explained in Sections 4.5 and 5.1 (12 and 18) of ENA Engineering Recommendation G12/4.


    Therefore, the published guidance from the IET recommends Regulation 722.411.4.1 for PME conditions is applied to TN-S from a public supply, and this would be the costs for most houses, unless the DNO confirms it's really TN-S and won't change (typically, therefore, only TN-S from a privately operated dedicated transformer is considered TN-S).

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    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.

    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
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    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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    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) ?

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