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Double wound safety transformer for EV supply.

SL1
Hi everyone, I have only posted once before so thanks to anyone who replies!


I am following on from the earlier "70 volt PEN conductor not allowed to exceed post", and looking into supplying a client with an electric vehicle power supply from a three phase isolating transformer BS 7671 722.413 (1.2): " The circuit shall be supplied through a fixed isolating transformer.."


The general consensus seems to be that an external IP box with an RCD (Type B) and a tethered lead is the standard to follow, and this may be the only option with a 230 volt domestic supply, but why not use a 3 phase 400 volt step down or tapped, safety double wound isolation transformer in a standard 100 -200 ampere or above industrial units/sheds?

( Subject to load and diversity).


The answer often stated when I have asked sparks/engineers is that in-rush current are too high but a type D CB BS 60898 will 'let through' the in-rush ( the transformer manufacturer agrees), and will still give at 5 seconds- (final circuit exceeding 32 A) 0.44 ohm EL ( 10oC) , so is achievable in many situations local to Birmingham.


I was then going to run a fused cable out to an external isolated IP 65 box with a Type 2 socket to IP44 or above ( 722.55.101).


Isn't it better to engineer a solution to the upcoming electric charger deluge, rather than buying (insert well known manufacturer name here), and lots of single phase loads usually dumped onto L1?


I would be interested in any thoughts or problems you may consider....





  • 0

    is not very tough so I wouldn't use it underground as per the You Tube video ZoomUp posted.



    Actually SWA isn't that touch either -  it's easily pierced even by a simple garden fork never mind any kind of mechanical excavator - it's read advantage underground is from a surrounding c.p.c. that'll activate ADS rather than its physical toughness. In some ways, a copper rather than steel 'armour' might work better - which is actually what most modern buried DNO cables have.

     

    Tyre Company workshops wired in a black version of YY cable. Not so much innovation as cutting corners I think, as they basically wired the place in flex.



    I suspect it's more a cause of someone using continental contractors and them just doing what they usually do - German factories are routinely wired in YY etc (submains and all) - all according to their standards (which are normally held in reasonable regard). It begs the question: what's wrong with using "flex" for fixed wiring. We used to have a reg that prohibited it - but that dated from the day when "flexible cord" was a flimsy unsheathed composition of fine wires, cotton and rubber and nothing like as tough as the fixed wiring cables of the day - but these days when flex is plastic insulated and sheathed and so almost identical to many types of fixed wiring cable, other than the conductors are stranded. The reg has long since been deleted, but the memory of it seems to persist in folk memory for some reason. If anything using flex is normally more expensive (because of the extra work drawing many more strands and then twisting them together) - so it's hardly as cost cutting measure. It can however be quicker and easier to install and often makes better & more reliable connections in some common terminal types (or can be ferruled for screw tunnel terminals). So I'd make the case for considering it.


       - Andy.
  • 0
    According to this recent article   https://www.voltimum.co.uk/articles/new-testing-and-certification-scheme

     SY/YY cable is soon going to be eligible for BASEC approval which might lead to the development of a new British Standard for it.
  • 0
    Well that would be sensible, there is enough of it about, and it actually works perfectly well, and experiments suggest that does not suddenly become a risk on import to the UK, and the rumours that coils of DIN  approved cable without a corresponding BS no. will  rise up, serpent-like spitting fire and sparks and strangle the Englisch users do seem to be unfounded.  Though it may be rather more fun to use if it did...

    From my side it would also be Ideal if they could  look to certify  a few LV style cables for use at voltages up to  690-0-690, and we could get more power from one end of site to the other in thinner cable- it would be easier to concede that modern insulation and ADS devices have improved in the last 50 years, and add some columns to allow higher voltages,  than try to impose the barycentric designs that nearly made it into the 18th.
  • 0

    AJJewsbury:




    is not very tough so I wouldn't use it underground as per the You Tube video ZoomUp posted.



    Actually SWA isn't that touch either -  it's easily pierced even by a simple garden fork never mind any kind of mechanical excavator - it's read advantage underground is from a surrounding c.p.c. that'll activate ADS rather than its physical toughness. In some ways, a copper rather than steel 'armour' might work better - which is actually what most modern buried DNO cables have.

     

    Tyre Company workshops wired in a black version of YY cable. Not so much innovation as cutting corners I think, as they basically wired the place in flex.



    I suspect it's more a cause of someone using continental contractors and them just doing what they usually do - German factories are routinely wired in YY etc (submains and all) - all according to their standards (which are normally held in reasonable regard). It begs the question: what's wrong with using "flex" for fixed wiring. We used to have a reg that prohibited it - but that dated from the day when "flexible cord" was a flimsy unsheathed composition of fine wires, cotton and rubber and nothing like as tough as the fixed wiring cables of the day - but these days when flex is plastic insulated and sheathed and so almost identical to many types of fixed wiring cable, other than the conductors are stranded. The reg has long since been deleted, but the memory of it seems to persist in folk memory for some reason. If anything using flex is normally more expensive (because of the extra work drawing many more strands and then twisting them together) - so it's hardly as cost cutting measure. It can however be quicker and easier to install and often makes better & more reliable connections in some common terminal types (or can be ferruled for screw tunnel terminals). So I'd make the case for considering it.


       - Andy.

     




    Hi Andy,Re: Continental contractors-

    No the guilty party was a local company although they may have foreign workers.


    I ahem heard the argument before that you could -in theory- wire a house in flex, but would you do it?  The testing bodies frown upon using any flexible style cables until they are to BASEC standards, and although many may be in the pipeline until a cable is certified it shouldn't be used at least in a final circuit, at present they are listed as: "These cables have a wide variety of uses in construction and for powering appliances and equipment."  ( Basec) link - https://www.basec.org.uk/cables/.

    But they are not listed as distribution or final circuit cables (yet).


    I'm not sure about your comments that you can pierce a steel armoured cable as easily as a flexible SY etc. The outer seating of an armoured is much tougher, and then there is the steel armour itself and then the inner insulation and then the cable insulation. A pretty good cable by any standards. The SY has a thin over-plastic sheath, then the stranded braid, then the grey plastic, no match in my book.

    I'm not saying I wouldn't install YY and SY, we do for cabinet and motor tails, but not as a final circuit until it is allowed to be included on an EIC ( by IET, NIC or ECA approval I mean). At the moment it would be a non-compliance.


    As for connecting up and terminating circuits, a lot of the push fit connections that are now employed by manufacturers for terminations ( LED down lights etc) are made for solid conductors.

    Loose strands leaking from  terminations of flexible cables are not uncommon when I inspect installations.

    Would using flexible cables encourage, indirectly, unskilled electrical works?

    I know that PVC/PVC is available at DIY sheds, but using 'flex' is a further step down the line to legitimising non-skilled installations.


    Regards



    Simon

  • 0

    I ahem heard the argument before that you could -in theory- wire a house in flex, but would you do it?



    I would a touring caravan (both distribution & final circuits) - because BS 7671 specifically demands it's done in flex - and all the normal compliance with standards & suitability rules still apply as normal.

     

    I'm not sure about your comments that you can pierce a steel armoured cable as easily as a flexible SY etc.



    I didn't mean to imply that SWA wasn't a bit tougher than SY etc - just that it's toughness (the physical armouring) still isn't good enough for proper mechanical protection underground - the mechanism that protects us is ADS rather than mechanical protection. The extra mechanical protection is to some extent wasted. I've done the experiment with a scrap length of 16mm2 SWA and a garden fork myself and the tine pushes apart the strands of the steel armour quite easily using little more than normal digging force in stony soil.


       - Andy.
  • Former Community Member
    0 Former Community Member

    Zoomup:

    More E.V. charging safety concerns.

    https://www.dailymail.co.uk/sciencetech/article-7086061/Electric-vehicle-drivers-UK-risking-death-electrocution-charging-car-home.html


    Z.




    Amongst the dangers claimed is fire risk from daisy chaining extension leads which is technically unconvincing. Worries about daisy chaining seem to originate from countries with unfused plugs where chaining multi-way leads can allow overloads. Coiled cables is a much more likely fire hazard and isn't even mentioned. The most significant danger in daisy chained leads is probably indoor-rated connectors in the middle of the chain getting rained on and associated shock risk but it's hardly unique to electric cars. Take a look outside at Christmas time and you'll see plenty of chained extension leads in people's front gardens, sometimes with plastic bags, tape or upturned buckets for "weather protection", often without.


    As Simon said the point of a 13A domestic socket is that unskilled people can plug things into them and have a reasonable expectation of safety. Electric cars are hardly the only thing an average person might plug in with a long-term 10 A load, which basically leaves you with worries about class 1 appliances outdoors on PME systems.

  • 0
    Perhaps cars should come with a power of opportunity adapter kit containing a lead ending in two insulated croc clips, some thick gloves, a jemmy bar and a selection of keys for lamp posts and street boxes and so forth, and maybe some wire cutters.  The ideal Christmas present for the intrepid EV owner. ?


    There is a disconnection in the  level of safety we expect for a traditional car where we happily use fuel pumps and jerry cans, when compared to the far higher level we expect for indoor electrics with ADS and all the rest.
  • 0

    mapj1:

    Perhaps cars should come with a power of opportunity adapter kit containing a lead ending in two insulated croc clips, some thick gloves, a jemmy bar and a selection of keys for lamp posts and street boxes and so forth, and maybe some wire cutters.  The ideal Christmas present for the intrepid EV owner. ?


    There is a disconnection in the  level of safety we expect for a traditional car where we happily use fuel pumps and jerry cans, when compared to the far higher level we expect for indoor electrics with ADS and all the rest.




    ?? This will probably happen if it hasn't already.


    Perhaps some form of utilisation of the street lighting system could be used in some way?


    I wonder if the REC or DNO have got something legitimate in the pipeline for this?


    A readily available  power supply infrastructure, I wonder if the the power supplied into the columns could be 'beefed up' and separate enclosures with RCD/Charging etc kit added?



    Regards



    Simon


     

  • 0

    mapj1:

    Some of us are quite happy to specify and use SY and similar in situations where we think it is the best technical solution. It  is important to realise that the braid is really best used as an EMC thing and not intended either as armour against attack by sharp objects nor are the fine strands good as a high current CPC. But for flexibility and shielding, ideal.

    Equally there are those of us who consider BS7671 to be a guide, and not the last word in wiring standards, though I suspect to many others that is tantamount to heresy.  However, I mainly work in a research environment, and as that is always pushing at non standard solutions, it probably colours my thinking somewhat.




    Also, note that most SY cable is not "submersible" or waterproof, and may not be UV stable.


    Therefore, even if in a dedicated duct, if the cable sits in water for some time, its insulation resistance suffers.

  • 0

    AJJewsbury:




    looking into supplying a client with an electric vehicle power supply from a three phase isolating transformer BS 7671 722.413 (1.2): " The circuit shall be supplied through a fixed isolating transformer.."



    I'd take a step back and think carefully about that approach first. Isolating transformers can be used in a couple of different ways - either to provide a separated supply (i.e. no deliberate connection to earth as per section 413) or with one pole of the secondary deliberately earthed (to form a local TN system as per 411).


    The trouble with the first approach is that many, if not most, electric cars check for a sound connection to earth (presumably a L-PE loop in practice) - and flatly refuse to charge if it's absent or has a high resistance. So a section 413 approach, although all very well and good and very safe in theory, probably isn't going to actually work in practice.


    The other approach of turning the secondary into a TN system (normally TN-S) means you need to obtain an earth connection to connect the secondary to - in theory that could be the a local electrode or even the c.p.c. of the primary circuit. If it's a PME supply then using the primary circuit's c.p.c. is obviously out, which leaves a local electrode (and the need to keep the EVSE system out of reach of anything connected to the PME earth)  - but if you're going for that you might as well just TT the EVSE and not bother with the expensive transformer at all.

     

    Agreed - some DNO's will require a minimum physical separation underground between metalwork (and electrodes) connected to the PME, and metalwork (and electrodes) connected to the new TN-S or TT system - this may exceed 3.5 m depending on the DNO.



    There have been suggestions along the lines of and isolating transformer with the EVSE's PE conductor connected to the secondary "neutral" but without any kind of Earth connection - but that approach is completely outside of BS 7671 methods and has all kinds of potential risks that would need to be addresses (e.g. the secondary PE being capacitively coupled to the primary and so floating up to a hazardous voltage) and you wouldn't be able to describe the installation as complying with BS 7671.


        - Andy.

     




    This is not a suggestion, but a recommendation of the IET CoP for EV Charging Equipment Installation (1st to 3rd Edition).


    This arrangement (provided it has a 30 mA RCD at the output of the transformer, and, importantly, is used to supply one vehicle only) can be taken as implementing separation described in 722.413, and 413 in general, but protection by the RCD is strongly recommended as it's required for all EV charging outlets anyway (except for DC outlets), and to provide disconnection if the mechanical integrity of the charging cable is broken.


    Thought on this being in conformity with 413: if we are talking about 413.3.3, live parts of the separated circuit in this arrangement are not connected to another circuit, Earth (with capital E), or a protective conductor of the electrical installation ... this is reinforced, almost by repetition, in 413.3.6. Worth also noting that 413 only requires simple separation.


    I agree it would be handy for BS 7671 to spell this out, as it's a little confusing, but essentially because it's application of the general rules, that would be guidance. As such, installers do have the IET Code of Practice to refer the arrangement to in the mean-time.

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