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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....





  • Former Community Member
    0 Former Community Member
    Like many people here I have yet to see a convincing technical justification for a vehicle body being a higher risk thing to connect to a TNC-S derived earth than a metallic illuminated bus shelter. Especially the kind of bus shelter popular around here with metal seats incorporated into the design ensuring that its users spend an extended period of time with their backsides connected to the metalwork and their feet firmly on the ground. They are both similar sized objects made mostly of painted steel, they're both usually on poorly conducting surfaces, what's special about the car? Or are both dangerous and bus shelter electrical installations should really be class 2 as telephone box lighting is/was? Perhaps the car manufacturer's are fundamentally at fault making the cars class 1 in the first place, after all other outdoor appliances have long been class 2.


    As a side note high-power AC charging seems to be dying, the main model to support it was the Renault Zoe and they have recently announced that future models will be restricted to 32 A per phase but 100 kW DC charging will be introduced in its place.
    https://insideevs.com/news/342860/renault-ends-sale-of-zoe-q90-with-43-kw-ac-charging-capability/
  • 0

    RichardCS2:

    Like many people here I have yet to see a convincing technical justification for a vehicle body being a higher risk thing to connect to a TNC-S derived earth than a metallic illuminated bus shelter. Especially the kind of bus shelter popular around here with metal seats incorporated into the design ensuring that its users spend an extended period of time with their backsides connected to the metalwork and their feet firmly on the ground. They are both similar sized objects made mostly of painted steel, they're both usually on poorly conducting surfaces, what's special about the car? Or are both dangerous and bus shelter electrical installations should really be class 2 as telephone box lighting is/was? Perhaps the car manufacturer's are fundamentally at fault making the cars class 1 in the first place, after all other outdoor appliances have long been class 2.


    As a side note high-power AC charging seems to be dying, the main model to support it was the Renault Zoe and they have recently announced that future models will be restricted to 32 A per phase but 100 kW DC charging will be introduced in its place.
    https://insideevs.com/news/342860/renault-ends-sale-of-zoe-q90-with-43-kw-ac-charging-capability/




    The answer is relatively simple, and lies in Table 6.2.15 of G12/4 (page 32) - it's to do with the load, and the required consumer earth electrode to help prevent earth potential rise in the event of a broken CNE conductor in the PME distrubution system. Additionally, the structure of the shelter may be able to act as a form of earth electrode depending on construction.


    See also section 7 of the IET's Code of Practice for EV Charging Equipment Installation, 3rd Ed - Table 7.1, page 48, and section 14.5, pages 98 and 99, of IET Guidance Note 5 Protection against electric shock, 8th Edition.

  • 0
    The use of a safety isolating transformer to provide an isolated supply seems to be being criticised here as unsafe in some way. An isolating transformer construction prevents any significant secondary voltage to any part of the primary circuit by class 2 insulation and an earthed screen between windings, so I cannot understand Andy's comment about capacitive coupling. An RCD on the secondary allows the neutral before the RCD to be connected to the earth connection of the car, as the only potentially dangerous scenario would be contact with both the car body and the cable live conductor, or an earth fault between the cable live and real earth AND contact from the car to earth by a person or livestock.  Both of these would trip the RCD, and must be considered as safe as any other equipment outdoors. A broken CNE would not be dangerous as far as the car is concerned, but could be at any other point in the installation where the installation earth and real Earth could both be contacted, although in theory this should not happen. Of course contacting BOTH live conductors in the charge cable would be just as dangerous as anywhere else.


    I have commented a number of times on the design error of cars, they should be class 2 and then all these problems go away at very low cost. I am also somewhat dismayed that a DC tripping RCD is needed for AC mains charge supplies as I see no way that a DC danger could occur, at least not if the batteries are isolated from the charge supply. None of the suggested supply arrangements are safe with PME supplies where the neutral is not to be trusted, and the COP is a pretty bad job in many domestic circumstances, where the car may be charged in or close to a garage with significant metalwork connected to the PME earth.

  • 0
    David,


    There has been, in the Forum, some debate as to whether providing the connection between the outgoing neutral and the earth contact on the outlet to the vehicle on the secondary complies with 413.


    Provided there's no connection to Earth (with capital E), or a protective conductor of another circuit, this should meet 413.


    There is, however, an unlikely fault scenario, if a cable is used to connect the output of the transformer to the vehicle. If you have a nail on your shoe, which penetrates the Line conductor of the secondary, but does not touch any other conductor on the secondary, and makes contact with Earth, there is a possible touch voltage of U0 to the "would have been exposed-conductive-parts of the vehicle - but aren't because it's a separated system". In all likelihood, the current flow through the body as a result would be quite low, but its magnitude can't be guaranteed. The RCD on the secondary is there to operate precisely in this condition.



    The situation that does arise, however, is high inrush current of the rather sizeable transformers. Yes, this can be overcome, but of course costs are now increasing, and we're competing against "forget it, I'll just plug it in to a socket-outlet", which of course is far less practical, and far less safe, but doesn't have the price tag attached.



  • 0

    Provided there's no connection to Earth (with capital E), or a protective conductor of another circuit, this should meet 413.



    I can't see how connecting the EVSE's PE to a separated live conductor complies with section 413 at all. 413.3.3 demands that live parts of the separated circuit are not connected at any point to another circuit or to Earth or to a protective conductor - as I read it that's any protective conductor, not just protective conductors of other circuits.


    Graham's wording seem to align more with 418.3.4 than 413 (which in a way would make more sense for a situation where we have multiple items of class I equipment on the same separated circuit) but the whole of section 418 is out of bounds for situations not overseen by skilled or instructed persons - so that's not really a goer either.

     

    The use of a safety isolating transformer to provide an isolated supply seems to be being criticised here as unsafe in some way. An isolating transformer construction prevents any significant secondary voltage to any part of the primary circuit by class 2 insulation and an earthed screen between windings, so I cannot understand Andy's comment about capacitive coupling.



    Maybe capacitive coupling was a bad example (although I've certainly seen some small transformers to BS EN 61558-2-series that don't even have PE connection so I'm pretty sure can't have an earthed screen) - and I note that 722.413.1.2's requirement is for a transformer to BS EN 61558-2-4 (isolating transformers) rather than BS EN 61558-2-6 (safety isolating transformers). But I still maintain that the approach of having an protective conductor & exposed-conductive-parts connected to a live conductor but isolated from Earth(*) doesn't fall under any of the approaches considered by BS 7671 - so not only can't be considered to be a BS 7671 compliant solution but could possibly (even likely) be subject to hazards that BS 7671 doesn't even consider.


    (* or equivalent substantial conductive part of the surrounding environment - e.g. mobile unit chassis for installations contained within that unit)


       - Andy.
  • 0

    AJJewsbury:




    Provided there's no connection to Earth (with capital E), or a protective conductor of another circuit, this should meet 413.



    I can't see how connecting the EVSE's PE to a separated live conductor complies with section 413 at all. 413.3.3 demands that live parts of the separated circuit are not connected at any point to another circuit or to Earth or to a protective conductor - as I read it that's any protective conductor, not just protective conductors of other circuits.


    Graham's wording seem to align more with 418.3.4 than 413 (which in a way would make more sense for a situation where we have multiple items of class I equipment on the same separated circuit) but the whole of section 418 is out of bounds for situations not overseen by skilled or instructed persons - so that's not really a goer either.


     


    How is it a protective conductor if it's not used for ADS because the means of protection against electric shock is Electrical Separation? (See definition of Protective Conductor in Part 2 - specifically, it's definitely not "PE" in this instance, more "FE".)


    The RCD is provided for Additional Protection, and has the advantage of being able to cover off the "spurious if unlikely fault condition" I mentioned in the last post.


    The wording I used is slightly robbed from 413.3.3 and 413.3.6 and not 418. I do see that, in fact, the argument of separation might indeed fall down if more than one vehicle were supplied from an individual separated source, but of course this is effectively prohibited by 722.413.1.2.


    The use of a "floating" system like this, although not specifically classed as electrical separation, is also permitted by BS 7430, but I would assert that is different because:


    1. it may supply more than one item of current-using equipment; and

    • it is supplied by an independent generator.

  • 0

    How is it a protective conductor if it's not used for ADS because the means of protection against electric shock is Electrical Separation? (See definition of Protective Conductor in Part 2 - specifically, it's definitely not "PE" in this instance, more "FE".)



    I'd say it's still a protective conductor - but as you say unearthed (so definitely not FE, more like keep the P, loose the E). I'd say it compares directly with the insulated conductor that section 418.3 demands connects exposed-conductive-parts together (but not to earth) on a separated circuit - so as to provide protection from shock on 2nd faults (otherwise we'd have a shock risk between two different exposed-conductive-parts on 2nd fault) - 418.3.4 it calls it a 'protective bonding conductor' (I might argue about the word bonding in there, but anyhow) it definitely refers to it as a protective conductor. ADS on 2nd fault doesn't stop the basic approach being separation (as per reg 418.3.7) although if you connect the protective conductor to a live conductor ADS should then happen on 1st fault rather than 2nd as we're in effect creating a separated circuit with the first fault already present.


    (It might have been simpler if 413 talked about one class I item rather than one item of current-using equipment - since the shock risks between exposed-conductive-parts are the same regardless of whether they're part of an appliance or part of an accessory)


      - Andy.
  • 0

    gkenyon:




    The situation that does arise, however, is high inrush current of the rather sizeable transformers. Yes, this can be overcome, but of course costs are now increasing, and we're competing against "forget it, I'll just plug it in to a socket-outlet", which of course is far less practical, and far less safe, but doesn't have the price tag attached.



     




    Hi Mr G Kenyon,


    The original idea of my questioning the possibility of using a Double Wound Safety Transformer was to look at an alternative, ( in an industrial and offices based environment), to installing the 'box on the wall with the Type 3 sockets/tethered lead and RCD'- insert manufacturer. Your last line of your quote (above) talks about using the transformer against a socket outlet and cost ( Mode 1). Cost is big driver of bad practises, and I have witnessed at office premises a coiled extension lead plugged into a socket to charge an EV.


    My original point was that 722.413.1.2 allows for the supply of one EV from one unearthed source, using a fixed isolating transformer complying with BSEN 61558-2-4.

    So Using mode 2 ( BEAMA Guide to electrical vehicle infrastructure), with a dedicated 3 phase circuit (regular charging), a circuit breaker rated for the transformer's in-rush current type D, could be rated at 7.4 KW for industrial use using a BS 60309-02 socket, source: BEAMA Guide, subject to the vehicle protocols.

    The secondary side earth cable PE would come from the secondary side of the transformer start point.

    The Mode 2 cable providing in cable and protection device (IC-CPD) and downstream RCD protection. 


    I have asked a transformer manufacturer for costs on this and in an industrial situation, using the safety transformer, and BS 60309-02, and not duplicating an RCD in a dedicated box, could prove cheaper to purchase and install than the units currently being supplied as a standard, and spreading load across three phases would assist energy efficient electrical installations compared to single phase load balancing.

    I would appreciate any comments on this idea.


    Regards


    Simon

  • 0
    An interesting reply Andy, but the earth connection to the car is not a protective conductor in the sense of a conventional circuit, it is simply there to enable the electronics to power the car charger up. If you think about it, it doesn't provide any protection to anything, as the supply is IT, and a double fault etc. is required to get any dangerous situation. It would ensure that the RCD tripped if the live faulted to the car bodywork, but even so this would not be a danger to any person or livestock. If there was a neutral fault the RCD would probably trip due to current diverted via the fault, but again could in no way be considered dangerous. The "even so" means that the RCD is not the primary protection of the circuit, it is additional protection against multiple faults, single ones not being dangerous.


    It has been extremely difficult to design a completely satisfactory and safe car charging installation with a PME supply, and particularly with the possibility of a broken CNE. This scheme is probably as safe as is possible when cars are class 1, and I would certainly recommend that all new car designs are class 2, because that change is fairly easy and cheap, and removes the charging dangers unless damaged flexible cables are involved.


    Interestingly no one has suggested a reason why a charging RCD should be DC sensitive.
  • Former Community Member
    0 Former Community Member

    gkenyon:



    The answer is relatively simple, and lies in Table 6.2.15 of G12/4 (page 32) - it's to do with the load, and the required consumer earth electrode to help prevent earth potential rise in the event of a broken CNE conductor in the PME distrubution system. Additionally, the structure of the shelter may be able to act as a form of earth electrode depending on construction.


     



    The problem with that approach (unless I have missed something) is it only offers protection from broken CNE conductors that supply only the installation in question and carry no load currents from other installations. Given that faults most often occur at joints and that a majority of the joints are usually shared it all seems a bit dubious. Yes, in that situation there will be some benefit from the PME electrodes and potentially some from load balance, but it seems rather optimistic to think that it will always, or even usually, hold the voltage down to an acceptable level.


     

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