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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....





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    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.
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  • 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.
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