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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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    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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  • 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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