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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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  • 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.
Reply
  • 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.
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