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Double wound safety transformer for EV supply.

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 gkenyon over 5 years ago

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.
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0 gkenyon over 5 years ago

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.
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Double wound safety transformer for EV supply.

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