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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 AJJewsbury over 5 years ago

Remember nothing on the secondary side of the isolating transformer is connected to real earth, nor any Protective Earth connection.

In a conventional separated system that's of course true, but if I've understood Graham's description of what the CoP says correctly, in our case there would also be a deliberate connection between the separated "N" and the EVSE's PE terminal (or c.p.c) - in order to defeat the EV's earth loop check. So what we have is at best a separated system with a first fault already present (or if you prefer an approximation to a TN system) - potentially referenced to true earth if any of the exposed-conductive-parts happen to be in contact with the ground (like that pole mounted EVSE pod). Yes the RCD should save the day but we're in effect relying on ADS then, not separation.

Moving to the block diagram, there is complete isolation between the charging supply and the charger output which is illustrated by the dotted line.

In that diagram yes - however I'm given to understand that's not the case with all EVs - as some don't have an isolating transformer in that position, but rather like some 'transformerless' PV inverters - use a direct connection or dc-dc converter - for reasons of both weight and efficiency. (I'll try to dig out some evidence for that.)

- Andy.
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0 AJJewsbury over 5 years ago

Remember nothing on the secondary side of the isolating transformer is connected to real earth, nor any Protective Earth connection.

In a conventional separated system that's of course true, but if I've understood Graham's description of what the CoP says correctly, in our case there would also be a deliberate connection between the separated "N" and the EVSE's PE terminal (or c.p.c) - in order to defeat the EV's earth loop check. So what we have is at best a separated system with a first fault already present (or if you prefer an approximation to a TN system) - potentially referenced to true earth if any of the exposed-conductive-parts happen to be in contact with the ground (like that pole mounted EVSE pod). Yes the RCD should save the day but we're in effect relying on ADS then, not separation.

Moving to the block diagram, there is complete isolation between the charging supply and the charger output which is illustrated by the dotted line.

In that diagram yes - however I'm given to understand that's not the case with all EVs - as some don't have an isolating transformer in that position, but rather like some 'transformerless' PV inverters - use a direct connection or dc-dc converter - for reasons of both weight and efficiency. (I'll try to dig out some evidence for that.)

- Andy.
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Double wound safety transformer for EV supply.

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