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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

RichardCS2:

Like many people here I have yet to see a convincing technical justification for a vehicle body being a higher risk thing to connect to a TNC-S derived earth than a metallic illuminated bus shelter. Especially the kind of bus shelter popular around here with metal seats incorporated into the design ensuring that its users spend an extended period of time with their backsides connected to the metalwork and their feet firmly on the ground. They are both similar sized objects made mostly of painted steel, they're both usually on poorly conducting surfaces, what's special about the car? Or are both dangerous and bus shelter electrical installations should really be class 2 as telephone box lighting is/was? Perhaps the car manufacturer's are fundamentally at fault making the cars class 1 in the first place, after all other outdoor appliances have long been class 2.

As a side note high-power AC charging seems to be dying, the main model to support it was the Renault Zoe and they have recently announced that future models will be restricted to 32 A per phase but 100 kW DC charging will be introduced in its place.
https://insideevs.com/news/342860/renault-ends-sale-of-zoe-q90-with-43-kw-ac-charging-capability/

The answer is relatively simple, and lies in Table 6.2.15 of G12/4 (page 32) - it's to do with the load, and the required consumer earth electrode to help prevent earth potential rise in the event of a broken CNE conductor in the PME distrubution system. Additionally, the structure of the shelter may be able to act as a form of earth electrode depending on construction.

See also section 7 of the IET's Code of Practice for EV Charging Equipment Installation, 3rd Ed - Table 7.1, page 48, and section 14.5, pages 98 and 99, of IET Guidance Note 5 Protection against electric shock, 8th Edition.
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0 gkenyon over 5 years ago

RichardCS2:

Like many people here I have yet to see a convincing technical justification for a vehicle body being a higher risk thing to connect to a TNC-S derived earth than a metallic illuminated bus shelter. Especially the kind of bus shelter popular around here with metal seats incorporated into the design ensuring that its users spend an extended period of time with their backsides connected to the metalwork and their feet firmly on the ground. They are both similar sized objects made mostly of painted steel, they're both usually on poorly conducting surfaces, what's special about the car? Or are both dangerous and bus shelter electrical installations should really be class 2 as telephone box lighting is/was? Perhaps the car manufacturer's are fundamentally at fault making the cars class 1 in the first place, after all other outdoor appliances have long been class 2.

As a side note high-power AC charging seems to be dying, the main model to support it was the Renault Zoe and they have recently announced that future models will be restricted to 32 A per phase but 100 kW DC charging will be introduced in its place.
https://insideevs.com/news/342860/renault-ends-sale-of-zoe-q90-with-43-kw-ac-charging-capability/

The answer is relatively simple, and lies in Table 6.2.15 of G12/4 (page 32) - it's to do with the load, and the required consumer earth electrode to help prevent earth potential rise in the event of a broken CNE conductor in the PME distrubution system. Additionally, the structure of the shelter may be able to act as a form of earth electrode depending on construction.

See also section 7 of the IET's Code of Practice for EV Charging Equipment Installation, 3rd Ed - Table 7.1, page 48, and section 14.5, pages 98 and 99, of IET Guidance Note 5 Protection against electric shock, 8th Edition.
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Double wound safety transformer for EV supply.

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