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RCD arrangements for EV charger

Alan B

I have an installation where for various reasons it's going to be impractical to fully comply with regs.

I am planning on adding to an installation where the existing CU is in the middle of the house, full and very limited room around it.

Moving the CU and the supply head would be prohibitively expensive so I am looking at teh next best option. Customer is adding the EV charger, some outside lights and also power to an extension that I am not involved with.

I think the best way forward for the EV charger and outside lights is to add a small DB on it's side just under the existing CU or ideally replace the existing CU and gain a couple of extra slots in the space available. Then run a sub main out to a separate DB for the outside lights and EV charger.

But for this question I want to focus on RCD protection. To get cables to the new DB located under the stairs they will have to run through the ceiling. The beams are 6 inches deep and I plan to use twin and earth for the submain as the route is quite convoluted and awkward to run SWA all the way.

 

From my understanding :-

Concealed cables should be protected by a 30mA RCD

The outside lights and the EV charger should also have individual RCD's protecting them in the sub DB.

This means I will have no selectivity between the 30mA rcd on the sub main and RCD's on the individual circuits. 

So do I use a 100mA type S RCD on the sub main or accept there will be a lack of selectivity. 

I am leaning towards using a 100mA type S on the sub main as the customer won't be happy if the ev charger fails due to lighting issue, what are your thoughts?

  • 0

    Your proposed installation of the sub-main in T&E protected by 100 mA RCD does not comply with BS 7671 if the T&E is “concealed” (as per the relevant particular Regs). Because of the history of the “concealed cables” issue, I would strongly recommend not deviating from the requirement.

    A solution is to use a compliant wiring method, such as SWA, or cables in earthed metallic conduit, or a cable route that avoids the “30 mA required because of concealed cables” requirements.

  • 0

    well the middle of a 6inch beam is both more than 50mm from the floor, and from the ceiling below, so maybe some cunning supports at the right height…

    If the load was smaller flexishield might be an option. If the DNO did it they would use split con, but on the consumer side we are not supposed to consider that an adequate armour.

    mike

     

     

  • 0

    Why not run the two circuits separately in T&E?

    Alternatively, stop worrying about selectivity. Tripping should be a very rare event.

  • 0

    Concealed cables should be protected by a 30mA RCD

    Only for soft-skinned cables concealed in walls - there's no 30mA RCD requirement for ceilings (but then there's no option for soft skin cables less than 50mm deep anyway - it's either 50mm from the top/bottom of the joist or comply with 522.6.204 (wiring system with a concentric c.p.c.).

       - Andy.

  • 0

    Hi All

    Thanks for all the input, regulation 522.6 is full of subtle differences. Andy pointing out that 522.6.201 means I do not need to RCD protect cables concealed in a ceiling 50mm from top or bottom is very useful as I am not concealing the distribution cable in the walls at all. 

    Not sure if I need to worry about the cable dropping a bit in the gaps between the joists, if it was a concern I could run it inside some plastic conduit or put some noggins in but I have never seen cables supported between joists in houses I have worked on in the past.

  • 0

    Agree, what you see is cable sitting on the plasterboard  or at most an odd clip to the side of the nearest joist as the standard methods. And in fairness, it works, - if anyone were to drive a spike up through the ceiling, it may be better if the cable is left free, as it will then push out of the way, rather than being damaged.

    Mike.

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