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Regulation stating a type AC RCD can not be upstream from a type A RCD

Alan B

Hi

I found an EV charger today with built in type A RCD + RDC-DD connected to a type AC RCD in the consumer unit, the AC RCD is also protecting 3 other circuits including sockets. I know this is incorrect because the type AC RCD could be blinded by DC currents, but I am struggling to find a regulation to reference when providing information to the customer?

Thanks

Alan

Parents
  • 0

    I think what you describe is probably OK, because in an unsmoothed DC fault that the AC type might fail on,  the A type will trip

    I think the problem we have is that AC type RCDs have no documented resilience to d.c. residual currents at all - while A types are meant to be resilient up to 6mA d.c.. So an A type plus and RDC-DD is OK since the the A type will be OK up to 6mA and the RCD-DD will trip above that. But on paper at least d.c. residual currents below 6mA could disrupt the operation of a AC type and nothing would trip to disconnect the problem.

    Individual designs very a lot - there are many AC type RCDs out there that seem to behave very much like A types - but nothing is guaranteed if it has an AC label. The actual danger also depends on what the upstream RCD is there for - e.g. ADS or just supplementary protection - a few mA change in tripping characteristics probably won't make much difference to ADS but could be more significant for supplementary protection where the residual current is likely very limited due to body resistance, footwear etc.

       - Andy.

Reply
  • 0

    I think what you describe is probably OK, because in an unsmoothed DC fault that the AC type might fail on,  the A type will trip

    I think the problem we have is that AC type RCDs have no documented resilience to d.c. residual currents at all - while A types are meant to be resilient up to 6mA d.c.. So an A type plus and RDC-DD is OK since the the A type will be OK up to 6mA and the RCD-DD will trip above that. But on paper at least d.c. residual currents below 6mA could disrupt the operation of a AC type and nothing would trip to disconnect the problem.

    Individual designs very a lot - there are many AC type RCDs out there that seem to behave very much like A types - but nothing is guaranteed if it has an AC label. The actual danger also depends on what the upstream RCD is there for - e.g. ADS or just supplementary protection - a few mA change in tripping characteristics probably won't make much difference to ADS but could be more significant for supplementary protection where the residual current is likely very limited due to body resistance, footwear etc.

       - Andy.

Children
  • 0 in reply to AJJewsbury

    Hi Andy

    All understood but where is it documented in BS7671 or the on site guide?

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