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My war against dual rcd boards

Newfutile
As each (RCD) Residual Current Device must not have more than 30% leakage current on it.
It's hard to see how dual (RCD) Residual Current Device boards can be fitted at all these days.
i come across so many dual rcd boards with solar, evse and heat pumps on them, these have all been recently installed.
i wonder if BS7671 should state: Dual (RCD) Residual Current Device boards shall not be fitted, unless it can be shown (and documented) that they are suitable for the combined leakage currents expected .
otherwise developers will keep specifying them and i will have keep educating them on the many reasons a type A rcd should not be shared with other equipment!.
maybe manuafactuers instructions should also state : not suitable for a shared rcd, for certain equipment.
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  • Do we not get into a merry-go round of confusing desired (designed in) 'leakage' such as that from EMC filters (tripping undesired), and the undesired 'leakage' fault currents, particularly accidental touch based leakages for which tripping is clearly desired?

    It wasn't clear to me what the 30% figure alluded to. Is it the lowest level of leakage (either style) that might trip the RCD? Could it be the maximum level of designed-in leakage from all the filters?

    The steady inclusion of more an more EMC (HF) filters can greatly increase the level of 'leakage' with the potential for nuisance trips. And that's before any of the Heat pump EMC filter problems (the ~1kHz band). 

    The modern house is definitely a complex system that needs some thought.

  • in reply to Philip Oakley

    I think the 30% comes from 531.3.2 - so avoiding "Unwanted tripping" - as RCDs can trip anywhere between 50% and 100% of their residual rating, I suppose it gives a bit of a margin.

    I'd be wary of blanket bans on "shared RCDs" - there are situations - i.e. RCD protection on a sub-main in TT systems, where one RCD "shared" across several final circuits in pretty much unavoidable - and can be OK if done carefully.

    By the looks of it BS 7671 already provides enough evidence that multiple high leakage appliances on a single 30mA RCD isn't compliant.

       - Andy. 

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  • in reply to Philip Oakley

    I think the 30% comes from 531.3.2 - so avoiding "Unwanted tripping" - as RCDs can trip anywhere between 50% and 100% of their residual rating, I suppose it gives a bit of a margin.

    I'd be wary of blanket bans on "shared RCDs" - there are situations - i.e. RCD protection on a sub-main in TT systems, where one RCD "shared" across several final circuits in pretty much unavoidable - and can be OK if done carefully.

    By the looks of it BS 7671 already provides enough evidence that multiple high leakage appliances on a single 30mA RCD isn't compliant.

       - Andy. 

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  • in reply to AJJewsbury

    Hmm, 10mA of idle state leakage on a 30mA RCD spanning many final circuits is not great design even so. Elsewhere (not the UK)  I have seen RCDs for general loads chosen based on 0.1%  - so 1mA per amp - 30mA RCD on a 30A circuit etc.
    Does not really work so well here as a rule of thumb with with ring finals and single phase 100A boards.

    Mike.

  • in reply to AJJewsbury

    I think that Andy has summed up the engineering succinctly.

    If the leakage currents in the dual-RCD boards are excessive, then there would be a lot of nuisance tripping. Is that actually the case?

    What is the economic argument? I doubt that margins are so fine on new-build developments that all-RCBO boards cannot be provided. With after-market solar PV installations costing £thousands, once again a few £tens more on a board change hardly matters.

    Of course, should nuisance-tripping materialize, the householder would have to stump up the cash to rectify the problem. That may seem like money well-spent, but would hardly be fair on a tenant.

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