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Omitting 30ma RCD Protection for single S/O in a domestic property

Former Community Member
Former Community Member
I installed a dedicated circuit for a hifi system for a customer last year. The customer requested a 6mm2 radial from a 16A MCB housed in its own independent consumer unit into a single, un-switched socket outlet. No problem, bit unusual but no worries.I wired it using a 3c 6mm2 armoured cable as I half anticipated the forthcoming...


The hifi equipment is causing the rcd to trip when started up. I haven't been over to have a look but I am assuming that the startup current for the many power supplies (he has told me there are ten!) coupled with electronic earth leakage is causing a CPC current that is sufficient to trip the RCD (perhaps only 16ma but enough). The earthing is high integrity having a 6mm2 cpc + armour and the Zs is sufficiently low enough that the 16A MCB can be used for fault protection. So, if this wasn't domestic I'd ditch the RCD (or replace with a 100ma) assuming that my assumptions to this point are correct.


The customer has now decided he doesn't want RCD anyway for 'reasons' but I'm still wary of removing it in a domestic situation, not because I believe the installation would become less-safe but just because it contravenes regulations.


Assuming there's no fault on the equipment and it is just a case of startup/inrush current and earth leakage, what approach would you take? Remove the RCD and write it up as a deviation from 7671 with a signed disclaimer/waiver from the customer? Install a 100ma RCD? Do nothing and walk away? Something else?


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  • 0
    I don't see how you can omit the 30mA RCD protection and claim either BS 7671 compliance or 'no less degree of safety' - without the RCD it lacks additional protection even if ADS is fine. A 100mA RCD isn't likely to be much better on that score either (no point in an 100mA RCD if 50mA can kill).


    Some 30mA RCDs can be better than others at handing inrush currents. A couple of different effects can cause some to trip - a high initial protective conductor current obviously, but some can also trip just due to a high L-N current - if the inrush current is well beyond the RCD's current-carrying rating (e.g. 40A or 80A, not 30mA) then the L & N coils can saturate the torroid and combined with manufacturing tolerances can mean the L and N coils don't quite balance as they should and the sense coil sees an imbalance larger than it really is.


    So things to try might include a 30mA RCD with a higher current carrying rating, (or just a different make/model), or a type with better transient resistance (e.g. Dopeke suggest that AKV, B, F and EV types do better than traditional AC and A types) - more expensive of course, but that's perhaps the market you're in?


    Alternatively, design out the problem by using a sequential start unit between the socket and various items of equipment (or just let the user switch things on one at a time).


      - Andy.
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  • 0
    I don't see how you can omit the 30mA RCD protection and claim either BS 7671 compliance or 'no less degree of safety' - without the RCD it lacks additional protection even if ADS is fine. A 100mA RCD isn't likely to be much better on that score either (no point in an 100mA RCD if 50mA can kill).


    Some 30mA RCDs can be better than others at handing inrush currents. A couple of different effects can cause some to trip - a high initial protective conductor current obviously, but some can also trip just due to a high L-N current - if the inrush current is well beyond the RCD's current-carrying rating (e.g. 40A or 80A, not 30mA) then the L & N coils can saturate the torroid and combined with manufacturing tolerances can mean the L and N coils don't quite balance as they should and the sense coil sees an imbalance larger than it really is.


    So things to try might include a 30mA RCD with a higher current carrying rating, (or just a different make/model), or a type with better transient resistance (e.g. Dopeke suggest that AKV, B, F and EV types do better than traditional AC and A types) - more expensive of course, but that's perhaps the market you're in?


    Alternatively, design out the problem by using a sequential start unit between the socket and various items of equipment (or just let the user switch things on one at a time).


      - Andy.
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