My war against dual rcd boards

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.

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. 

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.

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.

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.

Chris Pearson said:

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

Try and compare a Hager (other brands are available) split load CU against a Navits CU

https://www.navitascp.co.uk/product-page/10-way-100a-mainswitch-consumer-units-c-w-t2-spd

The economic argument goes out of the window (other Apertures are available) 

I see no reason why a fully RCBO and SPD type2 CU can not be designed and installed in the average UK dwelling

Sergio Fernandez said:

I see no reason why a fully RCBO and SPD type2 CU can not be designed and installed in the average UK dwelling

I entirely agree, but I would not condemn a dual-RCB on those grounds.

In theory, any board to BS EN 61439 should be satisfactory.

My recent shopping shows me that I can buy a turkey from Waitrose or Tesco for over £100. Currently, Aldi are offering a small turkey with accessories for £12 - link. There is no reason to believe that the Aldi turkey is less safe or less nutritious than the Waitrose or Tesco ones.

So what is better about the Hager, or Eaton, etc. board. They might last longer, but how else could you justify the extra cost?

Chris Pearson said:

In theory, any board to BS EN 61439 should be satisfactory

Give or take selection of it in respect of and division of installation (Section 314), and appropriate protective devices (Part 5). BS 7671 applies to the 'selection and application of the equipment in the installation concerned' (see Regulation 113.1).

Chris Pearson said:

I entirely agree, but I would not condemn a dual-RCB on those grounds.

The point I was trying to raise was that in this day and age a new build dwelling or a major alteration of an existing domestic dwelling should have a fully RCBO with SPD type 2 CU used.  However designers are still using Split load CU for brand new installs in 2025.  As a casing point I visited a new housing developement in Hemel Hempstead last month and the show homes had split load CU’s.  The development is planned to continue building dwelling at that site for the next 2 years which I assume means that they will still be using the same split load CU at that site for the next two years.  In my opinion I would say the split load CU should no longer be marketed and sold in the UK for the average domestic dwellings.  

Sergio Fernandez said:

In my opinion I would say the split load CU should no longer be marketed and sold in the UK for the average domestic dwellings.

I don't think that we are far apart, but as a libertarian, I would not ban them. They may have their disadvantages, but unless they are dangerous, their use should be permitted.

So let's propose a ban: what would the counter-argument be? It seems to me that it can only be economic, and trivial at that.

My argument in favour is libertarian - authorities (e.g. Parliament) should intervene in peoples' lives as little as possible, and people should be free to lead their lives as they see fit.

Apart from that and the economic argument, is there any other reason to favour dual-RCD boards?

In a fully RCBO CU each circuit has it's own (probably30mA) circuit.  Thus its own protection and no nuisance tripping affecting other circuits.  Some RCBOs may even be DP (Dual Pole)


I think designers need to stop looking at prices by companies like Wylex (Other brands are available) and start looking into what makes a better design.  In my personal opinion of engineering judgement a fully loaded RCBO is better than split load RCD and if the designers were open to reviewing their design then changing the housing estate/new development from lets say Hager Split load(other brands are available) to lets say Fusebox or Navitas not only would they save money but it would be a compliant install choice showing an attempt for future planning.  Eg should an electrician want to fit a Renewable/alternate energy source to the system.  
It seems that some Electrical designers are lazy and are happy for a rinse a repeat method rather than continuing to improve things. Designers need to think or be inspired more by the likes Percy Shaw, (cats eye) 

en.wikipedia.org/.../Percy_Shaw

Sergio Fernandez said:

In my personal opinion of engineering judgement a fully loaded RCBO is better than split load RCD

I agree, but I think that one must also accept that other opinions/judgements are valid. I find it difficult to see how a dual-RCD board could be the better one, but in most circumstances, it is just as good as an all-RCBO and accordingly, may be better value.

Getting away from Christmas turkeys, one might say that a Mercedes is a better car than a Kia, but they both get you from A to B in the same time (given speed limits and congestion) and they both comply with safety standards. However, it could also be argued that the Kia is better value, and on that ground, a better choice.

Chris Pearson said:

Mercedes

Would you class the SMART car as a Mercedes-Benz?  The name "smart" itself comes from "Swatch Mercedes ART," reflecting its origins as a collaboration between Swatch and Mercedes-Benz

No. No more than I would classify a Mini as a BMW.