Suggestions for better RCD discrimination

[Rockingit]

Have an existing old client with what's best described as a sprawling small-holding of a property.

There's an armoured external supply which branches into a submain for various sheds/workshops/stables (some of which are 150-200m) etc all of which have standard RCD final circuit protection and all the feeders are SWA, so there's not really a design need for an upfront RCD coming out of the house, except.... naturally the outgoing SWA only starts on the opposite side of the building to the CU, when the original builder put it all in there's standard 6242Y going from the CU through the fabric of the building to feed it - so that does require 30ma protection. Needless to say, this is leading to nuisance tripping of the feed RCBO in the CU.

Board is an all RCBO Easy9 that I updated to a couple of years ago, and Schneider naturally don't make anything so sensible as an S type RCBO for them (have asked technical). If I fit an external unit to the CU it will have to be a mess of replace RCBO for MCB > OUT > RCBO > Back in again and joint to the 6242Y. Equally, running a new length of SWA out and jointing on to the external feeder isn't really an option either as it would involve significant ground works.

Suggestions welcome! R.

 

[pc1966]

Do all of the SWA sub-mains leave in roughly the same place? I.e. could you run a SWA sub-main to there (even if around outside of house) and a 2nd CU with just OCPD per branch to divide that up?

If the internal wiring needs 30mA additional protection as buried in walls in unknown places you don't really have much else you can do. Of course if it is unreachable by 2" nails then you would need it but if in rodent territory you might still want 100mA or 300mA delay RCD for fire aspect.

 

[Rockingit]

Do all of the SWA sub-mains leave in roughly the same place? I.e. could you run a SWA sub-main to there (even if around outside of house) and a 2nd CU with just OCPD per branch to divide that up?

It's a 63A SWA run out to an external cabinet, branch circuits from appropriate OCPD from there. To replace that single trunk run would be £k's worth of work due to a variety of drives, ornate patio's, duck pond.....

If the internal wiring needs 30mA additional protection as buried in walls in unknown places you don't really have much else you can do. Of course if it is unreachable by 2" nails then you would need it but if in rodent territory you might still want 100mA or 300mA delay RCD for fire aspect.

This is my only thinking is that maybe I can second guess that the feeding run of T&E is actually running in ceiling void (which would sort of make sense) and then been dropped down inside the cavity, old school, before ending up at the junction point. I would, however, be making a guess.....

As for 100/300mA - again, limited by what Schneider make for those boards so it's back to an out/in/out/in scenario with an external box.

To be fair, by the time I've over-thought this several times I could have done the job already..... would just be better to have a neater solution!

 

[pc1966]

It's a 63A SWA run out to an external cabinet, branch circuits from appropriate OCPD from there. To replace that single trunk run would be £k's worth of work due to a variety of drives, ornate patio's, duck pond.....

OK so it is 63A from the current CU?

On a RCBO? Largest RCBO I have seen are 45A but maybe they do bigger.

I was thinking you have several SWA coming to the house then to the CU in T&E.

This is my only thinking is that maybe I can second guess that the feeding run of T&E is actually running in ceiling void (which would sort of make sense) and then been dropped down inside the cavity, old school, before ending up at the junction point. I would, however, be making a guess.....

Can you switch off every circuit but that and use one of the "live cable" locators to see if it can be found that way?

As for 100/300mA - again, limited by what Schneider make for those boards so it's back to an out/in/out/in scenario with an external box.

I don't know that range but at one time Hager did a 100mA RCBO, alas not delay, but for accumulated leakage nuisance trip rather than fault selectivity then any update in current would help.

To be fair, by the time I've over-thought this several times I could have done the job already..... would just be better to have a neater solution!

Maybe!

 

[Rockingit]

OK so it is 63A from the current CU?

On a RCBO? Largest RCBO I have seen are 45A but maybe they do bigger.

I'd need to dig up the old EIC (haven't been back yet) but from memory I think it was originally on a 63A MCB/RCD combo and is now a 50A RCBO. The whole install was a complete mess of stuff everywhere, Henleys, singles, SPV meters.. Took up nearly 2m of width on a wall and I managed to condense the lot into one large board!

Can you switch off every circuit but that and use one of the "live cable" locators to see if it can be found that way?

I could, to a point, but I'd have thought I'd still be second guessing what construction/depths were involved

I don't know that range but at one time Hager did a 100mA RCBO, alas not delay, but for accumulated leakage nuisance trip rather than fault selectivity then any update in current would help.

Some of the very far branch feeds have actually been PME divorced to rods, my ideal device would be a 100mA delay.

Maybe!

Definitely!!

 

[davesparks]

The problem is that if 30mA RCD protection is required then you can't substitute it for a higher tripping current or a time delay as it will not provide the required protection.

 

[happysteve]

Rather than burying SWA which you say is £k's, could you go aerial with some H07RN-F on a catenary?

 

[pc1966]

I would expect there is some way to run some SWA across the house, or maybe around it, and join to existing SWA so the RCD at source is not needed for "additional protection" forcing 30mA and the issues above.

Given the lengths mentioned above you might still want a delay RCD for fault protection if end of sub-man Zs is not low enough for the OCPD alone (especially if you go D-curve MCB to buy a tiny bit more fault selectivity).

 

[7029 dave]

Been reading this thread with interest, would it not be possible to run a suitably sized swa from the orgin of supply with switch fuse etc.Drill out up the building into the loft space and drop down externally to pick up the existing swa in the cabinet. Assuming there is a loft space.

 

[mainline]

Just thought I would mention the need to ensure that the earthing to some of these outbuildings are compliant as PME and Stables are involved.

 

[pc1966]

Good point from @7029 dave about having a fused-switch instead of taking off the CU, more space needed but better selectivity with downstream MCB/RCBO and if the fuse is 63A for 100A DNO (or 50A for 80A, etc) then it should be selective upstream as well.

 

[7029 dave]

Good point from @7029 dave about having a fused-switch instead of taking off the CU, more space needed but better selectivity with downstream MCB/RCBO and if the fuse is 63A for 100A DNO (or 50A for 80A, etc) then it should be selective upstream as well.

Yup IMO with what the OP has described, having a genuine sub main is better, a sub circuit off the main dB is asking for trouble.

 

[Rockingit]

Yup IMO with what the OP has described, having a genuine sub main is better, a sub circuit off the main dB is asking for trouble.

Am in total agreement..... had I only been there 20 years ago........

 

[Rockingit]

I would expect there is some way to run some SWA across the house, or maybe around it, and join to existing SWA so the RCD at source is not needed for "additional protection" forcing 30mA and the issues above.

Given the lengths mentioned above you might still want a delay RCD for fault protection if end of sub-man Zs is not low enough for the OCPD alone (especially if you go D-curve MCB to buy a tiny bit more fault selectivity).

Sadly there isn't! And just for extra giggles the T&E to SWA joint is buried underneath an ornate patio (or assumed to be, anyway). There's also no way in hell that the client would sanction an overhead catenary (it would need to be about 30m long and involve installing masts for extra height!).

I think quite possibly this may just be a bum deal and the customer has to accept the odd nuisance trip,

 

[mainline]

Sadly there isn't! And just for extra giggles the T&E to SWA joint is buried underneath an ornate patio (or assumed to be, anyway). There's also no way in hell that the client would sanction an overhead catenary (it would need to be about 30m long and involve installing masts for extra height!).

I think quite possibly this may just be a bum deal and the customer has to accept the odd nuisance trip,

I know not compliant, but I would be debating as to fitting 100ma s type at the source depending on if the loss of supply could result in an accident.

 

[Rockingit]

I know not compliant, but I would be debating as to fitting 100ma s type at the source depending on if the loss of supply could result in an accident.

So here’s a thing…. I need to get my head into the brown book and scour the regs but off the top of my head my recollection is that the requirement for cables in walls is simply 30mA, nothing said about S types at all??

 

[mainline]

So here’s a thing…. I need to get my head into the brown book and scour the regs but off the top of my head my recollection is that the requirement for cables in walls is simply 30mA, nothing said about S types at all??

It would not meet the disconnect time.

 

[pc1966]

Nobody makes a type S 30mA RCD as @mainline points out it would not meet disconnection times.

You have two issues for selectivity:

• One is when there is a 'hard fault' and so you need some delay to allow the downstream RCD to clear irrespective of the magnitude of the fault current.
• The other is for a 'soft fault' where the imbalance is small and rises slowly, or indeed is simply there all of the time due to accumulation of appliance leak currents, etc. For that you need a difference in trip threshold so again the downstream one always goes first.

So if designed you have no other choice then to go 100mA selective for upstream and then, to meet regulations, it has to be installed so accidental cable penetration is very unlikely. Yes, I know the BBC have some guidance limiting temporary supply RCDs to 100mA / 200ms max as it gives you some protection against a fatal shock, but that is not meeting the BS regs.

If you can clamp the sub-main and check the leakage you might find it is hovering in the 10-15mA region in which case it is not going to be cured by anything other then a change of upstream RCD to a higher current and all of the other implications of that. On the other hand, if it is normally below 10mA and the issue is the odd spike incident (like my oven that trips the RCBO a couple of times a year on switching off) then it might just be worth looking at ones of these:

A9R61263 - Residual current circuit breaker (RCCB), Acti9 iID, 2P, 63A, A-SI type, 30mA | Schneider Electric Global

Schneider Electric Global. A9R61263 - Residual current circuit breaker (RCCB), Acti9 iID, 2P, 63A, A-SI type, 30mA.

www.se.com

They claim to be less prone to false triggering while still meeting the overall low frequency trip current and times. Here is the FAQ comparing the A-SI type they call it with the normal type A:

What is the difference between A9D32620 and A9D33620 RCBO Acti 9 devices? | Schneider Electric Australia

 

[Rockingit]

Nobody makes a type S 30mA RCD as @mainline points out it would not meet disconnection times.

You have two issues for selectivity:

• One is when there is a 'hard fault' and so you need some delay to allow the downstream RCD to clear irrespective of the magnitude of the fault current.
• The other is for a 'soft fault' where the imbalance is small and rises slowly, or indeed is simply there all of the time due to accumulation of appliance leak currents, etc. For that you need a difference in trip threshold so again the downstream one always goes first.

So if designed you have no other choice then to go 100mA selective for upstream and then, to meet regulations, it has to be installed so accidental cable penetration is very unlikely. Yes, I know the BBC have some guidance limiting temporary supply RCDs to 100mA / 200ms max as it gives you some protection against a fatal shock, but that is not meeting the BS regs.

If you can clamp the sub-main and check the leakage you might find it is hovering in the 10-15mA region in which case it is not going to be cured by anything other then a change of upstream RCD to a higher current and all of the other implications of that. On the other hand, if it is normally below 10mA and the issue is the odd spike incident (like my oven that trips the RCBO a couple of times a year on switching off) then it might just be worth looking at ones of these:

A9R61263 - Residual current circuit breaker (RCCB), Acti9 iID, 2P, 63A, A-SI type, 30mA | Schneider Electric Global

Schneider Electric Global. A9R61263 - Residual current circuit breaker (RCCB), Acti9 iID, 2P, 63A, A-SI type, 30mA.

www.se.com

They claim to be less prone to false triggering while still meeting the overall low frequency trip current and times. Here is the FAQ comparing the A-SI type they call it with the normal type A:

What is the difference between A9D32620 and A9D33620 RCBO Acti 9 devices? | Schneider Electric Australia

So, here's a thing. Not conventional for a domestic, and not cheap, but an adjustable RCD could in theory be set for bang-on 300mS @ 30mA / 40mS @ 150mA and be compliant, basically relying on the downstream devices being inherently faster (which we all know they typically are, by a significant margin).

 

[pc1966]

So, here's a thing. Not conventional for a domestic, and not cheap, but an adjustable RCD could in theory be set for bang-on 300mS @ 30mA / 40mS @ 150mA and be compliant, basically relying on the downstream devices being inherently faster (which we all know they typically are, by a significant margin).

Yes - if the "adjustable" bit is continuous and not a set of pre-determined steps.

Last time I priced an adjustable RCD (actually separate Hager HR520 EL relay, sense toroid, and actuator to trip the Hager 125A 3P MCBs) it all totalled around £840 inc VAT at "trade" price.