UK Selectivity on concealed sub main supply

[Mark.fardon]

EICR: I’ve come across 30mA RCD supplying a consumer unit via a 63A MCB and 16mm T&E. connected via an isolator to the cutout PME. (Ground floor, outside meter cab on wall)


The T&E is concealed up the stairwell behind /within plaster into a 1st floor flat above a shop and into the CU so effectively has 6mm main earth.

(see pic for the bodge of an install of the RCD which has been wired incorrectly but functions - what would you code that ! )


The CU is a dual 30mA split load affair. Therefore no selectivity and any earth fault will likely trip the whole supply, as it did on RCD test

In addition NO bonding to gas or water which is another matter but related of course

I see options as below sny

1. rewire submain cable with SWA or surface mount with T&E (costly/messy/unsightly either way)

2. Replace Supply RCD with switch fuse but this breaches requirement of 30mA RCD for concealed cable -

3. Accept the selectivity/nuisance factor but the RCD supplying T&E is in an outside meter box with a lid that actually needs a tool to open

4. replace Supply RCD with time delay 100 mA RCD which does not comply with concealed cable requirements

4. submit EICR and leave for someone else to resolve !

 

[westward10]

As it stands you cannot remove the 30ma rcd as you remove the additional protection, fitting 100ma (s) rcd also removes the additional protection.
Submit the Report and worry about any remedial works if asked.
The cpc of the cable is also inadequate to support PME bonding.

 

[bill01803]

What are the water/gas mains made of?

 

[Mark.fardon]

What are the water/gas mains made of?

Copper. Gas is easy fix it’s in adjacent meter cab. Water not so easy - far end of flat. Suspect feed is from shop below. Need to talk to landlord on that. Might be an option to install a plastic section in water supply.

 

[Strima]

Is the water supply the same for the shop and flat?

 

[Mark.fardon]

Is the water supply the same for the shop and flat?

Not sure mate I need to find out. All I know currently there are no bonding conductors in the flat CU or in the meter cabinet supply origin. ?

 

[Strima]

Not sure mate I need to find out. All I know currently there are no bonding conductors in the flat CU or in the meter cabinet supply origin. ?

If the flat and shop share water supplies then they could be bonded back to a common MET.

Are the flat and shop electrical supplies bonded together or separated?

 

[Mark.fardon]

If the flat and shop share water supplies then they could be bonded back to a common MET.

Are the flat and shop electrical supplies bonded together or separated?

The mains supplies are separate. The metered supply for the flat appears to be from the DNO see pic

 

[pc1966]

If the feed is getting fixed in SWA then no need to worry about the bonding as most likely it would be 16mm 3-core and fine for PME, then as you say a switch-fuse and sanity returns.

I presume you took the photo with the cover off the MCB/RCD and it is not normally like that!

 

[Mark.fardon]

If the feed is getting fixed in SWA then no need to worry about the bonding as most likely it would be 16mm 3-core and fine for PME, then as you say a switch-fuse and sanity returns.

I presume you took the photo with the cover off the MCB/RCD and it is not normally like that!

That’s the point - fixed with SWA is the ideal solution but would mean SWA clipped up the stairs and along a hallway or hacked into the plaster / I don’t really want the job to resolve it so may just give a C3 on the selectivity issue and leave it to them with some supporting notes. The bonding/main earth though is another issue which until I find the water supply can’t comment - currently a C2. And no I’ve removed the cover to do the Ze

 

[SJD]

… fixed with SWA is the ideal solution but would mean SWA clipped up the stairs and along a hallway or hacked into the plaster ...

Is there a sensible route on the outside of the property, up/round an external wall instead?

 

[Strima]

Smart RCD at the supply end, expensive but saves walking down the stairs to reset it.

 

[pc1966]

Smart RCD at the supply end, expensive but saves walking down the stairs to reset it.

Have you any suppliers/part numbers for such a device?

 

[davesparks]

Smart RCD at the supply end, expensive but saves walking down the stairs to reset it.

What exactly do you mean by a smart RCD?

 

[John-SJW]

The 63A mcb is reverse fed. Is that correct? Need to check makers?

What size is the DNO's fuse? Can't the box with RCD and mcb be removed if DNO's fuse is 63A? Then direct from mainswitch isolator to remove CU using swa cable? Or does the main swa need additional protection as westward10 suggests?

 

[davesparks]

The 63A mcb is reverse fed. Is that correct? Need to check makers?

MCBs, and most RCCBs, work either way round.

 

[John-SJW]

Has to be checked with manufacturers of course.

 

[Mark.fardon]

The 63A mcb is reverse fed. Is that correct? Need to check makers?

What size is the DNO's fuse? Can't the box with RCD and mcb be removed if DNO's fuse is 63A? Then direct from mainswitch isolator to remove CU using swa cable? Or does the main swa need additional protection as westward10 suggests?

Yes as I stated earlier I think the RCD is wired upside down but operates correctly I assume once the double pole switch is closed it will function either way but whoever wired it made life difficult and the MCB doesn’t look designed to take 25mm csa. it’s a pig to get the cover back on too would fail the ip4x reg except it is not readily accessible but then thinking about it it is as the meter is designed to accessible to public.

DNO fuse is marked 60/80 not verified even at 60 the “tails” would then be about 10meters and the 6mm earth still an issue

 

[John-SJW]

Thx. I forgot about the tails having to be no more than 3m in length. So the 16mm main supply to the remote CU needs a protective break in it to be within the 3m. Hence a fuse, which will also solve the selectively between the mcb's at the remote CU and 63A mcb at the meter.

It appears best, as been suggested, to remove all, to have a fused switch, so all tripable protective devices are in one location inside the flat. The sub CU has two RCDs. So no going outside (lockable cupboard) to see what the problem is, or setset an RCD. Also the RCD has to be checked every six months by the user, so needs to be accessible.

 

[Strima]

What exactly do you mean by a smart RCD?

Smart RCD | Eurocraft Tech - https://www.eurocraft.co.uk/product-page/smart-rcd

 

[Mark.fardon]

Thx. I forgot about the tails having to be no more than 3m in length. So the 16mm main supply to the remote CU needs a protective break in it to be within the 3m. Hence a fuse, which will also solve the selectively between the mcb's at the remote CU and 63A mcb at the meter.

It appears best, as been suggested, to remove all, to have a fused switch, so all tripable protective devices are in one location inside the flat. The sub CU has two RCDs. So no going outside (lockable cupboard) to see what the problem is, or setset an RCD. Also the RCD has to be checked every six months by the user, so needs to be accessible.

But the problem remains of flouting the regulation requiring concealed cable protection by 30mA RCD ...

 

[westward10]

Submit the Report and worry about solutions if they are required by the client.

 

[John-SJW]

But the problem remains of flouting the regulation requiring concealed cable protection by 30mA RCD ...

It looks like earthing to gas and water pipes may not be a problem. To keep the existing 16mm sub main cable you need to have an RCD at the meter protecting it, as it is under plaster, removing the RCDs at the consumer unit. The flat would only have one RCD.

The cupboard would need a key which is also given to the resident, as they would need access to the meter and RCD to test it every six months and isolate the whole supply if needed. Even if a smart RCD is used, they still need access to fully isolate the supply.

I would assume some labelling stating what the equipment in the cupboard supplies (the flat, not the shop) and a clear label stating that the mainswitch isolates the flat.
[automerge]1591356214[/automerge]
What is the ideal solution, with all the given suggestions?

 

[pc1966]

Smart RCD | Eurocraft Tech - https://www.eurocraft.co.uk/product-page/smart-rcd

It sounds like a great idea, but only 40A rated so no use here, and hardly anyone seems to sell them. mostly eBay

 

[Strima]

It sounds like a great idea, but only 40A rated so no use here, and hardly anyone seems to sell them. mostly eBay

And most of the eBay ones have probably been nicked from BT...

I forgot to mention that their master and slave system is good. Master at source and the slave at the remote DB, only one will trip under fault conditions, so if your master trips it's the distribution circuit and if the slave trips it will be the final circuits.

Hopefully they will up their device ratings so this system could be more widely used in situations likes the OPs.

 

[John-SJW]

I forgot to mention that their master and slave system is good. Master at source and the slave at the remote DB, only one will trip under fault conditions, so if your master trips it's the distribution circuit and if the slave trips it will be the final circuits.

Hopefully they will up their device ratings so this system could be more widely used in situations likes the OPs.

As more outbuildings look to be used, the sooner the better.
As to this specific problem. One solution. Assuming the water pipe can be found and earth cable attached.

• Keep the RCD and 63A mcb at the meter cupboard with the main isolator.
• Use 16mm tails.
• Tidy up the wiring.
• Fit an SPD.
• Connect the earth wires.
• Remove the RCDs at the sub CU to eliminate selectivity problem with two RCDs in series.
• Give user a key to cupboard.
• Label the sub CU, and all inside the meter cupboard.
  

[automerge]1591364876[/automerge]

It sounds like a great idea, but only 40A rated so no use here, and hardly anyone seems to sell them. mostly eBay

Also,
Earth grounding systems: TT and TN-S only

 

[Mark.fardon]

Submit the Report and worry about solutions if they are required by the client.

I’d like to be prepared with a solution !

 

[westward10]

This in my eyes would be a C3 issue and most clients often don't want C3 issues to be overcome.

 

[davesparks]

Smart RCD | Eurocraft Tech - https://www.eurocraft.co.uk/product-page/smart-rcd

Ah I see, at first glance it looks like a cheap imitation of the schneider RED auto-recloser RCD

 

[pc1966]

Seems the Schneider option would be suitable, such as:

18685 - RCCB with re-closer - RED - 2 poles - 63 A - class A - 30 mA | Schneider Electric India

Schneider Electric India. 18685 - RCCB with re-closer - RED - 2 poles - 63 A - class A - 30 mA.

www.se.com

It is wider, so that nasty enclosure and related stuff would need to be changed, and ideally a switch-fuse put in for over current and then the RED auto-recloser to avoid the need for cable replacement, so a bit of space juggling, but it would address the selectivity issue to a large degree (or at least some of the annoyance factor).

 

[davesparks]

Seems the Schneider option would be suitable, such as:

18685 - RCCB with re-closer - RED - 2 poles - 63 A - class A - 30 mA | Schneider Electric India

Schneider Electric India. 18685 - RCCB with re-closer - RED - 2 poles - 63 A - class A - 30 mA.

www.se.com

It is wider, so that nasty enclosure and related stuff would need to be changed, and ideally a switch-fuse put in for over current and then the RED auto-recloser to avoid the need for cable replacement, so a bit of space juggling, but it would address the selectivity issue to a large degree (or at least some of the annoyance factor).

In my opinion that still would not comply, it will not minimise disruption in the event of a fault.
Yes it will auto re-close in the event of a nuisance trip, but that doesn't avoid nuisance tripping in the first place, or leave any circuits live during a genuine fault.

 

[pc1966]

In my opinion that still would not comply, it will not minimise disruption in the event of a fault.

Well in reduces it, but not eliminate it. But to some degree you can say the same about a split-RCD board as not complient due to the impact on multiple circuits of an RCD trip.

Still, it is up to someone else to decide if they want to improve matters and the cost they are willing to bear in order to do so.

The auto-reclose option is a lot cheaper than a new run in SAW and related redecoration, so someone else can decide what to do. Not having to leave the house to reset the MCB or RCD would be a major improvement in practical terms, having a 5 sec gap (or so, presumably depending on how it is configured) in power might be a trade off they are willing to accept.

 

[davesparks]

Well in reduces it, but not eliminate it. But to some degree you can say the same about a split-RCD board as not complient due to the impact on multiple circuits of an RCD trip.

, having a 5 sec gap (or so, presumably depending on how it is configured) in power might be a trade off they are willing to accept.

I can say it, and I do say it, twin RCD boards don't comply in my opinion.

It's only a 5 second gap for a nuisance trip, it's not going to come back on if there is a fault.

 

[pc1966]

I can say it, and I do say it, twin RCD boards don't comply in my opinion.

I agree in the sense that I would prefer to only see all-RCBO boards used, but the majority of opinion in the UK industry is split RCD boards are acceptable. After all, for most folk they sit for years without tripping.

It's only a 5 second gap for a nuisance trip, it's not going to come back on if there is a fault.

True, but for a fault at an end circuit the related house CU RCD will almost certainly have tripped, so at least the remainder of circuits would have power restored. Yes, there are fault cases that would not be covered (e.g. leak on non-RCD circuit, or on the feed cable) but with a simple fuse-fed SWA solution they would probably develop in to a MCB-tripping fault in the not too distant future.

 

[Ian1981]

Not sure why people think dual rcd boards are non compliant, other than regulation 314, as regulation 531.3.5.2 allows several circuits to be protected by the same device except to where particular restrictions for selectivity apply.

 

[John-SJW]

Just curious about the earthing here. Those more into regs can help. The 16mm T&E has a 6mm earth, when say the calc says 10mm is needed. If the CU has a 10mm earth to a water pipe at the far end will all be within spec?

 

[Mark.fardon]

Just curious about the earthing here. Those more into regs can help. The 16mm T&E has a 6mm earth, when say the calc says 10mm is needed. If the CU has a 10mm earth to a water pipe at the far end will all be within spec?

I’ve been thinking about this. The t&e is fed from a DB that has 16mm earth conductor correctly sized. This cable is feeding the DB/CU inside the premises . However the 6mm CPC of the cable is not an earth conductor so sizing relating to main earth do not apply . However does it need to be deemed a bonding conductor? If I can get the water and gas bonded where they enter the property in 10mm then it complies I think ?

 

[pc1966]

However does it need to be deemed a bonding conductor? If I can get the water and gas bonded where they enter the property in 10mm then it complies I think ?

That sound right, if you can bond them at the entry point to the whole building the T&E sub-main CPC would never see the full current of a PME fault so should be fine.

 

[John-SJW]

That sound right, if you can bond them at the entry point to the whole building the T&E sub-main CPC would never see the full current of a PME fault so should be fine.

That means getting a 10mm earth cable from the CU to a water pipe, which may be at the other side of the building. So that means the 6mm earth conductor in the 16mm T&E will not take all the fault current load.

John Ward has a vid doing the calulation of the earthing conductor. I was suprised how small they can be. In this case the impedance will have to be measured or known. Smaller than taking the size from a provided table - the table is for ball park convenience, which says below 16mm the CPC needs to be the same size. But calculate, then you can get it lower. He does the calcs.

View: https://www.youtube.com/watch?v=odnUIWyC0oU

 

[westward10]

The 16.0 cable has a cpc not an earthing conductor.

 

[John-SJW]

• The CPC (circuit protective conductor) is all the CPC conductors of all circuits that together are joined to the main earthing terminal near the DNO's fuse, joining all exposed conductive parts.
• The earth conductor connects to the DNO's earth or an earth rod from the main earth terminal.
• The bonding conductors connect extrenuous metal parts (gas pipe & water pipe, steel frame of building, etc) to the main earth terminal.

That separates them. into logical divisions. Realistically, the term includes the CPC, earthing conductor and the equipotential bonding conductors.

In this case the 6mm 'earth' wire in the 16mm T&E looks to be an earth, not a CPC as they in this installation are after the CU all joined at te CU's earth bar. Because it is in a T&E cable does not automatically make it a CPC.

 

[davesparks]

• The CPC (circuit protective conductor) is all the CPC conductors of all circuits that together are joined to the main earthing terminal near the DNO's fuse, joining all exposed conductive parts.
• The earth conductor connects to the DNO's earth or an earth rod from the main earth terminal.
• The bonding conductors connect extrenuous metal parts (gas pipe & water pipe, steel frame of building, etc) to the main earth terminal.

That separates them. into logical divisions. Realistically, the term includes the CPC, earthing conductor and the equipotential bonding conductors.

In this case the 6mm 'earth' wire in the 16mm T&E looks to be an earth, not a CPC as they in this installation are after the CU all joined at te CU's earth bar. Because it is in a T&E cable does not automatically make it a CPC.

The protective conductor in the T+E is the circuit protective conductor of the distribution circuit which feeds the CU. This may, or may not also need to perform the function of a main bonding conductor.

 

[Mark.fardon]

The protective conductor in the T+E is the circuit protective conductor of the distribution circuit which feeds the CU. This may, or may not also need to perform the function of a main bonding conductor.

Hi Dave please clarify - under which circumstances would athe CPC need also perform the function of a bonding conductor given this arrangement? ? We are not “exporting” the PME earth to an outbuilding here this is all internal to the same property.

 

[westward10]

Hi Dave please clarify - under which circumstances would athe CPC need also perform the function of a bonding conductor given this arrangement? ? We are not “exporting” the PME earth to an outbuilding here this is all internal to the same property.

You never export a PME earth you merely utilise a cpc which is derived from a PME earthing.

 

[pc1966]

It might be pedantic, but only the DNOs are allowed to run PME / TN-C networks.

 

[Mark.fardon]

You never export a PME earth you merely utilise a cpc which is derived from a PME earthing.

That’s why I put the word in inverted commas. But if, for example’, “utilising a cpc which is derived from PME supply” to a detached garage with say metal foundations needing to be bonded the cpc could also act as a bonding conductor if adequately sized for example by using 3core SWA

 

[John-SJW]

Back to the problem of this topic. The 16mm T&E cable that runs from the meter to CPU is an "earth". All the circuits in the CU (via their CPCs) run back to the DNO's earth terminal at the meter via this T&E. The T&E is extended meter tails, although split with protection, meter tails.

Bonding of the gas pipe near the meter does not use this T&E cable. If the water pipe is bonded at the other side of the building, this does not use it either. The gas & water bonding just gives gas and water pipes the same voltage potential as electrical system metal parts in a fault situation, so a full 240v is not running right across your heart killing you.

So, the size of the earth cable needs to be calculated - see John Wards vid that I posted. In his calculation I was surprised he came up with a size less than 6mm. If that calculation is applied to this installation with a result that the size is less than 6mm, then the cable can be left in place.

All it needs is RCD protection at the meter to protect the embedded T&E. Looks like a Smart RCD (auto resetable) will solve it with mcb's fitted at the CU. The 63A mcb can be ditched as long as the MCBs at the CU are not all far higher than they need to be, so the DNOs fuse does not blow.

 

[pc1966]

The 63A mcb can be ditched as long as the MCBs at the CU are not all far higher than they need to be, so the DNOs fuse does not blow.

No, if you have more than 3m then you must provide protection against a short circuit on the cable run. You cannot rely on the DNO fuse for that as it is not under your control and could be changed by them at any point in the future.

Ditching the MCB for a fuse would result in better selectivity so it is a good plan, but unfortunately there are virtually no fuse options to match typical CUs, and for currents above 32A typically they are 1.5 or 2 "modules" wide.

In that case your real problem becomes space to fit both a fused-switch and RCD, in particular as the only auto-recloser one to meet the 63A or so rating appears to be the Schneider one that is 4 modules wide already.
[automerge]1591563212[/automerge]
That said, if the DNO can arrange to pull the fuse, you could replace the DP isolator with a DP fused-switch, then there is probably enough room for a typical 4-module "garage CU" to house the auto-recloser RCD and tidy up the mess currently present.

 

[John-SJW]

OK, keep the 63A mcb adding a smart RCD, then: overcurrent, L to N fault and L to E fault protection. Have only mcbs at the CU. All providing the 6mm earth is fine. And install 16mm tails. Cold be conflict with CU mcbs and 63A mcb.

As T&E is under plaster, need RCD protection?

I thinking, is a way to keep the T&E which is a major job to replace.

 

[pc1966]

As T&E is under plaster, need RCD protection?

That is the underlying problem!

If it were in SWA then only the fused-switch for hard cable faults would be needed, so no real selectivity issues with the CU on MCB or RCD action.