Discuss Selectivity on concealed sub main supply in the Electrical Forum area at ElectriciansForums.net

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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 ...
 
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.
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What is the ideal solution, with all the given suggestions?
 
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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... :D

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.
 
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.
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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
 
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This in my eyes would be a C3 issue and most clients often don't want C3 issues to be overcome.
 
Seems the Schneider option would be suitable, such as:

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).
 
Seems the Schneider option would be suitable, such as:

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.
 
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.
 
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.
 
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.
 
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.
 
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?
 
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 ?
 
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.
 
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
 

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