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