Protecting CU tails - possibly one for ARW

[nothanks]

I need to install a second CU on the opposite side of an inner (cavity) wall to the side that has the: meter, isolator switch, Henley block and existing CU. Once the new CU is in place I'll be transferring several circuits to it and the old one will just be for the garage and outbuildings. Currently the walls are bare block but will either be plastered or dry lined later. The question is how to protect the tails between the Henley block and the new CU, and how to mount the CU. The distance will be about 3m.

The current plan is to sleeve the hole in the wall with waste pipe and to either use metal trunking or wooden boxing on the walls to protect the tails and provide access when the walls are plastered. Above the CU I plan to provide a boxed-in area (with a removable lid) for the cable drops.

I presume a flush-mounted CU is the way to go, but I'm open to other suggestions. Are any particular types recommended? I'll probably have a mix of RCBOs, MCBs and RCD ... I'd like to go all RCBO but the cost is very high.

[John Rumm]

If the tails will be covered in, then they need to meet the requirements for protection of concealed cables. i.e. buried deeper than 50mm or protected by earthed metal enclosure, or have RCD protection with a I presume a flush-mounted CU is the way to go, but I'm open to other

RCBOs are not that expensive now, so worth looking at. You could surface mount the CU on the finished wall but allow cable entry from the rear. That looks neater and there are no worries about adequately sealing access to the CU via the knock-outs at the top.

[stephenten]

You need to provide mechanical and/or electrical protection to the meter tails.

I did this by using 3mm thick stainless steel as (a) stainless steel blunts most drill bits apart from cobalt ones and (b) does not corrode when plastered over.

I also attached a 10mm earth wire to the stainless steel sheet and connected it to the met ( main earth terminal)

As for rcbos, I got these for ten quid each plus vat so filling a cu with rcbos is not really a bank breaker.

[arwadsworth2020]

Screwfix are selling BG RCBOs for £11.99 at the moment and BG main swi tch only CU with 10 spare ways is only £32.

[Fredxx]

SWA is perhaps an alternative. Given the bending radius I might go for 2 x 10mm^2 in parallel, which if clipped direct will be more that the company fuse.

The alternative is 3mm of steel for mechanical protection if the cable is hidden under plaster etc.

[Tim Watts]

There are issues with paralleling conductors and it's not standard practise.

I'd just surface run it in nice looking trunking (D-Line) or bendy flexi conduit (Kopex is very heavy duty, although plastic, it will survive some quite ferocious beating - does not count as mechanical protection for burying though).

[nothanks]

For some reason I was thinking that I might need access to the tails, but I was being dumb. My only concern about burying steel trunking is that the smallest regular rectangular stuff I can find is quite big and fairly easy to drill through, and the round conduit for 2 tails would be too large (or would need individual conduits). Is suitable rectangular trunking available? (Mr Google has not been helpful)

There will be 10-18 circuits (depending on whether/how I group them) so RCBO cost is not insignificant.

It's going to be quite a while until the rest of the room is ready for plastering so surface mount is not an option ... hmmm, unless I mount it on a temporary backing until the plastering is done (I hadn't thought of that until now).

[nothanks]

I understand about the need for protection, but this must be a common problem so I'm surprised that I can't find suitable "off the peg" trunking.

What make were the RCBOs? I'll need between 10 and 18

[nothanks]

I was going to steer clear of BG because a few people have said they've had problems with reliability. 10 ways is unlikely to be enough.

[nothanks]

I'm surprised that I can't find some suitable rectangular steel trunking

- my google-fu must be weak today.

[spuorgelgoog]

Put the lights on RCBOs (most disruption if they fail) and the cooker and immersion heater on RCBOs (most likely to have an element fail), and the rest can share a couple of RCDs.

Owain

[Tim Watts]

No - can't do that if using metal conduit - eddy currents from unbalanced conductors cause heating. All current carrying conductors on a circuit must pass through the same metal conduit or trunking or hole in a metal box (unless you cut a slot between 2 adjacent holes)

[Tim Watts]

Do you need metal trunking? Surface mounting not an option?

[nothanks]

I used to be involved with EMC so was very familiar with Mssrs. Lenz, Faraday et al but I hadn't considered that there would be a practical problem with conduit heating at the currents and distances involved. I don't plan to use individual conduits, but it would be interesting to understand more - any references to the problem?

[nothanks]

That's more or less what I'd planned, but with the fridge and freezer also on an RCBO.

[stephenten]

Denmans do a nice looking curve consumer unit that has a door which covers the main switch and all the rcbos.

I have the cur-m18 version and this just has the main switch and room for up to 18 rcbos. I now have 16 rcbos in it with room for two more.....

[Andy Burns]

17th ed 521.5.1 does say that, I seem to remember some discussion of the fancy insulated grommets with individual entries for tails within a larger knockout? e.g.

Are they standard with new all-metal consumer units?

But Mr Flameport showed it didn't make a lot of odds to use two knock-outs

[arwadsworth2020]

What problem?. Google for the John Ward eddy current video.

[John Rumm]

[John Rumm]

Probably not an issue with domestic kit and the typical currents involved. May prove relevant on industrial installs.

You may find this interesting: