Preferred method of extending ring main?

[tomsavage]

Reading the different posts about extending a ring main, it seems this can be done either by creating a localised 'loop' to and from a single point, or by taking out a link between two existing points and replacing this with a new cable that runs through new points.

I would like to add at least 3 new points within a 3 metre stretch between existing points, and have managed to trace which cable runs between them. I would just like advice on whether there is any preference to using either of the methods described above, and if so, what the reasons for this are.

I should point out that the ring main is currently wired in conduit wire for live and neutral, and the earth is run via the metal conduit. I know that this is not a satisfactory method, but until I have the money to get the house rewired, this is what I'm stuck with. Thus whatever method I use, I realise I will have to preserve the integrity of the conduit. I plan to use T&E cable to extend the ring, and to ensure a good connection of the new cable to the earth terminal in the back of the existing metal boxes.

Anyway, here is the current situation:

Here is the 'loop' solution:

And the other:

It will not be any more or less disruptive to use either method, as the wall is being replastered anyway. Your comments much appreciated, including on the earth-via-conduit situation.

 

[JohnD]

Your third drawing looks satisfactory.

It might be better to draw out the old cables that are no longer in use.

I don't think you've got it right about looping to a single point. A spur is not looped and can only have one outlet on it, unless it is a fused spur.

Incidentally, you might find you can poke an earth wire down the conduit if you are not satisfied with the current earthing. 2.5mm single would be ample. If the existing circuit is reasonably modern and was professionally installed, it will have brass glands and locknuts at every entry of conduit into a backbox, and will be OK unless someone has damaged or removed one. If you leave the conduit in place between the existing boxes it will continue to give earthing continuity through the conduit, which is no bad thing.

And, if you are not sure the current earthing is good, a socket Tester would be a start and should cost less than a tenner.

 

[Adam_151]

It would be so much nicer to do the additions in conduit

(but don't chase it in horzontally between points, AFAIK you are not suppose to chase more than 1/6 of the thickness of the wall horzontally, and 20mm conduit into 100mm block doesn't make it too easy... )

Conduit is perfectly acceptable to be used as an earth, just make sure its not corroded or loose at joints... and ideally you'd test ELFI as well (but if ELFI was ok when installed and its not corroded... I can't really see why it would be any different now) , it should not bee too difficult to pull an earth core into it if need be though, and I'd probably recommend to do this on any additional consuit you install

 

[Manty]

I've recently done something similar to this in my kitchen, but after reading the above post I'm a little concerned that I may have wasted my time and broken rules.

I have placed 2 double sockets (1 with a spur to an FCU above cabinets for lighting) and another FCU (feeds double socket under worktop) on this new part of the ring, in total the length from the socket where I'm planning on breaking in is about 2.5 metres.

I've not made the new part of the ring live yet, was going to join one end with the existing socket and the other end with a 30amp terminal inside the socket box.

The reason I ran it horizontally across is because I cannot get under the floor due to it being inaccessible... is it permitted in this case?

Or should I have chased down beneath worktop level, ran the cabling on the surface and then back up?

Edit: forgot to add, I buried the horizontal runs beneath lengths of steel capping.

 

[tomsavage]

I take your point about conduit Adam, but I'm not sure quite how I'd run the conduit if a horizontal chase is not allowed! It's into brick, and is a partition wall between us and next door, so I'm not sure I want to chop much more away than I have for the T+E. Thanks for the observations about conduit, it all looks in good condition, but I will get the earth tested (or get one of those gadgets JohnD recommends, thanks!).

To answer JohnD's point about my 'loop' suggestion... sorry for my poor description, no idea how to properly refer to what I was meaning! I guess I want to avoid using spurs, particularly as I am adding double sockets, so I thought I could use a connector block and localised 'loop' to extend the ring. i.e., like this:

As it is, I shall probably go for the third drawing above, as you suggest JohnD, it looks the better option.

Finally, as for removing the unused bit of cable, or pulling an earth cable through the conduit... I don't know if it's the cable, the conduit, or the way I'm doing it, but it seems pretty impossible to shift stuff that is in the conduit. The thought of feeding more cable through fills me with horror!

Thanks for the input.

 

[tomsavage]

Manty, if I understand Adam right, he is only saying that a horizontal chase is not permitted if it is deep (i.e., deep enough for the thick metal barrel conduit that my house has). A chase into the plaster for T+E is permitted as far as I know, and in fact this *should* run horizontally between points to ensure it is in the 'safe zone'.

 

[Manty]

Manty, if I understand Adam right, he is only saying that a horizontal chase is not permitted if it is deep (i.e., deep enough for the thick metal barrel conduit that my house has). A chase into the plaster for T+E is permitted as far as I know, and in fact this *should* run horizontally between points to ensure it is in the 'safe zone'.

Ah right, that makes sense then, sorry to hijack your thread.

 

[tomsavage]

Hijack away! It's often the interjections that bring up the interesting points.

Out of interest, what made you decide to use metal capping, neatness? As far as I understand it, plastic/metal capping/conduit is not required when chasing cables into plaster. However, if cables run outside of the safe zones (e.g. diagonally across a wall) then they should be protected by hefty steel conduit. So my plan is just to clip the cables in the channels then plaster straight over them! Hope this is acceptable

 

[Adam_151]

Capping serves a few purposes...

- It protects the cable from the plasterers trowel while the wall is being plastered.

- It protects the cable insulation from any chemical effects the lime in the plaster might have on it

- It makes it possible that if need be in future some slack can be pulled through or even a new cable pulled down in some cases, with plastering directly in you have no chance.


Capping does not offer mechanical protection, and the earthed metal screen idea* (if you earth it) is on decidedly shakey ground with capping[the criterion is, IIRC a screen thats suitable for use as a CPC, and you wouldn't use capping as a cpc...], so consider it as providing no additional protection and make sure capped cables are still in safe zones Though if you use metal capping I can see some advantages to earthing it, but its not required, and in the case of PVC capping, rather ineffective


*not sure I like the idea of running the likes of MICC out of safe zones although I'm pretty sure its allowed, its not going to be expected out side of zones, and a drill will go through it with ease and while I realise that the earthed metal screen critea which allows it means you arn't going to get a live fixing, it would be bloody annoying....

 

[Manty]

I used the capping more for the first two points, as it was me doing the plastering

 

[tomsavage]

I have cut channels to the depth of the plaster (10-15mm) which seems about right for the 2.5mm2 cable plus a layer of browning which I will carefully apply! It shouldn't then be an issue for the plasterer to run a skim over this, as the cables will already be covered. As far as I'm aware, the lime/corrosion issue doesn't apply to plaster used these days? But I could be wrong.

I think to use the the oval plastic conduit as mentioned would be nice, and would also allow for the 'leave some slack cable' suggestions, but I just can't face cutting sufficiently deep into the brick to allow the use of this stuff. Maybe I should though... Does anyone know if there is anything one can use to fix the conduit to the back of the channel and stop it raising proud?

 

[JohnD]

You can use plastic cable clips, I think the 10mm or 16mm T&E sizes fit, but these stick out a lttle more.

Provided the channel is deep enough for the conduit not to be trying to spring out, you can hold it in place temporarily, dab on some plaster except over your temp supports, them when it's hard, remove the temp supports and plaster over the gaps.

This is one of the things an amateur can do that a pro wouldn't, because the extra time isn't important.

If you find plaster difficult to work with, you can use sand and cement with PVA, provided you leave it below the level the plaster will skim over later.