Just curious!

[Allenm]

Hi there,

I'm curious to know the point of Plastic oval conduit when running cable in a plastered wall. I understand that it is what should be done, but can't really fathom why. A drill bit or an enthusiastically hammered nail is not likely to be stopped by a bit of plastic (or is it?), it just seems to be a bit more sheilding to fool the multitude of pathetically weak metal/volt detecters out in the DIY world.

Now please don't shoot me down. I am not asking to be in any way "smart" or agumentative, I am fully expecting an answer that is going to make me think "Oh yeah, of course, how stupid am I?". But if you don't ask these seemingly stupid questions, you never expand your knowledge.

Thanks
Merv

 

[Twist 'n' Tape]

The major advantage of oval conduit is to make a future rewire possible without disturbing the plasterwork/wallpaper.

 

[Screwi]

Something to do with allowing spiders to move house with ease...

 

[steveg1957]

If said nail goes through cable from enthusiastic DIY you will be very glad a new piece of cable can be pulled through the said conduit ! (Hopefully!)

 

[andy]

If said nail goes through cable from enthusiastic DIY you will be very glad a new piece of cable can be pulled through the said conduit ! (Hopefully!)

providing they have some common sense and remove screw/nail and put it somewhere else

 

[Allenm]

So if the point is simply to make future work easier and less destruptive to the decor, why is it mandated in the regs? Or did I simply read too much into what was advice?

There was me thinking that oval shaped plastic had some sort of magical strength!

Thanks again with your patience in answering my silly questions.

Merv

 

[Lectrician]

Oval conduit or capping is used to prevent damage from the plasterers trowl during plastering. As a bonus, it aids alterations.

You can ommit the protection if you can assure no damage will occur (If you are plastering yourself, or are using a dry-lining method.

 

[stevesey]

I remember being told years ago that plaster could, given enough time, etch into the outer sheathing. Sounds a bit of an old-wives tail to me, but probably has some truth if lime plaster and rubber cables are involved.

 

[sparkybird]

As I understand, it used to be that stuff in plaster would eventually 'attack' the protection around the cable, but this is no longer the case. However, I agree with Lectrician - it can save your cables from any potential damage from over enthusiastic plasterers.

As far as protection from nails etc goes, the Regs stipulate where cables can be buried at a shallow depth (eg vertical and horizontal runs from switches etc) which should tell Joe Bloggs where not to put a nail, although I've often wondered how many people know this!

As for pulling new cable through the old plastic conduit, I've never seen this done with any great sucess - steel maybe, but not plastic.

 

[BenStiller]

you can just about get your cables in them from the beginning, never mind if you have to replace them, came across a situation like that few weeks ago, cables were well and truly going nowhere

 

[HDRW]

you can just about get your cables in them from the beginning, never mind if you have to replace them, came across a situation like that few weeks ago, cables were well and truly going nowhere

I've found they go in reasonably OK if you pull them - trying to push them gets you nowhere!

Cheers,

Howard

 

[Diyisfun]

If you put washing up liquid on the new cable it will pull through a lot easier.

 

[mapj1]

If you can't re-thread it, it was probably installed ready threaded, and plaster has since got in, or it is more than the 50% or so filling factor, or its longer than the recommnded length without an opening.
Note that both oval conduit and capping are covered by the cable grouping factors if cables from different circuits share a conduit.

 

[ban-all-sheds]

Ah - but what if you have 2 cables from the same circuit, i.e. a segment of a ring final or a radial, going down the conduit to a socket, and then back up again?

Both cables will be carrying current....

 

[mapj1]

Now, now, B-A-S you and I both know the reg writers boobed on that one.
Yes, to the letter of the regs, its allowed, because it is the same circuit, and if the ring main is uniformly loaded it works out OK with the cable ratings really, (except for the drop to the CU of course where 2 fully loaded cables are side by side) but I agree there is a valid set of load positions that could overheat the cable and trip nothing.
But then all Ring mains have that problem anyway if all the load is at one end.
BUT the nearly nil incidence of problems caused by this even when no group9ng factors are applied, means that its not worth worrying about, and ring mains are not nearly as bad as they look on paper. (though there are many reasons why the rest of the world have made it easier on themselves by only accepting radials)