Polystyrene and plastic capping

[Johnboy54]

Hello to all.
Read the archives about polystyrene melting cables.
I don't seem to be able to find a definitive answer?
Can cables be covered with plastic capping or is that the same material as the cable.
It says conduit can be used, but is that steel conduit or plastic?
Would steel capping be just as good if it has to be steel?
Thank you.

 

[ColJack]

as long as the cable doesn't come into contact with the polystyrene then it won't affect it..

it's the cables melting the poly, not the otherway round..

don't forget to de-rate the cables to account for the insulation..

 

[bongos]

Hi John.

What exactly is it you want to do?

Polystyrene does cause problems for cables overheating.

But plastic capping is just that - plastic. It's commonly used to cover the cables to add a little extra protection - some argue to stop the plasterers taking gouges out of the cable. Doesn't stop the need for a RCD to be fitted if cables are less than 50mm from the surface.

Steel capping is much the same as plastic capping apart from the obvious... . Again you would still be required to install an RCD if cables are less than 50mm from the surface.

Steel conduit offers excellent protection for the cable but is less commonly installed in domestic properties because of having to chop it into the walls to be able to plaster over the top. Assuming this is correctly earthed then a RCD may not be required (on certain circuits).

 

[ColJack]

i think he wants it as a barrier to stopthe cable touching the poly and the chemical reaction that melts the poly..

 

[Johnboy54]

Sorry.
Having read your replies and my question, I realise I wasn't clear.
Yes ColJack, you're right. It's to stop them touching.
I'm dot dabbing with polystyrene-backed boards.
I've spent days trawling the net for an answer.
As for which affects what -
I came across loads of photos where the cables were melting to the point of needing a rewire - which can happen pretty quickly.
There were a few where the opposite was true.
Cables laid on polystyrene boards in lofts were melting through the boards but not affecting the cables.
My walls are sandstone, so I thought it would be a simple matter of clipping my cables to it and dot dabbing.
Now I know it's more complicated than that.
I can't find anywhere that says cables can't be covered in plastic capping or plastic conduit to stop the cables melting.
Then again, I can't find anywhere that says they can.
Anyone else come across this?
Thanks.
John.

 

[RF Lighting]

Part of the reaction between the polystyrene and the cable leeches the plasticisers out of the cable which leaves it very brittle.

Covering the cables with plastic cappling or conduit would be fine to stop any damage occouring to the cables.

 

[Johnboy54]

Thanks, RF, that's me sorted.
Ahd thanks to the others as well.
John.

 

[JBPElectrics]

You want 20mm oval conduit, 90p for 3 Meter lenghts.

CBW

 

[ColJack]

now.. is this actual polystyrene on the back of the boards ( white bubbly stuff ) ?
it's ususal for foam backed boards to hare PIR or PUR foam.. a yellow or pink coloured foam..

 

[ban-all-sheds]

If you haven't got to bury it use steel conduit, at least for all the lighting cables.

 

[Johnboy54]

Well, I thought I was sorted.
Don't know what type of foam, not seen it yet.
However, during my trawling of the net, BASEC say that even Kingspan etc can cause the same effect. They were in the process of consulting manufacturers.
The oval conduit suggestion sounds fine.
Not sure why I should use steel conduit for lighting though?
Is that because of the cable/thermal rating?
Thanks to all once again.
John.

 

[ban-all-sheds]

Not sure why I should use steel conduit for lighting though?
Is that because of the cable/thermal rating?

No, it's so you don't have to have your lights on an RCD.

 

[ban-all-sheds]

However, during my trawling of the net, BASEC say that even Kingspan etc can cause the same effect.

Ummm...

http://www.voltimum.co.uk/news/1275...protection-damaged-by-other-insulations-.html

??

 

[Johnboy54]

Thanks for the reply about lighting and RCD.

I looked at the BASEC link.
It's obviously a newer page then the one I found.
Although, it's not conclusive, is it?
BASEC say they are to undertake more research.
Whatever the outcome, at least you lot have managed to give the solutions to my immediate problem.
Thank you very much to all.
John.

 

[ban-all-sheds]

Aerogel