Polystyrene insulation and electric wires

[The Bear]

I seem to remember reading (not sure if here or elsewhere) that you shouldn't have electric wires up against polystyrene insulation. Something to do with the plastic around the wire melting I think??

Having spoke to an electrician who wired my shed last week he said there was no problem, "if the wires were getting hot enough to do that there's a problem with the electrics".

Anyone know if I can use polystyrene as insulation around these wires?

Mark

 

[Woody Alan]

A misunderstanding by your electrician, and he should know better. Polystyrene can give of a chemical that will turn pvc insulation into a jelly, it's nothing to do with the heat given off by the cable. (for reference all insulation will lower the load rating of the cable).
http://www.voltimum.co.uk/news/6419/s/E ... stems.html
There's loads on the web but that's a random quick link for you.

Alan

 

[head clansman]

hi

i think the electrician is right if the wireing can do that then you have a wiring problem . i think you should think along the line of polystyrene tiles like on the ceilings , when there a fire it wont be the fire that kills you but the toxic fume that comes from the melting polystyrene i can remember something like two minutes or so and your dead. hc

 

[The Bear]

As I suspected Alan, thanks.

I'll use the polystyrene elsewhere and use something else near the wires

Mark

 

[jhwbigley]

why not use celotex board?

John-Henry

 

[Night Train]

Adding insulation of any sort will affect the load capacity of a cable. Essentially the cable is supposed to be in free air for it's rated capacity in order to disipate heat. So if a surface clipped cable is running happily near its rated load then insulating it will cause it to fail due to overheating.

It is analogous to you working hard in the out doors in a tee shirt. You can do that for a long time quite happily. If someone wrapped you in a duvet and then expected you to do the same task then you will overheat pretty rapidly.

The wiring regs will give you formulae to work out the size of cable you need for your given load, over a given length, with the cable in free air and a selection of insulation types.
It varies for different cable types and the orientation of the cable too.

 

[Shadowfax]

Firstly, don't use polystyrene for anything that might lead to it being involved in a fire. It's just horrible stuff!
Secondly, I can't remember which way round it occurs but if you leave polystyrene in contact with PVC it is possible that polymers from one of them migrates to the other. This leaves the PVC weakened and it can crack (no jelly involved).
I actually contacted Pirelli Cables some time ago to ask about this problem and the boffin there told me that it is not such a big deal as it has been talked up to be. Basically, if the cable is not moving when it is in contact with the polystyrene there is very little chance of any relevant damage being done. On the other hand if movement is involved the cable insulation might be affected.
This could explain why some new appliances are delivered with defective flex where polystyrene packing has been in contact with the flex and the package has been moving about.
Food for thought, anyway.
Cheers,

SF

 

[Woody Alan]

weakened and it can crack (no jelly involved).

I dare say it can and will but that's no reason to dismiss what I said.

The company had other products to rely on, some in lucrative growth markets. After tires, a second key product was polyvinyl chloride, PVC. In experiments to improve the bonding between metal and rubber, company scientist Waldo Semon in 1926-1927 stumbled on a means to turn the polymer of vinyl chloride into a flexible, jellylike plastic. Managers did little to commercialize plasticized PVC until the late 1930s. World War II highlighted PVC's advantages, as the military funded rapid construction of production facilities. A key initial use was the coating of electrical wires and cables. By around the end of the war, Goodrich apparently had a capacity to produce annually over 10 million pounds of PVC. By 1966,

I have seen this effect on faulty telecommunication cables made by Pirelli and it has the result of altering the PVC to a more jelly like state.

Alan

 

[Shadowfax]

Alan, I didn't mean to dismisss what you said, However, it did come over that way on rereading my post. My apologies.

However, the guy I spoke to, (in a professional capacity rather than just a chat) did say that the effect (only concerning polystyrene) was one of minor cracking rather than anything else. At the time I was prepared to take his word for it because it would have a bearing on how I approached some crucial electrical works.
I do hope he was right, in fact. Otherwise, there must be some cables sitting around in various buildings in a very interesting state!
Cheers,

SF

 

[dedee]

I heard about this when I was insulating my workshop with 1" polystyrene insulation. I made a sandwich with an length of power cable sandwiched between to pieces of insulation and taped together. After 9 years there is no sign of deterioration of the cable.
I'm no expert and not saying degradation wont or cant happen.

Cheers

Andy

 

[wudbutcher]

Hi all

been lurking around the forum and using other peoples expertise for a while now thought I would empart some of my expertese working with volts for a living. it is true that PVC cable will deteriate in polystyrene insulation see below.

Question: Does PVC cable require to be inside conduit if passing through polyurethane insulation?

If PVC cable passes through expanded polysytrene insulation, it requires to be in conduit to prevent migration of the plasticiser from the cable to the polystyrene. Is the same true if it is in contact with polyurethane of polyiso (rigid polyisocyanurate)?
Answer: Only polystyrene thermal insulation is of serious concern for PVC sheathed cables. In general it is not required to place PVC sheathed cables in separate conduit if passed through polyurethane or polyisocyanurate thermal insulation. The thermal insulation polymers are different and do not have residual chemicals present that are found in polystyrene thermal insulation.
Thermal insulation manufacturers have confirmed this position with BASEC.

However I seem to remember that there is some cable available that will not deterioate in polystyrene. I will ask at the wholsaillers

Cheers

Gavin

 

[Tony Spear]

migration of the plasticiser from the cable to the polystyrene.

Cheers

Gavin

I know one shouldn't mention the word "fishing" on here, but any experienced fly-fisher will tell you that PVC loses it's plasiciser over time even in free air. The coating of most modern fly lines is basically PVC and after a couple of seasons they can actually crack quite badly. Luckily we can buy a goo that you wipe on the line to replenish the plasticiser and stop this happening. Not much good for miles of PVC/SWA/PVC though! :wink: