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Fundamentals: Are pvc cables rotted by polystyrene

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  1.  
    • CommentAuthortony
    • CommentTimeJul 17th 2015
     
    This idea does have some basis in that polystyrene does absorb plactisiser from the cable.

    This is a physical process, not a chemical reaction.

    We need evidence that it causes a problem, to date there is none

    Loads of pic cables have been laid touching polystyrene, I have seen some.

    There clearly is not a real problem or we would have these all over the world.

    As the plasticiser in the cable migrates into the eps, the eps shrinks back and in the vast majority of cases no longer remains in contact and deplastisation ceases.

    It would require a lot of effort to deplasticise a pic cable enough to make it brittle, it would not happen in the real world

    Any budding researchers reading this?
  2.  
    All residential cables here are sheathed in nylon, thus avoiding this problem.

    The conductors are insulated in PVC, but the sheaths are not - plus we get different colours that tell you the current ratings in a nice visual way :)

    Paul in Montreal.
    • CommentAuthorEd Davies
    • CommentTimeJul 17th 2015
     
    Previous discussion:

    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=1521

    I did a search before about this and didn't come up with any specific stories about it actually happening (really expected to see some horror pictures). However this, from a New Zealand cable manufacturer, seems pretty clear:

    http://www.generalcable.co.nz/getattachment/0ebda90b-b4f8-43ae-b622-c9280295116b/PVC-Cables-in-Contact-with-Polystyrene-or-Bitumen.aspx

    “Commonly used PVC plasticisers tend to exhibit solubility in polystyrene and polyurethane insulating
    materials, …”

    “The migration process would be expected to occur over a relatively long time frame, there is anecdotal evidence that rapid migration can occur in polystyrene block houses, portable buildings and chillers. This is due mainly to the large contact area between the insulating material and the cable.

    The service life of PVC insulated and sheathed cables in the above situation are dependent upon the conditions of exposure and many years may elapse before noticeable deterioration of the PVC occurs. However, cable lifetime could be significantly effected [sic].”
    • CommentAuthortony
    • CommentTimeJul 18th 2015
     
    Are they saying no evidence or no problem as the timescale is long?
    •  
      CommentAuthorfostertom
    • CommentTimeJul 18th 2015
     
    Even electricians confuse this issue with the one about having to uprate cables that are surrounded by insulation in general.
    • CommentAuthorEd Davies
    • CommentTimeJul 18th 2015
     
    Posted By: tonyAre they saying no evidence or no problem as the timescale is long?
    No, I think they're saying there is a potential problem and that there's anecdotal evidence that it can, in some circumstances, be quite rapid. Though I tried to extract the important bits I'd suggest reading the whole thing, it's not long.

    News, to me, was the possibility of problems with polyurethane and bituminsed building paper.
  7.  
    Yes, this is the first time I have heard of Pu being an issue. That's a bugger!
    • CommentAuthorowlman
    • CommentTimeJul 18th 2015
     
    Yes, I wonder how many cables have been "tacked" in place on initial fix with expanding PU foam.
  9.  
    Celotex say it is not an issue.
    • CommentAuthorbarney
    • CommentTimeJul 20th 2015
     
    I've seen plenty of PVC cables degraded by expanded polystyrene insulation - it migrates the platiciser out of the cable - it's prevalent in the refrigeration sector if you want real examples. You can also fine examples of "green goo" - although that is degraded di-isoctyl phthalate. Found in cables made from 1965-1971 used to uprate the temperature performance of the PVC cable - it's a reaction with the copper and not with any thermal insulation

    Most cables (today) either have a modified plasticiser or are cross linked polyethylene - so it's not an ongoing issue - but temperature is a trigger for damage

    Derating of cables when surounded by any thermal insulation is a different matter - that applies to any insulation type

    Common sense says do not put the cables in the insulation zone - use a service void

    Regards

    Barney

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