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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthortony
    • CommentTimeFeb 19th 2022
     
    When PVC cables come into contact with expanded polystyrene a physical reaction does take place. This is NOT a chemical reaction. What happens is that some of the plasticiser in the PVC migrates (technically probably diffuses) into the polystyrene which makes it appear to have been attacked. In the process if a gap forms the process stops.

    There is no evidence that this is dangerous as only a small portion of the plasticiser is lost and only from the outer sheath of the cable. The polystyrene is thinned a tad.

    Is anyone aware of a resulting problem?
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 19th 2022
     
    Thanks tony - reality check!
    • CommentAuthortony
    • CommentTimeFeb 19th 2022
     
    Two degrees in Chemistry help
    • CommentAuthorMike1
    • CommentTimeFeb 20th 2022
     
    Posted By: tonyIs anyone aware of a resulting problem?

    I seem to recall that the plasticisers used were changed years ago (70s?) to reduce the problem.

    Posted By: tonyIn the process if a gap forms the process stops.

    ...however if the cable was loose-laid on polystyrene - in a loft, for example - the cable may move as the polystyrene shrinks back. Or if the polystyrene becomes sticky, as it might, it could stick to the cable. Either could prevent the process stopping, in theory.

    It would be interesting to see if anyone has any real-world fire reports though.
    • CommentAuthortony
    • CommentTimeFeb 20th 2022
     
    I have been searching for such reports for years and never found any. Were there any I would expect them to be sufficiently publicised for us to know about them therefore surmise that there aren’t any
  1.  
    I have found many examples of EPS and XPS sticking to cables (or vice versa!) but the most surprising one was when we were working in an apex roof void and there were several sheets of 600 x 2400 x 50 XPS, presumably the result of good intentions to insulate sloping soffits, never enacted. We needed them out. I moved a couple then picked up another. It lifted about 200mm and then stopped. Couldn't work it out for a minute until I realised it was glued to 2 cables, very firmly. We had to prise it off, mm at a time. The PVC sheathing was 'shiny' but not, apparently, brittle.
    • CommentAuthorbhommels
    • CommentTimeFeb 20th 2022
     
    Hearing about this effect I have installed a soak test in my shed: a 200x200x200mm block of graphite EPS with a cable threaded through multiple times. The cable sits very snugly in the holes.

    4 years on (and counting),the cable is still fine. I can take it out, it is not sticky and I can bend it around without evidence of any cracking of the outer sheath.

    I am definitely not denying that the effect plays a role. As Tony says, it is likely driven by diffusion and that is strongly temperature dependent. My shed is shaded and does not get very hot in summer, it gets humid in winter, and is probably not the ideal site for accelerated tests, the timescales could be very long.

    I have installed conduits around all (but two!) cables in contact with EPS as I think it is good practice in any case. Better be safe than sorry.

    One bit of additional information that could be related: when trying to get my leftover graphite EPS recycled, one plant refused to take it on the grounds that it was produced before a certain year, and thus might contain a compound of which I forgot the abbreviation. If this compound has a high diffusion constant for common cable plasticizer it could be responsible for the effect. Then, it might not :-)
    • CommentAuthorowlman
    • CommentTimeFeb 21st 2022
     
    Could EMF intensify the effect?
    • CommentAuthortony
    • CommentTimeFeb 21st 2022
     
    No
    • CommentAuthorowlman
    • CommentTimeFeb 21st 2022
     
    Here is an interesting summary, the precaution checklist page 2 is worth a read.


    https://www.buildmagazine.org.nz/assets/PDF/BUILD65-26-ElecCableInPoly.pdf
    • CommentAuthortony
    • CommentTimeFeb 21st 2022
     
    Thanks for that Owlman, they say that samples take from cables in real buildings show minimal loss of plasticiser, so comforting to see.

    The last point about cable sizing may be the most important.
    •  
      CommentAuthordjh
    • CommentTimeFeb 21st 2022
     
    Agree it's a useful article. I'm not sure about one point though; it says:

    3. Don’t use EPS beads: Loose-fill EPS beads
    (‘beans’) must not be used as insulation since these
    could potentially maintain permanent contact with
    the cable (since gravity would keep moving new
    beads into contact with the cabling).

    That seems a bit over-protective. Firstly in many installations the beads are coated with glue so even when loosed-filled they won't move about afterwards. Secondly even if they can move they won't achieve anything near to close contact over the whole surface of the cable since there is in theory just a single point of contact between each bead and the cable. So I'd expect a lot less trouble than in many other circumstances.
    • CommentAuthorrevor
    • CommentTimeFeb 21st 2022
     
    Posted By: tonyThe last point about cable sizing may be the most important.


    BS 7671 17th edition 523.9 state that cables should not be installed in situations where they are possibly covered in insulation. If there is a situation where it unavoidable up to 500 mm is allowed for which there is a sliding scale derating factor for the cable depending on the length surrounded from 50 mm to 400 mm. 500mm and above of cable in insulation the cable is derated by 0.5 i.e. only allowed to carry half the current of that cable when clipped in the open.
    So bottom line don't cover with insulation.
    I can find no reference in there for EPS specifically.
    • CommentAuthorTimSmall
    • CommentTimeMar 4th 2022
     
    Might be worth asking on the IET wiring regs forum for this, there have been a few discussions of it there in the past I think.
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