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My living room has a 5.1 Dolby digital system; the audio cables for the three front speakers run through the walls for about 13 metres (about 42 feet) in a corrugated pipe with a diameter of 2.3 cm (a bit less than an inch).

A few days ago one of the speakers stopped working, after several years without problems. I traced the problem to one of the cables; I pulled all of them from the pipe and I was surprised to see that they seemed corroded in several points.

The insulation had failed - apparently scraped? - and the copper had corroded, even if I thought that copper oxide was green; the exposed parts showed a whitish dust instead.

The pipe runs indoor, it was never exposed to water and I doubt there are tiny 1" rodents in my place. The house is six years old.

Now I'm in the process of pulling a new set of wires through the corrugated pipe; it's the one with the green probe sticking out of it in the picture. I don't think it's relevant but it also has a perfectly working cat5 cable in it.

I'd like to know what happened to the old wires and how to prevent this in the future.

corrugated pipe corroded several points

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  • Mice is a possibility, they only need about a 1/4 inch opening. Check the edges closely for gnaw marks. The covering might have been damaged before being put in the pipe/conduit or there is a tight/rough spot in the conduit. Humidity in the house might have been enough if the cover is that damaged.
    – crip659
    Nov 5, 2022 at 15:50
  • manufacturing defect
    – asinine
    Nov 5, 2022 at 19:34
  • What is the wall made of? Concrete can cause some weird corrosion if the conduit and wire are damaged.
    – KMJ
    Nov 6, 2022 at 4:54
  • @crip659 no gnaw marks (also no other signs of infestation whatsoever in the house). The insulation looks like it burst from the inside - come to think of it, oxidized metals expand in volume.
    – Vorbis
    Nov 6, 2022 at 16:47
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    It looks like a bad cable or rough handling during installation. If that conduit is good it should not cause that damage. At least you had the conduit so replacement was easier than digging though walls.
    – crip659
    Nov 6, 2022 at 17:05

2 Answers 2

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Careless installation, and perhaps also unsuitable speaker cables for in-wall installation (apart from better fire ratings on in-wall cable insulation, the insulation itself is typically more durable than for speaker cables intended to be connected without any pulling involved.)

You might also want to plug the ends (once the wires are in place) with "duct seal", which is a non-hardening putty material that will block access to the duct, but can be removed years later if needed to pull wires out, or new wires in. Any place that carries electrical supplies should have that.

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  • @ecnerwall Indeed, the cables weren't rated as in-wall. Bonus point for mentioning duct seal, I never heard of it and actually needed it for a different corrugated pipe that had moisture problems. (Why would it be useful in this case, however? Both ends of the pipe are in the same room.)
    – Vorbis
    Nov 9, 2022 at 14:37
  • Prevents things other than the wires from getting in there.
    – Ecnerwal
    Nov 9, 2022 at 14:50
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Given the construction you described (brick and concrete) I think what you're seeing here is two problems that paired up to make a single more serious problem. I think there's probably a small cut in the conduit which is allowing the brick to make any moisture in the tube from condensation in to a slightly corrosive liquid. Paired with this, the wire may have been pulled too hard during installation, resulting in damage to the insulation. Give it many years and that spells corroded wire. If you were careful pulling the replacement wire and used sheathed wire, I would expect many years of service with little concern.

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  • I'd accept your answer too, but it seems that I can flag only one. Thanks for pointing out a (possible) explanation for the observed facts.
    – Vorbis
    Nov 9, 2022 at 14:40
  • Totally fine, it's just for imaginary internet points anyway.
    – KMJ
    Nov 9, 2022 at 15:44

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