Insulated copper wire turned into gray powder

[NascentOxygen]

However if we uncritically accept the facts as presented in this thread, exactly the damaged length of wire (no more, no less) was installed under the floor and not in the walls.

I don't believe that to be true, simply based on it being too coincidental. I suggest that the cable was in pretty much undamaged condition when installed, and that copper corrosion largely occurred subsequent to installation. Visual examination of the copper core in the red cable over the length where bulging is not evident should confirm/deny my hypothesis.

 

[Majorana]

I beg you pardon for NOT posting more photos, I have some problem with my camera (have not yet fully learned how to use the d@mn thing!) but will try to post a "picture in words". I cut open (longitudinally) three sections of red cable between swollen spots, and the copper strands are undamaged: corrosion - or whatever kind of degenerative process it is - only and neatly takes places in those 8-10mm spots. The second thing I discovered was ONE, and only one, corrosion spot on the BLACK-insulated cable. It was very difficult to see because the swelling, that is already small on the red cable (slightly more than 0.5 mm in diameter increase) was even smaller on the black cable, almost not noticeable, so it escaped detection until now. Important note: that single swelling on the black cable was exactly in correspondence with a swelling on the red cable.

 

[hutchphd]

Is the "black" output terminal from the amplifier held at local ground potential? This might explain the differential color response to a ground leakage (apologies if this has been considered previously).

 

[sysprog]

Some anaerobic bacteria can cause pure copper to corrode to copper sulfide, which is grey to black in color, and outgassing from the reactions combined with sulfide bond breaches of the insulator could account for the bulging. It could be that a section of a coil of wire was exposed to seepage of something that spottily penetrated the insulation at irregular intervals the length of which was partially dependent on the vagaries of contaminant distribution, and partially on the diminishing circumference of the coil as it was unwound.

 

[Majorana]

P.S.: the corrosion spot on the black cable went unnoticed until yesterday not only because of the less prominent "swelling", but also because there is no discoloration of the black PVC, whereas the red PVC is noticeably discolored where swollen.
@hutchphd the amplifier's negative output is insulated from the chassis, but I cannot say whether the two are internally connected, and hence grounded through the yellow/green wire in the power cord. But consider that the PVC insulation of the speaker cable is quite thick and was undamaged (the cable was new from the store when installed) until we cut it open to see inside, and the cable was housed in a plastic tubing embedded in this old concrete floor, so an electrical contact of some sort of the wires with the ground seems, if not impossible, at least quite difficult.

 

[Nik_2213]

Is the conduit wide enough for vermin damage ? My 'ex-Sparky' BIL described mice & rat nibbles thus.
IIRC, he once traced a frequent 'ring main' trip to a gnawed cable beneath a suspended floor. Despite fuse-board's 'safety breaker', the perp died of it. Had the system not tripped repeatedly, prompting timely investigation, the damage may have allowed condensation etc to penetrate...

I've seen similar, if milder damage in a UHF TV aerial's 'coax' down-lead due moisture ingress at abrasion pin-holes. When I removed the plug, the cable 'peed' on me. Stripping it back, I found much mesh shield had turned to greyish dust. Also explained why the distribution amp was 'misbehaving'...

FWIW, I recently had land-line phone service restored after months of intermittent problems. Some days okay-ish, some days dire. But, when I rang the service provider, even though their caller display identified me, my location and my valid account, call handler would not raise a ticket for my 'bad line' because the line was too noisy for him to take all the relevant details.
'Joined-Up Thinking', Not !

In the end, I used the service provider's web forum to plead for help. Which came very, very promptly...
Tech's binary-search revealed one of our cats had chewed the in-house wire in a very obscure place !

(OT: Boss-Cat has figured how to unplug un-shrouded network cables by clawing their tiny clip... )

 

[Majorana]

Is the conduit wide enough for vermin damage ?

I don't think so. Inner diameter of the conduit is about 1/2", put the cables in it and imagine how much is left... But especially, remember that the PVC insulation was swollen, but definitely intact!

(OT: Boss-Cat has figured how to unplug un-shrouded network cables by clawing their tiny clip... )

Hats off to Boss-Cat! (and to any kitty/cat/feline as far as I'm concerned =^.^=)

 

[Majorana]

I just found a lab to analyze the mystery powder! They would be available to check a sample in a week, next monday to be precise. Results should follow within two or three days. Stay tuned! ;-)

 

[Majorana]

Good day folks,

the analysis took considerably longer than I had expected, and the results are not as exhaustive as I hoped: the problem was the VERY small amount of powder that I was able to collect. Once I had removed all the pieces of residual solid copper (partially dissolved wire strands) from the powder, the yeld was perhaps less than 50 mg worth of gray powder. ANYWAY, the analysis reported that it was constituted of copper salts only. No trace of any other metals (aluminium...) was found.

Before and after the analysis, I had a bit of brainstorming with a chemical engineer, whose multi-decade experience with the chemistry of metals came in just handy. After a lenghty explanation about the "mystery cable", details of the installation, the bubbles etc etc, he theorized that a number of notable coincidences may have concurred to the degradation of metallic copper into a gray powder.

He supposed that, if the PVC insulation had some (almost)invisible cracks that put the copper strands in contact with the environment; if the environment was not only relatively moist [RH over here lies in the range 60%-85% most of the time] but also alkaline (the floor is made of concrete, so that's quite probable), and if the air was contaminated by certain elements that I can't remember right now (this is a 3.5M inhabitants city located in a very large depression with little natural ventilation, so smog and haze are almost constantly present), then the copper may have reacted with the oxygen and water in the air, and due to the alkaline pH and the other contaminants the outcome was not just the usual greenish-graysh-blackish coat typical of copper, but a complete degradation/disintegration.

It goes without saying that nor him or any of his colleagues had ever seen something like that before...

 

[anorlunda]

It goes without saying that nor him or any of his colleagues had ever seen something like that before...

That sounds like a rational speculation. But it sounds like a case where the explanation will never be established with certainty.

 

[Tom.G]

constituted of copper salts only.

Any (practical) way of finding out which salts?

 

[Majorana]

None "practical". Because the very small amount of powder in the few "bubbles" left would make further in-depth analysis very hard or impossible at all, AND because (I've been told) the labs out there that perform inorganic chemistry analysis - a tiny minority, since the vast majority are clinical analysis labs - are aimed to testing environmental-protection-related substances, which is not the case here. I guess some University lab could be a good candidate, but I don't have any hooks in there...

 

[Rive]

It was a wonderful follow-through for a seemingly minuscule, but actually quite interesting problem

If you still have the remaining cable then I wonder if you could put it back after violating it a bit? Just some trampling or so.
Maybe together with a printed explanation.

 

[Majorana]

It was a wonderful follow-through for a seemingly minuscule, but actually quite interesting problem

Yes, "interesting" is the word, if you looked at the pictures... it is a pretty unique case of catastrophic degradation of the conductor. Since it could have happened to theoretically any copper wire/cable in any potentially critical installation (and not just to a silly loudspeaker for ambient music...) I think it's imperative to understand the mechanism behind that.

I wonder if you could put it back

I am not completely sure of what you meant with "put it back", but there is good news for the sake of experimentation. The owner of that workshop has replaced the length of cable that was removed with another identical length taken from the very same roll of cable! The new length of cable has been thoroughly checked inch by inch and was intact at the moment of installation (another proof that the degradation takes place only when the cable is in place, and not just in the same room but over the floor instead of under it...).
Now we have to be patient and wait approximately another 18 months... so by the late spring of 2021 we should know. I wish to kindly ask the admins not to close this thread after a certain time, as it usually happens...

 

[Borek]

Sometimes older threads are closed automatically to avoid necroposting, feel free to contact Mentors to reopen the thread if it happens. We will be happy to see how the situation evolves.

 

[bhusebye]

If copper is left outside in the Sun the copper will harden. If moved after the copper hardens it begins to crack if moved, this is why they make Solid core and Twisted Strand (more stable from cracking) Cat 6 cabling, Solid core needs to be left alone once installed for the cracking reason. If this cable was preinstalled in an area with Windows it is possible that it took a sun hit, was stepped on several times. Just another theory. Many people will step on a cable as they roll it out, then let out some slack, and step on it again. Those spots are more then likely where someone stomped on it.

 

[Guineafowl]

If copper is left outside in the Sun the copper will harden. If moved after the copper hardens it begins to crack if moved, this is why they make Solid core and Twisted Strand (more stable from cracking) Cat 6 cabling, Solid core needs to be left alone once installed for the cracking reason. If this cable was preinstalled in an area with Windows it is possible that it took a sun hit, was stepped on several times. Just another theory. Many people will step on a cable as they roll it out, then let out some slack, and step on it again. Those spots are more then likely where someone stomped on it.

This cable was sheathed in PVC and under a floor, so sunlight is an unlikely factor.

 

[Majorana]

I do confirm. The roll of cable came straight from an electronics shop, and was stored in a closet in the office. It never saw any sunlight at all. The same goes for stomping: it was a length of about 6 meters total, of which 4 in the conduit under the floor and 1 out of the floor at each end. It was just cut from the roll and put through the conduit by hand, an 1-minute job.

 

[bhusebye]

This cable was sheathed in PVC and under a floor, so sunlight is an unlikely factor.

Your not understanding. This can also happen if you walk on the cable, even if coated with PVC crap. Think of it this way, when I was a plumber rolling out copper on the ground 3/4 pipe and it sat out in the sun too long, what happens when you go to try to bend it, so it comes up straight in the wall? It kinks is what happens.

I also installed cat 5 and cat 6 cabling, cat 5 would BREAK if you walked on it. And some of it was laying on the ground preinstalled and the links between switches and routers, or switches and another switch or pc would break. Twisted pair even has a coating on it. One substantially less brittle than that Red PVC coating on speaker cable. ANY added HEAT even through CONDUCTIVITY causes copper to get brittle and degrade over time, and when you run cable you tend to step on the wire on the floor not the parts in the wall.

Solid core cat 6 cable you can only install it one time. Over time it gets hard, moving it around will break it because the copper inside solidifies and is no longer pliable, just like the copper pipe, it will kink and break.
Cat 6 stranded you can tie in a knot when you first install it and it will still pass traffic and you can move it around after it is ran. You can't run cat 5 and twist it in a knot, it will not pass traffic.

Excuse me if I did not specifically talk about heat.Edit: post contained a formatting error that made it look like just a quote without any comment and was deleted as such. I have stumbled upon it by chance, corrected formatting and restored.

 

[bhusebye]

Not even worth arguing about anymore, my post was deleted and I give up trying to explain to you thermal properties of copper and introduction of breakages that can happen even with that pvc cover which somehow people seem to think is a magic non breakable prophylactic covering.

 

[ZG1K]

The powder is CuCl. PVC is 60% chlorine which provides the catalyst for the change, and the location of the wire provided the environment. The copper wire at some point heated to over 450C and reacted with the chlorine in the PVC. This is much more likely to happen in stranded wire, especially improperly protected stranded wire, as a pinch point or other mechanical force exerted on the wire can cause strands to break inside the sheathing, increasing the wire's resistance and decreasing its heat dissipation.

Interestingly enough, the combination of copper wire and PVC sheathing makes for an effective yet presumably unintentional fail safe. In the event of a runaway overload, the chlorine will react with the copper, opening the circuit before the copper can melt and the other layers of pvc reach its melting point.

 

[Majorana]

The powder is CuCl. PVC is 60% chlorine which provides the catalyst for the change, and the location of the wire provided the environment. The copper wire at some point heated to over 450C and reacted with the chlorine in the PVC. This is much more likely to happen in stranded wire, especially improperly protected stranded wire, as a pinch point or other mechanical force exerted on the wire can cause strands to break inside the sheathing, increasing the wire's resistance and decreasing its heat dissipation.

Interestingly enough, the combination of copper wire and PVC sheathing makes for an effective yet presumably unintentional fail safe. In the event of a runaway overload, the chlorine will react with the copper, opening the circuit before the copper can melt and the other layers of pvc reach its melting point.

Interesting possibility, provided that such heating of the copper wire could occur. But the cable was used to connect a SMALL loudspeaker to a SMALL amplifier, to provide background music (i.e. low volume) in a small reception office. The amplifier was not defective (or the loudspeaker coil would have burned loooong before the cable!), both the amplifier and the loudspeaker worked fine after the cable was replaced. No other wires in the vicinity, the loudspeaker cable was the only cable running in that conduit under the floor. Absolutely zero chances that the cable could heat above 40°C.

 

[Tom.G]

Any chance that the PVC was incompletely polymerized leaving som readily available Chlorine? If so, could the CuCl reaction take place at room temp?

 

[Borek]

Any chance that the PVC was incompletely polymerized leaving som readily available Chlorine? If so, could the CuCl reaction take place at room temp?

Doesn't make much sense chemically. No chlorine in the production of PVC. It it used in an earlier step, during production of a monomer, vinylium chloride - but any excess chlorine left before the monomer is used in polymerization would react with the double bond present, converting the vinylium chloride to just a trichloroethane and rendering it useless for the PVC production.

 

[ZG1K]

Doesn't make much sense chemically. No chlorine in the production of PVC. It it used in an earlier step, during production of a monomer, vinylium chloride - but any excess chlorine left before the monomer is used in polymerization would react with the double bond present, converting the vinylium chloride to just a trichloroethane and rendering it useless for the PVC production.

PVC (PolyVinylChloride) is C2H3Cl. There is certainly chlorine in it.

 

[sysprog]

PVC (PolyVinylChloride) is C2H3Cl. There is certainly chlorine in it.

Please re-read what @Borek said. He well knows that there's chlorine in PVC and didn't say that there wasn't. He was in an overviewing manner explaining some things about the PVC production process that were relevant to the question.

 

[David Lewis]

In the 1960s this was called black wire corrosion because the copper wire that disintegrated was more prevalent in wires with black plastic insulation. A tentative hypothesis put forward was corrosion caused by outgassing from the PVC insulation. It was never corroborated, however.

 

[tech99]

I am wondering if it is an attack by rodents. The copper was then exposed to atmospheric corrosion, maybe from ozone created by photocopiers.

 

[Tom.G]

I am wondering if it is an attack by rodents.

...cable was the only cable running in that conduit

I'm not sure I would even like to know there IS a rodent that big!

 

[tech99]

I'm not sure I would even like to know there IS a rodent that big!

Yes, I suppose the rodent is implausible - I had not previously realize Copper is attacked by Ozone. The biggest rodent is the Capybaras, by the way, which is a lovely animal.