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Chemical Cleaning of Blackened Copper Wires.

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After seeing the blackened C.P.C. in the metal back box in another post, I researched this matter and found this video clip showing the cleaning of blackened copper wires chemically, which should not remove too much copper as a mechanical abrasion might.

https://www.youtube.com/watch?v=TfEIEVrmzVw


Z.
  • 0

    mapj1:




    Does oxidation of the cores along the length of a multistrand cable affect its properties?




    Copper oxide generally becomes impervious to water and oxygen beyond a certain thickness, so once a patina has developed, corrosion is stopped by the oxide layer.

    (so plumbing pipes start bright, and then go dark, slowly reaching an equilibrium  with the environment.)

    It is a problem for very thin wires, as they may corrode right through or for applications with flexing, where the oxide layerr will keep being re-opened. In the case of this sort of wire, if you can get decent joints at the ends, I'd not be too worried about darkening in the bulk, so long as it is just oxide, and not aided by something more corrosive, like brown sauce.



     




    Loss of metal aside, are there any other effects that might affect the electrical properties of multistrand cable? Does it matter there's an effective layer of insulation between the strands?

  • 0

    Does it matter there's an effective layer of insulation between the strands?



    I would say yes - unless there's good contact between every strand and the terminal there isn't going to be anything like equal current sharing between the strands - and I don't see how you're likely to get good contact with the inner strand on a typical 7-strand wire in any event - likewise if you're relying on the end of the screw in a tunnel terminal to scrape off the oxide (or whatever it is chemically), it's only going to make contact with a few of the strands. Ditto for any faults if the oxidisation continues along the length of the conductor and say a nail or screw pierced the cable - a few strands alone would have to carry the full fault current and you'd be into an appendix 10 situation.


    Of course, smaller sizes of T&E have solid conductors, so the problem is avoided in most common domestic situations.


       - Andy.
  • 0
    When working with higher frequencies the skin effect tends to drive the current to the outer of the conductor, so the current carrying capaciity is reduced from the DC value.

    One way to beat this, without going to the aggro of flat palm braids or ribbons,  is to make a cable that is deliberately formed of many insulated strands, and then in the really expensive stuff with the very lowest losses, to plait the cores in a special way that they take it in turns to be inside or outside the bundle all equally balanced .

    Litzendraht or Litz Wire  to give it the proper name used to be common in the days of long wave radio, before electronics made amplification cheaper then fancy wire.

      Now mostly seen in induction hobs to make very compact low loss coils.

    Traditionally cotton weave, nowadays a self bonding varnish, so once the wire is wound on a former it is heated until the varnish sticks the strands together, and then the former can be removed leaving a self supporting coil.

    So, insulated strands do you no harm, but yes, at the ends, you do need to be sure the strands are all connected- the sort of job where 3 of the 7 strands go under the screw and the rest flap about will not do.

  • Former Community Member
    0 Former Community Member

    gkenyon:

    So, we've been discussing cleaning the copper at its termination.


    Does oxidation of the cores along the length of a multistrand cable affect its properties?




     

    Possibly no for the same reasoning, but we never used multi-strand bare copper cabling as the deep earthing electrode under buildings or for earthing at HV or LV in certain substations. This was down to not being able to inspect potential corrosion forming between strands. It always had to be a solid (tape) conductor.


    I'm not sure it would ever be a real problem in the particular application, but once you get a project adopting certain "rules" it' difficult to get change.


    Just for the amusement value, I have asked the question myself of design teams just to see the panic flutter across the face of a design discipline lead who hadn't really considered it.


    Regards


    OMS

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