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Strange C.P.C. attack and size reduction.

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Evenin' all,

                         today I attended a shop store at a beach location. The store is on the prom just above high water level.


I decided to replace a double socket with rusty screws that the owner has suspicions about. The socket is wired in old 2.5/1.0mm2 T&E copper cable. There were three 2.5s connected at the socket, a ring pair and a spur. The live of the spur cable was blackened at the point where the sheath had been removed. The copper C.P.C. of the spur cable immediately snapped off when I moved it. It was discoloured and pitted along its length, but the weird thing is that its diameter reduced, pretty much consistently, the closer the C.P.C. was to the remaining sheath. When removed, the copper C.P.C. resembled a needle, tapering down in diameter towards the place where it broke off at the point of sheath removal. Was this due to historical tracking between live and C.P.C? Or was it electrochemical corrosion and weird physics? The damaged length was about 50mm long,


Z.


  • 0
    The form of corrosion you're describing is usually due to DC in my experience, so electrochemical action is a strong contender.


    The pitting might suggest previous water ingress (salt water during a storm perhaps?)

    The 'rusty' part is telling... rust in contact with the copper, in the presence of salt... would do it
  • 0
    To become needle like as you describe material has to be dissolved more at one end than the other - that is indeed an electrolytic type thing, as the current density and therefore rate of solution rises in the regions that are nearer to whatever is acting as the other electrode. I'd be wondering if the backbox may have been full of salty water at some point in the past.
  • 0
    Indeed, Ancientmariner may be able to shed some light, given his maritime background... but yes, needle like does remind me of electrolysis experiments
  • 0
    Poor cable insulation stripping resulting in the copper conductor being stretched?


    Andy B.
  • 0
    I don't recall ever seeing a tapering effect on any cable. Generally if salt water had got in a junction box, ite would be a corroded mess.  The only odd corrosion that I recall was on a Stern Light (a small white light on the stern) not a high wattage. A quick Googles tells me that a stern light needs to be visible at 3 miles (in the dark!).  Anyway, this particular junction had different corrosion on each of the two terminals: one terminal was black, the other not, even though an a.c. supply.  ISTR that I put it down to the earth monitoring system impressing low voltage DC onto the system, and thus the cause of the differing corrosion was from that DC.  Incidentally, I recall that the red line on the earth leakage meter for the 220 v 60Hz domestic (including nav lights) was 220 k ohm basically 1 mA per volt. 

    Clive
  • 0

    Sparkingchip:

    Poor cable insulation stripping resulting in the copper conductor being stretched?


    Andy B.




    Quite possible, as the cable was only just long enough to terminate. It might have been puled very vigorously, but some considerable strength would be need I reckon to reduce 1.0mm2 copper wire. Or, possibly faulty cable from new with an overstretched earth wire?


    Z.

  • 0

    MHRestorations:

    The form of corrosion you're describing is usually due to DC in my experience, so electrochemical action is a strong contender.


    The pitting might suggest previous water ingress (salt water during a storm perhaps?)

    The 'rusty' part is telling... rust in contact with the copper, in the presence of salt... would do it




    The mounting pattress was secured by very rusty 70s steel screws, the slots of which had corroded away due to rust.


    We had a storm in 2013 that may have flooded the store area under the shop and allowed sea water to enter the socket pattress. The store is unheated as it only stores beach balls and deck chairs etc. It is damp.


    Z.




     

  • 0
    I am not sure that I could stretch 1.00mm2 copper wire just using a pair of pliers. Could you?

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


    Z.
  • 0
    Anyone for a dodgy roll of badly manufactured cable in the 1st instance as the root cause?

    What is the destination end of the cable like?
  • 0
    What did you replace it with an outdoor type given it is damp?

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