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Terminating Twin and Earth

nicemark
I have been arguing about the methods of separating out the ends of twin and earth cable.


An old accepted trade method was to simply grip the earth wire and tear it out of the insulation to split the ends open.

However this puts considerable strain on the earth wire and in the case of 1mm wire it is very easy to stretch a significant amount. 


I was wondering if you there is data or evidence to demonstrate that this method is not acceptable. 

  • 0
    mmm interesting question , that isnt methodised , I have never had a bare earth break on me , using this method , but recently a chap with a new tool made stripping sheathing from flat twin and earth much easier , with stripping pliers , works every time , and he had some special long nose strippers for earth , only problem is when you have to strip a long bit , but that isnt often on twin and earth . so basically right tool and you can do a better termination only problem was they were £40 each .
  • 0
    nicemark:

    I have been arguing about the methods of separating out the ends of twin and earth cable.


    An old accepted trade method was to simply grip the earth wire and tear it out of the insulation to split the ends open.

    However this puts considerable strain on the earth wire and in the case of 1mm wire it is very easy to stretch a significant amount. 


    I was wondering if you there is data or evidence to demonstrate that this method is not acceptable. 

     


    I have been an electrician for many years. I normally squeeze the P.V.C. T&E sheath with pliers so that it breaks at the end, then I recover the C.P.C. and use it as a cheese cutter wire to strip the cable back to the required length. I have never had a C.P.C. break or stretch. Some people use a knife and cut, or partially cut, down the middle of the sheath first. Only 1.0mm2 & 1.5mm2 T&E cables uses a 1.0mm2 sized C.P.C. others use a 1.5mm2 C.P.C. so it is stronger. The 1.0mm2 T&E or 3 core and C.P.C. has very thin weak sheathing so is easy to strip back. On site it is all about speed and efficiency. We do not need to clutter up our tool boxes with expensive unnecessary tools.


    Z.


     


  • 0
    I don't think you will have a problem, a quick back of a fag packet calculation tells me to stretch a 10cm length of 1mm2 by 1mm you would need 12.6 MN force which you would need a hefty machine to do that.  Furthermore, the force you apply to strip is not true tensile as your curling it as you strip therefore adding a sheer force too, so between tensile and sheer you will be perfectly OK.

    If you can stretch a length of 1mm2 copper wire with your hands you're a very strong lad!
  • 0
    Rob Eagle:

    I don't think you will have a problem, a quick back of a fag packet calculation tells me to stretch a 10cm length of 1mm2 by 1mm you would need 12.6 MN force which you would need a hefty machine to do that.  Furthermore, the force you apply to strip is not true tensile as your curling it as you strip therefore adding a sheer force too, so between tensile and sheer you will be perfectly OK.

    If you can stretch a length of 1mm2 copper wire with your hands you're a very strong lad!


    Something slightly wrong with your units there.


    I must get my entry in for The World's Strongest Man 'cos I just stretched a piece of 1 mm² wire and then it broke where it was wrapped around my pliers.


    Mind you, the force was considerably more than needed to cheesewire through PVC sheathing.


  • 0
    There is no significant diameter change when using the CPC as the "cheese wire", at least my micrometer which measures in microns cannot detect it (a micron is a thousandth of a mm BTW)
  • 0
    .. or grip with pliers or fingers and run a sharp knife down the earth wire at right angles to the flat area being careful not to nick it ofc.
  • 0
    "Something slightly wrong with your units there."


    12.6 Mega Newton force, 12.6 x 10^6.


    How much longer did the wire become after stretching it by hand, or did you just get out the kinks?  It broke at the pliers due to the edge of the pliers cutting into the soft copper, a significantly much lower force is required to do that.
  • 0
    Copper very much work hardens, and will snap off with very little force if kinked sharply back and forth more than a few times.

    Embrittlement at a sharp kink is almost certainly the failure mode where it snaps at the pliers, rather than ultimate tensile failure - which would be pulling in a straight line until it necks down and snaps. In the cold drawn state, oxygen free copper, as used for all good wiring, has an ultimate tensile stregnth of  ~40,000 PSI (1 PSI is 0.000703  kilos per square mm, so 1mm2 copper will snap at 28kg force) - for 1mm 2 core that is about the weight of a bag of cement - but you'd have to lift it with the wire perfectly straight, perhaps held rod-like  in some sort of drill chuck arragement, so the force was perfectly balanced on brand new wire with no twists or kinks, and even then,  bring the force on gently.

    At the tension it snaps, it has stretched in length  by 17 % or so, and so at the break point the useful area is typically reduced to 75 to 80% of the original.

    Note that the breaking strain can reduce by more than a factor of ten after  work hardening, and sets off already being that weak in cheap knock off cables if impurities are present.


    Round nose pliers exist to form wires into controlled curves to avoid the kinking issue. Good ones (not these) have plastic sleeves to protect the insulation. For semi- rigid coaxial cables we use something like a small plumber's pipe bender. (small bender for full sized plumber...) bigger tools for bending earthing straps.


    Mike

    M.

  • 0
    Rob Eagle:

    "Something slightly wrong with your units there."


    12.6 Mega Newton force, 12.6 x 10^6.


    How much longer did the wire become after stretching it by hand, or did you just get out the kinks?  It broke at the pliers due to the edge of the pliers cutting into the soft copper, a significantly much lower force is required to do that.


    That's the force exerted by gravity on about 1000 tonnes, so your wee copper wire will lift a small ship will it?


    I tried again. The 1 mm² wire stretched by 30% before it broke in the stretched part, not at the pliers.


  • 0
    I was pulled up about this, years' ago, on here for doing it just that way! ?


    I must admit, I have had sleepless nights', ever since, as I KNOW I reduced the cross section of the CPC, by 'stretching' it!! ?


    regards...


    Tom

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