conductor bend radius

Discussion in 'General Electronics Chat' started by wswilcox, Apr 14, 2006.

  1. wswilcox

    Thread Starter New Member

    Apr 14, 2006
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    Hi,
    I have been working for a manufacturing company that refuses to believe that the minimum bend radius for a current carrying conductor (CCCs) at 60Hz is a necessary code. I was wondering if anybody knows of any technical documentation online that would provide the physics or mechanical rational for this code. Also, I have seen that infrared photos of CCCs with below minimum bends that show hot spots at the bends. I would like to see those photos again. The hot spots obviously indicate an increased I squared R loss and since the current isn't increasing the R must be. Or could it be a skin effect due to deformation of the strands? This is important to me because one of the issues I am dealing with is excessive voltage drop and I would like to know if these below min bends are part of the problem.
    Thanks,
    Bill
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    As far as I know, observing a minimum conductor bend radius for a signal of a frqeuency as low as 60 Hz is done soley for mechanical conductor integrity. IPC-A-610D is a manufacturing standard that provides industry recognized guidelines for minimum bend radii as a function of the diameter of the wire involved. It is only a guideline but you may be able to use it to bolster your argument for avoiding excessive over bending of conductors whether the conductors are solid or stranded.

    I can imagine that over bending of conductors can introduce a slight localized change in the DC resistance of the wire at the site of the bend. Depending on the amount of increase in the resistance it is possible that under high current, some local heating might occur. I would think that for the heat increase to be detectable, the current flow would need to be on the order of tens or maybe hundreds of amps.

    hgmjr
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    Skin effect at 60 Hz? Not too likely. Unless there's some mechanical damage to the conductor/s, no bend is going to have a significant effect on the current-carrying capability of the wire. Of course, it's good practise to use a conductor rated above the designed current draw.
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    The mere act of bending a conductor causes damage if you don't follow the minimum bending radii. If the thermal imaging shows hot spots at the bends in the cable, there are IR Losses. The same happens across switches and relays when the contactors when they show signs of fatigue.

    At significant currents, that IR loss will manifests itself as a detrimental voltage drop.

    There are bending radii standards for all types of wire.

    A Google Search yields some interesting minimums based on usage.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    Assuming the bend has not caused the cross-sectional area of the conductor to change, like necking down, what is the mechanism for the increase in resistance?

    I ave never heard of this effect before now.
     
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Beenthere, I agree with what you said. Assuming someone didn't bend the wire too much [less than the minimum] you shouldn't have a problem.

    When one bends a wire, say a 12 awg less than the minimum bending radii, the inside is compressed and the outside is stretched. Of course that will be a tight bend, made with pliers, but it does change the physical characteristics of the wire at that point. Using thermal imaging, you will see the heat built up at that bend, as compared to a normal bend. Breaking a strand of wire obviously changes the cross sectional area.

    I've seen thermal imaging of contactors and connections that were good and some that were not so good about a decade or so ago. I wish I had pictures to show you. Those not so good showed the heat really well. Granted it's much easier to bend smaller wire less than the minimum bending radius and from the search at Google, it seems like a big problem in the network wiring and optical enviroment then elsewhere.

    Thirty or so years ago, I remember one of my military courses covering installations standards talked of minimum bending radii as 3 to 4 times the diameter of the wire. Some of the discussions today have 10 times for optical cable.

    The recommendation to use velcro tie downs for network cable vice tie wraps is a good example of how easy it is to damage some of those smaller wires.
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    From the standpoint of a person who has wired his own house, this is a bit interesting. I think all circuits are running well under the maximum current draw, but I am also aware that I paid no attention to bends while manipulating the wiring.

    The fact that the house has not caught fire after 5 years pretty much says that all conductors are not too damaged/deformed.

    Of course, 120 VAC/200 amp is a different situation from 480 VAC/800 amp service.
     
  8. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Hi back,

    I'm sure you subconsciously ensured the bends were greater than the minimums because you knew about "necking down" and physical damage.

    Only way to tell for sure is a thermal imaging scan.

    On the scan's I saw, about five years preivous to seeing them, my staff checked every electrical connection for integrity. There were alot of loose ones when we tested them.
     
  9. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    In a previous job where I used to teach this sort of stuff to MIL-SPEC standards we considered 6 times the insulation diameter to be the minimum bend radius for a standard conductor. Flexible RF cables were 10 times. Some other cables that had special insulation were also 10 times. There should be heaps of reference material on the web that can back you hotspot theroy up. Although it is not a therory it is a fact. If you look at all cables or wires that burn out you will find it is nearly always at a bend or a point where they have been cable tied (crushed).
     
  10. CATV

    Member

    Apr 17, 2007
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    The recommendation to use velcro tie downs for network cable vice tie wraps is a good example of how easy it is to damage some of those smaller wires.[/QUOTE]

    Another consideration in Cat 5 or Flexible Coaxial cables is the placement of cable ties at regular intervals will create Structural Return Loss due to periodicity. If you do use ties, space them at uneven distances.
     
  11. Salgat

    Active Member

    Dec 23, 2006
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    So if you bend a wire, and its cross sectional area decreases and its surface area increases, would this be the equivelant of the gauge of the wire decreasing at the location of the bend(thus decreasing its ability to carry current)?
     
  12. recca02

    Senior Member

    Apr 2, 2007
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    hey i have read a little abt strain gauges.
    not only does the length , dia (X-secn area) change but also the resistivity of wire changes with strain the effect is called piezo resistive effect.
    here is the eqn for it:
    dr/r = dl/l +2vdl/l +dp/p (r is resistance, v is poisson's ratio, p is resistivity)

    and consider the effect of localised heating of wire at the bend point will
    introduce some thermal strain and resistance change due to temperature as well.
    now to make matter worse in a bend the outer part is under tensile stress
    and inner part under compressive stress.
    man, i guess only a genius will be able to come up with an accurate answer.

    anyways most of those factors may not produce appreciable change perhaps.
     
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