Why does fuse wire always break in the middle?

Discussion in 'General Electronics Chat' started by p3t3rl1, Feb 1, 2008.

  1. p3t3rl1

    Thread Starter New Member

    Feb 1, 2008
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    Hello, I known fuse wire breaks to prevent overheating/overcurrent etc. but why exactly does the fuse wire melts in the middle? How does the middle of the wire gets hotter than the ends? I tried google for a while but couldn't get anything so I am wondering if you experts would know, thanks alot.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    The end attachments tend to act as heat sinks, so the fuse wire heats most in the center.
     
  3. p3t3rl1

    Thread Starter New Member

    Feb 1, 2008
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    Cool, thanks. Can you elaborate more on how exactly that works or give me a keyword to search on Google so I can read more? I tried heatsinks but I don't think that was specific enough? Thanks.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    You can see the effect in practice with a low voltage lamp. As you slowly increase voltage across it, the filament will begin to glow. It is always visibly hotter (brightest, whiter) in the center. The attachments do not get hot enough to glow.

    You might look up thermal conductivity in steel (the attachment material) and whatever material is used for the fuse wire.
     
  5. p3t3rl1

    Thread Starter New Member

    Feb 1, 2008
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    Cool, thanks for the help. :)
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    Some helpful search terms would be "heat transfer" and "thermal conductivity." Heat travels through different materials at different rates. Silver and copper conduct heat pretty well. Iron and steel less so. Glass even less still.

    The general rule of thumb (not universal in application): if it conducts electricity well, it will conduct heat well.
     
  7. t.yuvaraja

    New Member

    Feb 3, 2008
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    I say any fuse materials having the temperature limits, when short circuited or over loaded the fuse goes beyond the limit. During the heat fuse goes to higher tension thats why goes to center of the fuse. Because the ends not expanded.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    t.yuvaraja, I don't buy your theory.

    Materials generally expand when heated. In a typical cylindrical glass fuse with conductive end caps, as the fuse wire inside became heated, the wire would actually be under less tension due to the expansion of the length of the wire (positive temperature coefficient).

    Going back to the OP's Q,
    Fuses don't always part in the exact center, but that's where it tends to be.

    You can get an idea of what kind of overload caused the failure by close examination of the fuse. If the gap is rather narrow and well centered, it's very likely that the overload was just above the current rating of the fuse. The wider the gap, the more severe the overload was.

    I've seen cases where the fuse link was completely obliterated, which was caused by a dead short across the supply voltage. The rise in current was so rapid that there was no time for the heat generated to be dissipated via the end caps.
     
  9. RiJoRI

    Well-Known Member

    Aug 15, 2007
    536
    26
    It doesn't always break in the middle! I've had a fuse (glass tube with metal end caps) blow inside the cap -- the fuse looked fine, but it was really blown. After that, I always check "good-looking" :p fuses with a DVM.

    --Rich
     
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