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Why does fuse wire always break in the middle?

  1. Feb 1, 2008 #1
    Hello, I have been looking this thing up for a couple hours now on google and textbooks. So far, I got nothing... When you use a fuse wire in a circuit, why does the fuse wire always melt in the middle? I don't see how the middle of the fuse wire gets hotter than the ends unless the middle of the fuse wire is thinner than the ends. Which I don't think is the case. Well, time to ask the experts. Thanks.
  2. jcsd
  3. Feb 1, 2008 #2
    The only reasonable answer I see is the fact the ends of the wire are connected with bigger pieces of metal, which then cool for conduction the wire in theyr proximities.
  4. Feb 1, 2008 #3
    Hey, someone told me that too but I couldn't get much of a hint. How does connecting to bigger pieces of metal affect the fuse wire?

    I guess you said cool for conduction.. but I don't really know what that means, maybe I can google and see what i find, thanks.

    -- Edit: I read something about the heatsink capacity of the end attachments? The end acts as heatsinks but the middle doesn't so it heats up and melt. I don't get why the end attachments acts as heatsinks?
    Last edited: Feb 1, 2008
  5. Feb 1, 2008 #4
    Why does fuse wire always melt in the middle.

    First, it doesn't always melt in the middle but you're observation is generally correct. If we can assume that the thermal mass of the connection points on either side of the fuse wire are the same and there is REASONABLE thermal equilibrium in the general area around the fuse wire, it will melt approximately in the middle.

    Because of the increased resistance in the fuse wire, current flowing thru the wire heats the atoms in the wire much more than the circuit wires that connect that current to the fuse wire. That heating is more or less uniform across the entire length of the fuse wire. BUT, the atoms near the connector more easily transfer their heat energy to the connector and the connecting wires which are not being heated by the current. (for reference here, let's consider this is an in-line, cartridge fuse holder with a standard cartridge fuse installed)

    As the fuse wire heats, immediately all of the atoms start jostling around bumping into the atoms next to them and heating them by friction. The atoms in the middle of the wire are being jostled by atoms on both sides of them. And their heat has no place to go other than to radiate away into the fuse holder. The atoms nearer the ends, however, can more easily pass their heat out to the colder connectors and connecting wire at the ends. This happens simply because those hot atoms are closer to a cold mass that their heat can be conducted to. Now you can imagine that these atoms in the middle of this wire are just getting hotter and hotter but the hot atoms nearer the ends are just happily flowing some of that heat out to the cooler connectors which are then just sending it down the connecting wires.

    Most modern fuse wires are designed metalurgically so that the solid phase to liquid phase change (read, going from a solid wire to melted) happens very quickly. You want this fuse to stay intact while it carries its normal load and not get soft and sag. Then when the current reaches the failure point, you want the fuse to instantly turn to molten metal and interrupt the circuit. This is accomplished by alloying the metal so that it does not remain in a semi-molten state for long.

    Now, eventually that heat limit is reached near the middle and solid structure that supported the atoms in the wire in a rigid matrix fails and the middle sags or in some cases vaporizes. (there are other more complicated processes that take place here but are probably not required for this discussion)

    While this is by no means a complete analysis of the process, I hope it has been helpful.
  6. Feb 1, 2008 #5

    Andy Resnick

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    Seems to me that they are designed that way so that it's easier to see if the fuse is broken.
  7. Feb 1, 2008 #6
    You could be right. It would be interesting to see how they manipulated the laws of physics to make it work out that way. Industry is so creative.
  8. Feb 1, 2008 #7
    I just went to the shop and found four AGC fuses (two 1A and two 2A) that appeared to have uniform wire diameter. The 20A ones I found clearly had a narrow place in the middle so I couldn't use those. So, I hit the fuses with about 10ADC. One 1A and both 2A fuses melted at an end. The other 1A melted in the middle. Not a big sample, but all I had.
  9. Feb 2, 2008 #8
    Your sample is adequate. Maybe I was too narrow in my thinking. Fuse wire can fail under many different conditions and when I wrote earlier, I was thinking in very simplistic terms. I was simply thinking of an ideal situation with a perfect resistance wire brought somewhat slowly, maybe over 3 seconds or so, to the failure point. Too slow and the terminals and wires start warming and while the overall approach remains the same, it would get more and more difficult to know all the factors.

    If you pulse the fuse wire, i.e. just flip the switch on with a much higher than normal current, other factors could be involved. There is a magnetic component as the current thru the fuse tends to mechanically flex the wire. Very rapid heating of the wire may cause failure at the connector due to crimping or other termination factors. I suspect that due to the mass manufacturing nature of the cartridge fuse, there is probably not a lot of care given to uniform termination of the fuse wire to the end cap of the cartridge, which is connected in various ways including a solder ball.

    I suspect that this experiment would be more reliable if done with bulk fuse wire somewhat longer, maybe 2 to 3 inches with careful attention to the uniformity of the termination and a soft approach to failure rather than a pulse.

    Some final thoughts. While the labs I'm associated with, often deal with pulse currents of 10s and 100s of thousands of amps, failures at even a 2 thousand amps can fling a burned cable end across the bench and damage lab equipment. Your test current was far, far less than that but still the effect is there. Another possible factor is that when you pulse the wire with a fairly high di/dt, the wire expands throughout it's length, very rapidly, while the highest temperature is still likely to be near the center, the greatest mechanical stress is at the terminating points. You may notice a fuse that is pulsed (short duration pulse) well beyond it's normal failure point but not long enough to cause it to fail often exhibits a permanent bow in the wire. This is not so much due to the, sag, melt scenario I mentioned earlier, but to the fact that the fuse wire rapidly grows in length but it can't push the ends out.

    I can't think of a good reason why fuse manufacturers would want to force a fuse to fail necessarily in the center. Interruption is interruption as long as it's not an exotic application and in most cases they are not reusable. I rather suspect that by narrowing the wire at some point is a more accurate way to force the fuse to fail at a fairly precise temperature (current) while providing adequate mechanical support during normal moderately high current operation.

    Maybe we will attract the attention of a someone from Ferraz Shawmut or Bussman who would probably have a far more educated answer than my theoretical musings.
  10. Feb 3, 2008 #9
    You make an excellent point about the difference between 1000% overcurrent and, say, 150%. I thought about that but couldn't find any more fuses. This might be worthy of a posting on ENG-TIPS to see if anyone there does fuses.
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