How does a time delay fuse work?

Discussion in 'General Electronics Chat' started by strantor, Mar 29, 2012.

  1. strantor

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    Oct 3, 2010
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    I just had a bunch of wires melt in a control panel because a fuse didn't blow. It was a TR5R (5A time delay fuse) that was "protecting" a phase angle fired SCR heater controller. The heater had shorted out. I'm thinking that for those 14AWG wires to melt, they had to have well over 20A flowing through them for an appreciable amount of time. According to the TR5R data sheet, the fuse should have blown in <1 second if it had 20A flowing through it. the best I can come up with is that the fuse didn't see high enough current for long enough due to the "long" delay between AC cycles due to the phase angle firing. I don't have a lot of faith in this explanation, but like i said, I got nothing better. Any ideas?
     
  2. gerty

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    Should've blown, no doubt. Are you sure it's in the right part of the circuit?
    Is it a "REN" (renewable) type fuse with the wrong link in it?
    I cut one open that was in a similliar situation, didn't blow, and the fuse element was way thicker than another one of the same rating. In that case I think it was simply mis-marked
     
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  3. #12

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    I say, the fuse wasn't really wired in series with the fault. Look for human (wiring) error or mechanical interference like rubbing through a power wire before it got to its fuse.

    Time delay fuses have a bit of extra mass next to the filament to absorb heat, but a 5 amp slow fuse isn't going to absorb enough heat to melt a 14 gauge wire.
     
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  4. strantor

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    Yes I'm sure - it blew last week when the heater shorted out. I thought I fixed the heater good, but my repair failed & it shorted out again. I put the new fuse in, but this time it didn't blow.
    I don't see 'REN' stamped on it anywhere. I'm going to cut it open and see what I can see.
     
  5. gerty

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    A REN fuse has caps that screw on the ends of the body. That allows you to change (RENew) the fuse element, much cheaper than buying a new fuse.

    http://www.galco.com/buy/Bussmann/REN-30 here's a pic of one.
     
  6. strantor

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    I cut it open and the element is a tiny piece of copper. it should have melted long before the wires. Just for S&G I cut open another one and it was the same size element. oddly enough, the fuses say "dual element" but there's only one element in both the ones I cut open.
     
  7. gerty

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    What brand? Questionable quality, between the markings and the fact it didn't blow...
     
  8. strantor

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    Ferraz Shawmut - pretty common brand, I've never had a problem with them.
     
  9. strantor

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    Yeah I'm going to revisit this one. It just doesn't add up. After seeing that tiny element, I cannot believe, even if the fuse were incorrectly marked, that the wires would have melted before the fuse.
     
  10. mcgyvr

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    Are you sure this 14 AWG wire wasn't melted by the heaters or something else in the vacinity? Any 5A time delay fuse WILL blow WAY before any damage is done to a 14AWG wire.
    Have you measured continuity across the line/load fuse terminals without a fuse in place?
    These wires aren't directly attached to the heaters are they?
     
  11. strantor

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    No there are a few junction boxes between the panel and the heater. heater is probably 20ft away. I measured across the heater wires, at the heater (with the heater removed) and no shorts.
     
  12. GetDeviceInfo

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    Jun 7, 2009
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    you might want to check a couple of things. First, the application. Your overcurrent device protects the wiring, which in this case failed. TD fuses allow high inrush typical of inductive loads. Heaters, and more specifically thier circuit wiring would typically not be time delay, and, if done to code, are derated to 80% ampacity. Secondly, check for neutral connections in multi phase. Add ons or modifications can easily lead to overloaded neutrals.
     
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  13. jimkeith

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    I have heard that "dual element" is not necessarily two in parallel, but two in series--consisting of a normal fast blow (at a higher current rating for short circuit protection) in series with an element with a blob of solder on it--after a time delay, the solder blob melts and starts to alloy with the copper element thus increasing its resistivity so that it will eventually blow at about 135% of its current rating.

    Shawmut has very high quality--tops for semiconductor fuses

    Make sure that there were no ground faults present as they can do weird things if the input is fused incorrectly and /or the mains is not grounded as expected. e.g. for the convention of "B" phase grounded, is the fuse in "A", "B" or "C"?

    Also, heaters are subject to ground fault failure--just because it does not indicate a short at room temperature, does not prove what is happening at rated temperature.
     
    Last edited: Mar 30, 2012
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  14. GetDeviceInfo

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    good point. I've seen many heaters ohm out good cold, but short hot.
     
  15. #12

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    Depends on what kind of heater. The nichrome coils don't do that but the things I call, "Calrod" do. Resistive element in a steel tube with aluminum oxide between the core and the shell. Repeated heating cycles make the core wiggle until it touches the grounded shell. You can usuall tell what happened by the melted steel that was blown out of the contact area.
     
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