Where fuses melt

My guess would be the reflected heat and light from the metal base of the end of the fuses--The glass may let minute amounts of energy from the resistive element escape and the solid reflective ends won't------------???????????????

Hm mabe because the wire i thinner there? or the soldering to the contact makes the resistance of the wire slightly higher so when lots of current go through it, it is warmer there?

Oops. REDFACE alert !! I meant 10A not 1A. The 1A fuses which have an apparently uniform cross-section, squiggly wire don't do this. The 10A ones do. Sorry about the typo, but the question still is there.

Hello TVP45

OK, you are referring to AGC 10 Amp Glass tube fuses for low voltages.

Are you using the standard fuse, or the fast-acting ones?

These types of fuses are sometimes spot welded at the ends, inside the metal caps, and failure of the sopt weld occurs frequently, the fuse looking intact, but open-circuited.

It does seem to depend on the country of origin, German, English, Taiwanese and Japanese makes generally being OK, IMHO. (We don't usually get US AGC Fuses here, because they are much more expensive, and I don't now know of any US maker of these fuses).

There is really no way to 'test' a fuse, because such testing is destructive.

If you are referring to a plain copper wire with no reduxction in diameter, the wire is normally passed through a central hole in each of the caps, and then the external ends of each cap/wire is soldered.

The resistance of the soldered joint, is somewhat higher than the plain copper, thus on a slow blow of the fuse, the end with the higher resistance heats up more, and melts.

A higher current many times the fuse capacity, will lead to the wire heating up in the central portion of the glass tube, and it will thus blow there earlier, leaving a coating of fused copper on the central portion of the glass tube interior.

Kind Regards....

Good question, Sparky. I don't know how to tell the difference. And, since I've blown the whole supply and tossed them, I'm not sure. I'll see if I can find the box. BTW, I'm not trying to electrocute myself - I was doing an experiment

Excellent post Sparky,

I usually use US made BUSS and LITTLEFUSE Fuses because they are very precise, and seldom suffer fuse fatigue. Yes they are considerably more expensive (10 - 20 times as costly). But for critical circuits, it is worth the extra cost.

I have not seen any AGC fuses blowing at the ends, Mine seem to always blow in the middle. (Longitude, Latitude, Attitude, Altitude? ) I have no idea why.

> There is really no way to 'test' a fuse, because such testing is destructive.

Veto: We do test fuses and it IS destructive :-)

However, the DUT's are mostly for more then 10A, usually more than a 1000 times higher current.

When fault finding you can tell an awful lot of the type of fault by inspecting the blown fuse carefully.

If the fuse wire is just melted in the centre leaving little blobs of molten metal on the ends of the fuse wires - then you know the fuse was taken out by a surge of only a little above its rating.

If the molten blobs on the ends of the wire are not present then you can tell that the fuse may well have failed by vibration breaking the element.

I'm not going into all the other methods and types of overload you can tell from a blown fuse, or I could be sat here all day...

John

I don't know why the fuse always melts at the end, I would have thought the fuse caps would take heat away at the ends making it melt in the middle.

Anyway if your fuse is the type with soldered end caps, you can melt the solder to reveal a hole in the centre of each cap, then get a few strands from a multi-strand wire, poke them in and re-solder it. Voila! You just saved 10 cents. Next time though, you will find the fuse fails not only at the ends but in the middle too (with accompanying loud pop and flying broken glass). BRILLIANT!

"BRILLIANT!"

A good descriptor of the flash of light that will accompany the loud pop and flying glass shrapnel. (Don't try this at home, kids!)

Very interesting question. Our instincts (and the laws of heat transfer) tell us that the power dissipation per unit length is constant over the length of the fuse link. Because it is furthest from the 2 symmetrical end cap heat sinks, the thermal resistance of the link center to ambient is greatest in the center of the link, so that point should be the hottest. That's true, however, if the center of the link can dissipate more heat by some mechanism (think radiation) than the central portion, then another part of the link may get hotter than the center. Because fuse links are symmetrical, one would expect the hottest region to be on both sides of the center, and that's exactly what happens. Only rarely do both hot spots fail simultaneously.

I still can't answer Sparky's question. I wish I had saved an old paper Bussman catalogue; you can't tell beans from online catalogues. I can't find the box. I did find one remaining fuse from the lot. It just says AGC. The fusible element is jagged, like a lightning bolt. I have no idea if these were old; things in my shop are often 50 or more years old, so who knows?

Here is a 10A fuse that did not break at the end or middle. It created an internal plasma and glowed like a light bulb for a few seconds. I have a two of these. I don't care to repeat that experiment

Working military avionics I recognize that fuse LOL - Where did you find it ?

28VDC boo yah ampers LOL

Typically fuses burn about center if the material is uniform. Impurities cause it to burn at differing locations but ill leave that to the material engineers to explain, another caveat is the spot weld if they have impurities or fractures (which can happen in stuff that has a vibration) a little heating can burn out the end rather than a more central burn.

Besides the best way to replace a fuse is to insert a copper pipe or 2 gauge wire

(don't try this at home kids)

"Pop" "Fizzle" "Smoke" "Zerg" and then what you have replaced the fuse in no longer functions.

In Kuwait just after liberation we were in a villa that was struck by lightening, specifically one of the three main compressor units for the A/C - In Kuwait no A/C doesn't compute for this Yank, so out came the - and I quote "engineers" to fix it.

The first act of these "engineers" was to remove the street fuse that was burning out and replace it with a copper pipe. Hilarity ensued as the offending A/C unit control box ate itself whilst the pipe turned an amusing color of orange. The next step was to replace the relays as I was told in the control box- up they went - still no power and no A/C and accompanied glowing copper pipe- We left, my wife my new born son and myself went to the Sultan Center. We returned at 5pm and they were still working it. I decided maybe I would go up on the three story roof and look at it.

When I opened the control box I noted an acrid smell and a bright shinny relay set and then to my amusement directly to the right of those new shiny relays a black piece of charcoal ( that used to be a regulator) I asked have you tried this yet ? Silly question right ? The answer was funnier to me as it was "No". Imagine that. They replaced that part and it worked just like new. They then removed the copper tube at my insistence and replaced both the house main and street main with a fuse.

Ok so maybe Pipe isnt the answer.

Fuses I am convinced were the conception of the bean counters trying to save money . Buy a new something. It will be newer and smell better than the one that went pop.

Skin depth, proximity effect, eddy currents??? I've tried to imagine if there is a reason. Resistance--Is it equal through the length of the conductor? If it were 10' long and burned open, would it happen in a predicted spot? A variable resistor effects the voltage drop across it and the current increases and decreases proportionally, but is the heat that results even? The tap on the resistor dictates the total resistance so that point(at the end of the resistance) defines the total and would make me think that the heat could start to build there at the end of the resistive section--???? Or the beginning, or even across it?

Aneurysm time. I hope there is a really cool answer to this. HELP!

Thanks to all of you. I suspect, but can't prove, that Sparky may have the best answer in talking about where the fusible link is fastened to the end cap. This is the point at which I need an old Bussmann engineer.

The best way I have learnt to detect if a fuse like that has a "end fault" is to hold the fuse to 1 ear, and with the spare hand, gently flick the free end of the fuse, then turning the fuse around, repeating the process.

If there is a dull thud as your finger nail hits the fuse, generally the fuse is good.

If you hear a ring like a tuning fork, then generally, the fuse end has come away from the end cap. a Resistance check doesn't always pick this up, as the end of the fuse could be still contacting enough to make a short circuit.

Hey,

Thanks for the tips for the FUSE checking.

Once a fuse element melts in one spot, it doesn't need to melt anywhere else...

I would guess the fuse holder becomes oxidized and the connection becomes restrictive to the flow of electrons, when this occurs the connection tends to heat up which would in turn heat the end of the fuse...over time this would cause fuse link failure at the end connection....You might try smearing some conductive grease on the connection points of the fuse to prevent this from happening....