# Computing ampacity a of small fuse from change in temp on breakdown

I'm trying to design an experiment for an entry-level electronics course involving burning out fuses of an unknown rating and determining their rating by the temperature they were when they failed. Is this possible with a relatively cheap (say, below \$100) laser thermometer? How would I use specs of the fuse to determine theoretical failure temp?

I am aware of the serious physics problems involved in "converting" watts to a change in temperature.

• The temperature isn't likely to change depending on rating. The fuse material will have a certain melting temperature. The fuse wire will be sized to achieve that temperature at a different current, depending on the rating. So all fuses made from the same material will break at essentially the same temperature. Why not estimate the rating by measuring the current through the fuse when it fails? That seems much more direct. – The Photon Jan 24 '12 at 23:37
• Trying to work it into a discussion of ampacity. Since the only other experiment involves copper conductors, I was hoping to demonstrate relationshipo between heat and conductivity. I guess not, huh? – Joe Stavitsky Jan 24 '12 at 23:41
• @ThePhoton - That's an answer, not a comment :) – Kevin Vermeer Jan 24 '12 at 23:52
• "Laser" thermometers only use the laser as a pointer. The "sensing area" is anywhere from 0.5 inches in diameter when up close to many feet in diameter at a distance. Since the fuse is smaller than 0.5 inches the thermometer will be sensing more than just the fuse element. – user3624 Feb 24 '12 at 1:45
• I suggest doing the experiment anyway and posting your results. Keep in mind that glass often blocks the region of infrared a laser-style thermometer uses to read temperature, so you might want to try automotive fuses - you can see them directly from the bottom on the fuse. Since the area of fusing heat is smaller than the aperture of the sensor, the overall heat would be averaged to some degree. A larger fuse, though melting at perhaps the same temperature as the smaller, would still require a larger surface of that temperature to break, and perhaps given averaging would show a higher temp. – Adam Davis Feb 24 '12 at 2:51

@ThePhoton may copy stuff out of here into an answer as he sees fit if desired as it adds to his comment/answer. Copy out enough and I'll delete this answer.

M. Photon said:

• The temperature isn't likely to change depending on rating. The fuse material will have a certain melting temperature.

• The fuse wire will be sized to achieve that temperature at a different current, depending on the rating. So all fuses made from the same material will break at essentially the same temperature.

• Why not estimate the rating by measuring the current through the fuse when it fails? That seems much more direct.

I more or less agree.
But its worse.
Fusing occurs when the metal wire melts BUT the construction will affect this significantly. The wire length will affect its thermal resistance from its hottest point. size of end caps, attachment to end caps etc will affect this too.

A manufacturer can probably achieve fusing over a 2:1 + ratio (just guestimating) with the same wire by changing thermal aspects such as wire nength, end cap attachment points and means, wire forming etc.

Wire material does vary with fuse type (I read). Some slow blows are said to use low temperature melting point alloys.

You could probably surface treat wire to affect its radiation effect. How much this affects fusingh compared with conduction to end caps i know not.

BUT, pointing an IR thermometer at fuses could be interesting and useful.

Note that the LASER used is for pointing guidance and that an IR sensor is used for temperature sensing. My experience is that with small targets the LASER pointing is useless and you can best locate hot spot by slowly "waving" sensor aperture to and fro on 2D over target. Also, background temperature (objects in same room) can make a large difference when target is small so screening may be needed.

• Agree there are other factors than wire diameter that will affect the ampacity. But in the end, if the wire material is the same, the temperature when it melts will be the same. Also agree that not all fuses will use the same material, so there will be variation between different fuses, especially from different vendors, different lines, etc. Measuring the temperature as you ramp the current up (but not enough to actually blow the fuse), and using that to predict the ampacity, might be a nice experiment. – The Photon Jan 25 '12 at 2:31