OK, here is a "good challenge" for all of you out there...

I have a potential application where I have a piece of copper that is .004" thick. I need to cut a notch into this material to give it a fuse characteristic. If I am using a V notch, how wide does the copper need to be at the designated thickness in order to cause the copper to melt after 2 seconds at 25 Amps?

Does anyone know where I might even START looking for information on this? I have some documentation from Littlefuse that is a starting point, but really would like some formulas/theory/etc.

Thanks!

--Joe Jansen

 

I would start with http://www.micrel.com/_PDF/App-Hints/ah-25.pdf. Then
search http://www.ipc.org.

Fred Townsend

 

Responding to Joe Jansen's query. Here is a start or direction, if you will:

1) Convert area to kcmil.

2) Use analytical geometry to determine remaining area after deducting cut-away area.

3) Use I.M. Onderdonk's formula in Fink & Carroll's "Standard Handbook for Electrical Engineers." It can be found in Section 4, entitled "Fusing Current-Time for Copper Conductors and Connections"

4) As an alternative there is a Chart showing time vs fusing-current for commercial size USA conductors.

If the above std is unavailable to you, let me know.

Regards,
Phil Corso, PE
Boca Raton, FL
[[email protected]] ([email protected]) {[email protected]}

 

Once you found the melting temperature of copper (easy) and it thermal properties (temp rise / W, conduction Theta), then you need to solve the (differential equation) that determines the temperature rise rate over time, which is proportional to the difference between:
A. Heat input (W) of I xI x R (on one hand) and
B. Heat dissipation rate (W as well) which in turn has 3 components:
1. Conduction - from the hot spot to the left and right over the solid piece of wire (into the infinite length of wire or a major heatsink mass nearby at either end)
2. Convection - from the hot spot and its vicinity to the surrounding air
3. Radiation (black body, temp to the power of 4) from the hot spot (and its vicinity) to the space around You may well find that one or two of the three above are considerably lower than the one that is the major factor, and can be neglected.

Meir

 

Hi Joe,

I suspect trial and error is going to give you a much quicker answer. Just build one and make the notch bigger until it fails. Consider the safty issues however. Molten copper is very hot and may spray or explode when it fuses.

Consider also that copper is a very poor material for making fuses. We use copper in wiring because it doesn't melt easily. Not so much because of its melting point temperature but because of it's low resistance. It takes lots of amps to make it hot. 24G wire can handle maybe 5 to 10 amps before it gets hot enough to melt the insulation let alone melt the copper. Look at the electric cables used in moder cars. Even the headlight cables are very small gauge these days (to save on weight, space and copper) but use high temperature rated insulation so they can get away with it.

Normal fuses use lower melting point and higher resistance alloys than copper.

Regards

Peter Whalley
Magenta Communications Pty Ltd
Melbourne, VIC, Australia
e-mail: peter*no-spam*@magentacomm.com.au
delete *no-spam* before sending

 

Hi,

The ultracad site was interested in Printed Circuit Boards theory and calculators, including the fusing current of copper traces. Have a look at the graphs in http://www.ultracad.com/fusing.pdf
One of their examples is similar to your numbers.

HTH
Louis

 

Hi Joe

Agree with Pete here. I once did some study towards using pc traces as fuse links. Sounds easy right? Reproduceable, precise, etc, etc, In the real world, it was difficult to even get the things to open where they were supposed to without a very high constriction to conductor width ratio. And the current they opened at was a very broad range and outside the calculated range. This is another of those
areas where you can formulate and calculate and run your statistics. Then, when you turn the power on, it does as it damn well pleases. Minute metallurgical effects caused every event to be different at the instant of disjunction. At least, that was my story, and I'm sticking to it. :^) It's really quite amusing, many years down the road. As it turned out, stressed solder connections were much more
consistant.

Regards

cww