Equation for ANSI 37.42 K Type Maximum Melting Curve

Don't have a clue.

Have you asked Chance or Cooper for help?

Something like this?? From S&C's website, there are others, you may have to scale the numbers to fit the Type K characteristics, or do the research yourself.

I don't think the Mfrs generally publish that info on their fuse data sheets. The curves are what people generally work with to do coordination. Cooper Bussman says on their data sheets that the curves are plotted from test data at ambient starting temp of 25C, suggesting that although they may have started with a formula, the resulting fuse design (and curves) are based on what they measured in the test lab.

The following link gives some info on correlating different Mfrs' fuses. Maybe there's something that could be helpful to you:

https://www.fusetek.com/

The other place to check would be if you know someone with SKM PowerTools or a similar protective device coordination package. They may have formulae for the fuse curves - I seem to remember there was for some of the relay pickup curves.

Why?

The curves look approximately hyperbolic, which makes sense. If so, a formula like t = C/I should be close (where C is parameter depending on the nominal fuse rating).

Friend,

A Google search gave me this reference as a footnote to an on-line calculator:

References: The Preece and Onderdonk equations for fusible metals were found in the Standard Handbook for Electrical Engineers, 15th Edition 2007, Ed. by H. Wayne Beaty and Donald G. Fink. McGraw Hill 4-25

Of course, you have to consider the effects of the other materials surrounding the fusible element in a fuse, because the Onderdonk equation is only valid for a wire in free air without accounting for end-effects (conduction of heat away by the ends). Perhaps you could use the Onderdonk equations to calculate the theoretical time it takes to fuse the wire and compare this to the fuse manufacturers' data sheets at the different currents. This may give you an approximate correction factor to the equation. . .

--JMM