In the datasheet for fuses (for example, I'm looking at the Bel Fuse 0678L9150-02 -- datasheet at https://www.belfuse.com/resources/CircuitProtection/datasheets/0678L%20Jun%202016.pdf), they report the melting I²T integral (in A²·sec), but they say
@ 10 In --- not sure whether the first letter is a capital-i, or lowercase-L.
- What does In (whatever the right spelling may be) mean in this context?
- What does the Time-Current characteristic curve tell me? For example, in the above fuse model, if I look at the 10A curve (presumably that means the curve for the fuse with 10A rating), then 20A maps to 5 sec. Does that mean that if a fixed current of 20A runs through it, after 5 sec it will blow? (understanding that this is a "theoretical" / average figure). What doesn't seem right about this is that the curves get to very high amounts of time --- I mean, it doesn't make sense that the stress would be cumulative at such a slow rate until reaching an amount of accumulated stress that makes it blow.
I guess this brings me to a more fundamental doubt I have: what does the Amp rating really relate to? Is it:
- the point at which the fuse should blow? (meaning that normal operating current should be, say, half or a tenth of that value?)
- the maximum normal operating current? (meaning that the fuse will blow when the current exceeds that value by a factor of, I dunno, 5 or 10 times that?).
I suspect that strictly speaking it is neither, and that one has to do somewhat more involved calculations?