# Question on fuses specifications

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.

1. What does In (whatever the right spelling may be) mean in this context?
2. 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?

What does In (whatever the right spelling may be) mean in this context?

It is $$\I_n\$$ and it indicates that the measurements were taken at 10x the nominal current. The best example might be the 30 A fuse. A rating of $$\1000 A^2 s\$$ means that at 300 A it should blow around 0.011 s. That lines up roughly with the graph.

What does the Time-Current characteristic curve tell me? If I look at the 10A curve [...] 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?

Yes.

What does the Amp rating really relate to?

The amp rating on the face of the fuse is the nominal current. A 10 A fuse can operate indefinitely at 10 A. However, as you've seen, this 10 A fuse can also operate at a steady 15 A for ~1.4 hours. All fuses are designed to open during a short circuit but the time-current characteristic will give you an idea of the rest of its operation.

Try looking at the time-current characteristics of "fast-blow" and "slow-blow" fuses. You'll find fuses that can operate at 1 A indefinitely, but will blow at 2 A within 0.1 s. It all depends on your application.