Sorry Mike, i'm relatively new to this and you lost me there.

No prob - we all start somewhere. Are you happy with I squared t being a measure of the energy needed to melt a fuse of given mass  in a short duration fast overload ?

If not come back and we can look at that in more detail. 

The pre-arc energy is that below which it is absolutely going to remain intact and not going to operate, and the other limit, the one at which it always operates is almost 3 times higher, is the one in the curves, and then extends south at a constant I2t - so divide the time by 9 if the current is 3 times higher. 

At the north end, the curve becomes more or less vertical - so never trips even after hours - 1ksec is about 20mins, and 10kseconds is nearly 3 hours.

 Now a 60A fuse must carry 60A all day, but is not guaranteed to carry much more but may not blow all day until we reach more like 90A.

Note the ~ 2:1 ratio between the pre-arc (no blow) and the total for a 240v supply in the table. The all blow energy is higher for 415 V, as enough of the fuse has to melt to make enough opening  that the voltage could not sustain an an arc across the gap for the largest possible fault current.

Mike.

Consider the blow/no blow curves for the 13A plug fuse on the graph below.

Normally we only consider the right hand limit of the all-blow curve, but for any fuse it would have a form generally rather  like this. For the 13A fuse, note the vertical asymptote for the long term overload, could be between 20 an 25 amps for ever, while say a 100A fault may blow a 13A fuse as fast as 0.01 sec or as slow as 0,3 seconds.

Note that for these small fuses, even 10msec is not adiabaticlly fast, and the curve has not yet settled to an I2t form.