Great Question!

Two different things happening here.

D.7.1 is from IEEE 1584 and is the range of vailidy of the IEEE 1584 standard. i.e. calculations closely match the tests in this range. If the short circuit current is outside the range, most software will still use the formulas but with language that it is outside the range. For your case of a low short circuit current, using the formulas is still probably about your only option. - but I bet the protective device clearing time will be interesting!

D.7.6(H) is for Class R current limiting fuses up through 100A. There are other formulas for larger fuses and other classes. For RK1 up to a 100A rating, below 650 Amps (0.65 kA) the fuse is considered to not be in its current limiting region and therefore treat it as a non current limiting device i.e. use time current curves.

Above 650A a series of equations were developed based on a few tests to see what happens with let thru current, arc voltage etc. The 650 Amps lower limit of this specific equation is due to the small size of fuse for this case - 100A max.

However, you picked up on something I have not considered, the 650A limit of the RK1 fuse < 700A limitation of IEEE 1584 - oops!

I believe it is only for the smallest fuse - I don't think I would get too excited over 50 Amps - there are plenty of other areas that are more troublesome.

I'm thinking after the fuse tests were run and equations were developed, the low limit cut offs of the fuse equations were developed (650 Amps for RK1 100A max) even though it techincally falls below the overall 700A low limit of the IEEE 1584 document.