I have been confused on this issue.
Say you have a 100KA available at the service. Does using fuses at the main disconect (in lieu of a circuit breaker)protect down stream equipment?

The fuses in the MOCP provide xxxK at the 'load' side of the fuses. The next equipment downstream would be calculated with that 100K as a starting point. One of the calc methods is 'point-to-point', you may find one version of it at the Cooper Bussmann site.

Watch the ratings of the fuses.

100KA on line side of MOCP. ***KA on load side? 100KA or ***KA as starting point for calculation for the next equipment?

Let's see if I can sumerize this.
IF you have 100K at the line side, and have 100K fuses, then 100K is the first point. Keep in mind, this is based on a theoretical 100K from the service conductors.

The 'start' point is at the utility transformer terminals; the service conductors, length, & type of conduit are factored in, and the 'line' side terminals are the second point. You basically repeat the previous steps to all your points. IF you have motor loads, there are formulas to include the contributions.

The Bussmann 'Plan Review' Book has the complete scenario; I think you can view it online, & it is available to purchase at a small fee. IMHO, it is well worth the price. It will not make you an EE/PE, but it will provide acceptable data.

BTW, there is a fault current spreadsheet you can download over at Mikeholt.com Go to Free stuff and click on it.

Fuses may not protect down stream equipment, but then again they might.

You can never assume that a protective device, like a fuse, will protect another protective protective device, like a circuit breaker. Fuse-breaker and breaker-breaker combinations require actual testing.

You should never use the 'current limiting' effect of protective devices when performing short circuit calculations for equipment locations.

Let-through currents maybe used when considering the protection of specific non-active equipment (i.e. cables, switches, transformers, and electronics).