Dave,
Re: "So if I can prove imperically that the breaker doesn't open, can I use the current limitation of the fuse as proof of lowering the avilable fault current."
Why would you think that the breaker would not open? If a breaker is subject to a current equal or greater than it's trip calibration it will trip. If subjected to a fault current greater than its rating it probably would sustain damage. If great enough it could blow apart. That's where you must a consider series-conbination rating that you are so wisely exploring now.
If I may, and please correct me if I?m wrong, I would like to make a distinction between available fault current and current limiting.
Available fault current can be calculated if a qualified person has done the calculations and that the OCPD applied accordingly.
However, regarding a CL device as it would relate to protecting a down stream device by limiting fault current, curves and any calculations regarding same don't count. The devices to be used in series-combination (integrated rating) must be tested together and UL listed as such. No exceptions. Using the "up-over-down" method with "Prospective Peak Let-Through Current in Amperes" fuse curves may be invalid as they are based upon passive bus systems which do not take into account the dynamic nature of a circuit breaker. Always refer to the recommendations by the circuit breaker manufacture, which are based on actual test data.
It is interesting to note that CL fuses are focused on. But all breakers are also CL to some degree that makes them suitable for being series-combination tested with other breakers down stream. In addition there are specifically breakers which are UL listed and classified as current limiting that have been tested to limit the let through current to a specific level or less. As I can recall that level is less than 1/4 cyc.
Bottom line? Identify the available fault current, go to the GE series-combination rating chart as refered to by a previous poster and compare it to the exising gear. It may be that simply changing a breaker of two may be the solution dependent upon what the exisiting breakers are and if they are included on the series-combination rating chart of the kaic required.