We have a situation where we have old gear, old fuses and about 37k of available S.S.C.. at and MDP. In the MDP I have a 200a fused switch that feeds a small power panel nearby. Our recent survey tells me with the current fuse I have about 23k of available S.S.C at this panel. To help lessen the S.S.C. at the panel I have asked why we can't use the current limitation aspects of something like and LPN-RK200 fuse. That suggestion led off into a series rating conversation that didn't make sense. "I'm not saying they were wrong. It just didn't make sense." What am I missing? What is the purpose of the current limitation curves on a fuse like this if it must be series rated?

Thanks in advance for making me less ignorant.

For the most part these curves are now little more than marketing fluff. At one time they were important, but over the past several decades our industry has learned a lot about device interaction and the need for actual testing of devices in series ratings.
Molded case circuits breaker (since roughly the 60's) and definitely since the mid 80's have the ability to react and begin opening in just a few milliseconds. Search for information on Dynamic Impedance if you are interested.

The current limiting fuse curves and the "Up over and Down" Method can be used for static devices such as bus bar bracings on Switchgear and Bus Ducts. As mentioned in the previous post, this method can not be applied to circuit breakers because modern circuit breakers operate too fast. The breakers are interrupting at the same time the fuse is going into current limiting mode. The dynamic impedance makes it impossible to accurately calculate the reduce short circuit current. This is one of the reasons that manufactures pay to have the series tests done.

These are two equipment designs which are now rarely, if ever, performed by consulting engineers, end use customers and electricians. The method is important for equipment manufacturers that want to do custom designs. But seriously, probably 9999 out of 10000 people involved with breaker selection will actually be able to use it, so why is so much time spent teaching it? Shouldn't we focus on teaching series ratings instead, after all Modern molded case circuit breakers have been around for 50 years.

Mr. Fuhr is correct. A CB is protected by an upstream Current Limiting device (fuse or CB) only if the combination is tested per UL489 series ratings.

Two current limiting devices in series can interact with each other and cause very unpredictable stuff to happen, hence the up-over-&-down method, also called engineered ratings, should not be used!

As far a 200A fuse, that is fairly small and possible makes a good AF protector. But if you have under rated 20A 1 pole CB, relative to that fuse, the CB are small. The CB contacts could "pop" open before the fuse link melts... if that happens the CB is trying to protect the fuse, not the other way around... The CBs could be 50 years old or manufactured last year, would not matter.