A question was asked: “The letters, like a FRN-R or FLM or FNM before the Ampere rating (amp), are they referred to specified conditions or
is only manufacturers codes?”
Answer: Each manufacturer has their own blend of alphanumeric characters to signify a particular fuse. I am not sure if each character of a particular part number of a fuse serves as a code to explain particulars of that fuse. However, some characters do represent something special or different.
For example: Bussmann has a named fuse FRN. The amperage rating will follow these characters. Like this; FRN-1. One (1) is the amperage rating. I do not know if and what FRN stands for. These fuses are called Fusetron. Is it short for that? If so, then what would the competitor’s equivalent part number represent. Mersen (Gould/Ferraz Shawmut) has an equivalent part series TR. These are called Tri-onic. Again, the amperage rating follows the alpha characters; like TR2.
Now look at this. Bussmann has a fuse named FRN-R. The R after the hyphen is a symbol to represent that the particular fuse is a “rejection” type fuse. Their design is slightly different and have higher interrupt ratings than the FRN type fuses. You cannot physically use an FRN fuse in an FRN-R type fuse block. These fuse blocks are designed to accept only FRN-R type fuses. However, you can use an FRN-R type fuse in an FRN type fuse block. Mersen (Gould-Ferraz Shawmut) uses the symbol TR(amp)R; like this TR2R. The R after the amperage rating signifies “rejection” type fuse. Littelfuse names their equivalent fuse FLNR. Edison has ECNR. With these, I do not know the significance of the first three characters, but the R probably represents “rejection” type.
It appears that the manufacturers try to assign characters of a particular fuse that will distinguish themselves from their competitors, but at the same time have some kind of reference to the type fuse. But what does KTK represent? This is a Bussmann fuse called Limitron. How does two Ks fit into this? It is also fast-acting so a T, which on some fuses might represent time-delay, could not be applied here. Was this an arbitrary decision to name these fuses as such? The manufacturer should know. Littelfuse’s equivalent part symbol is KLK. Maybe the L represents Littelfuse.
So it appears that the alpha characters can represent specified conditions, such as “rejection” type and/or also signify a code for manufacturers’ parts. Whether these characters always have a logical signification, I am not convinced.