A three phase fused switch should have the same fuse sizes and types in all three phases. However, sometimes a different size fuse is used to replace a blown fuse - perhaps the correct size was not available in the middle of the night or other various reasons. So here is this week's question.

Have you ever seen different fuse SIZES in a fused switch?
Yes
No

Yes, in a three phase 4 wire 240/120V service entrance switch, where the high leg was smaller than the other phases.

And of course the is the ever common mix of RK1 and RK5 fuses as well as the 'fuses supplier of the day' situations.

Yes. We did an arc flash study at a warehouse were the MCC had fused switches. Several buckets had different size fuses, and several had the same size, but different model fuses. We modeled the worst case fuse to produce conservative arc flash results and made recommendations to our client. We suspect their maintenance people replaced the fuses with what ever they had available at the time to get the facility back up and running.

I will immediately acknowledge that TYPICALLY if you see different fuse sizes, one or more is probably incorrect. If they are even from different manufacturers or different models this might also be the case. The packages themselves lend themselves to quite a bit of variation in sizes as well as specifications. One particular manufacturer has been really aggressively marketing fuses with faster trip curves as "arc flash" fuses.

I have also run into the basic problem of having the correct fuse on hand or when warehouse stock gets "optimized", you have to make do with what you have on hand and fix it correctly at a later time. Certain manufacturers and fuses are worse than others. For example one fuse that has caused me countless problems is the SMC/DBU/CMU-20 fuse package. This is a relatively low cost medium voltage boric acid expulsion fuse. Up to 200 A they are almost interchangeable among manufacturers, all following the "E" or "K" rating system. Above 200 A only one manufacturer exists (S&C). "E" and "K" are generally interchangeable despite manufacturer claims except that you have to go up/down one size to get the curve correct. At times the fuses have taken up to 8 weeks to arrive. This problem also frequently comes up with semiconductor fuses.

I've also used different sizes in a 3 phase panel which was only feeding single phase loads to optimize each phase separately.

We see this a lot. Different sizes, speeds, brands and often non rejection type fuses.