The SCCR rating on the control panel's nameplate is 10kA, which is why my equipment evaluation is failing. Bussman KLU 800A fuses are utilized in an 800A fused disconnect that supplies the control panel with two 500 kcmil at a distance of fifty feet. An ABB XT6-N 800A main breaker with an AIC rating of 35K is located inside the control panel.

I've been looking for a way to reduce the fault current to less than 10kA. Adding a 750 KVA transformer after the disconnect was the only fix I could find.

Is there something I'm missing?

Did you model the drives to allow motor contribution through the drives? If they aren't regenerative drives and don't have bypasses or have bypasses that won't be used or can't be used without damaging the compressors or the process they are part of you could check or uncheck the boxes that allow motor contribution and see what that does. Probably will help some but not get you there. I looked up the noise filter and it says it is for applications using regenerative drives so you may be stuck if that is what you have. If you are allowing fault contribution through the drives, does the noise filter impedance have an effect on motor fault contribution? You'd have to model those to, to figure that out.

I don't see the sccr/withstand rating on the filter data sheet: https://coselusa.com/wp-content/uploads ... _TSD-S.pdf but I do see that rating on this page: https://www.cosel.com/en/product/noisefilter/TSD and here: https://www.cosel.com/en/lp/lineup/appl ... index.html

I spent enough time looking into it but it seems as though Cosel has had some of their filters tested in combination with some breakers and if they haven't the "assumed" value for that filter size is 10kA. Here is another link on this if you want to research further: https://www.cosel.com/en/coselplus/technical/111

Is this an existing installation or new design?

Reactors are another option.

Artificially increasing feeder length could be another option but I expect it would border on ridiculous.

Can you reduce the 2 sets of 750's to 600's or 500's or is voltage drop an issue? Your feeders are pretty long. Reducing those 750's to something smaller would help.

If this is a new project, could you locate B-2HA closer to the compressors and make it's feeder longer and the compressor feeders shorter? That may help or may make it worse. I don't know without modeling it.

B-2ha is existing. The medium compressor control panel and fused disconnect is new installation. The original feeders is (2) 500 kcmil at 50 feet. I was just changing the feeders to different types to see if it would make a difference. This is all I have for the compressor data.

Try using something other than a KLU fuse. I often found some Class L fuses to be have differently. I would typically look at fuses from Ferraz as an option.
It all depends on the TCC and your arcing current. Many times these large fuses were originally selected so they would never "blow".

This is something I come across frequently. Just recently I had to tell a contractor to re-pull the oversized feeders to a control cabinet with properly-sized wiring. They thought they were doing the customer a favor by using larger wire...unfortunately it elevated the AFC to more than the 10kA the panel was rated for.

It's frustrating there aren't simpler ways do deal with limiting fault current. I can't count the number of times I've had to tell contractors in the past year alone that no, putting a current-limiting fused disconnect ahead of the cabinet won't actually limit the current in a way that affects the calculations. They always respond, "Well, then what's the point of current-limiting fuses?" And all I can do is shrug.

I believe the code still allows the "over up and down" method for determining let-thru current via a fuse w/ the caveat "under engineering review" or something like that. I just don't know any engineers that want to sign off on that.

Mike

No major fuse manufacturer has advised the use of the Up over and Down method protecting anything other than static components for at least the past 35 years.

It definitely would not be suitable for something like arc flash.