No, I said you could drive it up to 281 A if your burden is negligible. If you have 0.1 ohm burden, the calc becomes 18/(0.064+0.100), or ~110A secondary, which gives some agreement with the C class rating.
Yes, the statement that "if burden is less than X you will get at least Y current before saturation" is an easy way to say your application is OK, in some cases at least. But what if I have a 200:5 CT with 0.05 ohm external burden and 10kA available? Given the CT knee point and Rct, I can get at least some idea pretty quickly. Or going the reverse direction, it tells me what knee point and/or Rct I need to look for.
I do the full calculation on every job, but "full" in this case is about 1 minute longer than the less full. I have to determine the burden on the CT no matter what, so by the time I have added up the burden, the calc I showed takes an additional 60 seconds, if the CT excitation curve and Rct is available. I am not sure if I am in the minority, but it depends on what crowd you count. Among those that I work with, I think I am just a typical engineer doing typical calcs.
The CT excitation curve, including Rct, comes in the approval package for every swgr, xfmr, bkr, etc that my company works on. It is a requirement of the specs I believe, but no bidder ever takes any exception that I am aware of. Me and my fellow engineers take a look at that data. We do the calc I mentioned and need Rct and the CT excitation curve. You will not be selling to the crowd I run with unless you decide to readily supply this data. Take a look at the ITI/GE lineup; you freely download such data.
I found your comment on wound primary CTs having a 1sec rating ~100xInom to be informative. I do not deal with wound primary CTs very often. I had not realized that they are more limited on fault withstand than window CTs.
I mentioned the matter on the CT short time rating to say that 281A will not destroy the CT, and you can go above 100A secondary, as long as you clear the fault quickly. The high CT current is a reason you have to clear the fault quickly.
Your response told me why the mfrs resist putting the short time rating on the CT. I had asked for it a few times, and got it, and like you said, it was off the charts, and have not asked for it since.