UL field inspectors are not by and large technically astute. It is not their job to train you or your people on how to design and build UL 508a panels, nor to answer technical questions.
There are good reasons why you can't use just any power supply in a UL listed control panel. They may not seem like good reasons to you, but they are good reasons, mostly. Most UL investigations involve just adding parts to your procedure and don't cost $10,000. You may have selected a power supply that they have to do actual testing on. But why would you want to use a recognized power supply anyway? There are plenty of listed power supplies available.
And this is where I have issue: if they're not technically astute, how can they be expected to properly interpret and understand a
technical standard? It also seems like few people at UL
can interpret their own standards, and finding someone who will return your phone call in less than a geological timescale is neigh on impossible.
To the OP's point: SCCR is a bit of a nightmare, and it's my belief that UL keeps it so in order to justify the way they do things. Either a fuse will limit current or it won't; there's no engineering rationale that can justify allowing it to be installed in the factory but not in the field. Either the math works or it doesn't.
I was quoted almost $10k to investigate the possibility of adding the power supply to our procedure; no testing required. As to why: it all came down to form factor and available space. Further, no one at UL could actually explain
why it couldn't be used in a 508A panel. It was a case of "just because we said so." If there is an actual engineering reason, so be it. That's a good reason, and they should be able to explain it. But it was just paperwork / money grab, and that's what I have a problem with.
SceneryDriver