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Comment by nemosaltat | Hacker News Reader

If that sounds silly, while working in commercial construction in Los Angeles, we encountered pushback for placing UL-listed devices in a UL-listed enclosure. The city determined that to be a “new assembly” and required UL listing for the entire unit. UL has a category for this situation; the enclosure is “UL recognized” because all of the devices and the enclosure are UL-listed. Their proposed solution, which UL was happy to oblige, was to come and field-inspect the installed units and have them certified in situ, at our (significant) expense, of course.

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I assume you know this, but depending on what the enclosure was made of, what frequencies the devices inside were broadcasting at, and where you placed them inside, you could very well have accidentally produced a resonating chamber, parabolic antenna emitter, or other type of structural frequency-specific gain-boosting mechanism.

They likely wanted to just make sure that you hadn't done that. (And to do that, they had to send some fellows whose time was very expensive, because it takes long experience to build up an intuition for these things to the point that you're confident enough in your answer to not have to bother with the "measure everything and plug-and-chug the dynamics in a simulator" step.)

In this particular case, it was a guest room control solution consisting of DIN rail mounted TRIAC dimmers 0-10V dimmers and other line-voltage & dry contact relays. Backbone was an RS485, 4-conductor bus. Yes, those components can produce various harmonica and frequencies, however they were tested extensively in a lab in various configurations. Each configuration varies slightly based on room layouts and number of zones, and obtaining UL-listing for every possible permutation becomes ridiculous (IMO).

TBH that doesn't sound ridiculous at all. "UL recognized" is not the same thing as "UL listed."

The former means that the assembly is assumed to meet UL specifications; the second means that it has been inspected by UL and confirmed that the assembly actually does meet specifications.

Making sure that things meet specifications is the entire point of the building department's inspections.

I acknowledged that in my comment. Part of my point here is that UL is not exactly a disinterested 3rd-party when it comes to endorsing additional layers of certification.

at our (significant) expense, of course.

When I see situations like this, it's often because someone on the other side, or someone related, benefits from the arrangement. They would never admit so, but that's how things tend to turn out like that.

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