I'm not so sure. There are two different problems: 1) What conditions equipment works under 2) What conditions equipment went through testing under, as per r

I'm not so sure. There are two different problems: 1) What conditions equipment works under 2) What conditions equipment went through testing under, as per regulatory regimes Equipment I worked on needed to be tested for a variety of failure modes (e.g. a power supply doesn't cause a house fire), EMI/EMC, and so on. I would guess that also varies internationally, as well as by use (UL-listing is not legally required, but it is required if you sell to customers who want to have fire insurance, for example). Some of this is wonky administrative overhead, but some of it is not. For example: - A DC-DC converter will have a different switching frequency if running from a different input voltage. This might work fine, but it will have a very different profile for EMI/EMC. Testing that might be $40k. If I know I have a 12V wall wart powering it, there's no reason to spend $40k to be able to label the box "5V-40V." - If a PCB has creepage / clearance for 120V, even if the power supply works fine for 240V, I can't sell it as 120V equipment. The reverse holds true the other way for wire gauge. These things wouldn't cause problems very often (indeed, it'd be exceptionally rare), but if we have a house fire 0.001% of the time people bought a TV set, we'd have hundreds of thousands of preventable house fires. That's why there's testing before products are sold.