I'm designing a new series of products in metal chassis containing plastic fans. The fans' power supply is referenced to the negative DC rail of a variable-frequency drive, and the fans are mounted to the grounded chassis. That means the fan is required to withstand a ~3kV hipot between its mounting holes and its internal circuitry. The fan is not marked with such a rating. However, we have a long history of products using this configuration. Those products are UL-listed, and every unit is hipot-tested before it goes out the door. We've never seen a failure.
Is there an industry-standard best practice in this situation? Would it typically be considered acceptable to rely on an unstated voltage withstand, as long as it was tested on each unit? Or would best practice call for the fan to be isolated from the DC- rail (or alternately, insulation to be added around the fan)?
EDIT: To clarify, our user I/O is floating, not referenced to anything. (We do prefer to keep our customers alive and not blow up their equipment.) It's only the fan that's in question.
The fan is blowing air into the box, so the "guts" of the fan are opposite the fan from the fan grill. And the voltage on the fan is stepped down to 24V from the incoming DC bus. The only high-voltage differential involved is between the fan's internal power supply and the unit chassis.
On one hand, I want to say that a history of thousands of units and UL's testing to get the units listed should be sufficient; I think it's pretty clear we don't have a safety issue as long as the user grounds the box and fuses the units as instructed. On the other hand, I hate to rely on an unpublished spec for the fan. Both arguments seem valid. The question is, which one is the more "correct" way of thinking?
FURTHER EDIT: The fan I'm planning on using is here: http://datasheet.octopart.com/4715KL-05W-B50-E00-NMB-datasheet-14433727.pdf However, I don't see its relevance. I'm not trying to identify a solution to a design problem. I have two solutions I'm trying to decide between. Which solution is better from a standpoint of best practice?