Field tested by a NRTL?
I suppose a lot depends on what they are testing. If this is assembled from U/R parts and they are only looking at the assembly methods it should not be that complicated but if the actual equipment is foreign and never evaluated at the component level there is no way the field test can be as extensive as the lab test. I have to say, I have only heard people talk about this. I never actually watched it.
OTOH, under a lot of pressure, I did certify a lot of stuff at the Ca D Zan (Ringling Mansion/Museum) in Sarasota without any protocols in place at all. This was antique equipment and for the most part I just assumed if it had not burned up in the last 70 years, it would last 70 more
I mostly was looking at the condition of the insulation, security of the attachments, chafing and other hardware issues. A lot of this was restored under supervision of the engineering firm the state hired. When possible they used listed components and modern fixture wire but the things you could see were original.
The guy I worked for in Tallahassee just told me "make sure it is safe" when I asked him what my job was.
It may have been the most interesting thing I have done in any of my careers.