NRTL Nationally Recognized Testing Lab. It would be U/L, ETL, TUV etc. They are labs that evaluate and list equipment so we have a standard we can trust. This is an independent test, not self certification like CE. Usually it s destructive testing. They will torture test things until they catch fire and make sure the fire is contained inside the case, grounding holds up, insulation survives and other things along these lines. There is also less destructive testing, like simply making sure the labeling is correct and the load is appropriate for the cord set. One of the guys I heard describing this from the manufacturing side said they kept asking him for more samples because they blew up all he had sent them. This was one of the first listed chlorine generators for a pool and they were making sure it was safe in any failure mode they could think of.
