A quick question, can you run double insulated wire directly on a wire cable tray AC or DC ?

why I ask the question is because I see a lot of solar installations with "exposed" (what is suppose to be double insulated) red and black wire.

Considering the DC from the solar panels are no longer "low voltage", some runs are over 400 VDC.

This discussion has come up in the past, but regarding "open" wiring.

The same rules would apply ?