Hotline, I must take some issue with your response.
It's what I was getting at when I said things were tested as an assembly. That is, a complete ceiling assembly.
Special fixtures? Ordinary fixtures? Ordinary fixtures wrapped in drywall? All of these are construction details specific to each ceiling. A cruise throught the UL Fire Resistance Directory *the 'orange books') will detail an enormous variety of ceilings.
Let me stress that the 'assembly' is more than just the ceiling grid. It is also the part of the building above the grid; construction details there - drywall thickness, etc.,- often are the real determining factors in the performance of the assembly.
In the actual fire tests, the real results seem to depend mpre on the grid holding the troffers in place, rather than whatever the troffer is made of. This is engineering trivia, though, and no substitute for following a tested design in its' entirety.
