NEC requires fixtures to be 'attached' to the grid, by mechanical means; screws, clips, rivets, etc. in most cases. This is the 'basic' NEC requirement...this is what an Electrical Inspector can enforce, but....
Ind. support is required in seismic areas.
Ind. support MAY be required by plans/specs.
Ind. support IS required in certain 'Fire Rated' ceiling assemblies (I have never seen one)
And; a 'COMMON SENSE' rule would be IF the fixture deflects the grid....install support!.
Who the enforcement falls upon...Electric Insp for NEC and Plans/Specs.; Building or Fire for seismic; Fire/Building for 'fire rated' assembly. (My understanding of the NJ UCC)
I'm bidding a "Family Dollar" store in Camden. The specs are "cookie cutter" for each store. They require 8' fixtures, surface mounted on a drop ceiling. (I know I mentioned troffers in my original post, but I believe the question still pertains.) I'm looking at Caddy "4G" series fixture mounts and wonder if additional support would be required.
They're of extra value when the layout of the fixtures, lighting track, etc. don't fall right on a main runner of the grid. The cross tees are comparatively weak and the IDS support clips can provide the support for the fixtures no matter what the layout.
Usually the GC will have the ceiling contractor shoot the additional support wires and seismic splay wires. If not, a case of beer or 2 to the ceiling guy seems to do the trick
I worked at an industrial plant outside Chicago and one of the processes was carried out in a room that had 2x4 troffers and a suspended ceiling. The process was apparently corrosive to the ceiling wires, because 1 day the whole grid let loose and dropped tiles on everybody.
The troffers stayed hanging by their small safety chains. If not for that little bit of extra protection, I'm sure that someone would have gotten a fixture dropped on their head that day.