Plenum fire protection, below and above?

[toledodude]

I am designing a fire alarm system for a 3 story building. Each floor is 800 by 300 feet and has a raised floor throughout with a supply air plenum and the entire ceiling is a return air plenum. The building has a complete sprinkler system with the sprinkler heads mounted in the ceiling tiles. It is my understanding that smoke detectors need to be installed in the underfloor air plenum, but I am unsure if smoke detectors have to be installed in the ceiling plenum as the sprinkler heads are installed below this plenum space. Thoughts?

 

[RonShap]

Unless the IT equipment space is using the more relaxed wiring rules found in the NEC Article 645 (2002 or later), there is no national code requirement for smoke detectors below the raised floor.
NEC 645 or not, install detectors at the point of return and no detectors required above the ceiling either.