Smoke Detector Above Suspended Ceiling

This doesn't make much sense. A smoke detector would be much more effective were it to be placed below the false ceiling, and not above it.

Is this space above the drop ceiling being used as a plenum?

Mainly yes. There is no duct for return air in it.

Then smoke and particulates will concentrate in this space and thus can easily produce more false, nuisance alarms. When a real fire incident happens this can also produce a false indication of the origin. I suspect the reason you cannot find a definitive answer on detectors in this space is that the NFPA does not wish to codify if this is a good idea or a bad idea. I don't have easy access to NFPA 72 codes so I cannot cite any content.

It's either a plenum or not. Look at smoke detectors designed to sample the air inside ductwork. they have a remote indicator that mounts below the drop ceiling.

..."NFPA 90A is specific to Air Conditioning and Heating Systems installation. The purpose of the NFPA 90A is to prevent the spread of smoke and fire through duct work and ventilation systems for air conditioning and heating systems with high capacities. Always ensure you check the latest codes for current information. Additionally, some municipalities have more stringent codes in place. Therefore, it is wise to check local code requirements in addition to current national codes.

Get your copy here: NFPA 90a: Standard for the Installation of Air-Conditioning and Ventilating Systems"...

https://highperformancehvac.com/duct-smoke-detectors-nfpa-90a-code-requirements/

There is no requirement for a smoke detector above a "dropped" ceiling.

If the building is frame (wood) built, there must be sprinkler protection above the ceiling.

Otherwise, there is nothing required.

Why are you involved in this project?

Joking right? Are you still trying to do this after a year. Lucky the place has not burned down. Or is this the best question your teacher asks of his new class each year and the easy route is to ask this yearly question here?

In generally, smoke detector are to be install under the ceiling but where some room you need to install false ceiling the degree of protection is limited between the false ceiling and the actual design build ceiling. Smoke can be generated above the false ceiling since most of the cable are running above the false ceiling.

Therefore, smoke and heat detector must be install above the false ceiling. If the height of the false ceiling and the actual building ceiling is less than 3.5 meter, then the smoke and heat detector should be install as NAPF 72 regulation. But if the height is higher you will need to add extra smoke and heat detector to it so that it can able to cover the entire area of the actual ceiling height.

This will need your experience and judgment for the installation of the smoke and heat detector above the false ceiling. It just happen to me as I have a control room with the actual building ceiling of 7.5 meter height and the false ceiling is 3.5 meter height. Therefore leaving me to have an extra 4 meter height between my false ceiling and the actual building ceiling height. Instead to install 2 smoke detector and 2 heat detector, I need to install 4 smoke and 4 heat detector to it so that the total area of the ceiling is fully covered.

Please refer me to the code which states those requirements.

An increase in room height will call for an increase in the density of heat detectors, but not smoke detectors (but this is NFPA code).

The requirement is depend upon the country to country. In my country it is the requirement of the fire and services department to install these smoke and heat detector above the false ceiling otherwise they don't approval the building plan.

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