Supply and Return Air Ducts from a Packaged Rooftop unit
This picture is from a departmental store. Notice the duct smoke detectors mounted on both supply and return air ducts. Duct smoke detector is needed to shut off the AC unit if smoke is detected in the space. One scheme is to have duct smoke detector on main return duct only; if you have smoke in the room, the sensor would sense the smoke and the AC unit would be turned off. But what if a motor burns in the AC unit? You can say that the smoke would go to the conditioned space but as soon as it is drawn in the return air duct, the smoke would be sensed and the unit would be shut off. The latest trend is to not rely on this indirect route and provide duct smoke detector in the supply air duct too.