The S200A multi-criteria, photo thermal detector uses thermal assistance to the core photoelectric smoke detector to give enhanced false alarm immunity and faster response to a wide range of incipient fires. The plug-in unit combines two separate sensing elements that are managed by embedded software to act as a single unit. The HM-PTSE conforms to EN54-7, a 58°C fixed temperature and rate of rise thermal assistance conforming to EN54-5.