Door Retainer 2 : A device, usually an electromagnet that is released by heat or smoke detectors, that holds a fire door open under normal conditions and releases it automatically when smoke or flame is detected
Ralph W. Burklin & Robert G. Purington, Fire Terms: A Guide to Their Meaning and Use, National Protection Association, Boston, MA 1980 USA. (Guide: Meaning and Use, Nat Prot Association Fr Srvc - Amer) {457}