A resettable fail-safe fire door release mechanism which allows normal powered operation of a fire door is disclosed. A solenoid having a first open state in the absence of an applied electric current and a second closed state in the presence of an applied electric current is incorporated into the motor-operator...http://www.google.de/patents/US5245879?utm_source=gb-gplus-sharePatent US5245879 - Fail-safe fire door release mechanism having automatic reset
