Motorized fire, smoke, and combination fire/smoke dampers and their testing requirements have come a long way since their introduction during the 1970s.
Originally, life-safety dampers were standard commercial control dampers with blade locks to hold them closed. They were Underwriter Laboratories-tested to the third edition of UL 555, the standard for fire dampers, and the first edition of UL 555S, a standard for smoke dampers. Testing consisted of fire, preconditioning and heat-degradation tests. Because Underwriters did not have a test procedure for actuators, damper manufacturers would list the actuator used in the test of their product. Actuators did not have to operate under fire conditions. Instead, a fusible link connected to the damper blades and a shaft connected to the actuator was used. During a fire event, the actuator would disconnect from the shaft, a spring would close the damper, and a locking device would secure the blades closed. Because of the fusible link, full access to a damper via an access door was required for visual inspection and testing.