How does an operating mechanism with simple fusible link work?

This kind of operating mechanism automatically unlatches the blade of the fire damper if the temperature in the duct exceeds a certain level (by default 72°C).

Due to the temperature rise the lead melts down and the fusible link opens. The internal torsion spring loosens and the blade closes. 

The correct functioning of the fire damper with fusible link can periodically be tested by manually unlocking and rearming it. By installing an end of range switch on the mechanism, the position (open or closed) of the fire damper can be monitored remotely. 

Once the fusible link has melted down, it needs to be replaced.