Curtain type: The blades are folded and held under spring tension using a bi-metal fusible link. Heat of 165°F (74°C) inside the duct is sufficient for fusible link to melt and break, thus allowing blades to shut like a curtain under the force of the springs attached. The fire damper acts as a barrier between fire and non-fire zones thus containing the fire in the compartment from which it originated. This gives sufficient time for occupants to escape and move into safety. There are three casing variants-all fully welded, Square / Rectangular, Circular and Flat oval with infinite sizing capability with in min/max dimensions. All are suitable for either low, medium or high velocity / pressure applications.