Constructed of galvanized steel sheet 0.9 mm . Thick , for frame , jambs and blades , linkages are designed to be outside the air stream ( Concealed in frame ) , very high structure for fire resistance rating up to 3 hrs ., in accordance with ULStandard 555 . Blades are hold at full open position by fusible link 165 F melting point Blades will be close and locked immediately by spring force and spring stap when the fusible link is melted . So it is versatile installation in any position . When installation to duct , should provide access door to duct nearest the damper for checking and servicing purp [ oses , one type ( FD ) . Application for mounting to duct line , building sidewwll ,shaft or floor that dust installed through for preventing fire spreads thrugh the building sidewall , shaft or floor .