Abstract: In order for a duct shut-off device to present the least possible resistance to flow in a duct when the device is in a non-activated state, a duct section (1) is provided with plates (3) which extend longitudinally in the flow direction and which are coated with an expandable material which reacts when the temperature, pressure or some other physical or chemical property of a substance flowing through the duct exceeds a given threshold level. When this threshold level is exceeded, the expandable material on the plates will swell and therewith reduce the cross-sectional area of the duct successively until the duct is completely close.