When there are two fans at a mine it is necessary to install isolation doors for each drift leading to a fan in order to prevent the working fan from drawing air from the outside atmosphere. With modern fan layouts, the fan drift may be 5 m or more square and pass 300 m₃/s of air and sometimes more. Modern fan drift doors can be fixed in any position from fully open to fully closed and can be manipulated by one person from outside the fan drift. Doors of the butterfly type are often used and can be opened manually or by power.