A fan belongs to an FEG grade (e.g. FEG85), if its efficiency, at best efficiency duty and maximum speed, is higher than the calculated value, at the impeller diameter, along the boundary, calculated in accordance with Annex A from the level of the immediately lower efficiency grade (e.g. 80 from FEG80), and equal to or lower than the value on the boundary calculated for the nominal efficiency grade (e.g. FEG85).