Gives the definition of nominal pressure when applied to components fo a metallic piping system, and states two series of PN ratings for use with these components. The nominal pressure (PN) is a numerical designation relating to pressure that is a convenient round number for reference purposes. The ratings for flanges shall be selected from: series 1: PN 10 - PN 16 - PN 20 - PN 50 - PN 100 - PN 150 - PN 250 - PN 420; series 2: PN 2,5 -PN 6 -PN 25 -PN 40.