De normen zijn gebaseerd op uitgebreide praktijkervaring in de jaren '70 en '80, maar missen vaak een moderne arbeidskundige onderbouwing.
. Below is a representative sample based on common data entries for piping and components: Component / System Size / Specification Norm Time (Minutes/Unit or Meter) Steel Pipe (Insulated) Steel Pipe (Insulated) Thick-walled Central Heating Pipe 24.0 min/m (0.400 hrs) Thick-walled Central Heating Pipe 24.0 min/m (0.400 hrs) Thick-walled Central Heating Pipe 28.0 min/m (0.467 hrs) Fittings / Flanges 35.0 min/unit Fittings / Flanges 40.0 min/unit Fittings / Flanges 54.0 min/unit Use in Modern Calculation While modern calculators prefer the CTN (Calculatietijdnormen) TABEL MONTAGE TIJDEN CONFORM GUSTAV ENDE