The following method is applicable to enclosed PTTA or partitioned sections of PTTA without forced ventilation. Note 1: The influence of the materials and wall thicknesses usually used for enclosures are negligible on the steady state temperatures. The method is therefore applicable to enclosures mad of sheet steel, sheet aluminium, cast iron, insulating material and the like. Note 2: For open-type and dead-front PTTA, no temperature-rise assessment is needed if it is obvious that no excessive air temperatures are likely to rise.