Applies to industrial furnaces heated indirectly with metallic heating elements in which the internal atmosphere is composed of air not intentionally modified. Standardizes testing conditions and methods in order to determine the essential parameters and technical data of batch chamber, bell, elevator, shaft and carriage furnaces, either with or without forced air circulation, as well as of furnaces of similar construction operating at below 500 V. The contents of the corrigendum of June 1999 have been included in this copy.