Applicable to associated power transformers for switch mode power supplies, single- or polyphase, air-cooled, including separating, isolating and safety isolating transformers with a rated supply voltage up to and including 1000 V a.c. and rated frequency of 500 Hz to 1 MHz, where the rated output is not in excess of 10 kVA for single-phase transformers and 16 kVA for polyphase transformers.