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