Applicable to stationary or portable, single-phase or polyphase, air cooled isolating transformers, associated or otherwise, with a rated supply voltage up to and including 1000 V a.c., and rated frequency up to and including 500 Hz, with rated output not in excess of 25 kVA for single-phase and 40 kVA for polyphase transformers. Also applicable to isolating transformers without limitation of the rated output; these transformers are considered as special transformers and party to an agreement between purchaser and supplier.