Replacement: This part of IEC 61558 deals with the safety of isolating transformers for general applications and power supply units incorporating isolating transformers for general applications. Transformers incorporating electronic circuits are also covered by this standard. NOTE 1 Safety includes electrical, thermal and mechanical aspects. Unless otherwise specified, from here onward, the term transformer covers isolating transformers for general applications and power supply units incorporating isolating transformers for general applications. NOTE 2 Switch mode power supply units are covered by IEC 61558-2-16. This part is applicable to stationary or portable, single-phase or polyphase, air-cooled (natural or forced) independent or associated dry- type transformers. The windings may be encapsulated or non-encapsulated. The rated supply voltage does not exceed 1 100 V a.c., and the rated supply frequency and the internal operating frequencies do not exceed 500 Hz. The rated output does not exceed: - 25 kVA for single-phase transformers; - 40 kVA for polyphase transformers. This part is applicable to transformers without limitation of the rated output subject to an agreement between the purchaser and the manufacturer. NOTE 3 Transformers intended to supply distribution networks are not included in the scope. The no-load output voltage or the rated output voltage does exceed 50 V a.c. or 120 V ripple-free d.c., and where applicable, does not exceed 500 V a.c. or 708 V ripple-free d.c. The no-load output voltage and the rated output voltage may be up to 1 000 V a.c. or 1 415 V ripple-free d.c. for special applications. This part is not applicable to external circuits and their components intended to be connected to the input terminals and output terminals of the transformers. Transformers covered by this part are used in applications where double or reinforced insulation between circuits is required by the installation rules or by the end product standard. NOTE 4 Attention is drawn to the following: - for transformers intended to be used in vehicles, on board ships, and aircraft, additional requirements (from other applicable standards, national rules, etc.) may be necessary; - measures to protect the enclosure and the components inside the enclosure against external influences such as fungus, vermin, termites, solar-radiation, and icing should also be considered; - the different conditions for transportation, storage, and operation of the transformers should also be considered; - additional requirements in accordance with other appropriate standards and national rules may be applicable to transformers intended for use in special environments. NOTE 5 Future technological development of transformers may necessitate a need to increase the upper limit of the frequencies, until then this part may be used as a guidance document.