This part of IEC 61558 deals with the safety of transformers for class III handlamps for tungsten filament lamps and power supply units incorporating transformers for class III handlamps for tungsten filament lamps. Transformers incorporating electronic circuits are also covered by this standard. NOTE 1 Safety includes electrical, thermal, mechanical and chemical aspects. Unless otherwise specified, from here onward, the term transformer covers transformers for class III handlamps for tungsten filament lamps and power supply units incorporating transformers for class III handlamps for tungsten filament lamps. This part is applicable to stationary or portable single-phase air-cooled (natural or forced) independent or associated dry-type transformers. The windings may be encapsulated or non-encapsulated. This standard is applicable to transformers and power supply (linear) with internal operational frequencies not exceeding 500 Hz. This standard used in combination with Part 2-16 for switch mode power supply (SMPS) units is also applicable to power supplies with internal operational frequencies higher than 500 Hz. Where the two requirements are in conflict, the most severe take precedence. The rated supply voltage does not exceed 1 000 V a.c., and the rated supply frequency and the internal operational frequencies do not exceed 500 Hz. Transformers for class III handlamps for tungsten filament lamps have the following additional characteristics: - the no-load output voltage and the rated outpur voltage do not exceed 50 V a.c. or 120 V ripple-free d.c.; - there is only a small difference between the no-load voltage and the rated output voltage. The output does not exceed: - 10 kVa. 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 2 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 3 Future technological development of transformers may necessitate a need to increase the upper limit of the frequencies, until then this Part 2-9 may be used as a guidance document.