This part of IEC 61558 deals with the safety of transformers for construction sites and power supply units incorporating transformers for construction sites. 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 transformers for construction sites and power supply units incorporating transformers for construction sites. NOTE 2 Switch mode power supply units are covered by IEC 61558-2-16. This Part 2-23 is applicable to stationary or portable, single-phase or polyphase, air-cooled (natural or forced) independent or associated transformers, being isolating or safety isolating dry-type transformers for the use on construction sites. 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 poly-phase transformers NOTE 3 Transformers intended to supply distribution networks are not included in the scope. Isolating transformers and power supply units incorporating isolating transformers for construction sites have a no-load output voltage and a rated output voltage exceeding 50 V a.c. and not exceeding 250 V a.c. Safety isolating transformers and power supply units incorporating safety isolating transformers for construction sites have a no-load output voltage and a rated output voltage not exceeding 50 V a.c. NOTE 4 This standard is applicable to transformers for the supply of electricity in locations as specified in IEC 60364-7-704. The latter also specifies the protection by using an earthed midpoint or starpoint of the output winding. Transformers and power supply units covered by this Part 2-23 are used in applications where it is required by the installation rules or by the appliance specification for protection purposes. When the transformers or power supply units are incorporated into low voltage switchgear and controlgear assemblies for construction sites as specified in IEC 60439-4, the additional requirements of IEC 60439-4 will apply to the assembly. NOTE 5 For transformers filled with liquid dielectric or pulverised material, such as sand, additional requirements are under consideration. NOTE 6 Attention is drawn to the following : -measures to protect the enclosure and the components inside the enclosure against external influences like fungus, vermin, termites, solar-radiation and icing should also be considered : -the different conditions for transportation, storage, and operation of the transformers and power supply units should also be considered: -additional requirements in accordance with other appropriate standards and national rules may be applicable to transformers and power supply units intended for use in special environments such as tropical environment . NOTE 7 Future technological development of transformers may necessitate a need to increase the upper limit of the frequencies, until then this standard may be used as a guidance document.