IEC 61558-2-4:2009 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. This second edition cancels and replaces the first edition published in 1997. It constitutes a technical revision. The main changes consist of updating this part in accordance with Part 1 (2005), and increasing the supply voltages up to 1 100 V to be in line with the standards of TC 14. This part has the status of a group safety publication in accordance with IEC Guide 104. This part is intended to be used in conjunction with the latest edition of IEC 61558-1 and its amendments. It is based on the second edition (2005) of that standard. It is the recommendation of the committee that the content of this publication be adopted for implementation nationally not earlier than 12 months from the date of publication.