IEC 61858-1:2014 lists the required test procedures for qualification of modifications of an established electrical insulation system (EIS) with respect to its thermal classification. This standard is applicable to EIS used in wire-wound winding electrotechnical devices. The test procedures are comparative in that the performance of a candidate EIS is compared to that of a reference EIS, which has proven service experience in accordance with IEC 60505 or has been evaluated by one of the procedures given in the IEC 61857 series. This first edition of IEC 61858-1 cancels and replaces the third edition of IEC 61858, published in 2008. It constitutes a technical and editorial revision. This edition includes the following significant technical changes with respect to the previous edition: a) this part is specifically for wire-wound winding EIS; b) new figures and charts support the contents.