This part of IEC 63093 specifies the dimensions that are of importance for mechanical interchangeability for a preferred range of ETD-cores made of ferrite and the essential dimensions of coil formers to be used with them, as well the effective parameter values to be used in calculations involving them. It also gives guidelines on allowable limits of surface irregularities applicable to ETD-cores. The specifications contained in this document are useful in negotiations between ferrite core manufacturers and users about surface irregularities. The use of derived standards which give more detailed specifications of component parts while still permitting compliance with this document is discussed in Annex A.