This part of IEC 63093 specifies the dimensions that are of importance for mechanical interchangeability for a preferred range of EFD-cores@ the essential dimensions of coil formers to be used with them@ and the effective parameter values to be used in calculations involving them. It also gives guidance on the allowable limits of surface irregularities applicable to EFD-cores in accordance with the relevant generic specification. The selection of core sizes for this document is based on the philosophy of including those sizes which are industrial standards@ either by inclusion in national standards@ or by broadbased use in industry. This document is a specification useful in the negotiation between ferrite core manufacturers and users about surface irregularities. The general considerations that the design of this range of cores is based upon are given in Annex A.