I. Introduction
Continuous development of modern measuring devices requires the use of increasingly better materials and structural solutions [1]. This development also includes studies of the usability of soft magnetic materials other than 80%Ni-Fe polycrystalline alloys (permalloy) [2], as these materials are currently very expensive. For that reason, Fe-based wound toroidal cores have become an alternative solution [3], [4]. They require—like permalloys—protection from mechanical stresses but they are less expensive and more useful for their applications in transformers.