In this thesis I’m writing about the reliability tests I performed on the products of Magnetec-Ungarn Kft. The samples were ferromagnetic cores made from nanocrystalline tape and current transformers based on these cores. I was investigating the degradation of the core material and the casing materials too. Most tests were made to compare different technologies, but some tests were made to ensure that the products can withstand the conditions specified by the IEC 62211 standard. I compared cores which had been subjected to different heat treatment, so they had different permeability. This was a long test containing five steps. I made a comparison between some cores with identical heat treatment, but different casing. I investigated the reliability of the insulation enamel on the copper wires of the current transformers. During the test of the plastic cased current transformers a lot of filling materials were compared by electrical and mechanical properties. The most used test methods were HAST and thermal shock, but I performed some THB and temperature cycle tests too.