This paper presents a study on the inductive current transformer behaviour when subjected to distorted actual waveforms. Typical tools adopted to deal with such a condition, as the well-known ratio error, phase error, frequency response, etc. not always provide satisfactory results in all the possible operating condition. In addition, Standards lack in detailing which actual distorted waveforms need to be used to test the transformers. To this purpose, the paper introduces both the use of particular actual fault current waveforms and a simple index for testing the performance of the transformer. Preliminary results show its helpfulness when applied to particular waveforms.