This dissertation for a master degree presents a methodology to analyse Instrument Transformers in general terms and several types of Current Transformers (CT s) for measurements, in details.At first, operation and characteristics of Potential Transformers (PT s) and thatof CT s are analysed, using traditional tools (as equivalent circuits and fasorial representation).The generalised treatment facilitates the comparison between PT s and CT s as well as a qualitative analysis of the parameters influencing the ratio and phase errors.After giving a general view, four types of CT s are studied in details. To analyse complex electromagnetic constructions, a non-traditional method is applied. By usingfasorial representations, calculus is based on defining separate groups of electrical and magnetic equations to describe a device. The result of the solution of this set ofequations is the complex error of the CT, which is being expressed exclusively in terms of parameters of the construction. The results facilitate the comparison of theperformance of various types of CT s, that exist or are to be designed. A simulation program, using language C, in the ambient of LabWindows/CVI - C for Virtual Instrumentation (National Instruments), was developed, in order tocalculate the errors, based upon the equations obtained. The program is capable of specifying the parameters and analysing the characteristics of four types of CT s,representing finally the ratio and phase errors, in terms of the variation of the current or that of the load. Characteristics of various magnet ic materials, dimensions ofstandardized toroidal cores and that of wires can be specified, stored and selected thereafter. The program facilitates experimenting with various combinations ofparameters in an interactive mode and to approximate optimised constructions in iterative steps. Efficiency of the method was verified and proved. The results are coherent and when are used in the simulation program, represent a new method to analyse anexisting CT or to design a new one, or to select the most adequate model for a specific purpose.