This paper discusses the basic technical characteristics of a current transformer for measurement and protection. The effects of short-circuit current and its DC component on transients and transformer saturation are presented. Due to everyday use of the electrical network and the need for electricity, a transformer saturation test cannot be performed while the transformer is connected to the network. The saturation effect was simulated in the EMTP-ATP program and MATLAB, where the negative impact of saturation is visible from the graphic display of the transformer secondary current signal. Secondary current distortion directly affects the operation of the connected protection on the secondary of current protection transformers. Transformer saturation needs to be monitored and controlled. In the practical part of the paper, measurements of the basic technical parameters of the current transformer were performed (primary and secondary insulation resistance to each other and to earth, primary and secondary winding resistance, transmission ratio and error, dielectric strength of oil, etc.). The results of each measurement method showed satisfactory transformer properties in terms of its correctness and remaining service life.