The role of this thesis is a detailed type test of numerical busbar differential relay protection, ensured by Protecta ltd. Primarily, it gives a comprehensive statement of the significance of busbars and a description of relay function. Thereafter the problems of bushbar differential relay protecting and the applied soulutions are collected. This includes the saturation of current transformers as a main question, as well as the incorrect operation caused by saturated transient waveforms. The further part of the thesis concentrates on testing busbar differential protection with transient waveforms. After explaining testing methods and the technique of generating transient testing files, it investigates the effect of internal and external faults. In chapter of testing with saturated transient waveform, the thesis specifies the incident of external fault. It details the effect of changing parameters as current magnitude, primary network time constant, residual flux and oversizing factor of current transformer, but balance characteristic slope also shared here. The series of tests end with a high-current measurement. All the results and diagrams are evaluated right after the tests. Finally with summing results the thesis gives a current transformer oversizing factor, which is recommended to these numerical busbar differential relay protection.