A new diagnostic tool nowadays available with digitally recorded waveforms by lightning impulse tests, is the so called transfer function. For the calculation of the transfer function, two waveforms are recorded. The first waveform is the impulse voltage and the second waveform is the impulse current. Both, waveforms recorded in the time-domain ve transformed to the frequency-domain and the transfer function is calculated. This paper proves that the transfer functions of a sequence of impulse tests with the same test circuit can be compared with each other. This issue has currently much attention all over the world. In our opinion, the differences found between transfer functions are mostly due to hard- and software which is not suitable for the determination of the transfer function. A statement about reliability is only possible if the used technology for determination of the transfer function is beyond any doubt. This paper will compare the transfer functions of the full impulse waveform and the chopped impulse waveform on the same terminal of the transformer and test circuit, which is the worst case condition for the comparison, to prove that comparison of transfer functions is possible.
