On the electrified metro-transit system of Rome some monitoring activities have been performed, in order to estimate the harmonic impact and the entity of some power quality events (such as sags, voltage RMS fluctuations, short interruptions) in the system. During several continuous monitoring sessions, at every service entrance of the system, some significant voltage transients have been recorded whose origin has been identified in periodic switching operations, performed on the dedicated distribution network. These surges present a front that is quite close to the standard one for very fast front overvoltages associated to the SF6 switches, but their values are out of the typical range. For this reason it seemed to be necessary to analyze and to separate the contribution of the measurement system used for their evaluation. Since voltage measurements have been performed on the 20 kV bus-bar of the terminal electrical sub station, at the secondary of instrument voltage transformer, that is well known to be a critical component for the accuracy of power quality measurements, it became necessary to try to characterize its behaviour. The present paper reports the preliminary results of the tests and measurements performed in laboratory in order to characterize the instrument voltage transformer behaviour in high frequency domain.