A power quality (PQ) and harmonics monitoring activity on the power systems in a metro-transit system has been performed by means of Instrument Voltage Transformers (IVTs) there installed. With the aim of identifying IVTs influence on the measurements, a wide laboratory survey has been carried out. The paper reports the present state of the work, in progress for many years, on the IVTs experimental analysis. In particular the paper deals with the main results obtained by the laboratory survey on two main types of IVTs: IVTs with equal constructive features and manufacture, IVTs with equal constructive features but different manufacture. The choice of these two types of IVTs is aimed at evaluating the impact of the production process and of the manufacture on IVTs behavior in switching transients.