The interface C4 of the Eurobalise subsystem is an optional interface. When it is implemented, it shall be used for transmitting the information that the Balise is powered by a train to the Lineside Electronic Unit (LEU). This then requires that the LEU shall not be allowed to switch a telegram for a certain time period. Paragraph 4.2.8.5.6 of the Subset-085 defines the test procedure for the verification of the optional Blocking of Up-link Telegram Switching functionality. At present, the verification of the impedance at the C4 interface is based on the adjustment of RL or RC equivalent circuits. This verification is based on a cumbersome algorithm for the determination of the corresponding circuit parameters which fit better to the measured values of voltage and current at the input of this interface. In this communication we propose a new procedure based on the use of voltage and current complex phasors allowing the instantaneous determination of the input impedance at the entry of C4 interface. The method is validated comparing this procedure with the traditional method in the measurements performed at two commercial Eurobalises.