## 1 Introduction
The electric traction power systems are of different types, depending on the current--direct current (DC) or alternating current (AC), the voltage, and the frequency of the contact line power supply. Over time, these power supply systems had continuous development depending on the technologies available at the time of their appearance. The first supply systems from the electric traction used the DC power supply, with the voltage values of 1.5 kV, 3 kV, or 6 kV. The next power supply systems used the AC single phase at the voltage value of 15 kV and the frequency 16 2/3 Hz or 25 Hz, respectively, the systems in the AC single phase at the voltage of 25 kV and the supply system of \(2\times 25\) kV with the frequency of 50 Hz. The evolution of the electric traction power systems was due, first, to the necessity of removing some disadvantages of the existing systems [27, 28, 29]. Currently, the power systems presented above are used on a smaller or larger scale; the most widespread supply system being the AC single phase at the voltage value of 25 kV and 50 Hz. The main elements of a power supply system in AC reported in [30, 31, 32] are:  
* overhead line or underground line to supply the electric traction substations;
* electric traction substations that ensure the reduction of the AC voltage level to the value required to power supply the contact line, 25 kV;
* electrical installations and equipment for supplying the contact line (power supply feeder, return feeder);
* the contact line, built along the railway, is an overhead electrical network that power supplies the consumers (locomotives and electric frames) from the electric traction;
* section post, subsection post for transverse, and longitudinal sectioning of the contact line;
* the railway tracks, which have a dual role, namely: of the track running and of the return conductor for the current to the electric traction substation;consumers (locomotive and electric frame, internal consumers);
* devices for monitoring, diagnosis, remote control, and remote signaling of the power supply system. An essential element of the power supply system from electric railway traction is the impedance bond from the return circuit, which is the subject of this research. Figure 1 shows the return circuit of the traction current from the power supply system of electric traction, where the impedance bond can be found. Generally, the return circuit is composed of all conducting paths (railway tracks, impedance bonds, connecting conductors between the railway tracks, and impedance bond, return feeders) that form the return path of the traction current to the power supply substation [32; 33]. In Romania, to increase the volume of freight transported, and the safety of the railway traffic, the railway track is divided into certain lengths called circulations (traffic) sectors. Passing the traction current from one sector to another is made through the impedance bond, which ensures the continuity of the current return circuit from the electrical substation. The impedance bond, besides ensuring the continuity of the return path of the electric traction current, is part of the track circuit, which allows controlling and signaling the running of trains [32; 34]. Moreover, another important issue is the large number of impedance bonds found on the power supply system of railway infrastructure to separate this system in circulation sectors of 1.6 km in length. If the impedance bond is broken, the consumers (locomotives) on that circulation sector are not supplied. This aspect runs to undesirable consequences in terms of the circulation of trains with economic and technical losses. Essentially, the impedance bond is a transformer that has a ferromagnetic core with a silicon sheet, on which are located two windings.