## 1 Introduction  
Railway transport is one of the main ways to move in the world. An efficient transport system is a fundamental condition for wellbeing and prosperity at national and international levels. In recent years, there has been a tendency to use railway infrastructure electrical equipment up to their limits, assuming the risk of failure, which leads to power supply interruption of the railway system. Those things lead to undesirable aspects in terms of train circulation (delays, injuries, etc. ), with economic and technical losses. Railway infrastructure generally uses scheduled maintenance activities. This type of maintenance is expensive and time-consuming since the activities have to be performed periodically. However, some failures will occur between the scheduled activities. Thus, the corrective maintenance performed after such an event has high costs. Traditionally, the return circuit (which contains impedance bonds) is monitored visually (manually) by railway maintenance workers. However, human monitoring is a subjective method, which is slow and unsafe for railway infrastructure. Timely detection and identification of faults in railway infrastructure are very important for the availability, reliability, and safety of the railway system. In order to reduce the failure risk in railway infrastructure, it is necessary to acquire, collect, and process detailed information about the technical condition of equipment and, thereby, enable prediction for any damage to the electrical equipment [1; 2; 3]. Faults detection and diagnosis is the next step after railway infrastructure condition monitoring. In this sense, some diagnosis methods of railway infrastructure have been proposed in the literature [2; 3; 4; 5]. The introduction of monitoring and diagnosis allows passing from corrective maintenance, respectively, maintenance based on predetermined criteria (time), existing in high percentage in the railway infrastructure, to condition-based maintenance, in order to increase the operational safety and the railway traffic, to ensure power supply continuity of the railway, to reduce the duration of non-power supply of the contact line, to minimize system losses, to reduce the risk of failure, to reduce the operating costs, etc. In [6; 7; 8], aspects of condition monitoring in the railway industry are presented, in which the sensors used for monitoring the railway infrastructure are highlighted. Moreover, the authors divide the railway infrastructure into two large components: fixed and movable [6]. The rail tracks, bridges, tunnels, rail track beds, and other infrastructure, are found in the fixed elements. The return circuit and the track circuit are found as part of the other railway infrastructure. On the other hand, the wagon, bogies, axles, brakes, pantographs, etc., are included in the movable components. Among the electrical equipment from the return circuit and track circuit, which need special attention, are the impedance bonds that are mounted in railway infrastructure [9; 10]. Its main functionalities are:  
* to ensure the return circuit of the electric traction current;
* to separate the signaling current of the track circuit from the traction current;
* to control and to signal the trains circulation;
* to ensure the protection of the installations from the contact line vicinity. In exploitation, the impedance bond is subjected to thermal, mechanical, electrical, and environmental stresses [11]. To avoid some faults, it is essential to monitor the relevant parameters in their incipient phase, which characterize the technical condition of the impedance bond. Impedance bond condition monitoring consists of characteristic parameter observation of equipment, to verify its performance through measuring one or more of these parameters (measurements performed with sensors and data processing devices, permanently connected to the equipment).