## 3 Condition Monitoring of the Impedance Bonds  
### Aspects about Monitoring and Possible Faults of the Impedance Bonds  
Monitoring and diagnosis of the electrical equipment technical condition from the supply system of electric traction are starting to be increasingly used due to the development of sensors and transducers, devices for monitoring, and diagnosis, respectively, new possibilities of data transmission acquired. The emergence and development of new technologies allow the development of devices for monitoring and diagnosis that meet the requirements of electric traction [6; 7; 8; 35]. The necessity to monitor the impedance bonds arose due to their importance in ensuring the return of traction current to the power supply, respectively, the control and signaling of the railway vehicles to achieve the safety and availability of the railway traffic. Thus, the monitoring and diagnosis of the impedance bonds will lead to the detection of incipient faults and reduce the delays in the train circulation. In this context, aspects regarding the problems/faults are presented, which can appear on impedance bonds from the power supply system of the electric traction and possibilities for monitoring and diagnosis of this kind of equipment. Thus, during the maintenance activities of the impedance bonds, there were often problems related to the return circuit from electrical traction, such as [32; 34]:  
* oxidized electrical contacts;
* insufficient tightening of the connection terminals;
* interruption (sometimes lack) of the connecting cables between the impedance bond terminals and the railway track;  
Figure 3: Image of impedance bonds: (a) simple in cast iron casing; (b) double in a non-metallic housing. Figure 2: Impedance bond electrical diagram in two constructive variants: (a) simple; (b) double. * interruption of the railway track;
* short-circuit in the winding of impedance bond;
* interruption of the track circuit. To detect the faults from the return circuit and to limit the negative effects, respectively, the reduction of the occurrence number of abnormal situations can be achieved by continuous monitoring and diagnosing the impedance bonds. In Table 1 are given some parameters and techniques to determine the technical condition of the impedance bond. Integrating a larger number of these parameters will allow extracting the real condition of the equipment with more accuracy. It was found that a multitude of characteristics and parameters must be monitored in order to diagnose the impedance bond technical condition. Thus, by monitoring the currents through the two primary impedance bond windings, the imbalance current can be easily obtained, which indicates the connection interruption between the impedance bond terminal and one of the railway tracks (at 100% imbalance). Knowing the temperatures in different areas of the impedance bond (the connection terminals A1, A2, and M, the connection cables, the coil housing, and the environment) allows us to identify the monitored areas status. In addition, correlating the current passing through the impedance bonds and a higher contact area temperature indicates an increase of connection terminals contact resistance (so, contact state deterioration). Knowing the coil housing temperature allows the evaluation of the sources of heat from inside coil housing or the cooling conditions. The heat sources are mainly due to the losses from the Joule effect in the impedance bond windings and the terminal connections in the impedance bond housing, or by hysteresis and eddy current losses in the magnetic circuit. A housing temperature increasing at the same current value, correlated with the ambient temperature, may occur due to: the inside contact areas deterioration of the impedance bond; the short turns appearance; magnetic circuit deterioration; low oil level (or lack of it).