## 4 Results
### Second Case
The thermal investigation was made with an infrared thermographic investigation device, FLIR T650 sc, for validation of the contact thermal stresses level between the terminal of the impedance bond and the connection cable. The thermographic camera resolution is 640 \(\times\) 480 pixels and accuracy of \(\pm\)1 \({}^{\circ}\)C or \(\pm\)1% for the temperature range -40 \({}^{\circ}\)C to 160 \({}^{\circ}\)C.  
Before viewing the infrared thermal image, the camera was configured according to the laboratory conditions. The values of the adjustment parameters for obtaining the thermographic image were the following:  
* ambient temperature: 20 \({}^{\circ}\)C;
* the emissivity index: this parameter has been set to the value of 0.9;  
Figure 9: Temperature variation from the contact area by impedance bond and one steel conductor at different currents (100 A, 150 A, 200 A). * reflected temperature: set equal to ambient temperature, 20 \({}^{\circ}\)C;
* relative humidity of the air: the value measured in the laboratory at the time of the experiment was 55%;
* the distance from the camera lens to the investigated connection was 1.5 m;
* the temperature range was selected from \(-\)30 to 160 \({}^{\circ}\)C.  
The obtained thermographic image was processed using FLIR Tools software, to identify the temperatures recorded on each contact area and connecting conductor. Thus, the surface points Sp1, Sp2 are shown the temperatures recorded for the copper conductor when current flow through to circuit at its value 100A, Figure 10. Moreover, the Sp3, Sp4 points give the temperature values of the steel conductor. The temperature obtained with the infrared device is close to that acquired with the experimental device for condition monitoring of the impedance bond. Thus, the temperature values on the contact area between the connecting ropes and impedance bond terminal are for the copper connection, 32.9 \({}^{\circ}\)C, as against 32.3 \({}^{\circ}\)C obtained with the monitoring device. To the steel connection, the thermographic temperature was 51.2 \({}^{\circ}\)C, as against 52.5 \({}^{\circ}\)C achieved with the monitoring device. Moreover, it can be observed that the contact area in the case with the steel cable is supplementary heated by the steel connecting rope, which has a higher temperature than the contact area. The steel connection rope is a thermal source for the contact area at the impedance bond terminal. Fourth Case--Faulty Condition with Insufficient Tightening (High Contact Resistance) of the Connection Terminals  
In the return circuit, in addition to the main elements, such as railway tracks, impedance bonds, connecting conductors between railway tracks, and impedance bonds, return feeders represent the electrical contacts between them. The electrical contacts introduce, in the traction current path, an additional electrical resistance. The appearance of the contact resistances in the return circuit is explained by covering the contact parts with disruptive films (oxidation, rust, etc.) and by changing the contact areas (loosening the tightening of screw nuts) between them. Moreover, to highlight the influence of the contact resistance measurements, these values have been modified through loosening the tightening of screw nuts. The tightening torque of the screw nuts, which, connecting the impedance bond terminal and the connecting conductor, was according to the connection standards, and having a value of 30 Nm, measured with a torque wrench. At this value of tightening torque, the contact resistance was 54.9 \(\upmu\Omega\). After losing the screw nuts, the value of tightening torque was 8 Nm, which corresponds with a 166.7 \(\upmu\Omega\) contact resistance value measured with the RMO 500 micro ohmmeter. Figure 10: Thermographic image at a test current of 100 A. This experiment was done when the connection between the impedance bond and the railway track was connected by one steel conductor (the worst case possible).