## 2 Discussion on Thermal Imaging Diagnostics in Electrical Power Engineering  
Some examples of applications of thermographic testing in electrical power engineering are given in Figure 1 as collected by the authors during the inspection of selected objects of high-voltage stations or lines. The places with increased temperature are caused by weak connections--Figure 1a,b,d--or leaking effects--Figure 1c. One of the most commonly used measurements is that of connectors on the 110, 220, and 400 kV transmission lines. The subjects of the tests are the cable clamps on the poles--Figure 1**d--and the mid-span connectors. Thermal imaging methods can also be used to determine the location of overheating at a disconnected or pressed line wire connection. The use of a helicopter to fly around the line makes it possible to inspect a significant length of the electric line at one time and obtain results in a relatively short time. When applying InfraRed Thermography (IRT) to the condition monitoring and diagnostics, it is strongly recommended that the severity assessment criteria be established [16]. In practice, no singular severity assessment criterion is universally applicable to the variety of situations that need to be considered. These criteria can be general categories that identify temperature levels versus levels of criticality or applied to specific components. The first approach is checking whether the measured temperature is lower or not than the absolute temperature limit given in the technical documentation for the tested element. It should be noted that the temperature of line components is very much influenced by atmospheric conditions changing the heat transmission between the object and surroundings. For example, Figure 2 shows the effect of wind on the temperature difference between the clamp and the electrical conductor. It can be seen that the wind velocity has a significant effect on the temperature difference for the same current load. It is extremely visible and significant for heavy loads. For example, for current load I = 500 A and wind speed v = 7.2 m/s, i.e., almost 26 km/h, the temperature rise can be six times lower than for no wind conditions. Another weather factor causing similar effects is rainfall or snowfall. Therefore, conclusions based on the temperature reading without considering other factors may lead to incorrect assessments and decisions. Furthermore, there are also other relevant factors related to the drawback of thermography like the influence of the emissivity of the surface of the tested object that can be polluted on temperature readings, the reflected radiation of other objects, a small difference between the object and ambient temperatures, the air transparency decreased by fog, some contaminating types of gases or smoke, pollution, and weather conditions if an inspection is carried out outdoors. Their influence on the temperature error was deeply analyzed, for example, in the book [1]. Before starting the inspection, the minimum requirements of the IR camera should be confirmed. They are at  
Figure 1: Overheated screw connection clamps on: (**a**) transformer insulator, (**b**) knife switch; (**c**) location of insulation weakening point; and (**d**) overheating of a cable clamp on the pole. least the following: spectral range, temperature range, operating ambient air temperature range, thermal sensitivity NETD, geometric resolution, absolute error of measurement, and adjustable parameters like emissivity, reflected temperature, adjustable focus, temperature level, and span, as listed in the standard [13] dedicated to testing photovoltaic systems. It seems that this standard is a unique example of a professionally and carefully prepared guideline for inspection practice and, in the authors' opinion, could be adapted to electrical power equipment and systems.