## 2 Discussion on Thermal Imaging Diagnostics in Electrical Power Engineering
Furthermore, typical mistakes must be avoided as the imprecise focus--in many low-cost IR cameras, the focus is fixed and can result in an image being unsharp, an incorrect emissivity setting as an object parameter, and the IFOV being too small (the object is too small or too far); therefore, temperatures can be overstated or understated [14]. It is illustrated in Figure 3. For example, this problem is common in the aerial thermographic inspection of photovoltaic plants [17]. The drone being equipped with an IR camera is still moving; therefore, first, the camera must be fast enough to avoid image blurring. Second, to obtain reliable measurements, the IR system must comply with the requirements of the inspection equipment as stated in [13] again. To be sure, the IFOV should be evaluated on information about the distance to the object, camera geometrical resolution, and optics angle. After that, the IFOV must be compared with the minimal object dimension, and good practice applied as given below and recalled here as a reminder because it is often neglected by camera users. Due to a phenomenon called optical dispersion, radiation from a very small area will not give one detector element enough energy for the correct value. The commonly used practice is making sure that the hot area where the spot value requested is at least 3 \(\times\) 3 pixels. Just multiply your theoretical spot size ratio called the IFOV by three, which gives you a spot size ratio of 3 \(\times\) 3 pixels instead of 1 \(\times\) 1. This number is going to be more accurate. Figure 3: The effect of IFOV value on the temperature indication: (**a**) distance too far, and (**b**) correct result. Figure 2: Dependence of the temperature difference \(\Delta\)T between the wire and the clamp on the current flow at different wind speeds v, m/s [18]. Thermal imaging measurements are most often used to determine the condition of an electrical wire after many years of use, when, due to aging effects, it becomes covered with an oxide or contamination layer of relatively high emissivity. In the case of thermal imaging measurements carried out on a new line, the thermogram obtained does not fully reflect the thermal condition of its components. There are much greater differences in emissivity coefficients and stronger radiation reflections in the new elements, recorded by the camera on the thermogram. This often masks the real technical condition. This causes considerable difficulty in relying on the absolute maximum permissible temperature criterion [16] due to the high uncertainty of such a measurement. Even if an inspection is performed in optimal conditions and mistakes are avoided, the IR camera accuracy is about 2 \({}^{\circ}\)C.  
In paper [19], the ambient, rated, and maximal temperatures for different materials used to build the electrical components such as PVC, polyethylene, silicone rubber, connectors and terminations (copper, copper alloy, or aluminum), fuses, coils, and relays are given and discussed. Therefore, it is most often not the temperature of the power line element under test that is important, but the temperature difference between a damaged and a good element. Another solution is comparing measurements with historical data taken for the same environmental and operational conditions. Generally, these temperature difference criteria are usually reported as the temperature rise of the anomaly above the temperature of a defined reference. By taking multiple measurements of similar components over time, a statistical analysis can be used to set operational limits for trending and predicting temperature performance [16]. We will discuss some approaches taken from published papers and standards. The practical criteria for evaluating the results of measuring the temperature differences between a good and an overheated clamp (contact or joint) are given in Table 1[18].