## 2 Discussion on Thermal Imaging Diagnostics in Electrical Power Engineering
### Low-Voltage Installations  
Slightly different recommendations and criteria presented in Tables 2 and 3 are given in the work [20]. The study assumes that the load remains constant for at least 45 min before measurements are taken, and, for the meanwhile in work [21], 15 min is suggested as being enough to obtain a steady state and constant temperatures. The German VDE standard [22] requires 15-30 min for low voltage and 15-60 min for high voltage on installations up to \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{3}{*}{
\begin{tabular}{c} **Line Load in \% of** \\ **Nominal Load** \\ \end{tabular} } & \multicolumn{4}{c}{**Assessing the Condition of a Component for Need of Replacement or Repair**} \\ \cline{2-9}  & \multicolumn{3}{c}{**Wind Speed \textless{} 2 m/s**} & \multicolumn{3}{c}{**Wind Speed \textgreater{} 2 m/s**} \\ \cline{2-9}  & **10–30 K** & **31–50 K** & **51–70 K** & **>70 K** & **10–20 K** & **21–35 K** & **36–50 K** & **>50 K** \\ \hline
40–60 & UR & IMM & IMM & IMM & UR & IMM & IMM \\ \hline
60–80 & UR & IMM & IMM & IMM & UR & IMM & IMM \\ \hline \textgreater{}80 & UR & UR & UR & IMM & UR & UR & IMM \\ \hline \hline \end{tabular} Abbreviations: IMM—immediately, and UR—urgently.  
\end{table} 
Table 1: Assessment of the need to repair clamps (contact or connection) and power line conductors [18]. 20 kV. You can see there exist at a least few proposals on it and, therefore, this issue needs further analysis and discussion. In addition, the measurements should be carried out at the time of the highest possible current load. The measurements cannot be performed if a tested element is conducting a negligible current, i.e., under 20% of the nominal load according to the authors [21]. Three general indications are commonly accepted, signaling the need to repair the equipment, i.e., if:  
* For a symmetrical three-phase load, a temperature difference between clamps on three neighboring phases \(\Delta\)T > 3 K;
* The clamp and ambient temperature difference T \(-\) T\({}_{\text{o}}\) > 35 K;
* The clamp temperature T > 70 \({}^{\circ}\)C.  
In many situations, the condition on the nominal load is not met. Furthermore, the temperature rise is affected by the condition of heat transfer to the surroundings. These factors are considered in work [20] and two methods are recommended for determining the severity of overheating. To apply them, the value of the current load I of the installation must be known. **Method 1. ** Table 2 is used in three-phase systems for the symmetrical loading of each phase line. The measured temperature difference between the overheated and good phase \(\Delta\)T, for the current load I, is converted into a temperature difference \(\Delta\)T\({}_{\text{n}}\), corresponding to the nominal load according to the formula:  
\[\Delta\text{T}_{\text{n}}=\Delta\text{T}\bigg{(}\frac{\text{I}_{\text{n}}}{ \text{I}}\bigg{)}^{\text{k}} \tag{1}\]  
where:  
k = 1.6 \(\div\) 2.0--the exponent depending on heat transfer conditions, i.e., weather conditions: >k = 1.6--good, k = 1.8--fairly good, and k = 2.0--poor heat transfer conditions. **Method 2. ** The second method is used in single-phase and three-phase installations when there is load asymmetry in individual phases. It can be used to determine whether the permissible temperature has been exceeded at a given clamp, for a given load I. In this case, the reference temperature is the ambient temperature T\({}_{\text{o}}\). If overheating occurs in a closed switchboard (panelboard switchgear), the switchboard must be kept closed until the temperature of the overheated clamp reaches a constant value. The switchboard is then opened and the temperature of the overheated terminal is measured immediately.