## 1 Introduction  
String insulators in transmission towers that operate in highly polluted areas require special handling. If this is not provided, it is possible that a flashover contamination that causes a phase to short circuit to ground could occur on the polluted insulator string. Furthermore, losses will arise, along with the discontinuity of electrical power transmission from the power plants to substations that are connected with many electrical loads. Insulator cleaning is one of the effective flashover mitigations against contamination [1]. However, the implementation of cleaning on the insulator strings installed in the power transmission tower cannot be carried out anytime, as strict licensing is required. In addition, the preparation of personnel and equipment is needed to meet proper cleaning requirements [2]. Effective guidance is needed to determine the cleaning schedule for insulator strings. In principle, the flashover mechanism due to contamination is a slow process [3]. Polluted insulator strings in a humid or drizzly environment (non-cleaning wetting) usually experience the partial discharge phenomenon first. More severe partial discharge will develop into a partial arc. If the partial arc has extended from the energized terminal to the grounded terminal, a flashover will occur. One indication of pre-breakdown phenomena, such as partial discharge and partial arc, is ultraviolet (UV) light [4]. Researchers conducted experimental studies in the laboratory to understand the characteristics of UV emission on insulators captured using an UV camera. Research [5] had found that the quantitative data generated by UV images were not only affected by the distance between the UV camera and the tested insulator, but also by the UV camera settings selected. There were two types of quantitative data, namely, the UV count which was shown in real time on the camera screen and UV pixels (facular area) extracted from the application of image processing on UV images (frames of video). It showed that the scalar area was better able to represent the dependent relationship between distance, gain, and discharge intensity than UV count. The use of image processing to calculate the facular area was supported by other researchers [6; 7; 8; 9; 10; 11]. Regarding the use of UV data as a reference for assessing the insulator condition level, the UV diameter as a severity level parameter for flashover condition was proposed in [6]. However, the object under study was only a cup-and-pin insulator. In addition, the UV pixels gathered to form a circle-like pattern. Since the circle formed was not perfect, the accuracy of calculating the diameter was affected. The development of UV emission on an insulator string in an artificial contamination test was investigated in [7]. Because the insulator string was very long (the string consisted of dozens of cup-and-pin insulators), the UV camera was fixed steadily only at the cup-and-pin insulators close to the energized terminal. This way caused a measurement window consisting of several cup-and-pin insulators. The insulator string voltage was varied from safe condition to flashover condition. The results showed that the UV count was not completely linear with the increasing voltage. In this case, the values fluctuated with voltage, so a clear threshold value could not be drawn. This nonlinearity behavior was also found in [12]. On the other hand, a very significant increase in the value of UV pixel (facular area) could be used as a reference that a partial arc had occurred. With the very long dimension of insulator strings, it would be unfair if the observation was only made on a few cup-and-pin insulators close to the energized terminal. In this case, the partial discharge may spread not only to the cup-and-pin insulators close to the energized terminal, but also on other sides, such as the middle side of insulator string. Therefore, observing the condition of each cup-and-pin insulator should be considered.