## 2 Material and Methods
### Equipment and Measurements  
Measurements were performed using two Pin type insulators, class 15 kV from the manufacturer Germer Insulators as shown in Figure 1. Both insulators are shown in Figure 1A side by side, and the insulator on the left, used in cases 1, 2 and 3 does not have any type of physical defect, as shown in detail in Figure 1B, while the insulator on the right, used in cases 4, 5 and 6 it presents a perforation, as shown in the detail of Figure 1C. The perforated insulator represents, for example, a component that has been in harsh conditions for a long time until an electrical perforation has developed. The physically damaged insulator was drilled in the laboratory using a bench drill. A drill bit of 3 mm in diameter was used, which passed through the insulator completely, as shown in Figure 1D. The perforated insulator may not have apparent flaws in nominal conditions, however, it will be prone to failure when certain conditions are present, such as humidity and contamination. To simulate an insulator under contamination conditions, the solid layer method present in NBR 10621/2017 [44] was followed, using Kaolin's surface in cases 2, 3 and 6. The 3 rd case differs from cases 2 and 6 with adding salt (NaCl) to Kaolin, following the same standard instructions. The NBR 10621/2017 [44] is national standard (Brazil) applied to high-voltage insulators to be used on alternating current (AC) systems under artificial pollution tests. The standard is applied to determine the tolerable characteristics of porcelain insulators or glass for external use, exposed to polluted atmospheres in AC systems. This standard is developed by the Brazilian electricity committee and is based on IEC/507 [45] (Artificial Pollution Tests on High-Voltage Insulators to be used on AC Systems). The ESDD was neither measured nor controlled during the experiment, as salty water was sprayed prior to sample high voltage application. However, given that the duration of ultrasound data acquisition was taken in a time window of about 20 min, the ESDD is not supposed to change. The water's conductivity used to spray the samples is 56 kg/m\({}^{3}\), corresponding to a middle-high contamination level (IEC-507). The analysis was performed in a chamber of acrylic to reduce interference in the recording of the ultrasonic signal. With sound insulation of the experiment, it is possible to carry out an adequate assessment, assuming that electrical equipment that may be close to the experiment can generate interference in the analysis. The recording of the experiment inside of the acrylic chamber is shown in Figure 2. To avoid capturing unwanted internal ultrasonic noise, it was decided to use insulating material to fix the insulator, as shown in Figure 3.  
Figure 1: Insulators used in the experiment: (**A**) the two insulators presented side by side; (**B**) bottom of the good insulator; (**C**) top view of the perforated insulator; (**D**) bottom view of the perforated insulator. The chamber is built in acrylic, and only one insulator is connected to high voltage and measured at a time. Supported on a stool, is the ultrasonic microphone connected to a computer to capture the ultrasonic signal. The experiment's voltage was supplied by a single-phase transformer class 15 kV to generate a 7.95 kV voltage phase-to-ground in the insulators, simulating the voltage of 13.8 kV phase-to-phase in which this type of insulator is normally exposed. The connection for grounding is made from the nut to the washer that are fixed on the screw. Figure 3: Insulator fixing using insulating material. Figure 2: Ultrasound recording from the electric voltage application. An ultrasonic microphone with a sampling frequency of up to 500 kHz was used to capture the data, being 0.4 m away from the insulators at the test time. The ultrasonic pickup recorded the insulators' ultrasonic noise separately, during 50 s, for each case.