# Evaluation of Pressure Effect on Accelerated Aging Tests of Polymer-Insulated Aircraft Wires
## 4 Experimental Setup  
The analyzed samples consisted of two AWG18 stranded tinned copper wires insulated with XL-ETFE (19 AWG30 strands, 1.52 mm outer diameter, M22759/32-18-9, Raychem-TE Connectivity, Menlo Park, CA, USA). The samples were deliberately damaged by removing 1 mm of insulation as shown in Figure 4a, according to the specifications of the EN 3475-603:2018 standard. The notches around the entire circumference of the insulation layer of the samples were made with a wire stripping tool (Knipex ErgoStrip, Wuppertal, Germany). Next, the wire samples were cleaned with isopropyl alcohol and secured with lacing tape. Rubber separators were inserted between the wires to prevent partial discharges from occurring outside the test area. During the electrical testing, one wire of the twin wire sample was connected to the high-voltage side of the source while the other was grounded. The wire degradation setup consisted of an adaptation of the aging process described in the European standard EN 3475-603:2018 for resistance to wet arc tracking. A programmable power supply SP300 VAC (0-300 V, \(\pm\)0.1 V, 15-1000 Hz, 600 W, APM Technologies, Dongguan, China) connected in series with a current limiting resistor (30 \(\Omega\), 2 kW) energized the sample wires. An electrolyte delivery system was connected to a needle (G18 gauge, stainless steel), which dropped an aqueous solution of 3% \(\pm\) 0.5% NH4 Cl at a rate  
Figure 4: Wire samples and experimental setup. (**a**) XL-ETFE insulated twin wire samples. (**b**) Surface discharge inception setup. (**c**) Sample undergoing the degradation process by applying a solution of NH4 Cl and voltage to the wires. of 100 \(\pm\) 10 mg/min. The application of this aqueous solution to the sample assisted in the generation of arcing activity and accelerated the degradation process of the insulation. The wire degradation experiments were conducted in a cylindrical low-pressure stainless-steel chamber with the following dimensions: height 375 mm; diameter 260 mm. This chamber had a sealed methacrylate lid, that allowed wireless transmission of the digital surface discharge images from the CMOS imaging sensor to an external computer. Pressure was regulated using a vacuum pump (1/4 HP, 0.085 m\({}^{3}\)/min, BA-1, Bacoeng, Suzhou, China), while the pressure was measured with a pressure sensor (digital, \(-\)1 to 0 bar, \(\pm\)0.5%, PSD-4, Wika, Sabadell, Catalonia). The high voltage was generated by connecting the SP300 VAC600W programmable AC source to a high-voltage transformer (50 Hz, 1:100 voltage ratio, 36 kV output, VKPE-36, manufactured by Laboratorio Electrotecnico, Cornella de Llobregat, Catalonia). High-voltage measurements were performed with a high-voltage probe (0-39 kV peak, with a ratio of 0-1000:1, accuracy \(\leq\) 3%, 0 Hz-220 MHz, CT4028, manufactured by Cal Test Electronics, Yorba Linda, CA, USA). A back-illuminated CMOS image sensor (ISOCELL HP2, 400-700 nm, 1/1.3\" sensor size, 0.6 \(\upmu\)m cell size, 200 Mpixel, Samsung, Korea) was used to acquire the digital images. The sensor supports raw images and includes a color filter (Tetra2 pixel, RGB Bayer Pattern). It is sensitive to visible light and partially sensitive to near ultraviolet light. Surface discharges were also evaluated using a UV sensor (solar blind, 185-260 nm, UVtron R9533, Hamamatsu Photonics, Hamamatsu City, Japan). The solar blind region was centered in the 240-280 nm UV band [35]. The UV sensor was driven by a quenching circuit (C10807, Hamamatsu Photonics, Hamamatsu City, Japan), which allowed the sensor to operate safely while minimizing the possibility of false detections due to the built-in signal processing circuit. The sensor was positioned facing the analyzed wires and was connected to a DAQ device (1 kS/s, \(\pm\) 26 mV, 12 bits, USB-6000, National Instruments, Austin, TX, USA). Note that both CMOS and UVtron optical sensors have been successfully used in previous research for the early detection of corona and surface discharges at standard and low-pressure conditions [32; 36].