# Evaluation of Pressure Effect on Accelerated Aging Tests of Polymer-Insulated Aircraft Wires
## 1 Introduction
The ionization efficiency of the electrons released from the surface discharges increases at lower air densities [28] because of the longer mean free path of the electrons, which also increases the potential for damage to the insulation. Therefore, it is important to have data on insulation failure at low pressures [29], so this paper focuses on this aspect. The results presented in this paper are based on a modification of the EN 3475-603:2018 European standard [30], which is used together with the EN 3475-100:2010 European standard [31]. The EN 3475-603:2018 standard relates to the resistance of electrical cables intended for aircraft applications to wet arc tracking. It uses purposely notched wires and an electrolytic solution to speed up the degradation process, which is introduced into the notches of the tested insulated wires by means of a dropping system. During the test, a 400 Hz alternating voltage was applied to the notched wires. In this paper, the degradation of the wire insulation was quantified using a low-pressure chamber operating at varying pressure conditions, in the range of 100 kPa-16 kPa. This pressure interval covers pressures from the ground level to the flight altitude of commercial aircraft [32]. The experimental degradation results presented in this paper are based on eight samples of XL-ETFE insulated wires. The degree of insulation degradation was quantified as a function of ambient pressure using three optical methods that detected the visible and UV light emitted by the surface discharges generated during the degradation process. Because of their readiness, reduced cost, and suitability for this purpose, back-illuminated CMOS and solar blind UV sensors were used for this specific purpose [33]. This paper contributes to the state of the art in several ways. Since there is a lack of data and experience in evaluating the performance of insulation materials under low-pressure conditions, this paper contributes to this area. This work dealt with the light emission from surface discharges, contributing to a deeper insight into how light emitted from discharges enables the quantification of insulation conditions and how commercial optoelectronic sensors can be used for this purpose. The results presented in this paper indicate that although aircraft wires must withstand low-pressure environments, measurements made at standard pressures provide better sensitivity for determining the condition of the insulation than those conducted at low pressure, despite the inherent variability in arc tracking tests. In addition, it is easier, faster, and less complex to perform degradation tests under standard pressure conditions. The results presented in this work show that optoelectronic sensors can potentially be used for insulation inspection, the detection and quantification of early insulation degradation, and predictive maintenance. They have potential applications for the testing and predictive maintenance of insulation systems at a competitive cost in the aerospace industry and other technology sectors where insulation integrity is critical.