# Evaluation of Pressure Effect on Accelerated Aging Tests of Polymer-Insulated Aircraft Wires
## 1 Introduction  
Aircraft electrification is challenging the behavior of insulation materials because of the severe ambient conditions and high electrical stresses they must endure. This electrification trend is being driven by several factors, including the need to meet decarbonization targets, reduce emissions, and increase the efficiency of air transport [1]. Increased electrical power has resulted in significant increases in distribution voltages, with operating voltages increasing from 28 V-DC to 230 V-AC and up to 1000 V-DC [2]. Aircraft power systems rely on insulated cables that are widely distributed throughout the aircraft [1]. Aircraft cables must withstand significant temperature and pressure variations that promote condensation, high levels of vibration, and exposure to contamination and the elements [3]. The decreased air density characteristic of flight altitude significantly reduces the dielectric strength of the air surrounding the insulation [4; 5], as well as the thermal exchange [2]. Due to the harsh environment and the higher operating voltages, insulation systems in more electric aircraft (MEA) are exposed to partial discharge, arcing, and electrical breakdown hazards under typical operating conditions, which can result in serious insulation failures. Thicker insulation helps reduce the risk of failure [6; 7]. However, it increases cable weight and makes thermal management more difficult [2]. Therefore, because insulation plays an important role in the stable operation of electrical systems [8], it is essential to develop lines of research focused on improving and optimizing insulation performance and developing detection methods to identify possible insulation failures such as partial discharges. Partial discharges (PDs) occur in insulation systems subjected to high electrical stress [9; 10]. Although PDs are localized and of relatively low intensity, they tend to degrade the insulation and cause premature aging [11]. Polymeric insulation materials are generally susceptible to chemical degradation, so prolonged surface PD activity willeventually create partially conductive paths of degraded insulation material that can lead to surface tracking and/or erosion [12; 13; 14]. Surface contamination combined with high electrical stress can lead to arcing along the partially conductive tracks, causing them to grow and penetrate into the insulation, facilitating the occurrence of major electrical failures [15]. Due to the low energy levels involved in the early stages of surface discharge activity, it is very difficult to detect. Surface discharge activity emits visible and ultraviolet (UV) light [16; 17; 18; 19]. Therefore, it is possible to detect surface discharge activity using appropriate optoelectronic sensors [20], which are readily available and inexpensive today [21]. Standard tests are used to test the dielectric performance and aging behavior of polymeric cable insulation. However, due to the nature of the specimens and the test itself, they tend to exhibit significant variability between specimens, challenging test laboratories, manufacturers, and end users [22]. The inherent variability in accelerated arc tracking tests is due to several factors, including changes in the composition and the quality of the insulation, surface contamination, changes in environmental conditions, changes in electrode geometry, electrode wear, measurement uncertainties, and the inherent stochastic nature of the arc tracking phenomenon [1; 23]. Therefore, to deal with the inherent variability, international standards for quantifying the resistance of insulating polymer materials to dry and wet arc tracking, and for qualifying such materials, often require that at least five samples of each wire be tested [24; 25; 26; 27]. Current standards for qualifying the insulation materials for aircraft use are based on tests conducted at normal pressure and therefore do not account for the lower air density found in aircraft systems.