###### Abstract  
The development of more electric aircrafts (MEA) and all electric aircrafts (AEA) inevitably implies an increase in electric power and a consequent increase in distribution voltage levels. Increased operating voltages coupled with low pressure in some areas of the aircraft greatly increase the chances of premature insulation failure. Insulation failure manifests itself as surface discharges, arc tracking, arcing, and disruptive or breakdown discharges, in order of increasing severity. Unfortunately, on-board electrical protections cannot detect discharges at an early stage, so other strategies must be explored. In their early stage, insulation faults manifest as surface and corona discharges. They generate optical radiation, mainly in the near-ultraviolet (UV) and visible spectral regions. This paper focuses on a method to detect the discharges, locate the discharge sites, and determine their intensity to facilitate predictive maintenance tasks. It is shown that by using small size and low-cost image sensors, it is possible to detect, locate, and quantify the intensity of the discharges. This paper also proposes and evaluates the behavior of a discharge severity indicator, which is based on determining the intensity of digital images of the discharges, so it can be useful to apply predictive maintenance tasks. The behavior and accuracy of this indicator has been tested in the laboratory using a low-pressure chamber operating in the pressure range of 10-100 kPa, which is characteristic of aircraft applications, analyzing a needle-plane air gap geometry and using an image sensor. The proposed method can be extended to other applications where electrical discharges are an issue. electric discharge; corona discharge; image sensor; state of health; fault diagnosis +
Footnote †: journal: Applied Sciences  
with supply voltage [5]. The persistent effect of the discharges in insulation materials induces chemical changes resulting from the flow of electrons at the discharge locations [6], increasing their temperature, thus progressively degrading insulation systems, which in turn favors discharge occurrence and eventually leads to major failures [7]. Partial discharges (PDs) occur between two electrodes insulated by a solid, liquid, or gaseous insulation when the local electric field strength exceeds the dielectric strength of the insulation material. PDs do not entirely bridge the insulation path between the electrodes [8]. In the case of gaseous insulation, they are known as corona, which are localized and luminous discharges that occur near an electrode, triggered by a highly inhomogeneous electric field [9] that exceeds a critical value [10; 11]. Contrarily, disruptive discharges completely bridge the insulation path between two electrodes [8]. In non-uniform air gaps, corona appears at a lower voltage than the one required to produce a disruptive discharge [12]. In the case of uniform electric fields in air, disruptive discharges appear without previous corona activity. Although in the short term persistent PDs do not usually pose a serious threat to polymeric insulation in cabling systems, in the long term, due to continuous chemical changes, because of the flow and bombardment of released electrons [6], they can cause serious damage. The chemical changes eventually create a conductive track in the polymeric insulation, which favors the flow of an electric current, thus raising the temperature in the track and causing further damage in the insulation. This sustained situation weakens the insulation and promotes arcing activity [13], which appears in the insulation between wires due to the formation of a carbonized track. Further discharges spread existing tracking paths, generating the arc tracking phenomenon [7], thus extending the damage and eventually causing a fire hazard [14]. Cruising altitudes of commercial jetliners are in the range of 10-13 km, while some other aircrafts have altitude ceilings greater than 15 km, so that operating pressure can be reduced to 10% of the sea level value.