## 1 Introduction  
The Electrical Power System (EPS) of an aircraft is generally designed to have many interactions with other aircraft systems. For example, the EPS provides electricity required for various valves in the aircraft like the fuel system, the Environmental Control System (ECS), and the pneumatic systems [1]. It is the conduit source of power for the avionics and provides power for all the instruments in the cockpit. The EPS is provided with the required power by the engines and the Auxiliary Power Unit (APU), which in turn generates electricity and provides for the valves and switches in both engines and the APU. The EPS is required for cabin lighting, as well as for the functioning of electrical and electronic appliances like cabin entertainment systems and sensors [2]. With the aircraft industry moving towards low CO\({}_{2}\) emissions, clean energy, and light weight, more focus is being given to the concepts of fully electric propulsion systems and all-electric aircraft. As the dependencies on the EPS grow within an aircraft, more importance is given for detecting and isolating faults in the EPS at an early stage. The effect of any fault in the EPS could easily propagate to other aircraft systems that the EPS interacts with. Some faults in the EPS result in common cause failures, affecting multiple aircraft systems at a given time. Electrical failures in the Boeing 747-4H6 in 2012 [3] and the Airbus A319-111 [4] in 2009 are classic examples of common-mode failures. In the case of the B747-4H6 event in 2012, the root cause of the event was a latent hardware fault on the bus tie breaker whereas, in the A319-111 event in 2009, the incident occurred due to an intermittent fault in the Generator Control Unit which led to the loss of the left electrical network. Both these faults resulted in intermittent blanking of cockpit displays, intermittent loss of power to one of the AC bus bars,and resultant degradation of multiple aircraft systems. In the case of B747-4H6, the failure led to the disengagement of all three autopilots and the pilots had to take over the landing manually. In A319-111, the APU generator took over from the failed generator, but many systems, including all means of radio connections, remained inoperative throughout the flight duration. Aircraft incidents like these show the necessity of developing diagnostics for the EPS at the system-level. This would not only detect and isolate the faults that affect the EPS but also prevent the possibility of their propagation to the interacting systems. The diagnosis also helps with the research focusing on health monitoring at the aircraft level, in order to isolate faults propagating to multiple systems [5]. In general, cascading faults result in effects that can be observed in systems other than the point of origin, increasing ambiguity, and resulting in extended downtime and prolonged maintenance as a consequence. Hence diagnostics developed at the systems level, combined with reasoning at the vehicle level, will enable isolation of such faults and their root causes. This will help in reducing the time and money involved in troubleshooting during maintenance [5]. Several research works have been dedicated to developing diagnostic methods for various faults in the aircraft EPS. The Advanced Diagnostic and Prognostic Testbed (ADAPT), an EPS testbed developed by NASA Ames Research Centre, has been used in multiple scenarios to evolve diagnosis and prognosis technologies [6; 7; 8; 9]. ADAPT evaluates diagnostic algorithms and software, enables the insertion of faults, and provides a mechanism for evolving the algorithms to be implemented into the vehicles [10]. However, the majority of work found in the EPS diagnostics literature has focused mainly on faults at the components level like wires [11] and relays [8], and the subsystems level like generators [12] and induction motors [13; 14].