Metallized film capacitors are a common cause of electronic system failure. Since some of their failures can lead to serious accidents and severe damage to their surroundings, such as explosions or fires, their fail-safe should be ensured independently of their intern structure. The aim of this paper is to investigate the runaway process leading to these catastrophic failures. Non-destructive accelerated aging tests to prevent unusual failures were carried out on 42 capacitors. Their key characteristics were measured periodically. Runaway indicators were found and analysed. Based on the results, knowledge about the runaway process have been deepened. Furthermore, a solution appears to avoid the catastrophic failures.