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Increasing operating voltages of insulated gate bipolar transistor (IGBT) modules results in higher demands on electrical insulation capabilities as well as partial discharge resistance. This paper discusses the insulation failure modes observed in high voltage power modules; most critical are water trees, partial discharge (PD) and electrical trees. Besides the review of insulation failure mechanisms, an experimental analysis method is discussed in order to identify possible failure source. Presented is a PD setup that includes a light sensitive CCD camera for optical inspection. Within the optical measurements, electroluminescence maps are also recorded in order to identify critical regions of high electrical fields. This method permits an analysis even before PD or electrical treeing begins. Optical PD inspections allow identifying PD failure root causes like protrusions or irregular edge shapes. In addition, investigations of PD failures on the ceramic substrate level have been performed and the distribution of inception voltages for one substrate type analyzed.