Standard accelerated lifetime tests on industrial parts such as polyimide-based digital isolators are usually performed under 60 Hz AC in a range below the partial discharge inception voltage (PDIV). However, new applications such as EV electrical motors fed by power inverters with fast power devices (i.e. SiC, GaN) and controlled with pulse-width modulation (PWM), involve higher working frequencies, potentially ranging up to 100 kHz with very high dV/dt >>10 V/ns. It is therefore important to know whether such conditions can impact the electrical reliability of polyimide. To establish the combined effects of high field and high frequency, a comprehensive investigation has been undertaken. It firstly assessed both the intrinsic properties of polyimide films at high voltage and high frequency in the sub-PDIV range. In addition, accelerated endurance experiments on fully built packaged units, also at higher frequency in square wave (50 kHz) were performed and are reported here.