Inductive voltage transformers are widely used in medium voltage networks to measure harmonic voltages. The accuracy of these voltage transformers at frequencies other than nominal frequency (50Hz/60Hz) is not yet defined by standards, but can have a significant impact on the overall accuracy of voltage harmonic measurements. Consequently, the accurate assessment of harmonic voltage levels, e.g. for verifying compliance with compatibility levels, planning levels or emission limits, requires knowledge about the frequency-dependent transfer characteristic of the used voltage transformers. This transfer characteristic of a voltage transformer is not constant, but depends on a complex set of influence factors, like temperature, burden, etc. Therefore, within the EMPIR project 19NRM05 IT4PQ, the effects of different external factors on the accuracy of inductive voltage transformers is systematically investigated. This paper shows the influence of selected external factors (temperature, burden and primary voltage) on the frequency-dependent transfer characteristic of resin cast medium voltage instrument transformers.