A number of changes in the power system have increased the risk for more serious resonances in the harmonic frequency range. The changes also result in an increased uncertainty with regard to the frequency and damping of those resonances. Uncertainties could be related to variations with time, uncertain future developments in the grid, and the modelling of individual components. In this article, a distinction is made between discrete and continuous stochastic uncertainties. This article further investigates uncertainties affecting resonant overvoltages. Several study cases investigating the impact of different uncertainties on resonances and temporary overvoltages, performed in PSCAD, are presented. The results show that some uncertainties may have a significant impact on the resulting impedance characteristics and potentially on the resulting overvoltage levels.