High-voltage transformers have tap changers to regulate the voltage in the high-voltage network when the load changes. Those tap changers are subject to different degradation mechanisms and need regular maintenance. Various defects, like contact degradation, often remain undetected and the probability of maintenance errors cannot be neglected. Preventive diagnosis of dynamic resistance can be used to determine the contact condition and to check the basic function of the tap changer. This can guarantee that the high-voltage transformer can be put back into operation safely after maintenance. In addition, the information obtained can be used for condition-based maintenance. The research of Jur Erbrink elaborates on the condition assessment of tap changers using dynamic resistance measurements. He describes in his thesis the different ways in which the measurements can be performed and how this influences the measurement results. In particular it was emphasized that the dynamic resistance measurements at a lower the test current are more sensitive for long term aging effects. It is also investigated which defects can be detected and how the results can be interpreted.