The reliability of operation of a nuclear power plant substantially depends on the reliability of its individual components. Electric cables, particularly medium-voltage cables, represent an important constituent of the nuclear island as well as of the balance of the plant. It cannot be considered as satisfactory that the redundancy of a particular subsystem is ensured. For this purpose, it is important to check the cable insulation condition, which is supposed to be carried out during the overhaul activities. The lifetime of medium-voltage cables is estimated to attain somewhere around 40 years. However, due to electrical, thermal, and mechanical stresses, it may be significantly reduced. In order to avoid its likely failures and following outages, the insulation must be tested regularly. Based on obtained testing results, the condition of a cable or cable termination can be assessed and repaired or a replacement made, if necessary. The paper presents some of the most important nondestructive diagnostic measurements on cable connections recommended to be done during the overhaul of a nuclear power plant following a good practice of asset maintenance management.