This paper presents international and domestic regulations related to standard voltages of electrical networks, types of overvoltages to which elements of electrical installations are exposed, and test voltages used for insulation testing. Overvoltages are defined, the classification of overvoltages is presented, the most common causes of their occurrence, and their basic characteristics. The most common cases are listed with respect to the locations of atmospheric overvoltages, as well as the ways in which switching overvoltages occur. Methods of generating test voltages are also described.