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Dielectric losses in solid insulating materials subjected to voltages containing 3rd harmonics are studied. By simple theoretical analysis, some curves are obtained showing the behaviour of the ratio between the dielectric losses in the distorted steady state and in a reference sinusoidal steady state, against 3rd-harmonic content. The reference sinusoidal voltage amplitudes were selected in two different ways; namely, such as to have either peak value or effective value equal to those of distorted waveform. Separate effects of harmonic content and of effective value of the applied voltage are shown through the correspondence of every 3rd-harmonic phase angle. Percentage differences between dielectric losses with sinusoidal and nonsinusoidal voltages are calculated in detail. Theoretical results are then shown to be in very good agreement with experimental data obtained in testing some solid dielectric materials. Emphasis is given to the implication of the results in problems of insulation involving thermal instability in EHV plants.