THD Measurement and Conversion "This number indicates the RMS voltage equivalent of total harmonic distortion power, as a percentage of the total output RMS voltage."
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IEEE 519 e outros padrões (draft): "distortion factor: The ratio of the root-mean-square of the harmonic content to the root-mean-square
value of the fundamental quantity, often expressed as a percent of the fundamental. Also referred to as total harmonic distortion."