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The motivation for the measurement of the harmonic emissions from electrical appliances is described. A discussion is given of the requirement to ensure that the associated harmonic analysis equipment is calibrated under realistic conditions including fluctuating harmonics. Calibrations performed under fluctuating conditions require measurement of non-repetitive waveforms including those that contain bursts of harmonics. A wavelet-based discontinuity detection system is presented in which the timing of harmonic step changes in a given signal can be detected. Using this information Fourier transforms can be used to give the harmonic content at each level. The limitations of this method are discussed and example results are given. The application to the calibration of harmonic analysers is presented.