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Electric energy meters have been always designed to account energy under sinusoidal conditions and therefore no uncertainty specification was provided outside these conditions. This was considered acceptable when voltage and current distortion levels were low and the old, well known induction meters were used. In fact, under these conditions, the measurement errors caused by distortion did not, generally, penalize the customers. The present situation is, however, quite different: distortion is not negligible any longer, especially in low voltage systems, and the modern electronic energy meters can be much more sensitive to distortion than the induction ones, depending on the implemented measurement algorithm. This paper proposes a new calibration procedure, based on a synthesizer of randomly distorted signals, capable of evaluating the measurement uncertainty of the energy meters under the distorted conditions that are likely to be met in their normal operating conditions.