Conducted power line emissions are produced as differential mode (normal mode) or common mode. Common mode emissions typically use the lines as a means to conduct emissions not related to the actual power consumed by the equipment. Since the lines are bundled together, we see that the emissions on both lines are the same.
Differential mode emissions are more commonly associated to power frequency harmonics. The differential mode emissions relate to current flowing in the loop entering on one line and returning on the other line (the normal mode). During test, one should review even compliant data for significant variance (more than the uncertainty allowance) between the phase and neutral line measurements. If a significant difference is observed, the fundamental power frequency should be checked and if the variance is high a problem with the design is indicated. If the return current is different than the phase current, assess where the current is flowing. If part of the current is returned by the protective earth (PE) conductor, corrective action is needed. The PE should only have current (other than leakage current) in a fault condition.
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