In order to measure the harmonic current emission of equipment in accordance with IEC standards, a measurement setup (voltage source, measurement equipment) with a specified accuracy and reproducibility is required. One possibility to verify its accuracy is the measurement of a set of reference loads with known harmonic emission and a comparison of measured and calculated values. However, laboratory measurements of different non-linear loads have shown that some loads can considerably affect the behavior of the measurement setup. The main objective of this paper is to identify in a systematic way the impact of different voltage sources, measurement equipment and loads on the accuracy of the harmonic current measurements. The paper also identifies the different factors which have considerable impact on the measurement results and evaluates the suitability of a system verification based on reference loads.