The power quality (PQ) problem is spreading among the distribution network (DN) since intelligent electronic devices (IED) are being installed. Their presence is crucial because they introduce several benefits for the network observability and management. However, the consequent power quality degradation has to be seriously tackled. To this purpose, measurement instruments should be capable of measuring the electrical quantities even in presence of distortion. Furthermore, the new added distortion has to be summed to the typical influence factors affecting the measurement instruments (like temperature, electromagnetic fields, humidity, etc.). Therefore, this paper presents a study on a specific low-power current transformer (LPCT), the Rogowski coil. In detail, the influence of external magnetic fields, on the Rogowski accuracy performance, is evaluated when the device is measuring distorted signals (with an increasing level of distortion). The aim is to study the combined effect of the magnetic fields and of the distorted currents on the measurement performance of a Rogowski coil. Therefore, an ad-hoc measurement setup and tests are developed in this work to assess 3 off-the-shelf Rogowski coils. From the results it can be concluded that the presence of external magnetic fields does not influence the harmonics evaluation, but it does influence the evaluation of the 50 Hz component.