Uncertainty Evaluation of the Electrical Transient Rise Time

Abstract

This paper deals with the uncertainty evaluation of the rise time of one pulse of an electrical fast transient burst (EFT/B), as carried out by the National Institute of Metrology, Quality and Technology (Inmetro) of Brazil. The main goal is to analyse the impact of two uncertainty sources not included in the uncertainty budget example in IEC 61000-4-4 standard, and not very well explored in the literature: Voltage Measurement and Horizontal Accuracy. The uncertainty sources considered in this evaluation were: the oscilloscope’s resolution and calibration of the voltage scale (Voltage Measurement) and the time scale (Horizontal Accuracy); the bandwidth of the measurement system; the oscilloscope sampling rate; and the repeatability of ten different measurements. Two sets of measurements were taken from two different oscilloscope settings: in the first one, the interpolation function was enabled, and in the other one was disabled. In both cases two components stood out for their huge relative contribution: time reading and repeatability. Considered together, these two components added up approximately 87% of the expanded uncertainty for interpolated samples, and 95% for non-interpolated samples. Also, the results indicate that the oscilloscope interpolation function (OIF), if available, should be used, as the expanded uncertainty decreases by 27% due to a better oscilloscope resolution. A discussion of the uncertainty budget example in Annex C of IEC 61000-4-4 is presented. In conclusion, once the OIF is enabled, the two additional uncertainty components discussed in this paper should be considered in the uncertainty budget. They should not be neglected since the combined relative contribution of these components is larger than the relative contribution of repeatability.