This paper presents the statistical analysis of variations in harmonic voltage in the time scale between 1 second and 10 min, a time scale that is rarely considered in studies but that is of relevance for incorrect operation of end-user equipment. The paper also introduces a set of novel power-quality indices quantifying these variations and it gives recommendations on how to include these indices in the power-quality-monitoring framework provided by IEC 61000–4–30. The statistics and the illustration of the novel indices are based on measurements of harmonic 2 through 40 with 1-second time resolution at 154 locations in 16 countries. The proposed power-quality indices are a compromise between storing large amounts of raw data with high time resolution, resulting in the different big-data challenges, and completely neglecting this time scale. Two important findings from this paper are that this time scale should not be completely neglected and that information on this time scale can be recorded without excessive increase in the need for data transmission and storage.