IEC 61000-4-30:2015+A1:2021 defines the methods for measurement and interpretation of results for power quality parameters in a.c. power supply systems with a declared fundamental frequency of 50 Hz or 60 Hz. Measurement methods are described for each relevant parameter in terms that give reliable and repeatable results, regardless of the method's implementation. This standard addresses measurement methods for in-situ measurements. Measurement of parameters covered by this standard is limited to conducted phenomena in power systems. The power quality parameters considered in this standard are power frequency, magnitude of the supply voltage, flicker, supply voltage dips and swells, voltage interruptions, transient voltages, supply voltage unbalance, voltage harmonics and interharmonics, mains signalling on the supply voltage, rapid voltage changes, and current measurements. Emissions in the 2 kHz to 150 kHz range are considered in Annex C (informative), and over- and underdeviations are considered in Annex D (informative). Depending on the purpose of the measurement, all or a subset of the phenomena on this list may be measured. This third edition cancels and replaces the second edition published in 2008. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: - the measurement method for current, previously informative, is now normative with some changes; - the measurement method for RVC (rapid voltage change) has been added; - the measurement method for conducted emissions in the 2 kHz to 150 kHz range has been added in informative Annex C; - underdeviation and overdeviation parameters are moved to informative Annex D; - Class A and Class S measurement methods are defined and clarified, while Class B is moved to informative Annex E and considered for future removal; - measurement methods continue in this standard, but responsibility...