IEC 61000-4-30 defines the methods for measurement and interpretation of results for power quality parameters in 50/60 Hz a.c. power supply systems. 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 voltage phenomena that can be conducted in a power system. 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 and rapid voltage changes. Depending on the purpose of the measurement, all or a subset of the phenomena on this list may be measured. IEC 61000-4-30 gives measurement methods and appropriate performance requirements, but does not set thresholds. The effects of transducers inserted between the power system and the instrument are acknowledged but not addressed in detail in this standard. Precautions on installing monitors on live circuits are addressed. IEC 61000-4-30 has the status of a basic EMC publication in accordance with IEC Guide 107. This second edition cancels and replaces the first edition published in 2003. This edition includes the following significant technical changes with respect to the previous edition: - Adjustments, clarifications, and corrections to class A and class B measurement methods. - A new category, class S, intended for survey instruments, has been added. - A new Annex C gives guidance on instruments.