The Class B performance class was originally included in the second edition of IEC61000-4-30 to help avoid making many instruments at the time the standard was published obsolete and to allow their measurements to still be useful. These instruments offer a level of accuracy that is typically used for qualitative surveys, trouble shooting and other applications where a high level of accuracy is not required. The uncertainties for Class B Power Quality Analysers measurements such as Frequency, Flicker and Harmonics are specific to the manufacturer and their own specifications.
The latest edition of IEC61000-4-30:Edition 3 published in 2015 no longer lists Class B in the standard as a performance class. Test equipment manufacturers and designers of new instruments were advised that Class B might be removed in this version of the standard to discourage them from using Class B methods that were defined in the now obsolete second edition of the standard.
Modern power quality analysers should now comply with Class A and Class S performance classes.