An original lack of standardisation of measurement methods has caused significant differences in values of individual mains parameters calculated with various devices.   However, efforts made by many engineers has resulted in the IEC 61000-4-30 standard concerning power quality. This standard (and related standards) provides very precise methods, mathematical relations and required measurement accuracy for Power Quality Analysers. Compliance with the standard (in particular, the class A and class S) guarantees repeatable and almost identical measurement results of the same magnitudes measured with devices from different manufacturers.