This guide is applicable to instrumentation intended for measuring voltage or current components with frequencies in the range of d.c. to 2 500 Hz which are superimposed on the voltage or current at the power supply frequency. This guide is also applicable to measurement instrumentation intended for testing individual items of equipment in accordance with emission limits given in standards as well as for the measurement of harmonic voltages and current in actual supply systems. The survey of harmonics in power supply systems is of particular concern.