IEC 61000-4-19 covers immunity to conducted differential-mode disturbances and to signalling voltages in the frequency range from 2 kHz to 150 kHz applied at a.c. power ports. The band has become a practical concern as the number of devices that deliberately inject signals or that switch at these frequencies has grown: power line communication systems, ripple control receivers, solar inverters, electric vehicle chargers and lighting drivers all operate there, and their emissions are not strongly limited by the standards that govern the higher frequency ranges. The test therefore has two purposes, to confirm that equipment is not disturbed by the differential-mode voltages generated by other equipment, and to confirm that a device intended to receive power line signalling is not upset by the disturbances present on the mains. The standard defines the disturbance waveform, which is a sinusoidal voltage whose frequency is swept or set at discrete points, the amplitude and the modulation used for the signalling test, and the coupling arrangement applied in series with the power port. The duration and repetition of each disturbance, the measurement of the applied voltage and the assessment criteria complete the document.