IEC 61000-4-34 extends the voltage dip, short interruption and voltage variation immunity tests to equipment that draws more than 16 A per phase. The test is technically more demanding than its lower-current counterpart because the generator must deliver the full load current of the equipment, which for large industrial drives, welding supplies, uninterruptible power systems and process control installations can reach hundreds of amperes. The standard addresses the consequences of this: the impedance of the test generator and the impedance of the supply network both influence the shape of the actual voltage seen by the equipment, and the changeover between the nominal supply and the reduced voltage must be accomplished quickly and precisely without introducing additional distortion. It defines the preferred test levels as residual voltage percentages and durations, the permitted transition times, the number of repetitions and the phase relationship between the dip and the supply waveform. The construction of the test generator, which is frequently based on a transformer with tapped windings or on a high-power electronic source, and the verification of its performance are described in detail, as are the monitoring and assessment procedures.