The purpose of this recommended practice is to provide information on surge voltages in low-voltage1 ac power circuits. With this information, equipment designers and users can evaluate their operating environment to determine their need for surge-protective devices. The document characterizes electrical distribution systems in which surges exist, based upon the data that have been recorded in interior locations on single-phase and three-phase residential, commercial, and industrial power distribution systems. There are no specific models that are representative of all surge environments; the complexities of the real world need to be simplified to produce a manageable set of standard surge tests. To this end, a surge environment classification scheme is presented. This classification provides a practical basis for the selection of surge-voltage and surge-current waveforms and amplitudes that may be applied to evaluate the surge withstand capability of equipment connected to these power circuits. It is important to recognize that proper coordination of equipment capability and environment characteristics is required: each environment and the equipment to be protected has to be characterized and the two reconciled. The surges considered in this document do not exceed one-half period of the normal mains waveform in duration. They may be periodic or random events and may appear in any combination of line, neutral, or grounding conductors. They include those surges with amplitudes, durations, or rates of change sufficient to cause equipment damage or operational upset (see Fig 1). While surge-protective devices acting primarily on the amplitude of the voltage are often applied to divert the damaging surges, the upsetting surges may require other remedies. Test procedures are described in IEEE C62.45-1987 [8],2 as a companion to the present document. Other surge-related standards are identified in Section 4; the present document is intended to complement these standards.