Information is provided to specifiers and users of surge-protective devices (SPDs) about the application considerations of SPDs associated with power distribution systems within North America. This guide applies to SPDs to be connected to the load side of the service entrance main over current protective device of 50 Hz or 60 Hz ac power circuits rated at 100-1000 V rms. The effects and side effects on the presence and operation of SPDs in low-voltage power distribution systems are described. The coordination of multiple SPDs on the same circuit is described.