Influence of different factors on coordination of two cascaded SPDs

Surge protective devices (SPDs) are the essential devices for the protection of low-voltage electrical equipment against in-rushing surges. This article presents numerical analysis of surge protection for electrical equipment with two cascaded SPDs; one at the service entrance of a building, and another at the equipment or in the socket. Coordination of these two cascaded SPDs is discussed by considering various influential factors. The influence of impulse waveforms on the coordination is the main issue addressed. It is found that the selection of SPD parameters might need the consideration of the impulse waveform of concern. Other factors such as separation distance are also discussed. Finally, this article also presents a case study on the cascaded SPD coordination when one ZnO varistor is substituted with a Class I spark-gap SPD.