Grounding for surge-protective devices ground resistance versus ground impedance

The aim of this paper is to address the often loosely used terms of impedance and resistance as related to grounding, and to educate the reader as to the significant difference between these two terms when contemplating the behavior of grounding systems impressed with transient impulse currents. It is not within the scope of this paper to provide a detailed guide on the practical aspects of constructing a ground system. However, by providing the reader with a better knowledge base as to how a grounding system responds to different injected current waveforms, the reader will be better positioned to design and implement effective grounding principles in the practical world. In particular, two important concepts are explained: the "impulse impedance" and the "30-meter rule" that differentiate the response of a grounding system to surges versus power frequency fault currents