Transient-Voltage Aspects of Grounding

Abstract

Lightning damages millions of dollars of electrical equipment each year. With the protective devices currently available, this should not happen. Unfortunately, in most cases, little consideration is given to the effects of grounding on the effectiveness of surge suppression. When the surge suppression device cannot be directly connected at the terminals of the equipment to be protected, the impedance of the connecting means must be examined. High ground resistance and lead inductance greatly diminish the effectiveness of surge suppression. This paper describes a circuit approach to lightning protection starting with a discussion of infinite ground, ground resistance, and lead inductance. These concepts are then applied to various types of pole-top grounding. Normally, the grounding terminal of transformer secondary windings connects to the common terminal of the lightning arresters. This is demonstrated to be a central cause for much of the ensuing damage. Usually, several surge suppression devices are connected to a single ground wire, and the transient voltages on that wire not only reduce the effectiveness of the devices but, because of their bilateral characteristics, can also actually cause damage. Multiple ground wires terminating on a primary low-resistance ground have proven very effective in minimizing equipment damage. The integrity of ground bonding for personnel safety is still preserved. The only alteration is how and where ground wires are connected. This solution greatly increases the efficacy of lightning protection, without sacrificing safety or code compliance.