Impulse and 60-Cycle Characteristics of Driven Grounds--III Effect of Lead in Ground Installation

In laying out grounds for protection against lightning, the inductance of the installation and its relation to the effective resistance of the ground are factors of importance. The lead and ground induction ance becomes particularly significant for rapid current discharges associated with direct strokes when the inductance drop may exceed the resistance drop. Examples show that the voltages developed from steep-front high currents in extensive grounds are considerably greater than where conditions permit shallow grounds. The simple method of analysis for determining the voltage across the ground installation, in which a current is applied to an equivalent circuit, is reasonably correct and practically satisfactory. Ground installations comprising multiple paths to earth are also amenable to simplified calculations. A survey of the terminal conditions in the earth should supplement the simple method of analysis in the more important installations of grounds. In assessing the insulation requirements, these should be considered in relation to the impulse voltages developed, which for ground installations are more complex than the ordinary standard waves.