Knowing the resistivity of the soil is important since the resistance of a ground electrode or a complete electrode system is directly proportional to soil resistivity. The most common method utilized for measuring soil resistivity is the four point method using the equally spaced Wenner arrangement. This method is commonly referred to as the dour pin method. The fall of potential method is most commonly used to measure the resistance of a ground electrode system, though in some situations, clamp-on style ground testers provide valuable information. The resistance of the installed systems is typically under five ohms.
Knowing the soil resistivity allows the designer to apply what is known about LP grounding conductors and electrodes. For example, if the upper layer is poorly conductive, larger radial wire or strip should be used to reduce the inductance between ground rods, and the spacing between the rods should be decreased. Changing conductor size and rod spacing is required due to the soil not shunting the radial inductance as it would if this layer were conductive. The impedance of the radial conductors can be greatly reduced by the addition of conductive ground enhancement materials around the conductors. Furthermore, high resistivity soils can cause a high concentration in the electric field around an electrode, which may cause arcing in the soil that can fuse the soil into a glass material (fulgurites). This glass material will no longer be conductive.
A periodic inspection program is needed to ensure that continuity exists throughout the ground system. Annual testing is recommended to verify that the system is operating at an optimal level. Regular inspections need to test both the electrical resistance of the system to remote earth as well as continuity within the system.
The impulse from a lightning stroke is comprised of both high- and low-frequency components. The wave shape of the impulse is characterized by a very steep rise in voltage and current followed by a long tail of excess energy content. The high frequency is associated with the fast rising front while the lower frequency component resides in the long, high-energy tail. Because of this steep rate of current rise, the inductance of the ground system becomes a central point of the design. The voltage rise, known as Ground Potential Rise (GPR), is dependent not just on the system resistance, but more importantly, the impedance (inductive reactance) of the system. The voltage rise can be expressed by the following formula, where I (A) is the instantaneous current, R (Ω) is the system resistance, L (µH) is the inductance of the system, and dI/dt (kA/µsec) is the current rate to peak of the impulse.