From the series of articles "Grounding in lightning protection - answers to frequently asked questions in the design".
The content of this section is unlikely to attract the expert, but it can be useful for electrical engineer who seeks to understand the methodology of project assessments of grounding devices. At the first stage of the development it is advisable to keep in mind the following considerations.
- Lightning rod grounding resistance in a practically important range of values does not affect the efficiency of lightning attraction and reliability of protection against a direct strike to an object.
- For a reliable assessment of grounding resistance one should have information about soil resistivity at a depth determined by the maximum overall dimension of the grounding device.
- Chemical treatment or replacement of soil in a limited depth in order to reduce its resistivity, effectively affects the resistance of ground electrode systems, the sizes of which is comparable with the depth of treatment; increase of the overall sizes of grounding devices significantly reduces the importance of treatment.
- Consequences of surface soil layer climatological changes are similar in their effects to the one noted in par. 3.
- When measuring grounding resistance of large ground electrode systems in the limited free space, it is advisable to place the auxiliary current and potential electrodes on the same line, and the allowed distance from the current electrode to the controlled ground electrode system can be reduced up to a maximum overall size of the latter, if the potential electrode is placed exactly between them. This measurement error won't exceed 10-15%.
- Usage of vertical rod electrodes for the reduction of grounding resistance of grounding contours of large area in the form of a mesh of horizontal buses is inappropriate and leads to unnecessary metal costs.
- Soil conductivity leads to the formation of electrical couples between different grounding devices and underground utilities. At distances, normalized in AD 34.21.122-87 and in EIC, due to these bonds, tens of percent of lightning currents can be put into the "insulated" ground electrode system.
- To make an individual ground electrode system, protected from electrical couplings with neighboring, it is reasonable to use vertical electrodes, isolated from the surrounding soil on the depth of 10 m or more. The insulation of the electrode should be counted for full voltage caused by lightning current spreading.
- Standard grounding devices recommended in AD 34.21.122-87 do not protect humans and animals from hazardous touch and step voltages. This equally refers to the reinforced concrete bases of typical residential and office buildings, which are recommended to use as natural ground electrode systems. Safety of people during spreading of lightning current requires special organization of buried ground electrode systems (see. P. 8), and (or) insulating coatings on the ground surface.
- In the mode of spreading of lightning pulse currents, the characteristics of lengthy grounding devices may significally different from those obtained under steady-state conditions. As a result, grounding resistance, step and touch voltage can grow by an order of magnitude or more. Especially dangerous is the uneven distribution of potentials over the area of the grounding contour, which can cause harmful electromagnetic interference in the underground utilities of the object.
E. M. Bazelyan, DEA, professor
Energy Institute named after G.M. Krzyzanowski, Moscow
Useful materials:
- Series of articles about lightning protection for beginners
- Series of webinars about grounding and lightning protection with Professor E. M. Bazelyan
- Elements of external lightning protection
- Consultations on the selection, design and installation of grounding and lightning protection systems
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