The effect of lightning rod grounding resistance on its effectiveness had been many times observed in laboratories. A metal rod, simulating a lightning rod, was connected to the grounded plane of a high voltage hall not tightly but through a resistor. However, even when the resistance of resistor is 1000 Ohm the rod had been effectively intercepting a multimeter spark, simulating a lightning, like in the case of a tight metallic bond. However, grounding resistance is reasonably believed as one of the most important parameter of internal and external lightning protection. There are enough reasons for that. Thanks to the grounding resistance, the lightning current rises to a megavolt potential of down conductors and lightning rods, creating storm surges this way. They are almost equally dangerous,both for overhead power lines with ultra-high voltage, and for a modest overhead highvoltage line 380/220 V supplying electricity to your home. The vast number of heavy damages for people and animals in the storm is the result of not a direct lightning strike, but step and touch voltages directly dependent on the grounding resistance. Due to the conductivity of soil a galvanic connection between the ground electrode system of a lightning rod and underground utilities is established, in some cases, it is extremely dangerous for the critical and expensive equipment of modern industrial facilities. However, for consumer equipment such connection also cannot be called desired.
The article studies features of lightning currents spreading in the ground and parameters of grounding devices, important for practical applications. Wherever possible, the labor-intensive analytical calculations are excluded. In order not to complicate reading of the article. In this situation the preference should be granted to the results of a natural experiment. However, a major obstacle here is exceptionally high cost, labor intensity and duration of many months, even with relatively simple field measurements. That's why they had to be replaced by a computer simulation. I think numerical experiments are also credible. The theoretical description of the DC electric field is entirely analogous to the theory of electrostatic fields, long and well developed both in principle relation and in the methodology of the specific practical calculations. Computer modeling used in this article is based on a completely reliable methodological basis.
It is necessary to say, that the calculation practice of grounding devices is not included into the article. They are supposed to be considered separately.
E. M. Bazelyan, DEA, professor;
Power Engineering Institute to the name of G.M. Krzhizhanovsky , Moscow;
a recognized Russian expert in the field of grounding and lightning protection.
1. Soil conductivity
This value can not be called suitable for environments where kiloampere lightning current is being spread. Specific resistivity even of highly conductive ground is about a billion times greater than the resistivity of conventional steel. At the arrangement of grounding, only a virtually unlimited amount of soil over which the current is distributed gets the professionals back.
Soil conductivity is extremely unstable. It strongly depends not only on its mineralogical composition, but also on humidity and temperature. That is why the guidelines have to give very approximate values of resistivity for different soils.
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The article includes answers to the following questions:
2. Measuring grounding resistance
This is supposed to be done in the process of completing the installation and before each stormy season. The principle of operation of any gauge comes down to the method of ammeter and voltmeter. A built-in generator of the device loads the controlled ground electrode system with the current of known value I, and the voltmeter measures the voltage on the ground electrode U3 with respect to the point at infinity. According to Ohm's law, the quotient of these values gives the grounding resistance. R3 = U3/ I. The problem of organization of measurrements is only connected to the circuit wiring. To apply the current, an auxiliary current electrode should be placed into the soil, and to measure the voltage, it is necessary to find the "infinity" point of zero potential, in order to put another auxiliary electrode into it for voltmeter connection. All this requires considerable space. Otherwise, mutual influence between the electrodes and their distortion of current spreading regime cannot be excluded.
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The article includes answers to the following questions:
3. Individual grounding contour - how real is it?
The requirement of an inpedendent grounding contour, not connected with anything, can be often heard from the users of expensive highly sensitive equipment. This requirement is fixed in EIC, for example, there is a ban on the placement of lightning rods on nearby portals of supply transformers. Is it really possible to make a ground electrode system with no galvanic connection with other underground electrodes? In a general case, the answer is definitely negative, because all underground electrodes are connected to each other through the finite conductivity of the ground. The only question is how strong this connection is, in the result of which a part of the lightning current from the lightning rod ground electrode system may not get to the ground electrode designated for this sensitive equipment.
4. Operation of the ground electrode system in a pulse mode
Any horizontal grounding bus or a vertical electrode posess inductance. This means that the current does not instantaneously penetrate to the ground electrode, but actually loads its elements with a very specific speed. As a result, not the whole ground electrode system instantly switches into the process of rapidly growring lightning current. Its areas enter into operation gradually and the further is the specific site from the place of the surge current input, the later it enters into operation. In such circumstances, it is useless to focus on the measurements or calculation of grounding resistance for a direct (or rather - a slowly varying) current. The actual conditions of spreading of the lightning current can be incomparably worse and will be characterized by completely different, multiply increased grounding resistances. Pulse characteristics of grounding devices require an individual analysis.
5. How to regulate grounding resistance in lightning protection
It is difficult to give a reasonable answer to this question, and I don't know a specialist who can do that. In the beginning of the article it was mentioned, that the change in grounding resistance of a lightning rod in any reasonable limits even to 2 orders of magnitude practically has effect on the efficiency of lightning attraction. Hence, it is necessary to focus on some other criterion, associated, for example, with electrical safety or acceptable level of surges in the electrical circuits of the object. An effort to shape the regulatory requirements on this basis is not meaningless, but it is bound to be related to the mass of unsolved problems. The main one among them - the maximum permissible level of touch and step voltage for humans and animals in a pulse mode.
6. What you need to know about the non-linear properties of soil
It is necessary to repeat that soil is a very poor conductor. At the resistivity of about 100 ohm m, even a moderate current density σ = 104 A/m2= 1 A/cm2 creates an electric field Egr = pσ = 106 V/m. This is enough to start the ionization processes in the soil. The classical idea of the effects of ionization comes down to the fact that a well conducting plasma bag is formed at the surface of the grounding electrodes, which increases the area of electrode's contact with the ground. As a result, the grounding resistance of the electrode is reduced greater, the stronger the current is and thus the radius of the formed plasma is greater.
7. Some results
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 phase of the development it is advisable to keep in mind the following considerations.
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