Webinar "How to make a good grounding?", page 3

Webinar #6 of Electric energy saving and quality improvement" series

Webinar text. Page 3

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Small-Sized Facilities

— Small-sized facilities, such as office buildings, detached houses or petrol stations have the indicated dimensions of 5x10 m. They do not differ much in terms of the earthing system used: three 5-meter earth rods are driven at 5 meters from each other. In this case, the resistance is 10 Ohm, which meets the requirements for earth arrangements as specified in the Electrical Installation Code PUE 1.7.103. By the way, this value is also sufficient for gas stations. Rules of RD 153-39.2-080-01technical document also state that 10 Ohm is a standard value for protection against statistical electricity. 

Earthing Standardization (1.2-1.4 Ohm)

— Such an option as using chemical earthing electrodes is also quite common. You can see 4 additional points here, where standard ZANDZ ZZ-100-102 rods (length: 2 and 4 meters, underground length: 0.6 meter) are installed. Installing just four rods reduces the earthing system resistance from 7.3 to 1.37 Ohms. This compact solution can bring the earthing system resistance to the standard recommended value for the efficient operation of the energy-saving hardware.

Oil storage farms grounding

— Let's turn to earthing of oil tanks, including tank farms and pumping stations, which, pursuant to the operating standards, also require the installation of an earth loop and a two-point connection to it. When using surge protection rods, the current should spread in three directions, and the rods should be installed at 3-5 meters from each other in these directions. 

Permafrost soil

— Solutions for permafrost. The situation tends to get more complicated with permafrost, since its resistivity can reach 1500 Ohm*m or even more. Then the 9-ohm resistance will be sufficient for such a high value achieved in ordinary soil with a resistivity of 100 ohm*m. An earthing grid should be made using an earthing kit with a vertical earth rod at each intersection point. This standard earthing system ensures resistance of 9 Ohms. If you wish to use chemical electrode kits, you'll need 6 such kits to achieve the required 9 Ohms. To achieve 1.4 Ohms, you'll need 50 chemical electrode kits. Therefore, calculations should be made in each individual case for each individual soil with its own resistivity, which will determine whether the earthing system used is efficient enough to achieve the required efficiency of the energy-saving hardware installed in permafrost.

Conclusion

— Finally, I would like to summarize all the conclusions we came to in the course of the reasoning. Firstly, before arranging artificial earthing, one should consider the possibility of using the building foundation, since it can act as a low-resistance earthing system suitable for energy-saving hardware installation. If it fails to comply with the rules, then artificial earthing is arranged. The surge protection circuit can also immediately provide the required resistance values.

Questions and Answers

Igor Krivulets is asking: "Are there any limitations for earthing resistance when removing static electricity from the roof?"

— As far as I know, earthing resistance is not standardized, but you can always use a standard value of 10 Ohm, which is the same for a protective earthing system. You should always bear in mind the standard value of 10 Ohms.

— Vitaly commented: "One can't arrange an earthing system using a horizontal anode groundbed only. Vertically driven earth rods contribute greatly to the current distribution in the ground. Therefore, they should be driven in the ground below the freezing depth".

— I agree with you in part, because vertical earth rods can significantly reduce the resistivity (by 30% as we could see above), while the real example shows the value up to 100%. 

— Vitaly commented: "In conditions of high soil resistivity, the earthing system should be designed in accordance with the effective touch voltage".

— That's quite right when we speak about substations with a voltage class from 110 kV or more and with a solidly earthed neutral. Then yes, it can be designed in accordance with the effective touch voltage. But to do this, you should comply with the standards established for this effective touch voltage value and use the necessary tools. Software is most often used to design a further system, when we take into account the relevant value of the effective touch voltage.

— Victor asked: " Hello. Why do you install lightning interception rods on brick air ducts on the roof in your projects? Isn't the earthing system too cost-ineffective when using a standard value of 10 Ohms and reducing the resistance to 0.93 Ohms? I think, it is reasonable for transformer substations only, but not for other facilities".

— 

— Vitaly asked: “What is the purpose for reviewing earthing systems in a webinar?”

— We speak about earthing systems at our webinars for the purposes of effective functioning of all energy-saving equipment components. According to the research and measurement results, the earthing system must have a resistance of 1.2 – 1.4 Ohms for the maximum possible energy efficiency.

— Vitaly asked: “What do you think of active interception rods?”

— In fact, this question is outside the webinar scope, as the webinar is about earthing systems, and the question regards surge and lightning protection. 

— Victor asked: “I've never seen the requirement to install the earth rod below the freezing depth. Where did you take it from? All that is required by regulatory documents is 0.7 m below the surface and allowance for the climatic coefficient. Unlike a water supply system or sewage system, an earth rod cannot break or tear when installed in the freezing zone, can it?"

— It is a horizontal groundbed that should be installed at 0.5 - 0.7 m below the surface, or even deeper depending on the circumstances. Regulatory documents do not limit the use of vertical earth rods to make the earthing system more compact. It will comply with all regulatory documents and will reflect the real conditions at the facility. Why not install vertical earth rods if we can? Besides, everything will be in compliance with regulatory documents. Any other questions?

— Alexander Ponomaryov asked: “Why is the linear circuit is used for vertical earth rod systems in almost all projects? In fact, earth rods are nothing more but a parallel connection of resistances. Will the resistance be lower if you use a closed loop of three elements?"

— Please see slide 5. I'll display the slide with the relevant formula.

— Alexander Ponomaryov commented: “The calculated values do not always coincide with the practical ones”.

— Yes, I agree with you here, because it is not always possible to assess the soil resistivity, or to define the soil type, or to survey, especially to carry out a sounding survey in order to get more accurate values. And even if we have the required design basis, it is not always possible to calculate the earthing system resistance, since soils may have a large number of separate layers and differ in depth. For example, the top layer may consist of loam, the lower layer of rubble, and the lowest one may be represented by rocky soils. Prior to any calculations, complex reduction should be used here in accordance with two- or three-layer models. Such faults may cause discrepancies in practical calculated values, but you can't get rid of them, anyway. I hope, I've answered your question. I will post an email address to the chat in case you have any further questions. Please feel free to send any questions to my email address, and I'll do my best to answer them. Chat link: dkrasnoborov@skomplekt.com

— Victor asked: "If the average soil resistivity is measured at the design stage, will the loop resistance be close to the actual value?"

— In this case, 

— Victor asked: “Do you mean to say that to calculate the loop resistance one should complete geological survey and measure soil resistivity first?”

— Yes, that's right. Geological survey is a must if you wish to calculate the resistance value as accurately as possible.

— Alexander Ponomaryov asked: “In our area, the connection of three 1.8-meter vertical earth rods in a triangle with 1.5-meter sides results in 1.5 – 1.8 Ohms, but the projects suggest an installation of 6 rods in a line. I'm speaking about the Krasnodar region."

— Such a low value of 1.5 – 1.8 Ohms may depend on the fact that it was measured at the moment when the groundwater level rose very high. It may seem quite true. There can be such an explanation that the standard soil resistivity value is accepted as 100 Ohm * m in designs, but in fact, this same soil may have a much lower resistivity. And also depends on what rates are used in the design. As I can see, there are three vertical earth rods here, which means that it is most likely 10 Ohms. Well, all I can say is it's a very good soil.

— Alexander Ponomaryov commented: "Measurements were taken in all four seasons. The error is about 0.2 – 0.3 Ohm."

— This is not an error, but a regularity. Perhaps, the measurements were taken incorrectly, or, as I said earlier, the soil is quite good indeed. The error of 0.2 – 0.3 Ohms in all four seasons means a very good soil.

— Victor asked: "If the ground water has a low salt content, can it act as an aggressive environment during the current distribution (electrolysis)?"

— Yes, it may be aggressive, or it may may be acidic instead. Then it will also be aggressive to steel and to other materials. The moisture content may be about 80% in an acidic environment as well. Besides, an aggressive environment can be neither acidic nor alkaline, but a large moisture content can negatively affect earth rods.

— Victor commented: "I got it. Could you please say some more words about installing a surge protection rod on the air duct?”

— There may be different circumstances here, but in general, a rooftop ductwork rises above the roof. As you know, the highest point should be protected against lightening, and in our case the rooftop ductwork is the highest point. A lightning protection rod of the required height should be installed in accordance with the relevant protection zone.

— Victor asked: "But why on the brick one?"

— For the same reason as on the metal one. 

— Victor asked: "Could you please tell us, how much they cost? Thank you".

— Their price may be different in each particular case, when the calculation results depend on loads, loops and circuits, the type of facility, and the currents available. They are pre-calculated according to the questionnaire filled in by the customer. The next stage includes an actual research with regards to the facility. Then the price is configured based on the customer's benefits. Thus, the price is configured as I've just said. Please feel free to send completed questionnaires for us to start the process. Then you'll know all the benefits and the price itself.

— There are no more questions left. Thank you for attention! Best regards and hope to see you at our next webinars.

Why Cannot Vertical Earthing Devices Be Installed Close to Each Other? Electrolytic Grounding in Permafrost Soils: Should Vertical of Horizontal Electrodes Be Used?