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Soil Resistivity Testing & Grounding System Design: Part II of II

The sole purpose of the design exercise for grounding systems is to make the performance predictable upon installation. For ground system designs, there are several options for the design process or the method of manipulating the data by formulas, spreadsheets, or software. Likewise, there are alternatives for the design result or the actual instructions showing locations, models and quantity of grounding electrodes required to achieve the grounding system performance indicated by the design.

The data required for a grounding system design is the soil resistivity information, the ground system performance requirement (5 ohms, etc.) and a site map. The map should indicate the lease area, the location(s) of the facility, shelter, tower and all other structures. Geo technical reports are also always beneficial. Particularly from an installation standpoint, it is good to know that rock is at three feet or water at ten.

A cautionary point is that even if the report contains soil resistivity data, it is nearly always unusable for grounding system design. Generally, the soil resistivity data provided is derived from a small sample, which is often saturated with water prior to testing. The disadvantage is that we want to know the resistivity of the whole area, not just the sample. Also, the natural moisture content and compaction are lost during the collection, shipping and testing process.

Design Methods
Methods of grounding system design vary from a Grounding Nomograph (AEMC – Understanding Ground Resistance) to simple formulas, not so simple formulas and software programs.

The nomograph is the simplest form of grounding design consisting of several exponential scales. By utilizing a straight edge, a resistance objective (5 ohms, etc.) and a soil resistivity number (15,000 ohm-cm, etc.), it can be used to determine the length and diameter of a grounding electrode needed to meet a specific performance requirement.

However, the nomograph is severely limited in several aspects. One, the range of soil resistivity is effectively limited to around 11,000 ohms-cm. Nearly everywhere, soils are higher in resistivity than 11,000 ohms-cm. Secondly, it can only provide the length/diameter of a single grounding electrode, which is inadequate to meet the ground resistance requirement for sensitive equipment. Utilizing the nomograph, achieving 5 ohms in 11,000 ohm-cm soil would take a 100-ft. driven rod. Multiple rods in a grid fashion would be a much easier solution but are not an option with the nomograph.

Complexity
Ground resistance formulas are available from several sources. The most reliable and thorough source is the IEEE Green Book (Std. 142), but even though this formula may not be very complicated, it is not really that simple. The complexity is required in defining the cross sectional area of the grounding system or a single electrode. Then, as you add electrodes and conductor, the formulas become exponentially more complex and still do not include the nearly impossible complexity of utilizing various soil resistivity values at different depths. These elements and real life situations often require the use of radial extensions for grounding systems and other odd configurations.

This is where software programs come into play because complex formulas require multiple iterations to solve. Modern computers are very efficient at this task, which is mainly number crunching a complicated formula.

Although there are a few software programs available for ground system resistance calculations, we use a commercially available program (CDEGS), which is produced by Safe Engineering Services of Laval, Canada. We believe that CDEGS does the most accurate ground system design and is also used for our substation work with step and touch calculations as specified by IEEE 80.

Evaluating Data
The design process really starts with the engineer evaluating soil resistivity data, and the more data available the more confident we become about a grounding system design. The data is evaluated for obvious outliers. Sometimes, for example, a metallic object (sewer/gas pipe) may be located parallel to the soil resistivity testing. When this happens the data collected along that line of tests will be suspiciously good and should not be used in the modeling. After all the selected resistivity data is inserted, the program will model this section of earth and result in numerous options of how many layers to utilize, whether to use vertical layers or volumes.

After the soil model is developed, the engineer then designs the grounding system around the constraints presented by the site, lease area, geotech report, available construction equipment and customer. For instance, if good soil is available at deeper depths, the engineer might use deep electrodes. If the site is inaccessible to drilling rigs, deep electrodes would not be an option.

Many ground system designs are “geometric,” or simply made to be geometrically pleasing and consistent with the facility. In nearly all of these cases, the design is made without regard or thought toward ensuring the ground system performance meets the facility requirement. With an understanding of the importance of the grounding systems, the methodology utilized in an effective design process, and the various options available, it is possible to have the knowledge to select the appropriate system for each particular installation.

About the Author: John Howard, Vice President of Business Development at Lyncole XIT Grounding, has been with Lyncole since 1991. Utilizing his training in electricity and electronics, he has worked with both national and international telecommunications carriers and municipalities, developing and refining their grounding procedures, testing methods and electrical protection designs. Howard is also a frequent speaker at conferences and companies on grounding system design and proper testing.

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Friday, 14 August 2020

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