21 Sep Achieve Effective Grounding With Soil Resistivity Testing
In 2019, SAE was approached by an electrical utility in the Midwestern United States to engineer an effective grounding solution for a new distribution substation. The substation site sits on highly resistive dolomite bedrock with minimal overburden. This combination created an extremely challenging scenario to achieve sufficient grounding with traditional grounding methods.
In order to measure the soil resistivity at the site, SAE performed testing to a depth of 80m (~260’). The resultant soil model reflected what was previously known of the site’s geology: low resistivity soil for the top 7.5m (~25’) and highly resistive bedrock below. The actual top layer at the substation site is less than 1m (~3’) in-depth due to excavation. This posed a greater challenge in achieving the required results.
However, during the second test, SAE field engineers increased the scope of the test to reach depths of nearly 300m (~985’) in an attempt to gather information about the underlying layers. The test successfully indicated that the high resistivity layer gave way to a much lower resistivity layer at depths exceeding 115m (~380’).
By utilizing the lower resistivity layer in the grounding design, SAE engineers were able to dramatically decrease the grounding requirements in order to achieve the target R-value and mitigate all step & touch voltage hazards. An original substation grounding grid design featured an extensive grid expansion with over 1300m (~4400’) conductor to be installed. SAE was able to design a grid within the substation footprint that was supplemented with a Conducrete® enhanced vertical electrode penetrating into the lower resistivity layer identified by the deep soil profiling. The result was a much more cost-effective grounding system that will perform during fault events and a satisfied customer.
For more information on SAE’s engineering services or to learn more about Conducrete® and SAE’s other specialized grounding products, visit www.saeinc.com.