What matters most
A substation soil-resistivity survey places multi-spacing Wenner arrays at representative locations within and around the proposed ground-grid area. The program should account for site size, earthwork, cut and fill, imported material, groundwater, rock, nearby lines or pipelines, and the grounding engineer’s model requirements.
Key takeaways
- Coordinate the survey before mass grading and grid installation when possible.
- Represent native ground, cut, fill, and expansion areas separately.
- Use spacings and stations appropriate to the grid size and expected layering.
- The field survey supplies inputs; the grounding engineer completes the safety design.
Earth resistivity is one grounding-design input
Substation analysis also needs grid geometry, conductor size and depth, fault current and split, clearing time, surface layer, equipment layout, fences, nearby utilities, and applicable safety criteria.
The soil survey should be designed with the grounding engineer so the spacing range and station coverage support the intended earth model rather than a generic field package.
Existing ground may differ from the finished substation pad
Mass grading can remove a surface layer, expose rock, place borrow, or create several fill zones. A survey completed too early without grading context may characterize material that will not remain around the grid.
Provide the grading plan, geotechnical information, borrow source, pad limits, and anticipated grid extent. The report should identify whether each line represents native, cut, fill, or uncertain future conditions.
Plan around transmission and conductive infrastructure
Existing lines, pipelines, fences, foundations, rails, buried utilities, and operating grounding systems may affect safe access or the measured current path. A greenfield interconnect site is generally easier than an energized expansion.
Use several candidate line directions and document unavoidable features. For expansions, clarify whether the work occurs outside the existing grid or requires a specialized installed-system test plan.
Send the one-line, site plan, grading plan, proposed grid footprint, requested spacing series, and receiving grounding engineer’s data format before the field date.
This page provides general educational information, not a grounding-system design, cathodic-protection design, installed-system acceptance test, or parcel-wide resistivity conclusion. A Wenner result is apparent soil electrical resistivity for the documented array, spacing, location, orientation, and field condition. The responsible grounding or corrosion engineer must evaluate the complete dataset, earth model, asset geometry, governing documents, and uncertainty.
Frequently asked
Questions about this topic
How many Wenner stations does a substation need?+
There is no universal count. Site area, variability, grading, grid geometry, expected layering, and the grounding engineer’s model determine coverage.
Should testing happen before grading?+
Often yes, but the program must consider what material will remain. Some projects need additional testing after grading or of imported fill.
Can a survey be performed at an energized substation?+
Potentially, but safety, interference, installed grounding, and limited clear space require a different review than a greenfield survey.
Primary references and further reading
- ASTM International — ASTM G57-20 Official record for the Wenner four-electrode field and four-electrode soil-box method.
- IEEE Standards Association — IEEE 81-2025 Current IEEE guide for measuring earth resistivity, grounding-system impedance, and earth-surface potentials.
- Soil Tests Laboratory — Wenner Four-Pin Testing Calicore Engineering’s testing division and field-service information.