What matters most
Soil resistivity characterizes the ground’s opposition to current flow and is commonly measured before design with a Wenner array. Ground resistance or grounding-system impedance describes an installed electrode or grid and depends on soil, geometry, conductor arrangement, connections, depth, frequency, and the extent of the system. The terms should not be used interchangeably.
Key takeaways
- A Wenner survey does not directly measure an installed grid’s resistance.
- An installed-system test does not automatically provide a clean soil profile.
- The requested service should name the asset and project stage.
- IEEE 81 addresses several related but distinct measurement families.
Soil resistivity is a ground-property measurement
Before construction, an equally spaced four-electrode array measures the earth response at selected locations and spacings. The dataset can support a uniform or layered-earth model for design.
The value is tied to the field condition and influenced volume. It is not the resistance of a future rod, grid, foundation, fence, or substation.
Ground resistance and impedance belong to an installed system
Once electrodes or a grid exist, the measured system response includes conductor geometry, burial, bonds, connected neutrals or shields, parallel paths, remote-earth assumptions, test frequency, and current distribution.
Large interconnected systems can require specialized test methods, safety planning, outages, current injection, or modeling beyond a new-site Wenner survey.
State the requested measurement precisely
For a proposal, identify whether the need is predesign soil resistivity, installed electrode resistance, ground-grid impedance, continuity, step and touch potential, or troubleshooting. Include drawings and facility status.
Calicore’s standard nationwide offering is in-situ Wenner soil-resistivity testing. Other installed-system measurements should be discussed and scoped separately.
“Ground test” is not a complete scope. Name the measurement, project stage, facility status, method or guide, and intended decision.
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
Will a Wenner test tell me the resistance of my ground rod?+
No. It provides soil apparent-resistivity data. Rod or grid performance also depends on geometry, depth, connections, surrounding layers, and the installed-system test method.
Can I perform Wenner testing after a grid is installed?+
Sometimes, but the installed conductors may influence the current path. Predesign testing in undisturbed or representative ground is generally easier to interpret.
Does lower soil resistivity always mean a safe grounding design?+
No. Personnel safety and system performance require a complete design using fault, geometry, surface-layer, duration, bonding, and applicable criteria.
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.