Soil Resistivity and Ground Electrode Resistance - Field Measurement

What it is

This field form documents the measurement of soil resistivity across a site using surface probes or driven rods (Wenner, Schlumberger or driven-rod methods) and the measurement of ground electrode or grid resistance using one of several techniques: the fall-of-potential method with the 62 percent check, the plateau average, the slope method, two-point, three-point or clamp-on measurement. The form captures the geometry each method requires - probe spacing, burial depth or rod length, and the current probe distance - the test signal and frequency, the resistance read at each spacing with any reciprocal reading, the computed apparent resistivity at each spacing, and the resulting electrode resistance. It also records the soil model derived from the resistivity data, the result of each method's own validation check, site conditions such as soil type, moisture, season and the weather of the three days before the test, the instrument and its calibration date, and the design or owner maximum resistance value that establishes the acceptance threshold for the electrode or grid under test. Current injection, touch and step surveys, surface aggregate and transient tests are out of scope, and bonding integrity stays on the grounding systems form.

Why it is performed

Soil resistivity varies with depth, season, moisture and mineral content, and it directly affects how quickly current can leave a grounding electrode and disperse into the earth. A ground grid or rod that meets its design resistance value only in summer or only in wet soil may not meet it during drought or freeze, and a site survey that samples resistivity at only one location or in only one direction may miss a buried layer of high-resistivity material that could trap the current path. IEEE 81-2025 guides the measurement process so that the resistivity traverses cover more than one direction and a range of spacings, the soil model reflects the actual layering, and the electrode resistance is measured with a geometry and a method check that make the value trustworthy. Recording all probe positions, spacings, method checks and site conditions allows the result to be reproduced and the design resistance to be validated for the actual soil the electrode must work in.

When it is performed

Before a grounding system is first placed in service, to confirm that the electrode or grid resistance meets the design specification at the soil and moisture conditions present on the day of test, and on a design-stage job before the electrode system is designed, when ANSI/TIA-607-E asks for the soil resistivity to be tested first. The test may be repeated during maintenance, after a major system alteration, or when there is doubt about whether seasonal or long-term soil changes have caused the resistance to rise above the design value. IEEE 81-2025 recommends at least two resistivity traverses in different directions; the form holds up to four, and fewer than two, or all on one line, raises a review flag so that the soil model is not skewed by a localized anomaly. Readings taken with different methods, layouts or reference points are not comparable from one visit to the next.

How it is typically performed

With the test location marked and the design or owner maximum resistance value noted with its source document, the field crew reviews the safety plan - a written test plan reviewed with the crew, a job safety briefing, test leads treated as energized, routed and supervised, no testing near lightning without a risk assessment, no surge arrester ground opened on an energized line, the crew kept clear of the probes at higher test voltages, and utility locates done before probes are driven. For resistivity, a Wenner, Schlumberger or driven-rod array is set up and the instrument reads a resistance at each spacing; the form computes the apparent resistivity from the spacing and the reading, and the spacings are extended until the apparent resistivity flattens on the plot. A reciprocal reading, a repeat at a second frequency and the probe contact resistance show whether the reading can be trusted. For electrode resistance, the fall-of-potential method places the current probe several times the largest electrode dimension away and reads the resistance with the potential probe at 52, 62 and 72 percent of that distance; the outer readings must agree with the 62 percent reading for the value to count. The plateau average takes about nine readings along the line and averages three consecutive consistent ones; the slope method takes three readings and computes a coefficient that decides where the potential probe belongs; two-point, three-point and clamp-on measurements are entered with their own conditions. On a fall-of-potential or three-point measurement the electrode must be disconnected from external ground paths, and a clamp-on reading counts only with a confirmed low-resistance parallel return. All spacings, frequencies, probe positions, instrument settings and the results of the method checks are recorded in the form so the work can be audited, as IEEE 81-2025 describes.

What gets recorded

For each of up to four resistivity traverses: the method (Wenner, Schlumberger or driven rod), bearing, center location and probe burial depth, and at each spacing the spacing values, the measured resistance, any reciprocal reading and the computed apparent resistivity, with flags for reciprocal agreement, the Schlumberger spacing ratio, the total span and plot flattening. For the electrode or grid: type and largest dimension, method, whether external ground paths were connected, current probe distance and its ratio to the dimension, potential probe direction and second direction, probe type and depth, the readings the method needs (52, 62 and 72 percent, plateau, slope, two-point, three-point or clamp-on with its frequency), the resulting resistance with its method check, and the lead routing. Also: the design or owner maximum with its document and revision, the drawing reference, season, soil moisture, type and temperature, days since rain, frost depth, nearby conductors, the three-day weather history and the number of test locations, the soil model layers, interpretation method, software and engineer, the instrument make, model, serial and calibration date, test signal and frequency, probe contact resistances, ambient conditions, attachments, comments and deficiencies.

How results are evaluated

The grading rests on two conditions. First, the safety plan items, drawn from IEEE 81-2025 clauses 5.1, 5.2 and 7.5.1, must all be marked Satisfactory or Not Applicable with at least one Satisfactory; one Unsatisfactory fails the record, and an unanswered or all-not-applicable plan leaves the record recorded, not graded. Second, the electrode resistance must be at or below the design or owner maximum recorded on the form; a value equal to the maximum passes, and with no maximum entered the resistance is recorded, not graded. The electrode value is graded only when the selected method's own check holds: the 52 and 72 percent readings within 3 percent of the 62 percent reading, a consistent plateau of three rising readings, a slope coefficient of 2 or less, a disconnected electrode on a fall-of-potential or three-point method, auxiliary electrodes not much higher in resistance than the subject, and a confirmed low-resistance parallel return for clamp-on. A failed check is an invalid measurement, not an electrode failure: no value is reported, the record reads indeterminate and an issue line names the check. IEEE 81-2025 is a guide and states no acceptance value for soil resistivity or for electrode resistance; resistivity is recorded to build the soil model and is never graded. Review flags only, never a fail: a test frequency outside the guide's example ranges, a reciprocal reading differing by more than 5 percent (an UltraDb convention), a current probe closer than five times the largest dimension, plateau readings outside the usual window, fewer than two traverses or all on one line, frozen or saturated soil, a high probe contact resistance, lead coupling at low resistances, a clamp-on frequency above 1 kHz, and a missing three-day weather history or fewer than two test locations, which ANSI/TIA-607-E asks for. See IEEE 81-2025 for the complete method requirements.