Ground Resistance Testing Methods Flashcards

When performing a fall-of-potential ground resistance test on a large grounding electrode system, the auxiliary current electrode must be placed at a minimum distance to ensure the resistance areas of the electrodes under test and the current electrode do not overlap. For a complex grounding grid with a diagonal measurement of 50 meters, what is the minimum recommended distance from the edge of the grid to the current electrode C2?

Answer: 250 meters

IEEE Standard 81 recommends the current electrode C2 be placed at a distance of at least 5 times the maximum dimension (diagonal) of the grounding system being tested. For a 50-meter diagonal grid: 5 × 50 = 250 meters. This ensures the resistance areas do not overlap, which would cause significant measurement error.

A technician performs a staged-fault ground resistance test and notices that the measured resistance increases significantly as the potential probe P2 is moved from 40% to 62% of the C1-C2 distance, but then stabilizes and slightly decreases beyond 62%. This behavior most likely indicates:

Answer: The current electrode is too close and the resistance areas are overlapping

In a properly conducted fall-of-potential test, the resistance curve should show a flat plateau region around the 61.8% distance point. An increasing then stabilizing/decreasing pattern indicates the resistance hemispheres of the current electrode C1 and the electrode under test are overlapping. The plateau is shifted or distorted because the two resistance areas are not truly independent, invalidating the measurement and requiring C2 to be moved much farther away.

A NETA technician is tasked with testing the ground resistance of a substation grounding grid but cannot access the physical grid connection point. Instead, the measurement must be taken at a remote structure grounded to the grid via a 200-meter buried counterpoise conductor. The two-point (dead earth) method is used as an alternative. What is the primary source of error in this measurement approach?

Answer: The resistance of the counterpoise conductor and its contact resistance are included in the measured value, overstating grid resistance

The two-point (dead earth) method measures the total loop resistance including the electrode under test, all interconnecting conductors, and the earth return path through the reference electrode. The 200-meter counterpoise conductor adds its own DC resistance and soil contact resistance to the reading, making the result a composite value that significantly overstates the true grid resistance. This method is only acceptable for quick comparative checks, not accurate absolute measurements.

During a Wenner four-pin soil resistivity test, a technician obtains the following apparent resistivity values at increasing pin spacings: 15 Ω·m at 1m, 22 Ω·m at 2m, 31 Ω·m at 5m, and 48 Ω·m at 10m. This progressively increasing apparent resistivity with depth most likely indicates:

Answer: A two-layer soil model where a lower-resistivity top layer overlies a higher-resistivity bedrock or caliche layer

In the Wenner method, each pin spacing samples an approximate depth equal to the spacing distance. A consistent increase in apparent resistivity with increasing spacing is the signature of a two-layer soil structure where the surface layer has lower resistivity than the deeper layer. This commonly occurs when conductive topsoil or clay overlies high-resistivity bedrock, caliche, or dry sandy subsoil. This profile significantly impacts grounding system design, as deep driven rods may not achieve the expected resistance reduction.

A technician uses a clamp-on (stakeless) ground resistance tester to measure a ground rod that is part of a multipoint grounded system. The clamp-on instrument reads 3.2 Ω. Later, the same rod is isolated and measured using the fall-of-potential method, which reads 85 Ω. What is the correct interpretation of these two measurements?

Answer: Both readings are valid; the clamp-on measures the parallel combination of all other ground paths, while fall-of-potential measures the single rod in isolation

Both readings are technically correct but measure fundamentally different things. The clamp-on tester injects a signal and measures the total loop impedance, which equals the electrode under test in parallel with ALL other parallel ground return paths (other rods, building steel, utility grounds, etc.). The fall-of-potential method measures only the isolated electrode's resistance to remote earth. A single isolated rod reading of 85 Ω is perfectly consistent with a clamp-on reading of 3.2 Ω if many other low-resistance parallel paths exist. Neither reading is wrong; they answer different engineering questions.

When applying the selective (attached-lead) clamp-on ground resistance test method at a telecommunications tower with multiple ground rods bonded together with a perimeter ground ring, the technician clamps the injection clamp around the down conductor to one specific rod, then places the measurement clamp around the same conductor below the ring bond connection point. The purpose of this two-clamp selective technique, compared to a single-clamp measurement, is to:

Answer: Isolate and measure only the resistance of that specific rod without being affected by other parallel ground paths in the ring

In the selective two-clamp method, the injection clamp drives current down the conductor toward the rod, while the measurement clamp — placed between the ring bond and the rod — measures only the voltage drop through the section of conductor and rod below the bond. Because the ring bond provides a very low-impedance return path that bypasses the individual rod, current is forced to flow only through that specific rod to earth and back through the soil. This isolates the individual rod's resistance contribution from the parallel combination of all other bonded electrodes, giving a meaningful individual electrode reading without physical disconnection.