NETA Ground Resistance Testing Methods 5 — Questions and Answers
Question 1: When using the two-point method for ground resistance measurement, what does the second reference point typically represent?
- A remote earth connection with known resistance
- A water pipe or structural steel with assumed zero resistance (Correct answer)
- A calibrated reference electrode buried at a specific depth
- The neutral conductor of the power distribution system
Correct answer: A water pipe or structural steel with assumed zero resistance
The two-point method connects the tester between the electrode under test and a convenient reference such as a water main or building steel, assuming the reference has negligible resistance.
Question 2: What soil condition produces the lowest earth resistivity and therefore the easiest ground electrode installation to achieve low resistance?
- Dry sandy loam with low moisture content
- Moist clay or loam soil with high mineral content (Correct answer)
- Dry gravel with large particle size
- Rocky terrain with shallow soil overburden
Correct answer: Moist clay or loam soil with high mineral content
Moist clay or loam with high dissolved mineral content has the lowest resistivity because moisture and ions are the primary charge carriers in soil.
Question 3: A NETA technician is testing a ground rod in soil with high resistivity. Which chemical enhancement method is acceptable for permanently reducing ground resistance at that electrode?
- Pouring common table salt solution around the rod each year
- Installing a ground enhancement material (GEM) or conductive concrete in the electrode bore (Correct answer)
- Connecting additional copper wire to the rod and burying it horizontally
- Injecting high-pressure water to fracture surrounding rock
Correct answer: Installing a ground enhancement material (GEM) or conductive concrete in the electrode bore
NETA and IEEE 142 recognize ground enhancement materials such as conductive grouts and GEM compounds as permanent, non-corrosive methods to lower soil contact resistance.
Question 4: In the four-potential method (Wenner array) for soil resistivity testing, what does the calculated apparent resistivity represent?
- The resistivity of soil at the exact depth equal to electrode spacing 'a'
- A weighted average of soil resistivity to an approximate depth equal to the electrode spacing 'a' (Correct answer)
- The minimum resistivity found anywhere within the test grid
- The surface resistivity only, independent of depth
Correct answer: A weighted average of soil resistivity to an approximate depth equal to the electrode spacing 'a'
The Wenner array measures an apparent resistivity that represents a weighted average of all soil layers from the surface down to approximately the electrode spacing distance 'a'.
Question 5: Why must all parallel ground paths be disconnected or the electrode isolated before performing a fall-of-potential test?
- Parallel paths increase the injected test current beyond instrument limits
- Parallel paths provide alternate return routes for test current, causing the instrument to read lower than the true single-electrode resistance (Correct answer)
- Parallel paths generate interference frequencies that corrupt the measurement
- Parallel paths cause the potential electrode to float at an unpredictable voltage
Correct answer: Parallel paths provide alternate return routes for test current, causing the instrument to read lower than the true single-electrode resistance
If parallel grounding paths exist, test current returns via multiple routes, and the instrument measures the parallel combination of all paths rather than the resistance of the isolated electrode.
Question 6: According to NETA maintenance testing standards, how frequently should ground resistance tests be performed on critical substation grounding systems?
- Every 6 months or after any ground fault event
- In accordance with the facility's maintenance schedule, typically every 1 to 3 years per NETA MTS (Correct answer)
- Once at commissioning; no periodic re-testing is required
- Only when visual inspection reveals corrosion on ground conductors
Correct answer: In accordance with the facility's maintenance schedule, typically every 1 to 3 years per NETA MTS
NETA MTS recommends periodic ground resistance testing on a schedule appropriate to the facility's risk level, generally every 1 to 3 years for critical systems.
Question 7: A technician observes that a ground resistance reading increases significantly when the current electrode lead is oriented toward a buried metallic pipeline. What corrective action should be taken?
- Accept the higher reading as the worst-case scenario for the electrode
- Reposition the current electrode in a direction away from the pipeline to avoid current path interference (Correct answer)
- Switch to a higher test frequency to overcome pipeline interference
- Use the clamp-on method instead, which is immune to buried metal interference
Correct answer: Reposition the current electrode in a direction away from the pipeline to avoid current path interference
Buried metallic structures can conduct test current and distort fall-of-potential results; repositioning the current electrode away from the pipeline eliminates the interference path.
NETA Ground Resistance Testing Methods 5
When using the two-point method for ground resistance measurement, what does the second reference point typically represent?