NETA Ground Resistance Testing Methods 4 — Questions and Answers
Question 1: When performing a slope method ground resistance test, what does a flat slope in the resistance curve indicate?
- The test electrode spacing is too close together
- The current and potential electrodes are properly placed in resistivity-free soil (Correct answer)
- The ground electrode under test has a very low resistance
- The soil has extremely high resistivity
Correct answer: The current and potential electrodes are properly placed in resistivity-free soil
A flat or plateau region in the slope method curve indicates the potential electrode is outside the mutual resistance zones of both the current and ground electrodes, confirming valid placement.
Question 2: Which NETA standard specifies the acceptance criteria for ground grid resistance at electrical substations?
- NETA ATS Table 100.1
- IEEE 80 in conjunction with NETA MTS
- NETA ATS Section 7.13 (Correct answer)
- ANSI C2 with NETA overlay
Correct answer: NETA ATS Section 7.13
NETA ATS Section 7.13 addresses grounding systems and provides acceptance criteria and test procedures for ground resistance measurements.
Question 3: What effect does frozen ground have on earth resistance measurements using the fall-of-potential method?
- Frozen ground decreases soil resistivity, giving falsely low readings
- Frozen ground increases soil resistivity, giving falsely high readings (Correct answer)
- Frozen ground has no effect because current bypasses frozen layers
- Frozen ground only affects measurements below 1 Hz test frequency
Correct answer: Frozen ground increases soil resistivity, giving falsely high readings
Ice formation in frozen soil dramatically increases resistivity because water, the primary conductor in soil, becomes non-conductive ice.
Question 4: In a clamp-on ground resistance test, what is the primary limitation compared to the fall-of-potential method?
- Clamp-on testers cannot measure resistance below 10 ohms
- Clamp-on testers require the ground rod to be disconnected from the system
- Clamp-on testers cannot isolate a single electrode; they measure the parallel combination of all grounding paths (Correct answer)
- Clamp-on testers are only accurate above 100 Hz
Correct answer: Clamp-on testers cannot isolate a single electrode; they measure the parallel combination of all grounding paths
Clamp-on testers inject current into the ground loop and measure the total impedance of all parallel grounding paths, making it impossible to isolate a single electrode's resistance.
Question 5: A technician measures ground resistance using the 62% rule. The current electrode (C2) is placed 100 feet from the ground electrode (C1). Where should the potential electrode (P2) be placed?
- 38 feet from C1
- 50 feet from C1
- 62 feet from C1 (Correct answer)
- 76 feet from C1
Correct answer: 62 feet from C1
The 62% rule places the potential electrode at 62% of the distance between the ground electrode and the current electrode, which is 0.62 × 100 = 62 feet from C1.
Question 6: What is the purpose of using a low-frequency AC signal (typically 128 Hz) in modern digital ground resistance testers?
- To increase the depth of current penetration into the soil
- To reject power frequency (60 Hz) interference and stray currents (Correct answer)
- To comply with NETA requirements for substation testing
- To measure soil resistivity rather than electrode resistance
Correct answer: To reject power frequency (60 Hz) interference and stray currents
Using a test frequency different from 60 Hz allows the instrument to filter out power frequency noise and stray ground currents that would otherwise corrupt the measurement.
Question 7: During a staged fault test for ground grid evaluation, what measurement validates that the ground potential rise (GPR) is within safe limits?
- The GPR must not exceed 5,000 volts at the grid perimeter
- The touch and step voltages must remain below IEEE 80 calculated tolerable limits (Correct answer)
- The ground resistance must measure below 1 ohm during the fault
- The fault current must be limited to less than 10,000 amperes
Correct answer: The touch and step voltages must remain below IEEE 80 calculated tolerable limits
IEEE 80 defines tolerable body current limits that are used to calculate maximum allowable touch and step voltages, which are the true safety metrics for a ground grid.
NETA Ground Resistance Testing Methods 4
When performing a slope method ground resistance test, what does a flat slope in the resistance curve indicate?