NETA Ground Resistance Testing Methods 2 — Questions and Answers
Question 1: The resistance of a driven ground rod is most significantly reduced by:
- Increasing the rod diameter from 5/8 inch to 3/4 inch
- Doubling the rod length (depth) (Correct answer)
- Using copper instead of steel for the rod material
- Installing the rod in a gravel bed for drainage
Correct answer: Doubling the rod length (depth)
Doubling rod length approximately halves ground resistance (per the Dwight formula), while doubling diameter produces only a small improvement due to the logarithmic relationship.
The resistance of a driven rod electrode is given approximately by R = (p/2*pi*L) x [ln(4L/d) - 1], where p is soil resistivity, L is rod length, and d is rod diameter. Doubling the length cuts resistance approximately in half, while doubling the diameter only reduces the logarithmic term slightly. NEC requires a minimum 5/8-inch diameter rod. For difficult soils, increasing rod depth by stacking rods or using longer ground rods is far more effective than increasing diameter.
Question 2: When multiple ground rods are installed in parallel, their combined resistance is not simply R/n (where n is the number of rods) because:
- Parallel rods must be connected with resistance wire that adds impedance
- The resistance areas of the rods interact (mutual resistance effect), reducing the benefit of additional rods (Correct answer)
- Parallel rods create opposing magnetic fields that increase total resistance
- NETA standards require a safety factor of 2 for parallel ground rod calculations
Correct answer: The resistance areas of the rods interact (mutual resistance effect), reducing the benefit of additional rods
Adjacent ground rods share overlapping resistance areas in the soil. This mutual resistance effect means each additional rod provides diminishing returns — the improvement is less than 1/n.
When two ground rods are installed adjacent to each other, their resistance zones overlap in the soil. This mutual coupling means the combined resistance is higher than the simple parallel formula R/n suggests. For two identical rods separated by a distance equal to the rod length, the combined resistance is approximately 60% of one rod's resistance, not 50%. IEEE 80 provides combination factors for multiple rods. To maximize the benefit of multiple rods, they should be separated by at least the rod length. For large substation ground grids, similar considerations apply, which is why ground grids are designed to cover the maximum area possible.
Question 3: Ground resistance measurements are best performed during which soil conditions for most conservative results?
- During wet seasons when soil moisture is maximum, giving lowest resistance
- During dry periods when soil moisture is minimum, giving highest resistance — representing worst case (Correct answer)
- At moderate temperature (15 to 20 degrees C) for most consistent readings
- During summer when soil bacteria activity is highest
Correct answer: During dry periods when soil moisture is minimum, giving highest resistance — representing worst case
Dry soil has higher resistivity than moist soil. Testing during dry conditions gives the highest (worst-case) ground resistance reading, which is the most conservative measurement for determining if the grounding system meets requirements.
Soil resistivity (and therefore ground electrode resistance) varies significantly with moisture content — values can change by a factor of 10 or more between wet and dry conditions. For safety and performance verification, ground resistance measurements should be taken during the driest season (typically late summer) when soil resistance is highest. This represents the worst-case condition for ground fault protection. Measurements taken after heavy rain may show acceptable values that would fail during dry conditions. NETA and IEEE standards recommend noting soil conditions with test results.
Question 4: The purpose of measuring transferred ground potential in extended grounding systems is:
- To verify that the ground potential rise does not exceed safe levels at remote locations connected to the grounding system (Correct answer)
- To measure the voltage transferred between primary and secondary grounds
- To test the continuity of the grounding conductor between buildings
- To verify that telephone cables do not carry ground fault current
Correct answer: To verify that the ground potential rise does not exceed safe levels at remote locations connected to the grounding system
Transferred ground potential occurs when a grounded metallic system (pipelines, cables, rails) extends beyond the grounding system into areas of lower ground potential, potentially carrying dangerous voltages to remote locations.
When a major ground fault occurs at a substation, the ground potential rise (GPR = Ig x Rg) elevates the local earth to a high voltage relative to remote earth. If metallic conductors such as communication cables, pipelines, rail lines, or fences are connected to the high-voltage ground point and extend to remote areas at lower earth potential, they carry the high voltage away from the substation, creating hazardous voltages at locations far from the fault. Mitigation measures include isolation transformers, drainage coils, separating bonds, and gradient control wires.
Question 5: IEEE 80 recommends that a grounding system's safety is verified by calculating:
- Ground resistance less than 1 ohm as the sole criterion
- Tolerable touch and step voltages compared to actual design touch and step voltages under maximum ground fault conditions (Correct answer)
- Ground fault current magnitude at the substation only
- Soil resistivity at multiple depths to verify uniform soil model
Correct answer: Tolerable touch and step voltages compared to actual design touch and step voltages under maximum ground fault conditions
IEEE 80 safety verification compares computed touch and step voltages during maximum fault conditions against the tolerable voltage limits based on body weight and fault clearing time.
IEEE 80 establishes that safety is verified by: calculating maximum ground fault current (Ig), calculating ground potential rise (GPR = Ig x Rg), modeling earth surface potential distribution, computing mesh voltage (worst-case touch voltage within the grounded grid) and step voltage (maximum step voltage at the grid perimeter), and comparing these to tolerable limits that account for surface material, body weight, and fault clearing time. Ground resistance below 1 ohm is a guideline but is not sufficient alone to confirm safety.
Question 6: What is the purpose of testing ground grid integrity using low-voltage high-current injection after installation?
- To verify the ground grid can carry the full fault current without melting
- To verify electrical continuity of all connections and conductors within the grid by identifying open connections through current distribution measurements (Correct answer)
- To measure the resistance of each individual conductor in the ground grid
- To test the insulation between the grid conductors and buried pipes
Correct answer: To verify electrical continuity of all connections and conductors within the grid by identifying open connections through current distribution measurements
Ground grid integrity testing injects test current into the grid and measures the resulting current distribution or voltage response to identify poorly connected or open sections that would limit fault current flow.
After installation and burial of a substation ground grid, integrity testing verifies that all conductors and connections are properly bonded. Methods include injecting DC or low-frequency AC current at one point and measuring current distribution or voltage at multiple points, thermal imaging after current injection to identify poorly bonded connections, or using time-domain reflectometry. Poor connections create high-resistance paths that limit fault current distribution, potentially causing dangerous local GPR concentrations. NETA acceptance testing specifications include ground grid integrity verification for new substation construction.
NETA Ground Resistance Testing Methods 2
The resistance of a driven ground rod is most significantly reduced by: