NETA Ground Resistance Testing Methods 1 — Questions and Answers

Question 1: The fall-of-potential method for measuring ground electrode resistance uses a minimum of:

One auxiliary electrode and two meters
Two auxiliary electrodes (current and potential) and a ground resistance meter (Correct answer)
Three auxiliary electrodes to triangulate the measurement
No auxiliary electrodes — it measures from the electrode to a reference point

Correct answer: Two auxiliary electrodes (current and potential) and a ground resistance meter

The fall-of-potential method requires a current electrode (C2) to drive current into the earth and a potential electrode (P2) to measure voltage, plus the instrument connections to the ground electrode being tested.

The fall-of-potential method (also called the 3-point or 3-terminal method) is the most common ground electrode resistance measurement technique. A test current is passed between the ground electrode under test (C1) and a remote current electrode (C2) driven into the earth at a distance. A separate potential electrode (P2) is placed between them to measure the voltage gradient. The instrument calculates R = V/I. The potential electrode is moved along the current path (typically from 20% to 80% of the C1-C2 spacing) to find the flat region of the voltage gradient curve, which indicates the true ground resistance.

Question 2: For the fall-of-potential test to provide accurate results, the current and potential electrodes should be placed at minimum distances because:

Short distances increase the test current, improving accuracy
The resistance spheres of influence of the electrodes must not overlap (Correct answer)
Shorter distances reduce the effect of soil resistivity variations
Short distances are required by NETA testing standards only

Correct answer: The resistance spheres of influence of the electrodes must not overlap

Each electrode has a resistance area or sphere of influence in the surrounding soil. If electrodes are too close, their resistance areas overlap, causing mutual interference and incorrect readings.

Every grounding electrode has a zone of earth resistance surrounding it (the resistance area or sphere of influence) where significant voltage gradients exist. The fall-of-potential method requires placing the current electrode (C2) far enough from the test electrode that their resistance areas do not overlap — typically a minimum of 10 times the depth of the test electrode, and the potential electrode approximately 62% of the C1-C2 distance. If electrodes are too close, the measured resistance includes interaction effects, resulting in artificially high readings. For complex grounding systems like substations, specialized techniques may be required.

Question 3: The Wenner 4-pin method is used to measure:

Ground electrode resistance of a specific electrode
Soil resistivity, which is used to design grounding systems (Correct answer)
Contact resistance of buried ground conductors
Ground fault current in the grounding system

Correct answer: Soil resistivity, which is used to design grounding systems

The Wenner 4-pin (4-electrode) method measures soil resistivity (ohm-meters) rather than electrode resistance. Four equally-spaced probes are driven into the earth; current is injected through the outer probes and voltage measured by the inner probes.

The Wenner method (ASTM G57) uses four equally-spaced electrodes in a straight line at spacing 'a'. Current is injected through the outer two electrodes, and voltage is measured by the inner two. Soil resistivity p = 2 x pi x a x R, where R is the measured resistance and a is the electrode spacing in meters. By varying the electrode spacing, resistivity at different depths is measured. This data is used to design grounding systems per IEEE 80 (substations) and IEEE 142 (industrial). High soil resistivity requires more extensive grounding systems to achieve required ground resistance values.

Question 4: A ground resistance test shows a reading of 50 ohms for an industrial substation grounding system. This value is:

Acceptable per NETA standards for all industrial installations
Potentially too high — IEEE 80 and NETA recommend less than 1 ohm for substation grounding systems (Correct answer)
Excellent for a ground rod in high-resistivity soil
Normal for a grounding system in sandy soil

Correct answer: Potentially too high — IEEE 80 and NETA recommend less than 1 ohm for substation grounding systems

IEEE 80 and NETA standards recommend substation grounding resistance of less than 1 ohm (ideally much lower) to ensure adequate fault current flow and limit touch and step voltages during ground faults.

IEEE 80 (Guide for Safety in AC Substation Grounding) specifies that ground resistance alone is not sufficient — the critical parameters are tolerable touch and step voltages during ground fault conditions, which depend on fault current magnitude, clearing time, and ground resistance. General guidelines suggest substation ground resistance should be less than 1 ohm. The 50-ohm reading for a substation is clearly problematic: during a 10 kA fault, this creates a 500,000 volt ground potential rise, dangerous for personnel and connected equipment. Ground improvement with additional rods, ground enhancement material, or deep-driven electrodes would be required.

Question 5: The clamp-on ground resistance test method is advantageous because it:

Requires no auxiliary test electrodes and can test electrodes without disconnection from the grounding system (Correct answer)
Provides more accurate results than the fall-of-potential method
Can be used to measure soil resistivity simultaneously
Tests the ground electrode at full fault current levels

Correct answer: Requires no auxiliary test electrodes and can test electrodes without disconnection from the grounding system

The clamp-on method measures ground resistance by clamping around the ground electrode conductor without disconnecting it or driving auxiliary electrodes, making it fast and practical for production testing of multiple electrodes.

The clamp-on ground resistance tester uses a transformer clamp to inject a test current around the ground electrode lead and another clamp to measure the resulting voltage, calculating resistance without driving auxiliary electrodes. This allows testing in congested areas, does not require disconnecting the electrode from the grounding system, and is much faster for testing multiple electrodes. However, it requires at least two parallel ground return paths and cannot be used on isolated single electrodes. It is widely used for testing building ground rods and electrical infrastructure ground points.

Question 6: Touch potential and step potential are safety concerns during ground fault conditions because:

They describe the voltage a person could experience, which may cause ventricular fibrillation (Correct answer)
They are measures of the ground electrode's ability to dissipate fault current
They determine the maximum fault current the grounding system can handle
They describe the mechanical forces on grounding conductors during faults

Correct answer: They describe the voltage a person could experience, which may cause ventricular fibrillation

Touch potential is the voltage between a person's hands and feet when touching a grounded structure during a fault. Step potential is between two feet at different earth potentials. Both can cause dangerous body currents leading to ventricular fibrillation.

During a ground fault, a ground potential rise (GPR) develops across the grounding system, creating voltage gradients in the earth. Touch potential is the potential difference between the grounded metal structure a person touches and the ground surface where they stand. Step potential is the voltage between two points on the earth's surface separated by one step (typically 1 meter). IEEE 80 calculates tolerable body current limits based on body weight and fault clearing time, then back-calculates allowable touch and step voltages. Grounding design must ensure these values are not exceeded.

NETA Ground Resistance Testing Methods 1

The fall-of-potential method for measuring ground electrode resistance uses a minimum of: