10.1 Ground-Electrode Testing: Fall-of-Potential & Clamp-On

Why Ground-Electrode Resistance Matters

Every protective relay, surge arrester, and equipment grounding scheme assumes a low-impedance path to earth. A ground electrode that has corroded, dried out, or lost contact with surrounding soil can leave fault current with no return route, raising touch-and-step voltages and defeating protective device coordination. ANSI/NETA ATS and MTS therefore require a ground-electrode resistance measurement at acceptance and at periodic maintenance intervals, performed per IEEE Std 81-2012 (Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System).

The two field methods you must know for NETA Level 2 are the fall-of-potential (FOP, 3-point) method and the clamp-on (stakeless) method. They use completely different physics and have different limitations.

Fall-of-Potential (3-Point) Test Setup

The FOP method uses three contact points:

1. Electrode under test (E) - the ground rod, grid connection, or grounding electrode being measured.
2. Current probe (C, sometimes H) - an auxiliary stake driven into earth at a distance D from E. The tester injects a known AC current between E and C, forcing current through the soil.
3. Potential probe (P, sometimes S) - an auxiliary stake moved along a straight line between E and C. At each position the tester measures the voltage between E and P and calculates R = V/I.

The current probe distance D should be at least 5 times the maximum dimension of the grounding system under test so the two hemispherical "voltage funnels" around E and C do not overlap too heavily. For a single 8-ft rod, D of 80-100 ft is typical; for a substation grid, D may be several hundred feet.

Plotting the Resistance Curve

Moving P from near E outward toward C in equal increments (say 10%, 20%, 30%... of D) and recording R at each point produces a curve that ideally looks like an S-shape with a flat middle section - the plateau. The resistance value on that plateau is the true resistance of the electrode under test.

If there is no clear plateau, the current probe is too close, soil is highly non-uniform, or a buried metallic object is distorting the field. Move C farther out or repeat the test in a different direction.

The 62% Rule Explained

Annex C of IEEE 81-2012 derives the 62% rule (more precisely 61.8%). Under three assumptions - (1) uniform (homogeneous) soil, (2) hemispherical electrodes, and (3) the distance D large enough that E and C each behave as isolated hemispheres - the equation for the voltage measured between E and P equals the true ground potential rise of E alone when:

x = 0.618 x D

At that exact point, the overlap of the two voltage funnels cancels in such a way that the measured resistance equals the true resistance of the electrode under test, free of the mutual resistance contribution from C. In a perfectly uniform soil, a single reading at the 62% point gives the same value as the plateau.

Why 62% and Not 50%?

At 50% of D, the potential probe sits symmetrically between E and C, so the measured voltage includes equal contributions from both electrodes' ground potential rises - it does not isolate E. Pushing P to 61.8% moves it past E's zone of influence while keeping it short of C's, landing in the sweet spot where the reading reflects E alone.

Assumptions and When the Rule Breaks

The 62% rule is a quick-check shortcut, not a guarantee. It fails when:

• Soil is non-uniform - layered earth, buried rock, water table, or nearby buried metal shifts the plateau location.
• The grid is large or irregular - a substation grid's electrical center may not be its geometric center; use the slope, intersecting-curves, or four-potential method instead.
• A parallel return path exists - transmission towers with overhead ground wires, pole grounds bonded to the neutral, or any interconnected grounding that lets test current return without going through C. Here I_E does not equal I_H, and the 62% point must be corrected with a coefficient k_i = I_E / I_H (for k_i = 0.4 the probe moves to about 46%). IEEE 81 Chapter 8.2.2.6 covers this correction.

Best field practice: run the full FOP curve when you can, and use the 62% position as one validation point inside the plateau.

Clamp-On (Stakeless) Ground Tester

A clamp-on ground resistance tester (Fluke 1623/1625, AEMC 6417, and similar) looks like a large clamp ammeter. It clamps directly around the ground conductor or rod and requires no auxiliary stakes. Inside the clamp head are two transformers:

1. A voltage (drive) transformer that induces a known AC voltage into the loop formed by the electrode and earth.
2. A current (sense) transformer that measures the resulting current flowing in that loop.

The tester divides induced voltage by measured current and displays resistance directly. The earth itself, plus any parallel ground paths back to the source, forms the return loop.

When Clamp-On CANNOT Be Used

The clamp-on method requires a closed loop with a parallel return path through earth. It will not work on:

• An isolated single ground rod with no other grounding connection - there is no return path, so no current flows and the reading is invalid or shows an open circuit.
• A building whose main bonding jumper is open or where the neutral-ground bond is missing.
• A recently driven rod before it is bonded to the rest of the grounding system.

Clamp-on is ideal for pole grounds, transmission tower grounds, and service entrances where the utility neutral provides the return path. Always verify the reading by clamping at a second point or comparing to a prior FOP value.

Acceptance Values

NETA ATS requires the measured value be compared to the owner's specification and to the design value. A reading significantly above the baseline warrants investigation of corrosion, soil drying, or broken connections.