What Is the Fall-of-Potential Method Used to Measure?

The fall-of-potential method is used to measure the electrical resistance of a grounding electrode (ground rod, grid, or plate) to the surrounding earth mass. It is the definitive, industry-standard technique for verifying that a grounding system can safely dissipate fault currents and lightning strikes into the soil.

The physics are straightforward: the tester injects a known alternating current (I) into the earth through the ground rod and a remote outer probe. It then measures the voltage drop (V) between the ground rod and an inner potential probe. Using Ohm’s Law (R = V/I), the meter calculates the resistance. Because the voltage gradient drops sharply near the electrodes and flattens out in the middle, the method relies on precise probe spacing to find that "flat" zone, ensuring you are measuring the true earth resistance rather than just the localized soil contact resistance.

⚠️ SAFETY & CAT RATING REQUIREMENT: Ground testing involves injecting current into the earth and working near service entrances. While the test voltage is low (typically 25V–50V AC at 128 Hz to avoid 50/60Hz interference), you must use a CAT III 600V or CAT IV 600V rated ground tester when working near energized substations or main panels. This protects you from transient overvoltages on the grounding conductor. Always de-energize the main panel and physically disconnect the grounding electrode conductor from the rod before testing to isolate the electrode from the facility's parallel ground paths.

Meter Setup and Probe Placement Protocol

Accurate fall-of-potential testing lives or dies by your probe placement. The industry relies on the 62% Rule, which positions the potential probe in the flattest part of the voltage gradient between the electrode and the current probe.

Meter Setup Block

  • Dial Position: 3-Pole Ground Resistance (often labeled as RE 3P, ρ, or 3P).
  • Lead Jacks:
    • E / C1 (Green): Connect directly to the isolated ground electrode under test.
    • S / P2 (Yellow): Connect to the inner potential probe.
    • H / C2 (Red): Connect to the outer current probe.
  • Range: Use Auto-ranging on modern digital testers. If using a manual analog meter, start at the 200Ω range and step down to 20Ω or 2Ω for better resolution.
  • Test Frequency: Set to 128 Hz (or the meter's default non-mains frequency) to prevent the meter from reading stray 50/60Hz ground currents as test voltage.

Numbered Steps for Probe Placement

  1. Isolate the Electrode: Unbolt the grounding electrode conductor from the ground rod. If you leave it connected, you will measure the parallel resistance of the building's bonded water pipes and structural steel, rendering the test useless.
  2. Place the Outer Probe (H/C2): Drive the red current probe in a straight line, at least 100 feet (30 meters) away from the electrode under test. For large ground grids, this distance must be 5 to 10 times the maximum diagonal dimension of the grid.
  3. Place the Inner Probe (S/P2): Drive the yellow potential probe in the exact same straight line, precisely 62% of the distance between the electrode and the outer probe (e.g., 62 feet away if H is 100 feet away).
  4. Drive Depth: Drive auxiliary probes at least 18 inches deep into bare, moist soil. Never drive them through concrete, asphalt, or gravel.
  5. Execute Test: Connect the color-coded leads, press TEST, and record the value. Move the inner probe 10 feet closer and 10 feet farther to verify the reading is stable (if the reading changes by more than 5%, your outer probe is too close; move it further out and repeat).

Expected Readings: Good vs. Bad Ground Resistance

What constitutes a "good" reading depends entirely on the application. NEC Article 250.56 sets the legal baseline for residential and commercial made electrodes, while IEEE and Fluke application guides recommend stricter tolerances for sensitive equipment.

Application / Facility Type Target Resistance Standard / Status Expected Meter Reading
Residential Service (Single Rod) < 25 Ω NEC 250.56 Minimum 10 Ω – 24 Ω (Pass)
Commercial / Industrial Panel < 5 Ω IEEE 142 (Green Book) 2 Ω – 4.9 Ω (Pass)
Telecom Central Office / Data Center < 1 Ω TIA-607 / IEEE 1100 0.5 Ω – 0.9 Ω (Pass)
Substation Ground Grid < 1 Ω IEEE 80 0.1 Ω – 0.8 Ω (Pass)
Any Facility (Reading > 25 Ω) > 25 Ω NEC Fail 26 Ω – 500+ Ω (Fail / Remediate)

Common Mistakes That Give Misleading Readings

When field technicians get bizarre readings, it is almost always due to one of these three physical setup errors:

  • Mistake 1: Overlapping Spheres of Influence (Probe Spacing Too Close). If the outer current probe is only 20 feet away from an 8-foot ground rod, their respective voltage gradients overlap. The meter will display an artificially low or highly erratic reading. Fix: Maintain the 100-foot minimum spacing for single rods.
  • Mistake 2: High Probe Contact Resistance. Driving probes into dry, sandy, or frozen soil prevents the test current from entering the earth. Modern meters will halt the test and flash High Rc (Current probe error) or High Rp (Potential probe error). Fix: Pour 1–2 gallons of water mixed with a pinch of salt or Epsom salt around the auxiliary probes to lower contact resistance. This does not alter the actual ground rod's resistance reading.
  • Mistake 3: Buried Parallel Paths. Testing a ground rod that is located 5 feet away from a buried metallic water pipe or a concrete-encased UFER ground will result in the meter reading the combined parallel resistance, masking a failing rod. Fix: Always unbolt the ground wire from the rod before testing.
💡 Pro-Tip for Rocky Soil: If you cannot drive the auxiliary probes into rocky terrain, lay them flat on the ground, cover them with metallic mesh or aluminum foil, and pour saltwater over the mesh. The meter will still complete the circuit and yield a valid fall-of-potential reading.

Decision Tree: Pass, Fail, or Remediate?

Use this decision path to determine your next physical action based on the numeric reading on your meter's display.

If Meter Reads... And Application Is... Then Execute This Action
< 5 Ω Commercial / Industrial Pass. Reconnect grounding conductor, torque lug to manufacturer spec (typically 25-30 in-lbs for #4 AWG), and log the value.
5 Ω – 24 Ω Residential / Light Commercial Pass (NEC Compliant). Reconnect and log. Note: If sensitive electronics are present, consider adding a second rod to drop below 5 Ω.
25 Ω – 50 Ω Any NEC-regulated facility Fail. Drive a second copper-bonded ground rod (e.g., Harger 102508 8ft rod) exactly 6 feet away from the first. Bond them with a continuous #6 AWG bare copper wire and re-test.
> 50 Ω High-resistivity soil (rock/sand) Fail. Multiple rods will not suffice. Terminate the decision path by excavating around the rod and installing a Harger GEM25A Ground Enhancing Material kit (or equivalent conductive cement). This permanently lowers soil resistivity and reliably pulls the reading below 5 Ω.

Tool Recommendations for Field Testing

To execute the fall-of-potential method correctly, you need a dedicated 3-pole ground tester. Standard multimeters cannot inject the required test current or filter out soil noise.

  • The Industry Standard (High-End): Fluke 1625-2 KIT (~$3,800). This is the benchmark for commercial and industrial testing. It offers 3-pole and 4-pole fall-of-potential, selective testing (no need to disconnect the ground wire in some configurations), and soil resistivity (Wenner method) testing. It is CAT III 600V rated and built for daily abuse.
  • The Prosumer / Residential Pick (Budget): Kyoritsu 4105A (~$450). An excellent, no-frills 3-pole tester that perfectly executes the fall-of-potential method. It lacks the advanced filtering and selective testing of the Fluke, but for verifying residential ground rods against the NEC 25-ohm threshold, it is highly accurate and cost-effective.
  • What NOT to Buy for This Method: Clamp-on ground testers (like the Fluke 1630-2). While fantastic for testing multi-grounded systems without disconnecting wires, clamp-on meters cannot perform the fall-of-potential method and will give infinite/open readings on an isolated, single ground rod.

Final Verdict: If you are a residential electrician or DIYer verifying a service upgrade, buy the Kyoritsu 4105A. If you are testing commercial facilities, data centers, or telecom sites where IEEE 5-ohm and 1-ohm thresholds apply, the Fluke 1625-2 is the only tool that provides the resolution and probe-rejection diagnostics required to pass an inspector's audit.