Ground resistance testing measures how easily fault current can flow from a grounding electrode into the surrounding earth, and under the National Electrical Code a single rod, pipe, or plate electrode must read 25 ohms or less to stand on its own. Industrial sites following IEEE 142 typically aim much lower, often 1 to 5 ohms. The measurement itself is straightforward when done correctly, but probe placement, soil conditions, and a few code details trip up even experienced technicians.
What the Reading Has to Be
NEC Section 250.53 sets the performance requirement. For rod, pipe, and plate electrodes, the code defaults to requiring two electrodes. A single electrode is allowed only if testing confirms its resistance to earth is 25 ohms or less.
Read that sequence carefully, because many installers reverse it. The baseline expectation is two electrodes. The 25-ohm test only matters if you want to justify installing one instead of two. Once a supplemental (second) electrode is in place, the NEC does not require the combined system to meet 25 ohms. If your first rod tested at 40 ohms and you add a second rod at least six feet away, you are code-compliant without retesting. Contractors and some inspectors chase a number the code no longer demands in that configuration.
Industrial and commercial facilities hold themselves to tighter targets. IEEE Standard 142 recommends resistance values well below the residential 25-ohm threshold, often in the range of 1 to 5 ohms depending on equipment sensitivity.1IEEE Standards Association. IEEE 142-2007 – IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems Data centers and facilities with sensitive electronics may target 1 ohm or less.
Running a Fall-of-Potential Test
The fall-of-potential method is the standard technique. It works on any electrode, isolated or connected, and produces the most reliable reading when the probes are placed correctly. You need a dedicated earth ground tester, two auxiliary metal stakes, and the color-coded lead set that comes with the instrument.
Start by disconnecting the grounding electrode from the rest of the electrical system. You are measuring the electrode’s connection to earth, not the building’s wiring, and leaving everything tied together will skew the reading. Attach one lead directly to the electrode under test.
Drive the current probe into the soil at a distance roughly ten times the length of the electrode. For a standard 8-foot rod, that means placing the current probe about 80 to 100 feet away. This spacing lets the current probe create its own zone of influence in the soil without overlapping the electrode you are testing.
Place the potential probe in a straight line between the electrode and the current probe. The mathematically optimal position is 61.8% of the distance to the current probe, rounded in practice to 62%. If the current probe is 100 feet out, the potential probe goes at the 62-foot mark.2Metrel. Earth Resistance Measurement and 62% Rule At that location the voltage gradient in the soil flattens out and the reading represents the true electrode-to-earth resistance rather than an artifact of probe placement.
Activate the tester and record the reading. Then take two more measurements with the potential probe moved to roughly 52% and 72% of the distance. If all three readings are close, the potential probe was sitting in the flat zone of the resistance curve and the 62% reading is valid. If they diverge, the current probe is too close and its zone of influence is overlapping the electrode. Move the current probe farther out and repeat.
Check every lead wire connection before and during the test. A loose clip or corroded contact adds intermittent resistance that corrupts the measurement without showing anything obvious on the display.
Clamp-On and Two-Point Methods: Where They Fit
Two alternative methods exist, and each has real limits worth knowing before you rely on the number it gives you.
A clamp-on tester wraps around the grounding conductor without disconnecting anything and without driving auxiliary stakes. It induces a voltage signal and measures the resulting current through the complete grounding circuit. Speed is the appeal. The catch is that it only works where multiple ground paths exist in parallel, because the tester needs a return path through the soil. On an isolated single-rod system it will not produce a valid reading.3AEMC Instruments. Fall-of-Potential vs. Clamp-On Ground Resistance Testing Clamp the meter above a continuous metallic path instead of below the point where current enters the soil and the signal travels through wire rather than earth, producing a falsely low reading, often under one ohm.
The two-point method uses an existing low-resistance ground such as a water pipe as a reference electrode. It suits paved urban sites where you cannot drive auxiliary stakes. The reading is the sum of both resistances, so the reference electrode must be much lower than the one being tested. This method cannot reliably measure resistances below about 10 ohms and is best treated as a rough check rather than a compliance measurement.
Why Soil and Season Change the Answer
The electrode is only half the circuit. The soil around it decides how easily current flows into the earth, and soil resistivity varies enormously with composition, moisture, and temperature. Clay and loam hold moisture and conduct well. Dry sand or gravel can be a thousand times more resistive. Bedrock is worse still.
Moisture matters as much as soil type. During dry seasons, resistivity can increase by a factor of ten compared to wet conditions. Freezing has an even sharper effect: the top meter of soil can jump one or two orders of magnitude in resistivity when frozen. A system that tests perfectly in July may perform poorly in January.
Test during a wet season and the measured value may significantly understate the worst-case resistance your system will face the rest of the year. Engineers designing permanent systems apply seasonal correction factors, or they simply schedule testing during dry or cold periods to capture conservative numbers.
Safety When You Disconnect the Electrode
Disconnecting a grounding electrode to test it temporarily removes a safety system. OSHA regulation 29 CFR 1926.962 covers this directly: employers must ensure employees use insulating equipment when removing grounds, that workers are isolated from any hazards that could arise if the previously grounded equipment becomes energized, and that additional protective measures are in place for the duration of the test.4Occupational Safety and Health Administration. Grounding for the Protection of Employees 1926.962
On systems above 600 volts, the grounding device must be detached from the line using a live-line tool before the ground-end connection is removed. On systems at 600 volts or less, insulating gloves and other equipment can substitute for a live-line tool, provided the employer confirms the line is de-energized at the time of disconnection or demonstrates that employees are otherwise protected.
Verify de-energization with a voltage tester before touching any conductor. Wear rubber-soled footwear. Do not test during active thunderstorms; a nearby strike can induce dangerous voltages through the soil without a direct hit.
When the Reading Is Too High
If the measured resistance exceeds your target, three approaches can bring it down without starting over.
Driving deeper is often the most effective single change. Doubling the length of a rod reduces its resistance by roughly 40%. Reaching the water table, where soil is permanently saturated, delivers the biggest gain. Sectional couplings allow driving standard 8-foot rods to 16 or 20 feet where geology permits.5AEMC Instruments. Understanding Ground Resistance Testing
Adding parallel rods helps, but not proportionally. Two rods do not cut resistance in half unless separated by several rod lengths. The recommended minimum spacing is one rod length between each pair, arranged in a line, triangle, or square. Rods placed inside the perimeter of an existing group contribute little; the useful ones sit on the outer edge.
Ground enhancement material (GEM) backfilled around the electrode can lower resistance by 40% to 80% depending on the product and soil. Bentonite clay, carbon-based compounds, and specialized chemical backfills retain moisture and hold their conductivity as surrounding soil dries out.6MDPI. Ground Enhancement Materials for Grounding Systems: A Systematic Review of Factors, Technologies and Advances Older salt-solution treatments work short-term but cause corrosion and need replenishment; modern GEM products are designed to be permanent and non-corrosive.
Recording Results and Retesting
Every test should be documented with the date, weather, soil conditions (wet, dry, frozen), the specific equipment tested, the serial number of the testing instrument, and the name of the person performing the measurement. A measurement without documentation has limited value during an inspection, even if the installation is sound.
Historical readings matter as much as any single number. If a rod read 8 ohms three years ago and now reads 18, something has changed underground: corrosion, soil disturbance, or a dropped water table. Trends catch failures before they arrive.
NFPA 70B sets a baseline retest interval of 60 months for grounding systems in good condition with no history of problems. Systems that have shown deterioration, missed maintenance cycles, or operate in harsh environments should be tested every 36 months. Visual inspections of accessible connections should happen annually regardless. Facilities with critical loads such as hospitals, data centers, or chemical plants often test annually even where standards would allow longer.
What Non-Compliance Costs
OSHA can cite employers for grounding deficiencies under its electrical safety standards. The maximum penalty for a serious violation is $16,550 as of January 2025, adjusted annually for inflation.7Occupational Safety and Health Administration. OSHA Penalties Willful or repeated violations carry penalties up to $165,514 per violation. Grounding failures are among the most frequently cited OSHA violations in construction and general industry.
Local building inspectors can fail an installation that does not meet NEC requirements, delaying occupancy permits and triggering re-inspection fees. Municipal penalties for electrical code violations vary by jurisdiction and can reach several thousand dollars for repeat offenses.
Insurance is the quieter risk. Most commercial and homeowner policies assume the building meets applicable electrical codes. A grounding deficiency that contributes to a fire or equipment failure may give the insurer grounds to deny or reduce a claim. Current test records demonstrating compliance are among the simplest defenses against that outcome.