Ground Electrode Resistance Testing: Fall-of-Potential and Clamp Methods

What it is

A ground electrode resistance test measures the resistance between a grounding electrode, or a whole grounding system, and the remote earth. The electrode may be a driven rod, a buried ring, a concrete-encased electrode, a ground grid or a combination of these, and the number reported is the opposition the earth offers to current leaving the electrode. It is performed on the main grounding electrode of switchgear and switchboard assemblies and panelboards, on the grounding of low-voltage surge protective devices, on the grounding system of solar photovoltaic arrays, and on substation and building grounding systems generally. The measurement is made with a dedicated earth tester, or with a clamp-on instrument in situations where the electrode is part of a multi-point system. It is not the same as a continuity check of the bonding conductors inside a facility: the test of interest here is the connection to the earth itself, which is influenced by the soil and by the geometry of the electrode, as described in the guide IEEE 81-2025.

Why it is done

The ground electrode is where fault current, lightning and surge energy finally leave the electrical system. If its resistance is too high, a ground fault produces a higher voltage rise on the grounded equipment, which raises touch and step hazards for people and makes protective devices slower or less sure to operate. Surge protective devices depend on a low-resistance path to dump energy, and a poor ground weakens their protection of sensitive loads. Solar arrays need a sound ground for protection against lightning and for safe operation of ground-fault detection. A ground that was adequate at installation can degrade through corrosion of connections, drying or freezing of the surrounding soil, theft or damage of conductors, and construction work that disturbs the grid. Because these changes are invisible from the surface, the test is the only direct way to know whether the grounding system still performs as designed. Periodic testing also reveals seasonal swings, which matter for planning when an installation is most vulnerable.

The principle

Current injected into the ground spreads out from the electrode in all directions, and the opposition it meets is concentrated in the soil closest to the electrode, where the current density is highest. Far from the electrode the current passes through so large a volume of earth that additional resistance becomes negligible. That is why a ground resistance has a definite value, even though the earth is enormous. The classic fall-of-potential method places an auxiliary current probe at a distance and drives a test current between the electrode and that probe, while a second potential probe is moved along the line between them and the voltage is read. The potential at the correct position gives the electrode's resistance by the ratio of voltage to current. A plot of readings against probe position flattens in a region where the potential probe sits outside the influence of both the electrode and the current probe, and the flat part indicates a valid result. Clamp-on methods work differently: they induce a voltage in a loop formed by the electrode and the other parallel grounds in the system and measure the resulting current, which gives the loop resistance of the tested path. Soil moisture, temperature, salts and layers of different resistivity all alter the reading.

How it is performed

The electrode or system to be tested is disconnected from the equipment it serves where the method requires it, which means the test is done with the equipment de-energized or with the electrode safely separated under the owner's lockout procedures, because disconnecting a live system's ground is hazardous. The auxiliary probes are driven into firm soil in a line, a suitable distance from the electrode, with distances scaled to the size of the grounding system as the guide describes: a single rod needs a much shorter spread than a large grid. The earth tester is connected with its current and potential leads kept apart so they do not couple with each other, and it injects an alternating test current, usually at a frequency chosen to avoid power frequency interference. The potential probe is moved to several positions and a reading taken at each, so that the plateau can be checked and the result confirmed. Where space or access prevents driven probes, a clamp-on or stakeless instrument is used on an electrode in a multi-ground system, taking care that the measured loop is the intended one. Soil condition, recent rainfall and temperature are noted, and repeat tests are made at similar times of year so that results are comparable. All leads are discharged and removed, and the electrode connection is restored before the equipment is returned to service.

What is recorded

The record shows the measured resistance, the method used, the probe spacing and the layout relative to the electrode, along with a note of how the readings behaved as the potential probe was moved. It holds the soil condition and weather at the time, including recent rain or frost, the identity and calibration status of the instrument, and the as-found state of the electrode and its connection, for instance clean, corroded or recently disturbed. A sketch or description of where the probes were placed lets a later tester repeat the layout exactly, which matters because a different layout can change the result.

How results are judged

This page states no numeric limits. Results are judged by the shape of several criteria used together. The first is comparison with the design or specification target for the installation, which depends on the type of facility and the protection role of the ground and is set by the engineer of record, the owner and the governing codes. The second is trending against earlier results on the same electrode, taken with the same method, probe layout and season, because a rising value means deterioration even if it is still within target. The third is checking that the reading is valid: a plateau in the fall-of-potential plot, repeatability and agreement between methods where two were used. The fourth is seasonal context, since a reading taken in dry or frozen ground may be higher than the yearly average. The test methods are described in IEEE 81-2025, and the test is called for in ANSI/NETA ATS-2025 for acceptance and in ANSI/NETA MTS-2023 for maintenance, for the equipment classes named above. Refer to your own licensed copies of these documents for the governing values and wording.

Variations

Acceptance testing proves that a newly installed grounding system meets its design target and records the baseline, ideally before the soil has settled and before the equipment is connected, when the electrode can be isolated. Maintenance testing is done at intervals and compared with that baseline, with clamp-on methods often used for convenience on multi-ground systems. For switchgear and panelboards the question is the main grounding electrode of the building. For surge protective devices the grounding path feeds directly into protection performance. For solar arrays the ground system may span a large field of racking, and the test may cover the array grounding as well as the equipment ground. Small rods and large grids behave differently, so the method and the probe spacing change with the size of the system. Where soil is very dry, rocky or frozen, readings can be hard to obtain and are interpreted with the soil conditions in mind. The related continuity test checks the bonding between points inside the facility and complements this measurement.