Earthing System Testing: Ground Resistance and Soil Resistivity is the set of field measurements that confirm an earthing (grounding) system can carry and dissipate fault and lightning current fast enough to keep touch and step voltages safe and to let protective devices operate. It covers electrode resistance to remote earth, the resistivity of the soil the electrodes sit in, and the continuity of the bonding that ties equipment together. Good numbers on paper mean nothing if the connections have corroded or the water table has dropped, which is why this is a periodic test, not a one-time commissioning check.
An earthing system does three jobs: it provides a low-impedance path so overcurrent devices trip on a fault, it clamps equipment enclosures near earth potential so people are not exposed to dangerous voltage, and it gives lightning and switching surges a route to ground. All three depend on a resistance value that engineers cannot see. Soil dries out, freezes, or is disturbed by excavation; buried electrodes and clamps corrode; bonding jumpers get removed during maintenance and not replaced. Testing is the only way to know the as-found condition. Reference practice is set out in IEEE Std 81 for measurement technique and IEC 60364-6 for installation verification.
The fall-of-potential test is the reference method for a single electrode or an isolated array. The electrode under test is disconnected from the system. A current probe (C) is driven into the soil a long distance away, and a potential probe (P) is moved between them while the instrument injects a known current and reads the voltage. Plotting resistance against P position gives a flat plateau; the electrode resistance is read at the plateau, which for a uniform soil falls near the 62 percent point of the C distance. The C probe must be far enough out that the electrode and current probe resistance areas do not overlap, often 30 to 50 metres or more for large systems. This is the most accurate method but needs space and probe access.
Where an electrode cannot be disconnected and sits in a multi-grounded network (utility neutrals, telecom, interconnected building steel), a clamp-on tester is practical. The clamp induces a voltage on the loop and measures the resulting current, returning the loop resistance, which is dominated by the electrode under test when the rest of the network is many parallel low-resistance paths. It is fast and non-intrusive and keeps the system energised, but it measures a loop, not a true electrode-to-earth value, and it can mislead on an isolated single electrode. Use it for routine screening and the fall-of-potential method for definitive figures. When an elevated reading points to a buried cable defect rather than the electrode itself, pair the survey with cable fault location to find the fault.
Soil resistivity drives electrode design before anything is buried. The Wenner four-pin array uses four equally spaced pins in a line at spacing a. Current is injected through the outer pins and voltage measured across the inner pins. When the pin depth is small relative to a, apparent resistivity simplifies to rho = 2 x pi x a x R, where R is the measured resistance. Repeating with increasing spacing probes deeper strata, because spacing a roughly corresponds to the depth being averaged. The resulting resistivity-versus-depth curve tells the designer how deep to drive rods and whether a grid, deep well, or chemical electrode is needed.
| Test | What it measures | Primary purpose |
|---|---|---|
| Fall-of-potential (3-point) | True electrode resistance to remote earth | Definitive electrode acceptance value |
| Clamp-on loop | Loop resistance in a multi-grounded network | Fast routine screening, live system |
| Wenner four-pin | Apparent soil resistivity vs depth | Electrode and grid design input |
| Bonding continuity | Resistance of jumpers and connections | Confirm equipotential bonding integrity |
Targets depend on the installation. NEC 250.53(A)(2) requires a single made electrode (rod, pipe, or plate) reading above 25 ohms to be supplemented by a second electrode. IEEE 142 practice for larger industrial and commercial systems commonly aims for 5 ohms or lower, and sensitive or high-current sites often specify 1 to 5 ohms. Lower resistance means a larger fault current for a given voltage, faster operation of protection, and lower touch voltages. It also supports selective, correctly graded ground fault protection. A low earth resistance further improves the performance of installed surge protective devices (SPD): types and coordination, since surge energy is only diverted effectively into a low-impedance earth.
Electrode resistance is only half the picture. Equipment bonding, main earthing terminals, and jumpers must present a low, stable resistance so the whole installation stays at one potential during a fault. A micro-ohmmeter or a bonding tester passing a defined current (commonly 10 A or more) verifies each connection. Loose lugs, painted contact faces, and corroded joints raise resistance and are a frequent as-found defect. Continuity testing should confirm that structural steel, cable armour, and enclosure earths are actually connected, not just assumed.
Because soil and connections change, earthing tests are repeated on a schedule, typically annually for critical sites and after any major electrical work or earthworks. Trending results over time is more valuable than any single reading: a slowly rising electrode resistance flags corrosion or drying soil before it becomes a hazard. Logging each measurement, probe geometry, weather, and soil condition against the asset in a CMMS keeps the history auditable. Book a Fabrico demo to see how earthing tests, intervals, and trends can be scheduled and tracked as recurring maintenance tasks.
Commonly once a year for critical and high-risk installations, and after any excavation, electrode replacement, or major electrical modification. Trend the readings so a gradual rise in resistance is caught early rather than at the next failure.
In uniform soil the resistance curve between the electrode and the current probe flattens where the two resistance areas no longer overlap, which occurs near 62 percent of the current-probe distance. That plateau gives the true electrode resistance; if no flat region appears, the current probe is too close.
It depends on the code and application. NEC requires supplementing a single made electrode above 25 ohms, while IEEE practice for larger systems targets 5 ohms or less, with sensitive sites specifying 1 to 5 ohms. Lower is always better for protection speed and touch voltage.
Yes, with a clamp-on tester on a multi-grounded network, which measures loop resistance while the system stays live. For a definitive isolated electrode value, use the fall-of-potential method, which does require disconnection.
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