Earthing Resistance Measurement: Why Driven-Rod Tests Often Give Misleading Results

The fall-of-potential (FOP) method, described in IEEE 81-2012, Clause 8.2 and AS/NZS 3000:2018, Clause 5.3.3, is the most widely used technique for measuring the resistance of an earth electrode to “remote earth.” The setup is:

1. The earth electrode under test (E) is disconnected from the installation.
2. A current electrode (C) is driven into the ground at a distance (typically 30–50 m) from E.
3. A potential electrode (P) is placed between E and C at various distances from E.
4. A test current is injected between E and C, and the voltage is measured between E and P.
5. Resistance = V(E-P) / I(E-C) at each P position.

The “62% rule” states that the true earth resistance is read when P is placed at 61.8% of the distance from E to C. This is mathematically exact under one critical assumption: the soil is homogeneous (uniform resistivity in all directions to infinite depth).

The assumption of homogeneous soil is almost never valid in practice. Soil resistivity varies with:

• Depth: Typical sites have 2–5 distinct soil layers with resistivities varying by 1–2 orders of magnitude.
• Lateral position: Rock outcrops, clay pockets, fill material, and underground water courses create lateral variation.
• Moisture content: The top 1–3 m of soil varies dramatically with rainfall and water table depth.
• Temperature: Frozen ground has resistivity 5–10× higher than unfrozen ground (less relevant in tropical locations).

When the soil is non-homogeneous, the 62% rule produces an incorrect result. The error can be in either direction — the measured resistance may be higher or lower than the true value, depending on the resistivity profile between E and C.

Industrial and mining sites contain extensive buried metalwork that affects earth resistance measurements: reinforced concrete foundations, buried water pipes, buried cable armour, structural steel connected to ground, abandoned electrodes, and underground metal structures. This metalwork creates parallel earth return paths that reduce the measured resistance below the true electrode resistance.

The effect is insidious because the test instrument cannot distinguish between current flowing through the soil (the intended measurement path) and current flowing through buried metal (an unintended short circuit). The result appears to show an excellent earth resistance, but the actual electrode-to-soil contact resistance may be much higher.

Field example — Processing plant substation:

The true electrode resistance was 9.1 Ω — nearly 3× the initial measurement. The difference was entirely due to buried reinforced concrete footings from a decommissioned equipment pad, which were providing a low-resistance return path that made the electrode appear better than it was. The footings were not documented on any as-built drawings.

Per IEEE 81-2012, Clause 8.2.4, test probe placement should avoid known buried metalwork. But undocumented buried structures cannot be avoided if their presence is unknown. This is a fundamental limitation of the method that no procedural control can fully mitigate.

We recorded quarterly earth resistance measurements on a grid of 8 test electrodes at a mine site in eastern Indonesia over a 3-year period. The site has a distinct wet season (November–April, 250–400 mm/month rainfall) and dry season (May–October, 20–80 mm/month).

Seasonal earth resistance variation (8-electrode average):

The peak-to-peak variation is 2.4 Ω to 4.8 Ω — a ratio of 2.0×, or a 40% swing above the annual mean. This variation was measured on the same electrodes with the same test equipment, eliminating measurement error as a factor.

The practical implication: an earth resistance measurement taken in February (wet season) that reads 2.4 Ω would read 4.8 Ω if taken in August. If the design requirement is 5 Ω (per AS/NZS 3000:2018, Clause 5.6.2.2), the installation passes comfortably in the wet season but approaches the limit in the dry season. An installation that just meets the requirement in the wet season (say, 4.5 Ω) would fail in the dry season (projected 9.0 Ω).

IEEE 81-2012, Clause 7.1 notes that measurements should be taken during the period of minimum soil moisture to obtain worst-case values. In practice, measurements are frequently taken during commissioning, which may coincide with any season, and the seasonal correction is rarely applied.

The Wenner 4-pin method, described in IEEE 81-2012, Clause 9.3, measures soil resistivity rather than electrode resistance. Four equally spaced probes are driven into the ground in a straight line. Current is injected through the outer two probes, and voltage is measured across the inner two. The apparent resistivity at a depth approximately equal to the probe spacing is:

ρ = 2πa × R

Where a is the probe spacing (m) and R is the measured resistance (Ω). By repeating the measurement at increasing probe spacings (typically 1, 2, 3, 5, 7, 10, 15, 20, 30 m), the engineer builds a soil resistivity profile with depth.

Advantages over the 2-pin FOP method:

1. Reveals soil layering. The resistivity vs. depth profile shows distinct layers that the FOP method cannot detect. A low-resistivity clay layer at 3 m depth, underlain by high-resistivity rock, produces dramatically different earthing performance than homogeneous clay.
2. Less susceptible to buried metalwork interference. Because the measurement uses short probe spacings (1–30 m), the influence zone is smaller and more controllable. Multiple traverses at different orientations and locations average out local anomalies.
3. Predictive capability. With a calibrated soil model (2–4 layer), the engineer can predict the resistance of any electrode geometry (rods, grids, rings) before installation, optimizing the design. This is not possible with a simple FOP measurement, which only tells you what exists now.
4. Multiple traverses provide statistical confidence. A minimum of 4 traverses (N-S, E-W, and two diagonal) at each measurement location is recommended per IEEE 81-2012, Clause 9.3.3. Divergence between traverses indicates lateral soil variation or buried metalwork.

Wenner survey results from the mine site (10 traverses, averaged):

This profile shows a 45 Ω·m surface layer over high-resistivity rock — a common pattern in tropical mining sites. The shallow low-resistivity layer means that earth rods longer than 3 m provide diminishing returns, as additional length penetrates into 340+ Ω·m rock. Horizontal earth grids or radial electrodes in the surface layer would be more effective than deep-driven rods.

A rigorous earthing study uses both methods: the Wenner survey characterizes the soil, the FOP test verifies the installed electrode. When the two methods give consistent results, confidence in the measurement is high. When they diverge, something is wrong — and it is almost always the FOP measurement that is compromised.

Common causes of divergence and diagnostic indicators:

At the mine site, we established a protocol: any FOP measurement that diverged by more than 25% from the Wenner model prediction was flagged for investigation. Over 3 years, 18% of FOP measurements triggered this flag. In every case, the investigation revealed either buried metalwork, electrode degradation, or measurement error (probe in fill material rather than natural soil).

1. Always conduct a Wenner 4-pin soil resistivity survey before designing the earthing system. Per IEEE 81-2012, Clause 9 and AS/NZS 3007:2013, Clause 13.2 for mining installations. A minimum of 4 traverses at each measurement location, with probe spacings from 1 m to at least twice the maximum electrode depth.
2. Take FOP measurements during the driest period of the year. Per IEEE 81-2012, Clause 7.1. If measurements are taken during the wet season, apply a seasonal correction factor. For tropical sites with distinct wet/dry seasons, a correction factor of 1.5–2.0× is appropriate based on our field data.
3. Take FOP readings at a minimum of 3 azimuths (120° apart). If the readings diverge by more than 20%, buried metalwork or non-homogeneous soil is present, and the lowest reading should not be trusted.
4. Compare FOP results against Wenner model predictions. Divergence exceeding 25% warrants investigation before accepting the measurement.
5. Document the measurement conditions. Record date, recent rainfall, soil moisture (if measured), ambient temperature, probe locations (GPS), and instrument serial number. This allows future measurements to be compared on a normalized basis.
6. For critical installations (substations, processing plants), install permanent test electrodes. Flush-mounted test pits with pre-installed current and potential probes at fixed positions allow repeatable measurements without the variability of manually driven probes.

Standards referenced: IEEE 81-2012 (Clauses 7, 8, 9), AS/NZS 3000:2018 (Clause 5.3, 5.6), AS/NZS 3007:2013 (Clause 13), IEC 60364-5-54:2011 (Clause 542), BS 7671:2018+A2 (Regulation 542).