Why Your Earth Electrode Resistance Test Passes — But Your Earthing System Fails

The scene repeats on every project I've worked on. The electrician drives an earth rod, connects the resistance tester, reads 4.8 Ω, writes "PASS" on the test sheet, and moves on. The design called for "less than 5 Ω" and the measured value is less than 5 Ω. Job done.

Except it was measured after three days of rain. In the dry season — which in Sumbawa, Indonesia lasts six months — that same electrode will read 30–50 Ω. The earthing system that "passed" in February will fail every test from June to November. More importantly, it won't perform its safety function during a fault.

This is one of the most common and dangerous oversimplifications in electrical installation practice: treating earth electrode resistance as a single number rather than a range that varies with soil conditions, and treating a passing resistance value as proof that the earthing system is adequate.

What the Resistance Test Actually Measures

The standard earth electrode resistance test — the 3-point fall-of-potential method — measures the resistance between the electrode under test and the mass of earth surrounding it. A test current is injected between the electrode and a remote current spike, and the resulting voltage is measured between the electrode and a remote potential spike.

This gives the electrode-to-soil resistance — the resistance of the soil immediately surrounding the electrode. For a single vertical rod, approximately 90% of this resistance is concentrated in the soil within 1–2 metres of the rod surface.

BS 7430, Clause 10 — Measurement of earth electrode resistance

What this test does NOT measure:

• The resistance of the earthing conductor from the electrode to the main earth terminal
• The impedance of the earth fault return path back to the source
• The touch voltage or step voltage at the electrode location
• How the resistance changes with seasonal soil moisture variation

The Seasonal Variation Problem

Soil resistivity is dominated by moisture content and temperature. The same soil can have dramatically different resistivity values depending on the season:

The IEEE 80 standard recommends applying a seasonal correction factor to account for this variation. AS/NZS 3000 and BS 7430 both note the importance of testing under worst-case (dry) conditions or applying correction factors.

IEEE 80, Clause 13.4 — Soil resistivity measurements and seasonal variation

The Correct Approach: Design First, Then Verify

The proper engineering approach to earthing is:

Step 1: Measure Soil Resistivity (Not Electrode Resistance)

Use the Wenner 4-pin method to measure soil resistivity at the installation site. This method measures the apparent resistivity of the soil at depth, not just the surface layer.

Where a is the pin spacing (equal to the measurement depth) and R is the measured resistance between the inner pins.

Take measurements at multiple spacings (1m, 2m, 5m, 10m) to build a soil resistivity profile. Take measurements in both wet and dry conditions if possible.

Step 2: Design the Earthing System

With the soil resistivity data, calculate the required earthing system using standard formulas:

Where ρ is soil resistivity, L is rod length, and d is rod diameter.

Use the DRY season resistivity for design — this ensures the system works year-round, not just when it rains.

Step 3: Verify the Complete Earth Fault Path

The electrode resistance is only one component of the total earth fault path. Verify that:

• Earthing conductors are correctly sized for fault current and duration
• All connections are tight and corrosion-free
• The total earth fault loop impedance (Zs) meets the requirements for automatic disconnection

AS/NZS 3000, Clause 5.6 — Earth fault loop impedance verification

Why Multiple Electrodes Don't Simply Divide the Resistance

Engineers often assume that two electrodes in parallel give half the resistance of one electrode. This is approximately true only when the electrodes are separated by a large distance (more than twice the electrode length).

In practice, electrodes installed close together have overlapping "resistance zones" — the soil between them is shared, and each electrode's current field interferes with the other's. This is called the shadow effect or proximity effect.

The actual reduction factors for parallel rods (at a spacing equal to the rod length):

BS 7430, Clause 8.3 — Parallel earth electrodes and spacing requirements

Touch and Step Voltage: The Real Safety Criterion

Earth electrode resistance in ohms is a convenient number to measure, but it's not the actual safety criterion. What matters is whether a person standing near the electrode (or touching equipment connected to it) during a fault can experience a dangerous voltage.

Touch voltage is the voltage between a person's hand (touching faulted equipment) and feet (standing on the ground). Step voltage is the voltage between a person's two feet, one step apart, standing on the ground above the earthing system during a fault.

The permissible touch voltage depends on the fault duration:

• 0.2s fault duration: ~220V permissible (the body can tolerate higher voltages for shorter durations)
• 0.4s fault duration: ~65V permissible
• 5.0s fault duration: ~50V permissible (long duration, low voltage limit)

IEC 60364-4-41, Clause 411.3 — Protective provisions for TT systems

This is why a 5Ω electrode that seems "fine" might still be dangerous. If the fault current through that electrode is 200A (from a 1000V MV system), the electrode potential rise is 1,000V — and the touch voltage could be several hundred volts, persisting for the full fault clearance time.

Ring Earth Electrodes vs Rod Electrodes vs Grids

The choice of earthing arrangement depends on the soil conditions and the application:

Rod electrodes (vertical, 2.4–6m long):

• Best for sites where deep soil is more conductive than surface soil
• Limited by rod length — driving rods deeper than 6m requires sectional rods
• Each rod's resistance zone is small, so multiple rods need wide spacing

Ring earth electrodes (horizontal conductor buried at 0.5–1m depth around the perimeter):

• Best for large buildings and substations
• Provides uniform earth potential across the structure footprint
• Less affected by deep soil conditions — uses the surface layer
• Required by AS/NZS 3000 for new installations in some jurisdictions

Earth grids (mesh of conductors buried under the installation):

• Used for substations, industrial plants, and sites with high fault currents
• Provides the best touch and step voltage performance
• Most expensive, but necessary where fault currents are high

AS/NZS 3000, Clause 5.6.3 — Types of earth electrodes

The Mining Context: Volcanic Soil Conditions

At a large copper-gold mining operation in Indonesia, the earthing design was one of the most challenging aspects of the electrical installation. The site sits on volcanic soil — highly variable resistivity, ranging from 50 Ω·m in saturated alluvial areas to over 1,500 Ω·m in dry weathered andesite.

We measured a rod electrode at 2.8 Ω during the wet season (December). The same electrode measured 35 Ω in August. The specification called for <1 Ω at the main substation — achievable in December, impossible in August with simple rods.

The solution was a combination approach:

• Ring earth electrode (bare copper, 50mm², buried 600mm) around the substation perimeter
• Vertical rods at 3m spacing driven to 6m depth
• Bentonite backfill in each rod borehole (bentonite retains moisture, stabilising the resistance)
• Horizontal radials extending 30m from the substation in 4 directions

The final result: 0.35 Ω in wet season, 1.8 Ω in dry season — a 5:1 ratio instead of the 12:1 ratio we'd have seen with rods alone. The bentonite and ring electrode provided the stability that individual rods could not.

Practical Recommendations

1. Never design an earthing system from a single resistance measurement. Measure soil resistivity using the Wenner method, and account for seasonal variation.

2. Test during the worst case. If you can only test once, test during the driest period. A "pass" in the dry season is reliable. A "pass" in the wet season is not.

3. Verify the complete earth fault path, not just the electrode resistance. The earth fault loop impedance (Ze + R₁ + R₂) determines whether faults are cleared safely.

4. Consider touch and step voltages for installations with high fault currents (substations, industrial plants, MV systems). Electrode resistance alone doesn't guarantee safety.

5. Use ring earth electrodes where possible — they provide more stable, lower resistance than individual rods and better touch voltage performance.

6. Document the soil conditions and design basis. Future modifications to the installation may change the fault current, and the earthing system adequacy needs to be re-evaluated.

Related Resources

• Buncefield: Lightning Protection Calculation — Why earth electrode resistance matters for lightning protection
• Sayano-Shushenskaya: Earthing System Failure — Earthing failure in a real-world incident
• Earthing Systems: TN-S vs TN-C-S vs MEN — How earth electrode requirements differ across standards
• View all worked examples →