What is an acceptable earth electrode resistance value?

BS 7671 does not specify a single maximum earth electrode resistance value — instead, it requires that RA x IΔn does not exceed 50V, where RA is the earth electrode resistance and IΔn is the rated residual operating current of the RCD. For a 30mA (0.03A) RCD, the maximum RA is 50 / 0.03 = 1667 ohms. For a 100mA RCD, the maximum is 500 ohms. For a 300mA RCD, the maximum is 167 ohms. However, in practice, many competent person scheme providers and local authority building control departments prefer RA values below 200 ohms for TT installations, and some specify maximum values of 100 ohms. Lower RA values provide a greater margin of safety and reduce touch voltages under fault conditions.

Why is my earth resistance reading too high?

The most common cause of high earth resistance is dry or sandy soil, which has high resistivity. Other causes include insufficient rod depth (the rod has not reached moist soil or a low-resistivity layer), poor contact between the rod and the surrounding soil (air gaps caused by loose backfill or soil shrinkage), corroded or damaged earth electrodes, loose connections between the earth electrode and the earthing conductor, and seasonal variations (soil moisture reduces in summer, increasing resistance). If your reading is unexpectedly high, first check all connections for tightness and corrosion, then consider soil conditions and rod depth.

How do I measure earth electrode resistance?

Earth electrode resistance is measured using the fall-of-potential method (also called the 3-point method). You drive two temporary test spikes into the ground at measured distances from the earth electrode under test. The instrument passes a current between the electrode and the furthest spike (current spike) and measures the voltage between the electrode and the nearer spike (potential spike). From the current and voltage, it calculates the resistance. The potential spike should be positioned at 61.8% of the distance between the electrode and the current spike for the most accurate result. Most modern multifunction testers include an earth electrode resistance function that automates this calculation.

Can I use multiple earth rods to reduce resistance?

Yes. Connecting multiple earth rods in parallel reduces the overall earth electrode resistance. The rods must be spaced at least 2.5 times their driven depth apart to be effective — if placed too close together, their resistance zones overlap and the reduction is minimal. As a rough guide, two rods spaced correctly will reduce resistance to approximately 60% of a single rod (not 50%, because the rods influence each other). Three rods will reduce to approximately 40% of a single rod. In practice, this means that if a single 1.2-metre rod gives 200 ohms, two rods correctly spaced should give approximately 120 ohms, and three rods approximately 80 ohms.

How deep should an earth rod be driven?

The deeper the earth rod, the lower the resistance — because the rod reaches more moist and compacted soil layers. Standard earth rods in the UK are available in 1.2-metre lengths that can be coupled together. A single 1.2-metre rod may be sufficient in moist, clay-rich soil. In dry or sandy soil, 2.4 metres (two coupled rods) or 3.6 metres (three coupled rods) may be needed. The rod should ideally reach permanently moist soil below the water table or seasonal variation zone. Typical resistance values range from 10 to 50 ohms in wet clay, 50 to 200 ohms in damp loam, 200 to 500 ohms in dry sandy soil, and 500 to 5000+ ohms in rock or gravel.

Does earth resistance change with the seasons?

Yes, significantly. Earth electrode resistance varies with soil moisture content, which changes seasonally. During wet winter months, soil moisture is high and earth resistance is at its lowest. During dry summer months, soil moisture reduces and earth resistance increases — sometimes by a factor of two or more. BS 7671 requires that the earth electrode resistance is adequate under the worst-case conditions (typically late summer). If you measure earth resistance during a wet period and obtain a marginal reading, it will almost certainly exceed the limit during dry weather. This is why building a margin of safety into your earth resistance values is important — a reading of 100 ohms in winter might rise to 300 ohms or more in summer depending on soil type and rod depth.

ESSENTIAL GUIDE

High Earth Resistance
Causes and Solutions

High earth electrode resistance compromises protective device operation and increases touch voltages. This guide covers why high readings occur, how soil conditions affect resistance, electrode types, practical methods to reduce RA values, and how to record results on certificates.

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14 min readUpdated 2026-06-10Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

1Earth electrode resistance (RA) must be low enough to ensure that the RCD protecting the circuit can disconnect within the required time. For a 30mA RCD, RA must not exceed 1667 ohms (RA x IΔn must not exceed 50V). In practice, values below 200 ohms are preferred for TT installations.
2High earth resistance is most commonly caused by dry, sandy, or rocky soil with high resistivity. Soil moisture content, mineral composition, temperature, and compaction all affect earth electrode resistance significantly.
3The three main electrode types used in UK installations are driven rods (most common), earth plates (for areas where rods cannot be driven), and earth mats or tapes (for shallow soil over rock). Each has specific advantages depending on soil conditions.
4Methods to improve earth resistance include driving rods deeper, using multiple rods in parallel, using chemical earth enhancement compounds, selecting a better soil location, and using earth plates or mats where rod driving is impractical.
5Elec-Mate includes an earth rod resistance calculator and captures RA values on all certificates. The app validates the RA x IΔn product against BS 7671 requirements and flags non-compliant values automatically.

01 · Essential Guide

What Is Earth Electrode Resistance?

This guide is written and maintained by qualified electricians holding City & Guilds 2391 (Inspection, Testing and Certification) and AM2 assessments, and reviewed against BS 7671:2018+A4:2026 using Elec-Mate's verified BS 7671 content. Regulation citations link directly to the relevant clauses of the current Wiring Regulations.

Earth electrode resistance (RA) is the resistance between an earth electrode (such as a driven rod) and the general mass of earth. It determines how effectively fault current can flow from the installation into the ground and back to the supply transformer, which is essential for the operation of protective devices.

In a TT earthing system — where the installation has its own earth electrode rather than using the supply company's earth — the earth electrode resistance is the critical factor in determining whether RCD protection will operate within the required disconnection time. The lower the RA value, the more effective the earth path and the faster the RCD will trip under fault conditions.

Earth electrode resistance is not the same as earth fault loop impedance (Zs). Zs is the total impedance of the earth fault loop, which includes the supply transformer impedance, the line conductor impedance, and the earth return path impedance. For TT installations, the earth electrode resistance is typically the dominant component of the earth fault loop impedance because the earth return path through the ground has much higher resistance than a metallic conductor.

BS 7671 Reg 411.5.3 — Precise Definition of Ra

BS 7671 Regulation 411.5.3 defines Ra as the sum of the resistances of the earth electrode and the protective conductor connecting it to the exposed-conductive-parts (in ohms). Ra is not simply the rod resistance alone — it includes the resistance of the earthing conductor between the rod and the main earthing terminal (MET). This distinction matters: a high-resistance connection or undersized earthing conductor will increase Ra above the rod resistance and may cause the RA × IΔn product to exceed 50 V even when the rod itself tests acceptably.

The earthing arrangement of an installation directly determines the significance of earth electrode resistance. For TN-S and TN-C-S systems, the supply company provides the earth path and RA is not relevant. For TT systems, RA is a critical measurement that must be tested and verified.

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02 · Essential Guide

Why High Earth Resistance Is Dangerous

High earth electrode resistance is dangerous because it limits the fault current that can flow under earth fault conditions. If the fault current is too low, the protective device (typically an RCD in a TT system) may not operate, or may operate too slowly, leaving the user exposed to electric shock for an extended period.

BS 7671 Regulation 411.5.3 requires that for TT systems, the product of the earth electrode resistance and the rated residual operating current of the RCD must not exceed 50V:

RA x IΔn ≤ 50V

Where RA is the earth electrode resistance (ohms) and IΔn is the rated residual operating current of the RCD (amps). For a 30mA RCD: RA must not exceed 50 / 0.03 = 1667 ohms. For a 100mA RCD: RA must not exceed 500 ohms.

The 50V limit ensures that the touch voltage across the earth electrode does not exceed 50V — the maximum extra-low voltage considered safe for prolonged contact. If the earth resistance is too high, the touch voltage under fault conditions exceeds this limit, and the RCD may not trip quickly enough to prevent a dangerous shock.

High RA Means Higher Touch Voltages

If a line-to-earth fault occurs on a TT installation with high earth resistance, the voltage appearing on the exposed metalwork of the faulty equipment can be dangerously high. The higher the RA, the higher the touch voltage and the greater the risk of fatal electric shock — especially in wet or outdoor locations where body resistance is reduced.

Reg 531.3.5.3.2 — Selecting the Right RCD: Table 53.1

The RA × IΔn ≤ 50 V formula (Reg 411.5.3) tells you whether a given RCD is safe for a measured Ra value. But Regulation 531.3.5.3.2 imposes a separate, normative requirement: when specifying an RCD for a TT system, the rated residual operating current (IΔn) shall not exceed the value corresponding to the maximum seasonal Ra as shown in Table 53.1. This means you must account for worst-case summer resistance (when soil dries out and Ra is highest), not just the value measured on the day. Table 53.1 examples:

Maximum Ra up to 50 ΩMax IΔn 1 A

Maximum Ra up to 167 ΩMax IΔn 300 mA

Maximum Ra up to 500 ΩMax IΔn 100 mA

Maximum Ra up to 1667 ΩMax IΔn 30 mA

Source: BS 7671:2018+A4:2026, Reg 531.3.5.3.2, Table 53.1

03 · Essential Guide

Causes of High Earth Resistance

High earth electrode resistance can be caused by a range of factors, most of which relate to the soil conditions at the electrode location or the physical condition of the electrode itself.

Dry soil — soil moisture is the single most significant factor affecting earth resistance. Dry soil has high resistivity because water provides the conductive ionic path between the electrode and the general mass of earth. During drought or extended dry periods, earth resistance can increase dramatically.
Sandy or gravelly soil — sand and gravel have poor moisture retention and high resistivity compared to clay or loam. Installations in coastal areas, heathland, or sandy subsoil regions often have challenging earth resistance conditions.
Rocky ground — rock has extremely high resistivity. In areas with shallow soil over rock (common in parts of Wales, Scotland, and the West Country), driving rods deep enough to reach low-resistivity layers may be impossible.
Insufficient rod depth — a rod that has not been driven deep enough will be in the zone of seasonal moisture variation. Deeper rods reach permanently moist soil and give more stable, lower readings.
Corroded electrodes — over time, earth rods can corrode, particularly in acidic or chemically aggressive soils. Corrosion reduces the effective surface area and increases contact resistance. Copper-clad steel rods resist corrosion better than bare steel.
Poor connections — loose, corroded, or inadequate connections between the earth electrode and the earthing conductor increase the total resistance. All connections must be tight, clean, and protected from corrosion. Clamp connections should be inspected during every periodic inspection.

04 · Essential Guide

Soil Conditions and Resistivity

Soil resistivity is the primary factor determining earth electrode resistance. Different soil types have vastly different resistivity values, which directly affect how easy or difficult it is to achieve an acceptable earth resistance.

Typical Soil Resistivity Values

Wet clay or marshy ground5 to 40 ohm-metres

Garden soil / loam10 to 150 ohm-metres

Chalk60 to 400 ohm-metres

Dry sand / gravel200 to 3000 ohm-metres

Rock (granite, sandstone)1000 to 100,000+ ohm-metres

Understanding the soil type at the installation site helps you plan the earthing strategy before arriving on site. If the property is on sand, gravel, or chalk, you may need to budget for multiple rods, longer rods, or alternative electrode types. Use the earth rod resistance calculator to estimate the expected resistance based on soil type and rod dimensions.

05 · Essential Guide

Earth Electrode Types

BS 7671 recognises several types of earth electrode, each suited to different installation conditions. The choice of electrode type depends on the soil conditions, available space, and the required earth resistance value.

Driven Rods

The most common electrode type for UK domestic installations. Copper-clad steel rods (typically 15.8mm or 19mm diameter) are driven vertically into the ground using a rod driving tool or SDS hammer. Available in 1.2m lengths that couple together for deeper installation. Cost-effective and efficient in most soil types except rock.

Earth Plates

Copper or galvanised steel plates buried horizontally in the ground. Used where rods cannot be driven due to rock or other obstructions. The plate provides a larger surface area in contact with the soil. Requires excavation and is more labour-intensive to install. Typically buried at least 600mm deep.

Earth Tapes / Mats

Bare copper tape or strip buried horizontally in a trench. Suitable for areas with shallow soil over rock. The tape is laid in a trench at least 500mm deep and covered with low-resistivity backfill material. Provides good contact area in shallow soil conditions. Can be arranged in radial or ring configurations.

In addition to purpose-built electrodes, BS 7671 also recognises the use of structural metalwork (such as building foundations) and the lead sheath of underground cables as supplementary earth electrodes. Note that Regulation 542.2.6 of BS 7671 imposes an outright prohibition: the metallic pipe of a water utility supply shall not be used as an earth electrode under any circumstances. Other internal metallic water pipework may only be used as an earth electrode where precautions against its removal have been taken and its suitability has been formally considered — and the water authority must be consulted beforehand.

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06 · Essential Guide

How to Improve Earth Electrode Resistance

If your earth electrode resistance measurement exceeds the acceptable limit for the installation, several methods are available to reduce it. The most effective approach depends on the specific cause of the high reading.

Methods to Reduce Earth Resistance

Drive the rod deeper — the most effective single action. Each additional 1.2m section coupled onto the rod reduces resistance by approximately 30 to 40 percent as the rod reaches moister, more compacted soil layers.
Use multiple rods in parallel — connect two or more rods together with a bare copper bonding conductor. Space the rods at least 2.5 times their driven depth apart. Two rods will reduce resistance to approximately 60% of a single rod value.
Use earth enhancement compound — proprietary low-resistivity compounds (such as bentonite or Marconite) can be packed around the earth rod to reduce contact resistance. The compound absorbs and retains moisture, providing a consistent low-resistivity zone around the rod.
Relocate the electrode — if the soil at the current location is unsuitable (rock, gravel, building rubble), moving the electrode to an area with better soil conditions (garden soil, clay, near a water course) can significantly reduce resistance.
Switch electrode type — if rods cannot be driven deep enough, switch to an earth plate or earth tape configuration that provides a larger surface area in the available soil depth.

Earth Rod Resistance Calculator

Elec-Mate's earth rod resistance calculator estimates the expected RA value based on soil type, rod dimensions, and number of rods.

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07 · Essential Guide

Earth Resistance Testing Methods

Earth electrode resistance is measured using specific test methods that determine the resistance of the electrode to the general mass of earth. The standard method is the fall-of-potential (3-point) method.

For a detailed step-by-step procedure, see the earth electrode test guide. The key principles are:

Fall-of-Potential Method

Disconnect the earth electrode from the installation earthing conductor (to ensure you are measuring only the electrode resistance, not the parallel path through the supply earth).
Drive two temporary test spikes into the ground in a straight line from the electrode under test. The current spike (C) is placed at a distance of at least 10 times the electrode depth (typically 20 to 30 metres). The potential spike (P) is placed at 61.8% of the distance from the electrode to the current spike.
Connect the instrument — connect the earth electrode to the E terminal, the potential spike to the P terminal, and the current spike to the C terminal.
Take the reading — the instrument injects a test current and measures the voltage, calculating the resistance in ohms.
Verify the reading — move the potential spike 10% closer and 10% further from the electrode and repeat the measurement. If all three readings are within 5% of each other, the result is valid. If not, increase the distance to the current spike and repeat.

Test Under Worst-Case Conditions

BS 7671 Regulation 531.3.5.3.2 requires that RCD selection for TT systems accounts for the maximum seasonal Ra, including soil drying and freezing. Wherever possible, earth electrode resistance should be measured after a prolonged dry period (late summer) to capture the worst-case value. If testing during wet conditions, note the soil conditions on the certificate and apply a seasonal uprating — a reading that appears acceptable in winter may exceed the limit during a dry summer. A common site failure is measuring in favourable wet conditions without recording that the reading is not worst-case.

Most modern multifunction testers include an earth electrode resistance function, and dedicated earth resistance testers are available from manufacturers such as Megger and Fluke. The instrument must comply with BS EN 61557-5.

08 · Essential Guide

Recording Earth Resistance Values

Earth electrode resistance values are recorded on the electrical installation certificate (EIC) or the electrical installation condition report (EICR) in the section for supply characteristics and earthing arrangements.

Legal Mandate to Measure and Record RA

BS 7671 Regulation 643.7.3 (Part 6 — Inspection and Testing) requires that where the earthing system incorporates an earth electrode, the electrode resistance to Earth (RA) shall be measured as part of initial verification. This is not optional — it is a normative test requirement for TT installations. GN3 Regulation 5.9 and OSG Regulation 7.28 further require that the EICR records details of the installation earth electrode (type, location, and measured RA) including any relevant soil conditions at the time of test.

For TT installations, the following information should be recorded:

Earth electrode type (rod, plate, tape, etc.)
Earth electrode location
Measured earth electrode resistance (RA) in ohms
RCD rating (IΔn) in mA
Confirmation that RA x IΔn does not exceed 50V

The EICR should also note any seasonal factors that may affect the reading. If the test was performed during a wet period, note that the reading may increase during dry weather and that periodic retesting during summer is recommended.

Earth Resistance on Every Certificate

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High Earth ResistanceCauses and Solutions

Key Takeaways

What Is Earth Electrode Resistance?

BS 7671 Reg 411.5.3 — Precise Definition of Ra

Get the BS 7671 A4:2026 Cheat Sheet — free

Why High Earth Resistance Is Dangerous

RA x IΔn ≤ 50V

High RA Means Higher Touch Voltages

Reg 531.3.5.3.2 — Selecting the Right RCD: Table 53.1

Causes of High Earth Resistance

Soil Conditions and Resistivity

Typical Soil Resistivity Values

Earth Electrode Types

Driven Rods

Earth Plates

Earth Tapes / Mats

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How to Improve Earth Electrode Resistance

Methods to Reduce Earth Resistance

Earth Rod Resistance Calculator

Earth Resistance Testing Methods

Fall-of-Potential Method

Test Under Worst-Case Conditions

Recording Earth Resistance Values

Legal Mandate to Measure and Record RA

Earth Resistance on Every Certificate

Frequently Asked Questions

What is an acceptable earth electrode resistance value?

Why is my earth resistance reading too high?

How do I measure earth electrode resistance?

Can I use multiple earth rods to reduce resistance?

How deep should an earth rod be driven?

Does earth resistance change with the seasons?

Continue reading

Earth Electrode Test Guide

Earth Rod Resistance Calculator

Earthing Arrangements Guide

Ze Values UK

RCD Types Explained

BS 7671 18th Edition

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Trusted by electricians across the UK

Calculate and Validate Earth Resistance On Site

High Earth Resistance
Causes and Solutions