Every TT installation depends on its earth electrode. If the resistance is too high, the RCD cannot disconnect the supply fast enough to prevent injury. This guide covers the fall of potential method, acceptable values, instrument setup, and how to record results on your EICR or EIC.
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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.
Disconnect the electrode from the installation earthing conductor, then use the fall of potential (3-pin) method: drive a current spike well clear of the electrode and a potential spike at 62% of that distance, and read RA in ohms. Under BS 7671 Regulation 411.5.3, RA x IΔn must not exceed 50 V; for a 30 mA RCD that is 1667 ohms maximum, but aim well below 200 ohms for stability.
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An earth electrode is a conductor (usually a copper-clad steel rod) driven into the ground to provide a connection between the electrical installation's earthing system and the general mass of earth. It is the foundation of the TT earthing system, where the electricity supplier does not provide an earth terminal and the installation must create its own earth path.
TT systems are common in rural areas, overhead supply lines, older properties, and installations where the supplier's earth is unreliable or has been removed. They are also used for temporary installations, construction sites, and caravan parks. In a TT system, the earth fault current must flow through the soil to reach the supply transformer neutral — and the resistance of that path depends entirely on the earth electrode.
If the earth electrode resistance is too high, protective devices (particularly RCDs) may not operate fast enough to clear a fault within the disconnection times required by BS 7671. This means the installation is unsafe. Testing the earth electrode resistance is therefore a critical part of both initial verification and periodic inspection for any TT system.
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Earth electrode resistance must be tested in the following situations:
For TN-S and TN-C-S systems, earth electrode testing is not normally required because the earth path is provided by the supply network (the cable sheath or combined neutral-earth conductor). Earth electrode testing is specifically a TT system requirement.
The fall of potential method is the standard technique for measuring earth electrode resistance. It is described in GN3 (Guidance Note 3: Inspection and Testing) and is the method required by BS 7671 for accurate electrode resistance measurement. Here is how it works:
The 62% rule is based on the mathematics of the voltage gradient around a hemisphere electrode in uniform soil. At 62% of the distance between E and C, the potential spike sits in the "flat zone" of the voltage curve where the reading is least sensitive to exact spike placement. This gives the most accurate result.
The 3-pin method is another name for the fall of potential technique described above. The "3 pins" refer to the three connections: the earth electrode under test (E), the potential spike (P), and the current spike (C). Some texts call it the "3-terminal method" or "3-pole method" — they all describe the same measurement.
The key difference between the 3-pin method and simpler 2-wire measurements is accuracy. A 2-wire measurement (connecting the instrument between the electrode under test and a single remote spike) includes the resistance of both the electrode and the test spike in the reading. The 3-pin method separates the measurement: the current path goes through E and C, while the voltage measurement is taken independently at P. This eliminates the test spike resistance from the reading.
Site access tip: On confined sites where you cannot achieve 30 metres between E and C, try to get the maximum distance possible and use the 52%/62%/72% verification readings to check accuracy. If the three readings do not converge, note the limitation on the EICR and record the best reading you could achieve with a note about the constraint.
Some modern instruments also offer a "stakeless" or "clamp-on" method that does not require test spikes at all. This works by injecting a signal through a clamp placed around the earthing conductor and measuring the return current through an existing parallel earth path (such as a metallic water pipe). The stakeless method is useful for quick checks on existing systems, but it is not a substitute for the 3-pin method when commissioning a new electrode.
Probes in hand? Speak your earth electrode reading — 'RA 47 ohms' — and Elec-Mate fills in the schedule of test results.
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The maximum acceptable earth electrode resistance depends on the type of protective device and its rated residual operating current. The fundamental condition for TT systems comes from BS 7671 Regulation 411.5.3:
RA x IΔn ≤ 50 V
Where RA is the sum of the resistances of the earth electrode and the protective conductor connecting it to the exposed-conductive-parts (in ohms), and IΔn is the rated residual operating current of the RCD (in amps). 50 V is the touch voltage limit for normal dry conditions. Regulation 411.5.3 also confirms the requirement is met where the earth fault loop impedance meets Table 41.5.
BS 7671 Table 41.5 — non-delayed and time-delayed ‘S’ type RCDs to BS EN 61008-1 / 61009-1, U0 = 230 V. Disconnection within the times of Table 41.1.
* Note 2 to Table 41.5: the resistance of the installation earth electrode should be as low as practicable — a value exceeding 200 Ω may not be stable (see Regulation 542.2.4). So although the arithmetic limit for a 30 mA RCD is 1667 Ω, the practical target is far lower.
While the tabulated maximums look generous (especially for 30 mA RCDs), there are good reasons to aim much lower. An earth electrode with RA comfortably below 200 ohms provides a margin of safety and keeps the reading stable across the seasons. Electrodes in the range of 20 to 100 ohms are common in clay and loam soils; sandy, rocky, or chalk soils may produce higher values that need longer electrodes or multiple rods in parallel. Regulation 542.2.4 also requires the type and embedded depth of the electrode to be chosen so that soil drying and freezing will not raise its resistance above the required value.
Lowest and most stable resistivity. A single 1.2 m rod will often read well within limits and hold its value through dry spells.
Higher resistivity and drains quickly, so readings swing with the weather. Expect to need a longer rod or two rods in parallel.
Highest resistivity and hardest to drive into. Multiple electrodes, deep driving, or an earth mat with conductive backfill may be required.
Space rods at least twice their driven length apart — closer than that and the resistance zones overlap, so the second rod adds little benefit.
Use the earth loop impedance calculator in Elec-Mate to check whether your measured RA value meets the disconnection time requirements for each circuit.
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Try the certificate tools freeSetting up the earth electrode tester correctly is essential for accurate results. Here is the procedure for common instruments:
Select the 3-pole earth test mode. Connect the green lead (E) to the electrode under test. Connect the yellow lead (P) to the potential spike. Connect the red lead (C) to the current spike. Set the test frequency to avoid mains interference (the instrument auto-selects on later models). Press TEST and wait for a stable reading. The instrument displays RA in ohms. Check the noise indicator — if mains interference is high, the reading may be unreliable.
Select 3-pole mode on the rotary switch. Connect H (current) to the current spike, S (potential) to the potential spike, and E (earth) to the electrode under test. The Fluke 1625-2 also supports stakeless testing using the included clamp accessories. For the 3-pole method, press TEST and read the resistance in ohms from the display. The instrument filters out mains noise automatically.
A compact and affordable dedicated earth tester. Select the 3-wire mode. Connect the E, P, and C leads to the electrode, potential spike, and current spike respectively. Press MEASURE. The display shows resistance in ohms with automatic range selection. The KEW4105A also supports 2-wire simplified measurement for quick checks.
Before testing, check that the test leads are in good condition, the spikes are clean (free from paint or corrosion), and the soil at the spike locations is damp enough to make contact. Dry, hard ground may give erratic readings — pour a small amount of water around each spike to improve soil contact if needed.
Earth electrode testing seems straightforward, but several common mistakes can produce inaccurate or misleading results:
If you get an unexpectedly high or unstable reading, work through these checks systematically before recording the result. An accurate earth electrode reading is essential for confirming that the TT system is safe.
The earth electrode resistance value (RA) is recorded on the schedule of test results for the EICR or EIC. It should be entered in the "Earth electrode resistance" field in the supply characteristics section of the certificate.
Regulation 643.7.2 (Chapter 64 of BS 7671) requires that, where the earthing system incorporates an earth electrode, the electrode resistance to Earth is measured — this verifies TT system compliance with Regulation 411.5. Where measuring RA is not practicable, the standard permits the measured external earth fault loop impedance to be used instead. When recording the result, note the following:
If the earth electrode resistance exceeds the maximum permitted value for the installed protective devices, record an observation code. A dangerously high RA that prevents disconnection within the required time is a C2 (Potentially Dangerous) defect. The remedial action is to reduce the electrode resistance (longer rod, additional electrodes, soil treatment) or install additional RCD protection.
Record earth electrode results, Zs values, IR readings, and RCD trip times — all by voice. The schedule of test results fills itself in while you work.
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