Testing earth resistance on an operational transmission tower is not the same job as testing a substation earth electrode or a building earth rod. The overhead ground wire changes the measurement entirely. Most engineers who have not worked specifically on pylon networks discover this when their first set of readings comes back looking plausible but wrong, and they cannot work out why.

This article covers what the UK standards actually require, why the standard Fall of Potential method fails on live towers, and what equipment and approach produces readings you can stand behind.

The 40-Ohm Limit and Which Standards It Comes From

The primary resistance limit in the UK sits at 40 ohms. This comes from ENA Engineering Recommendation G12 Issue 4 (Amendment 1, 2015), which covers the earthing of electrical distribution and transmission infrastructure. Your tower footing readings need to come in under that figure. If they do not, that is a compliance failure and a documented one if you have a testing record.

The test methodology sits under BS 7430:2011+A1:2015, the Code of Practice for Protective Earthing. BS 7430 specifies the Fall of Potential (FoP) method and the Wenner four-pin method for soil resistivity, with readings taken to a depth of 60 metres where the site conditions allow. For towers that also form part of a lightning protection system, BS EN IEC 62305 (updated 2024) requires electrical testing at a minimum of once every 12 months. Higher-risk sites need more frequent checks. ENA TS 41-24:2018 covers design, installation, testing, and maintenance of earthing systems more broadly, and EN 50522 is the HV-specific earthing standard for installations above 1kV AC.

Why the Fall of Potential Method Fails on Live Pylon Networks

The FoP method works by injecting current between the earth electrode under test and a remote current electrode driven into the ground some distance away. A potential electrode placed between them measures the voltage drop, and from that the instrument calculates resistance. On an isolated earth electrode, this gives reliable results. On a transmission tower with an overhead ground wire still connected to the rest of the pylon network, it does not.

The overhead ground wire provides an alternative current path. The current you inject does not stay in the soil directly around the tower footing. Some of it disperses into the interconnected pylon network instead. The voltage reading you measure reflects the combined impedance of the tower footing and the broader network behind it. The tower can appear to pass when its footing resistance is actually out of tolerance.

The second problem is the test frequency. Standard FoP testers operate at frequencies close to 50Hz, chosen to minimise interference from the mains supply. On a live transmission tower with an energised overhead ground wire, those frequencies pick up induced voltages from the power system. The noise floor rises and the measurement becomes unreliable. You are effectively trying to hear a quiet signal in a loud room at the same pitch as the noise.

How Frequency-Based Testing Solves This

The approach described in EN 50522:2011, referenced in IEEE 81 and the CIGRE guidance on measuring earth resistance of towers equipped with earth wires, is to test at a frequency high enough that the overhead ground wire's impedance rises to the point where it no longer provides a meaningful parallel current path. At sufficiently high frequencies, the current stays in the soil around the tower footing rather than travelling up into the network. The reading reflects the tower footing alone.

This is where specialist equipment becomes necessary. The Chauvin Arnoux CA6472 Earth and Resistivity Tester analyses frequency behaviour from 41Hz to 5,018Hz. In automatic mode it selects the optimal test frequency based on the site conditions. In expert mode the engineer sets it manually. Either way, the result accounts for what is actually happening at the tower rather than producing a figure that looks clean on paper but misrepresents the earthing system.

The CA6472 does not handle tower testing alone. For pylon networks specifically, it works in combination with the CA6474 Tower Tester as part of the CA6474 Tower Test Kit. The kit includes the CA6472, the CA6474, and the earth probe set needed for the measurement geometry a lattice tower requires. The CA6474 handles the pylon-specific measurement configuration; the CA6472 handles the frequency analysis and resistivity work.

Soil Resistivity and Why Your Readings Will Vary

Tower footing resistance is not a fixed number. The same tower tested in February and then again in August will often produce different readings. Soil resistivity changes with moisture content and temperature, and the effect is significant enough to push a borderline tower above or below the 40-ohm limit depending on the season. Clay soils show the most variation. Sandy and gravelly soils tend to be more stable but often have inherently high resistivity, which pushes tower footing resistance up regardless of conditions.

BS 7430 requires soil resistivity measurement at multiple depths to build a profile of the soil column down to 60 metres where practicable. The CA6472 supports both the Wenner and Schlumberger four-pin methods for this. Running a resistivity profile alongside your tower footing readings gives you a more defensible record, because it explains why the reading is what it is and helps predict whether seasonal variation is likely to cause a compliance problem between scheduled test cycles.

Data Logging and What an Audit Trail Actually Needs

ENA TS 41-24:2018 implies documented test records as part of any credible earthing maintenance regime. If an incident occurs on a pylon network and your test records are incomplete, that gap does not stay theoretical for long. Readings logged by hand and transcribed into a spreadsheet are harder to defend than records produced directly by the instrument, particularly if there is any question about which tower was tested, when, and under what conditions.

The CA6474 Tower Test Kit stores up to 512 test results internally, with USB optical transfer to PC and Chauvin Arnoux's Ground Tester Transfer software for data management. The record includes the reading, the test parameters, and the date. That is the format an audit trail actually needs, not a figure written next to a tower reference in a site notebook.

If your work covers a broader range of earth electrode types across different site categories, the CA6472 Earth and Resistivity Tester is also available as a standalone instrument. For questions about calibration of your testing equipment and when it is required, the GNW calibration guide covers the key considerations.

Browse the full electrical installation testers range for other Chauvin Arnoux instruments stocked by GNW Instrumentation. View the Chauvin Arnoux CA6474 Tower Test Kit or contact us for more expert guidance.