Most of the earth measurements taken on South African sites trace back, whether the person holding the meter knows it or not, to one document: IEEE Std 81. It is the guide for how you measure the ground — and it has just been revised for the first time in over a decade. If your field practice is built on the previous edition or on whatever your instrument does by default, this is the moment to check that the numbers you sign for still mean what you think they mean.
What IEEE 81 actually governs
It helps to be clear about what this standard is and is not. IEEE 81 is not a design code — it does not tell you how big your earth mat should be. It is the measurement guide beneath the design codes: how to measure earth resistivity, how to measure the ground impedance of an installed system, and how to measure the earth surface potentials — touch and step voltages — that decide whether a grounding system is safe. When a SANS 10142-1 or SANS 10313 compliance decision rests on an earth measurement, IEEE 81 is usually the method behind it. That is why the edition you work to matters.
What the 2025 revision brings into focus
Two themes stand out in the revision, and both reward attention:
- Transient (surge) impedance to remote earth. The revised guide gives explicit treatment to measuring how a grounding system behaves under a fast transient — not just its steady, low-frequency resistance.
- The factors that distort a measurement. It reviews, more fully, the many things that can quietly corrupt a reading — so you can recognise and measure around them rather than trust a clean-looking but wrong number.
Transient impedance: the number a lightning strike actually sees
Here is the practical point behind the first theme. A grounding system's resistance, measured slowly, is not the same as the impedance it presents to a lightning surge. A fast transient has high-frequency content, and a long buried electrode has inductance; under a strike, current does not distribute the way it does at 50 Hz. So a system with a comfortable low-frequency resistance can still present a higher impedance to the very event — the lightning strike — that the earthing was installed to handle. For any site where lightning is the design driver, treating the steady resistance as the whole story understates the risk.
The factors that quietly distort a reading
The second theme is the unglamorous heart of good measurement. A fall-of-potential result depends on getting the probe spacing right and the current probe far enough out to reach the plateau — the flat region of the curve where the reading reflects the true resistance rather than the influence of the probes. Get that wrong and the number is confidently incorrect. The same is true of buried metalwork and parallel paths that offer the current another route, of coupling between test leads, of stray currents in industrial ground, and of soil that reads very differently wet and dry. IEEE 81's value is that it names these effects, so a competent engineer measures around them instead of being fooled by them.
Touch and step voltage — the life-safety output
The third leg of the guide is earth surface potentials, and it is the one with lives attached. Ground potential rise and the touch and step voltages that follow are the direct safety output of an earthing design, yet they are too often assumed from a calculation rather than verified by measurement. Measuring them properly — to the method in the current guide — is how you show the design is not just adequate on paper but safe in the ground.
Fitting it to SANS practice
None of this displaces your SANS obligations — it strengthens them. SANS 10142-1 and SANS 10313 tell you what compliant looks like; IEEE 81 is how you measure to find out. Working to the current edition of the measurement guide keeps the numbers underneath your compliance certificates current too. An assessor, an insurer or a court asking how a value was obtained is far better answered with “to the 2025 guide, here is the method” than with a reading whose provenance stops at the instrument.
A new edition of the guide is not paperwork. It is a standing invitation to check that the way you measure the ground still produces numbers you would be willing to defend.
This article is a practitioner's orientation, not a substitute for the standard. Measure to the current published IEEE 81 and the applicable SANS requirements.