Earth Resistance Testing: How to Do It and What the Numbers Mean

Why the reading moves

Earth resistance is not a fixed property of the installation — it is a property of the soil around it, and soil changes. The same pit that reads 3 ohms in August can read 12 ohms in May. This is normal and it is why testing should be done in the dry season, when the reading is at its worst. A pit that passes in the monsoon and fails in summer was never actually compliant.

Three-pin and four-pin testing

The three-pin fall-of-potential test measures a single electrode: the electrode under test, a current spike well away from it, and a potential spike between them. The four-pin Wenner test measures soil resistivity rather than an installed electrode, and is what you use before designing the system. Both need the spikes far enough away that their resistance zones do not overlap — placing them too close is the most common cause of a wrong reading.

Disconnect before you measure

If the electrode is still bonded to the rest of the earthing system you are measuring the whole network in parallel, not the electrode, and the answer will look better than reality. Isolate at the test link first. This is exactly why test links are installed and exactly why they are so often left out of the installation.

What the numbers should be

Under 5 ohms is a common general requirement. Substations and sensitive equipment often specify under 1 ohm. Lightning protection earths are typically specified under 10 ohms. Check the requirement in your drawing or the applicable standard before deciding whether a reading has passed — the target varies by application, not by habit.

When a pit fails

First check the joint and the conductor, because a corroded connection reads as a failed earth far more often than a failed electrode. If the electrode itself has degraded, options are adding compound around a new bore, driving a coupled extension deeper, or installing an additional bonded electrode at proper spacing. Pouring salt into a failing pit is a short-term reading improvement that accelerates corrosion — it makes the underlying problem worse.

Keep the records

An annual reading logged against the same pit tells you whether the installation is degrading and roughly how fast. A single reading tells you almost nothing. For facilities under audit, that record is also what proves compliance.