How to Measure Earth Resistance: Step-by-Step Guide
Measuring earth resistance correctly confirms that an earthing system will safely carry fault current and protect people and equipment. This step-by-step guide explains how to measure earth resistance using the standard methods, what a good reading looks like, and how to get reliable results. For the right instrument, see the MarMonix earth resistance testers.
What You Are Measuring
Earth resistance is the resistance between an earth electrode and the general mass of earth. A low value lets fault current flow safely to ground and lets protective devices operate quickly; a high value is a shock and fire risk.
Method 1: 3-Point Fall-of-Potential Test (Most Accurate)
- Isolate the installation and confirm it is safe to work on.
- Disconnect the earth electrode under test from the installation.
- Place two auxiliary spikes in a straight line — the current spike farthest away, the potential spike around 62% of that distance.
- Connect the tester terminals to the electrode and the two spikes.
- Run the test and record the reading.
- Confirm stability by repeating with the potential spike moved slightly; a stable reading means a valid result.
- Reconnect the earth electrode immediately after testing.
Method 2: Clamp-On (Stakeless) Test
A clamp earth tester measures loop resistance by clamping around the earth conductor — no spikes or disconnection needed. It is fast and ideal for multi-earthed systems, though it requires a complete return path through other earths to work.
What Is a Good Reading?
Lower is better. Many installations target 5 Ω or less, while sensitive sites aim lower still. Always compare against the requirement for your specific installation and standard.
Tips for Accurate Results
- Drive auxiliary spikes firmly into moist, undisturbed soil.
- Keep test leads spread out to avoid coupling.
- Test in normal (not unusually wet or dry) soil conditions where possible.
- Use a tester with good noise rejection on live industrial sites.
Frequently Asked Questions
What value should earth resistance be?
As low as practical — many installations target 5 Ω or lower; follow your site’s specific requirement.
Can I measure earth resistance without disconnecting?
Yes, with a clamp-on (stakeless) earth tester on multi-earthed systems.
Why does my reading keep changing?
Usually poor spike contact or spacing; firm the spikes into moist soil and confirm a stable reading by moving the potential spike slightly.