How to Measure Earth Resistance: Step-by-Step Guide
Measuring earth resistance correctly is essential for electrical safety and compliance. This step-by-step guide shows you how to measure earth (ground) resistance accurately using both the fall-of-potential and clamp-on methods.
What You’ll Need
An earth resistance tester (such as the Marmonix MEG825 or MEG725), auxiliary stakes and leads for the fall-of-potential method, and basic PPE.
Step-by-Step
- Choose the test method — 3-pole for isolated electrodes; clamp-on for connected systems.
- De-energise and disconnect the electrode for fall-of-potential testing.
- Position the stakes — current stake 30–50 m away, potential stake at ~62%.
- Connect the tester to E/P/C terminals, or clamp the conductor for stakeless testing.
- Run the measurement and use the flat region of the fall-of-potential curve.
- Record and compare the result against your target (≤25 Ω general, ≤5 Ω sensitive sites).
Tips for Accurate Results
Test in representative soil conditions, keep leads untangled, and repeat periodically to track trends. If resistance is too high, drive the rod deeper, add bonded rods, or use conductive backfill, then retest.