There's no room for guesswork in earthing: earth resistance is measured with dedicated instruments and standard methods, and is repeated periodically because soil is a constantly changing medium. Periodic measurement is part of earthing system maintenance, no less important than its initial installation.
The Principle: Dedicated Instruments, Not an Ordinary Ohmmeter
Earth resistance is not measured with an ordinary ohmmeter, but with dedicated earth testers that inject an alternating current at a distinctive frequency into the ground and measure the resulting voltage drop, calculating resistance while rejecting stray earth currents and noise that would corrupt a regular measurement.
The Classic Method: Fall-of-Potential (Three-Electrode Method)
- The instrument is connected to the earth electrode to be measured (after isolating it from the network following a safe procedure).
- An auxiliary current electrode is driven at a sufficient distance (tens of meters), and the instrument injects current between it and the electrode under test.
- An auxiliary potential electrode is driven between them (at roughly 62% of the distance in the well-known method), and the instrument measures the voltage drop at that point.
- Resistance = Voltage ÷ Current, and the position is validated by slightly moving the potential electrode: a stable reading confirms the measurement.
Newer methods also exist: clamp-on testers for multi-path systems without disconnection, and selective measurement instruments — each has validity conditions that must be understood before trusting its readings.
Why Is Periodic Measurement a Must?
- Soil conditions change with the weather: moisture and temperature vary across seasons, and so does resistance — see Influencing Factors.
- Connections corrode and conductors may be damaged by excavation or theft — only measurement reveals this early.
- Document every measurement with its date and conditions (moisture, season) in the system record to track trends.
Disconnecting an earth electrode from an in-service system for measurement is a sensitive procedure: a ground fault during the disconnection makes the conductors hazardous. Follow approved isolation procedures, and use no-disconnect measurement methods where their conditions apply.
Sample answer: Because earth resistance is not constant: soil conditions change with the weather — moisture fluctuates between seasons, raising resistance during dry periods — and connections corrode over time while buried conductors may be damaged by excavation. Periodic measurement using dedicated instruments, with date and condition documentation, is what reveals degradation early and ensures the system remains capable of performing its protective function — it is part of earthing system maintenance, no less important than its initial installation.
Placing the auxiliary measurement electrodes too close to the electrode under test or to each other. Overlapping zones of influence give a falsely low, deceptively reassuring reading — adequate spacing and verification by moving the potential electrode are conditions for a correct measurement.
Want to understand substations step by step?
Follow trainer Fahad Refai's Substations and Electrical Maintenance courses — a practical walkthrough from maintenance fundamentals to SCADA systems.
Browse Fahad Refai's Courses