Earthing (grounding) is one of the most critical safety measures in an electrical installation, even though it is rarely visible. It lets fault current flow safely into the ground, keeps touch voltages from reaching dangerous levels and allows protective devices (residual-current devices, fuses) to operate in time. But earthing is not something you install once and forget: soil moisture, corrosion and loose connections change its resistance over time.
Why measure it?
- Safety of people: High earthing resistance can cause dangerous voltages on metal enclosures during a fault.
- Protective devices: If the fault current is too small, the protective device may not trip.
- Equipment and data integrity: Industrial automation, communications and sensitive electronics depend directly on good earthing.
- Regulatory compliance: Earthing regulations set measurement and periodic inspection obligations. Check the current regulations and your project specification for valid limits and intervals.
How is it measured?
The most common method is fall-of-potential, using the earth electrode under test and two auxiliary electrodes (current and voltage probes):
- Disconnect the electrode under test from the installation as safety rules allow, and connect it to the meter.
- Drive the current probe far enough from the electrode.
- Place the voltage probe between the electrode and the current probe, typically around 62% of the distance between them.
- The instrument injects a known current and calculates resistance from the voltage drop.
- For confidence, move the voltage probe a little to two or three other positions and repeat; if the readings agree closely, the measurement is reliable.
Clamp-on testers can also help assess connections and the integrity of an earthing network. They are practical where closed loops exist, but they cannot replace the classical method in every case.
Interpreting the results
- Compare readings in dry and wet seasons; seasonal variation is normal.
- Keep auxiliary probes away from metal pipes, cables and other earth electrodes.
- Record results with date, weather/soil condition and instrument details; the trend over time says more than a single value.
- If the value exceeds the limit set in the project or regulations, improve the electrode: add rods, increase depth, use soil-improving material or check connections.
Conclusion
Regular earthing measurement is a cheap and quick check, while neglecting it can cost lives and equipment. Have measurements done by competent people, keep records, and re-measure after any change to your installation.
This article is for general information; for any specific installation, the project, regulations and a competent person's assessment prevail.