An earth pit cannot be judged by looking at it. The only way to know it works is to measure its resistance. This guide explains the standard test, what can make a reading wrong, and how often to test. It is written for facility managers and contractors; the test itself should be done by a qualified electrician with a proper earth tester.
What you need
- An earth resistance tester, either a three-terminal or four-terminal instrument.
- Two auxiliary spikes and the test leads supplied with the tester.
- Access to the earth pit and its test point, which is why every pit should have a cover rather than being buried.
- A record sheet, or last year’s readings for comparison.
The fall-of-potential method
This is the most widely used test for a single electrode or a small group of electrodes.
- Disconnect the electrode under test from the installation at the test link, so other earth paths do not affect the reading. Make sure this is safe to do first, and reconnect it afterwards.
- Drive the current spike into the ground in a straight line away from the electrode. It must be well outside the electrode’s area of influence; a common starting distance is several times the electrode length.
- Drive the potential spike on the same line, at about 62% of the distance to the current spike.
- Connect the tester as its instructions show and take the reading.
- Check the reading by moving the potential spike a short distance towards and away from the electrode and measuring again. If the three readings agree closely, the result is reliable. If they differ a lot, move the current spike further out and repeat.
What can make a reading wrong
- Spikes too close: the most common error. It produces a reading that looks better than it really is.
- Buried metal nearby: pipes, cables or other earth pits along the test line can distort the result. Choose a line away from them.
- Not disconnecting the electrode: you end up measuring the whole installation in parallel, not the pit itself.
- Weather: a reading taken after heavy rain will be lower than one taken in the dry season. Note the conditions every time.
Other test methods
A clamp-on earth tester measures without disconnecting the electrode, which is convenient on sites with many interconnected pits, but it only works where the system offers other return paths. For new projects, the Wenner four-pin method measures the soil itself before any electrode is chosen. Large grids and substations need specialised methods set out by the designer, following standards such as IEEE 81.
What value is acceptable?
There is no single number for every installation. The acceptable value is set by the designer or the specification, with IS 3043 as the Indian reference. Lightning protection, substations, telecom and data installations usually need lower values than a single building. If you do not know your target, ask whoever designed the system before deciding a pit has failed.
How often to test
- At handover, for every pit, as the baseline.
- At least once a year, preferably in the driest part of the year, when resistance is highest.
- After any work that could disturb the pits or conductors, and after a known lightning strike on the building.
Keep a simple record
For each pit, note its location and number, the date, the weather, the method, the reading and who tested it. A reading that rises steadily year on year warns you long before the pit fails. Our earlier guide on choosing an earthing method explains why some pits drift more than others.
Frequently asked questions
Can earth resistance be tested with a multimeter?
No. A multimeter or a test lamp can show whether there is a connection, but it cannot measure the resistance of an earth electrode to the ground. That needs an earth resistance tester and auxiliary spikes.
Why does the reading change from one test to the next?
Moisture in the soil changes with the weather and the season, and the position of the test spikes affects the result. Note the conditions and use the same method each time, so readings can be compared fairly.
Who should test the earthing?
A qualified electrician or earthing contractor with a calibrated earth tester. Disconnecting an electrode from a live installation needs care, so it should not be done by someone without training.
We test existing installations and install new pits where readings are too high. See our testing and maintenance service, or book a test.