What is earth-resistance testing?
A check that an earth electrode makes a good connection with the ground, using a tester and two temporary probes, so a fault current has a low-resistance path to earth. The value becomes the baseline for later tests.
Earthing only protects people and equipment if the earth electrode makes a good connection with the ground. Earth resistance is the resistance between the electrode and the general mass of earth. The lower it is, the more easily fault current finds its way into the ground, and the lower the touch voltage on equipment at the surface.
It is measured with an earth-resistance tester using the fall-of-potential method. The tester drives an AC test current between the electrode under test and a current probe, and a potential probe, about 62% of the way along the line, reads the voltage drop. AC is used because DC would polarise the soil and give a false value. The electrode is disconnected at its test link so that only it is measured. Never test during a thunderstorm, and keep the loop de-energised and isolated while the earthing is open.
- Prepare. Take the design value and the specified limit, check the tester is in calibration, and choose a clear line away from buried cables, pipes and other earth electrodes.
- Disconnect. With the loop isolated and locked out, open the test link so that only the electrode under test is measured, never the whole network in parallel.
- Set the probes. Drive the current probe in line at the spacing the tester instructions give, and the potential probe about 62% of the way along. Both go into undisturbed soil, and the leads are kept apart.
- Measure. Run the test and read the resistance in ohms.
- Check. Move the potential probe a little each side and repeat. The readings should agree. If they do not, the probes are too close, so increase the spacing.
- Restore and record. Remove the probes, close the test link, and log the value, date, probe spacing, soil and weather as the baseline.
What the readings suggest
At or below the design value
The electrode is effective. Record it as the baseline for later tests.
Higher than specified
Suspect dry or stony soil, a rod that is too short, poor contact, or a loose or corroded connection. Improve the earth, then re-test.
Readings disagree as P moves
The probes are too close or the line passes buried metalwork. Increase the spacing or test along a different line.
Noisy or unstable
Stray currents or nearby cables are interfering. Change the line, keep the leads apart, and repeat.
Varies with the season
Wet soil lowers the value and dry soil raises it. Note the conditions so tests are compared like with like.
Always record
Log the pit, value, probe spacing, date, soil and weather, and instrument. Later tests are judged against this figure.
Frequently Asked Questions
Q1What is earth-resistance testing in AGL commissioning?
It measures the resistance between an earth electrode and the general mass of earth, using an earth-resistance tester and two temporary probes. It shows whether the electrode is effective, so that fault current has a low-resistance path into the ground, and it sets the baseline for later tests.
Q2How does the fall-of-potential method work?
The tester drives an AC current between the electrode under test and a current probe placed some distance away. A potential probe in between measures the voltage drop caused by that current, and the tester works out the resistance from the voltage and the current.
Q3Why is the potential probe placed at about 62%?
Along the line between the electrode and the current probe, the measured value levels out near 62% of the distance, which gives the true electrode resistance. Moving the probe a little each side should give the same reading. If it does not, the spacing is too small.
Q4Why is AC used instead of DC?
DC passed through soil polarises it, producing a false reading. The tester uses an AC test signal, usually at a frequency that avoids interference from mains and stray currents.
Q5Why is the electrode disconnected before the test?
If the electrode stays connected to the rest of the earthing, the tester measures the whole network in parallel and hides a poor individual electrode. The electrode is opened at its test link, with the loop isolated, so that only it is measured.
Q6What resistance value is acceptable?
It depends on the design, the soil and the project specification, so this page gives no fixed figure. Use the value stated in the specification, and compare the result with the design value and with earlier records.
Q7Why do readings change through the year?
Soil moisture and temperature change how well the ground conducts. Wet soil lowers the resistance and dry or frozen soil raises it. Record the conditions with every test, and compare results from similar conditions.
Q8How does this differ from insulation-resistance testing?
Insulation-resistance testing checks that the cable and transformers do not leak to earth, and a high value is good. Earth-resistance testing checks the earth electrode itself, and a low value is good. Both are needed before energisation.
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